Tests for Dysautonomias

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Tests forDysautonomias

There are four types of tests doctors use to diagnosedysautonomias—physiological, neuropharmacologic,neurochemical, and neuroimaging.

Physiological tests involve measurements of a bodyfunction in response to a manipulation such as standing,tilt-table-testing, or a change in room temperature.

Neuropharmacologic tests involve giving a drug andmeasuring its immediate effects.

Neurochemical tests involve measuring levels of bodychemicals, such as the catecholamines, norepinephrineand epinephrine, either under resting conditions or inresponse to physiological or neuropharmacologicmanipulations.

Neuroimaging tests involve actually visualizing parts ofthe autonomic nervous system, such as the sympatheticnerves in the heart.

This section describes examples of each type of test.Each type has its own advantages and disadvantages.Most centers that carry out autonomic function testinguse more than one type of test, but none use all of thetests described in this section.

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Physiological tests usually are simple, quick, painless,and safe. The main problem with them is that there arealways several steps between the brain’s directing achange in nerve traffic in the autonomic nervous systemand the physiological changes that are supposed tomeasure the autonomic changes. As a result, the resultsof physiological tests are always complex and indirect ,and they may or may not identify a problem correctly.

Neuropharmacologic tests are relatively simple andquick, but they depend on drug effects on how the patientfeels or how the body functions. This means that therealways is at least some risk of side effects. In addition,neuropharmacologic tests are somewhat complex andindirect. For instance, a neuropharmacologic test of therole of the sympathetic nervous system in a person’s highblood pressure might include measuring the effects of adrug that blocks sympathetic nerve traffic on bloodpressure, because a large fall in blood pressure wouldsuggest an important role of the sympathetic nervoussystem in keeping the blood pressure high. But ifblocking the sympathetic nerve traffic activated anothersystem compensatorily that also increases blood pressure,then the sympathetic blocking drug might not decreasethe pressure, and the doctor might mistakenly think thatthe sympathetic nervous system wasn’t involved with thepatient’s high blood pressure.

Neurochemical tests involve measurements of levels ofcompounds such as norepinephrine in body fluids suchas plasma. These tests can be done while the patient is at

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rest lying down, during a physiological manipulationsuch as exercise or tilting on a tilt-table, or during aneuropharmacologic manipulation such as blockade ofsympathetic nerve traffic by a drug. Neurochemical teststhemselves are safe, but the type of body fluid sampling,such as arterial blood sampling or cerebrospinal fluidsampling after a lumbar puncture, can involve some risk.

A major disadvantage of neurochemical testing is thatthere is no test of parasympathetic nervous systemactivity. This is because acetylcholine, the chemicalmessenger of the parasympathetic nervous system, isbroken down by enzymes almost as soon as it entersbody fluids such as the plasma.

Neurochemical testing based on plasma norepinephrinelevels also can be problematic, because those levels aredetermined not only by the rate of entry ofnorepinephrine into the plasma but also the rate ofremoval (clearance) of norepinephrine from the plasma.In addition, plasma norepinephrine levels are determinedcomplexly by a variety of processes in the sympatheticnerve terminals. Neurochemical testing by plasmanorepinephrine levels requires a carefully controlledtesting situation and expert technical analysis andinterpretation. Few clinical laboratories measure plasmalevels of catecholamines such as norepinephrine andepinephrine, and laboratories vary in the validity of theassay methods they use.

Neuroimaging tests, which are relatively new, involveactually depicting the autonomic nerve supply in body

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organs such as the heart. As yet there is no acceptedneuroimaging test to visualize parasympathetic nerveterminals. Sympathetic neuroimaging is done inrelatively few centers, and although this type of testingcan produce striking images of the sympatheticinnervation of the heart, this provides anatomicinformation about whether sympathetic nerve terminalsare present, and it is still unclear whether sympatheticneuroimaging can provide information about whetherthose terminals are functioning normally or not.

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Brain

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Physiological tests involve measurement ofa body function, such as pulse rate or bloodpressure.

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Neuropharmacologic tests involve using adrug that affects a function of the nervoussystem.

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Neurochemical tests involve measuring achemical produced in the nervous system,such as norepinephrine, the chemicalmessenger of the sympathetic nervoussystem.

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Receptor Binding


Feedback


Neuroimaging tests involve visualizing partof the nervous system, such as the autonomicnervous system.

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Physiological TestsThe Valsalva ManeuverDespite its apparent simplicity, the Valsalva maneuvertest is one of the most important clinical physiologicaltests for autonomic failure.

In the Valsalva maneuver, the patient blowsagainst a resistance for several seconds andthen relaxes.

The maneuver consists of blowing against a resistancefor several seconds and then relaxing. Often the patientblows into a tube connected to a blood pressure gauge,moving the gauge’s needle to a particular pressure andkeeping the needle there for 10-15 seconds.

The instant the patient begins to blow, the suddenincrease in chest and abdominal pressure forces blood outof the chest and down the arms. This increases bloodpressure briefly (Phase I of the maneuver). The increasein blood pressure in Phase I is mechanical and not part ofa reflex.

Soon afterwards, however, the amount of blood ejectedby the heart with each beat (stroke volume) plummets,because the straining decreases entry of blood from the

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veins into the heart. Blood pressure progressively falls(Phase II). The brain immediately senses this fall, due todecreased input to the brain from stretch receptors(baroreceptors) in the walls of the heart and major bloodvessels. The brain directs a rapid increase in outflows inthe sympathetic nervous system to the blood vessels and arapid decrease in outflow in the parasympathetic nervoussystem to the heart. The increase in sympathetic nervetraffic leads to more release of norepinephrine from thenerve terminals, and the released norepinephrine tightensblood vessels throughout the body. The total peripheralresistance to blood flow in the body goes up, just liketightening the nozzle at the end of a garden hoseincreases the pressure in the hose. Therefore, normally, atthe end of Phase II the blood pressure increases from itsminimum value, even though the amount of blood ejectedby the heart remains low.

When the patient relaxes at the end of the maneuver,briefly the blood pressure falls (Phase III)—a mirrorimage of the brief increase in Phase I. Blood rushes backinto the chest, and within a few heartbeats the heart ejectsthis blood. The blood pressure increases (Phase IV).Since the blood vessels are constricted, the normalamount of filling of the constricted vessels produces anovershoot of blood pressure, just as pressure in a gardenhose attains higher levels if one turns on the faucet withthe nozzle tightened. Finally, in response to this Phase IVovershoot of blood pressure, sympathetic nervous systemoutflow to blood vessels falls and parasympatheticoutflow to the heart increases. This causes a rapid returnof blood pressure and heart rate to normal.

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In a patient with sympathetic neurocirculatory failure,during Phase II the blood vessels fail to constrictreflexively, and so blood pressure falls progressively anddoes not increase toward baseline at the end of Phase II.

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Abnormal blood pressure (BP) and heartrate (HR) responses to the Valsalvamaneuver, indicating parasympathetic andsympathetic nervous system failure.

During Phase IV, because of the lack of tightening ofblood vessels, there is no rapid increase in blood pressureand no Phase IV overshoot of pressure. Instead, the bloodpressure gradually increases slowly back to the baselinevalue.

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The responses of pulse rate to the Valsalva maneuverdepend mainly on changes in parasympathetic nervoussystem outflow to the heart via the vagus nerve. In PhaseII, the pulse rate increases, and in Phase IV the pulse ratereturns rapidly to baseline. In parasympatheticneurocirculatory failure or baroreflex failure, the pulserate remains unchanged both during and afterperformance of the maneuver.

Note that one must monitor the beat-to-beat bloodpressure changes in order to diagnose sympatheticneurocirculatory failure based on the Valsalva maneuver.Until recently, such monitoring required insertion of acatheter into an artery. Since neurologists rarely feelcomfortable doing this, they usually settle for recordingonly the peak and trough pulse rates during and afterperformance of the maneuver. This may enable adiagnosis of parasympathetic neurocirculatory failurebut cannot diagnose sympathetic neurocirculatoryfailure.

The recent introduction of special testing devices hasprovided non-invasive means to follow blood pressurebeat-to-beat and detection of sympatheticneurocirculatory failure.

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Forearm Blood FlowThis non-invasive test measures the rate of blood flow inthe forearm. From the forearm blood flow (FBF) and theblood pressure (mean arterial pressure, MAP), theforearm vascular resistance (FVR) can be estimated. Inthe garden hose analogy, the FVR would correspond tothe extent of tightening of the nozzle. One of the mainways the body has to regulate blood pressure is byregulating vascular resistance, using the sympatheticnervous system.

When a person stands up or is tilted on a tilt table as partof tilt-table testing, the amount of blood ejected by theheart per minute falls, due to the force of gravity, whichtends to pool blood in the legs and lower abdomen anddecreases venous return to the heart. The brain directs anincrease in sympathetic nervous system outflows, whichincreases peripheral resistance to blood flow and helpskeep the average blood pressure (mean arterial pressure)normal, despite the decrease in cardiac output.

To measure forearm blood flow, a blood pressure cuff isattached around the upper arm, and a special bracelet-likedevice called a strain gauge is attached around the upperforearm. The strain gauge measures stretch verysensitively. For a measurement of forearm blood flow,the blood pressure cuff is inflated to just above thevenous pressure but below the arterial pressure.

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The steeper the slope,the higher the flow

Blood flow in a limb can be measured non-invasively using a blood pressure cuff and abracelet-like device around the limb.

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This is like tightening a tourniquet around the upper arm,for obtaining a blood sample. Because the cuff pressureis above the venous pressure, blood in the forearm andhand can’t get past the cuff, and because the cuff pressureis below the arterial pressure, blood can still enter theforearm and hand. In this situation, the volume of theforearm expands slightly, and the strain gauge senses theincrease in volume. If the rate of blood flow into theforearm is high, then the volume of the forearm increasesrapidly after the cuff is inflated; and if the rate of bloodflow is low, then the volume of the forearm increasesmore slowly. By a simple calculation we can estimate theblood flow into the forearm, from the rate of increase inthe volume of the forearm after the cuff is inflated.Usually, measurement of forearm blood flow is done atleast five times, to obtain a reliable average value.

Once the rate of forearm blood flow (FBF) is known, theforearm vascular resistance (FVR) can be estimated fromthe average blood pressure (mean arterial pressure,MAP) divided by the forearm blood flow. This is asimilar calculation as for measuring total peripheralresistance (TPR) from the mean arterial pressure (MAP)divided by the cardiac output (CO). When a personstands up or is tilted on a tilt-table as part of tilt-tabletesting, the forearm vascular resistance normallyincreases. A failure of the forearm vascular resistance toincrease during standing is a sign of sympatheticneurocirculatory failure.

Incidentally, the logo of the National DysautonomiaResearch Foundation is based on the signal used to

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measure forearm vascular resistance. Just before apatient faints, the forearm blood flow increases. The logoshows the increase in the forearm blood flow on repeatedmeasurements, as in the background the patient’s skincolor turns from a healthy red color to a deathly blue!

Power Spectral Analysis ofHeart Rate VariabilityThis test is much simpler than the fancy name suggests.Normally, a person’s heart rate is not constant. The pulserate increases when the person breathes in and thendecreases when the person breathes out. This means thatthe pulse rate normally oscillates in a wave-like pattern.

The change in pulse rate from breathing is sometimescalled respiratory sinus arrhythmia. This sounds like anabnormal heart rhythm, but it actually is a sign of ahealthy heart. Respiratory sinus arrhythmia is thought toresult from changes in parasympathetic nervous systeminfluences on the heart.

If one graphs the size of the oscillation as a function ofthe frequency of the heartbeats, then at the frequency ofbreathing, there is a peak of “power.” In people whohave failure of the parasympathetic nervous system, thereis little or no respiratory sinus arrhythmia, and there isno peak of power at the frequency of breathing. This sortof analysis has revealed a second peak of power, at alower frequency than the frequency of breathing.

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H E A R TR A T E( b p m )

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Power spectral analysis is a non-invasiveway to interpret heart rate changes.

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Researchers have thought that this low frequency poweris related to sympathetic nervous system influences on theheart.

Power spectral analysis of heart rate offers theadvantages of being safe, technically easy, and fast. Themain disadvantage is that the meaning of low frequencypower as an index of sympathetic nervous system activityin the heart remains in dispute.

Tilt-Table TestingTilt-table testing is done to see if standing up(orthostasis) provokes a sudden fall in blood pressure(neurally mediated hypotension), an excessive increase inpulse rate (postural tachycardia syndrome, POTS), orfainting (neurally mediated syncope).

Tilt-table testing is used to detect POTS orneurocardiogenic syncope.

The testing itself is simple. The patient lies on astretcher-like table, straps like seat belts are attachedaround the abdomen and legs, and the patient is tiltedupright at an angle. The exact angle used varies fromcenter to center and may be from 60 degrees to 90degrees. The tilting goes on for up to many minutes (thisagain varies from center to center) up to about 45minutes. If the patient tolerates the tilting for this period,

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then the patient may receive a drug, such as isoproterenolor nitroglycerine, which might provoke a sudden fall inblood pressure or loss of consciousness. As soon as thetest becomes positive, the patient is put back into aposition lying flat or with the head down, and sometimesfluid is given by vein. Patients usually recoverconsciousness within a minute or two.

Tilt-table testing is a form of provocative test. Thedoctors are hoping to reproduce the patient’s problem ina controlled laboratory situation. The testing is quite safewhen done by experienced personnel, in a setting whereemergency backup is available.

There are several disadvantages of tilt-table testing. Oneis the issue of false-positive results, especially when adrug such as isoproterenol is used. In a false-positive test,the results of the test are positive, but some healthypeople can have a positive test result, so that a positivetest result might not actually mean that anything really is“wrong.”

Another disadvantage is that most tilt-table testing doesnot provide information about disease mechanisms. Thismeans that, beyond verifying the patient’s complaints,the testing does little or nothing to suggest treatments thatmight be effective.

Tilt-table testing is not useful in patients with a persistentfall in blood pressure each time they stand up (orthostatichypotension), because the results are a foregone

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conclusion: the blood pressure will fall progressivelybeginning as soon as the tilting starts.

Sweat TestsSweating is an important way people regulate bodytemperature in response to external heat. The brainincreases sweating by directing an increase insympathetic nervous system traffic to sweat glands in theskin. The chemical messenger, acetylcholine, is released,and the acetylcholine acts on the sweat glands tostimulate production of sweat.

Sweat tests evaluate a particular part of“automatic” nervous system function.

There are several ways to measure sympatheticcholinergic sweating in response to external heat(thermoregulatory sweat test, TST). One is fromsprinkling starch with iodine all over the body. When thestarch-iodine combination is wetted, the powder turnsbrown. One can then photograph the body and see whichparts sweated. Sometimes other powder-dyecombinations are used. When the skin becomes sweaty,the ability to conduct electricity increases dramatically,because of the salt and water in the sweat, and one canmonitor the electrical conductivity. Sweat increases localhumidity, and one can also monitor the humidity in achamber attached to the skin.

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Another way to test sweating is from the galvanic skinresponse (GSR) or skin sympathetic test (SST). Thegalvanic skin response is part of polygraphic “liedetector” testing. When a person is suddenly distressed,or a small electric shock is delivered, increased activitiesof the sympathetic nervous system and theadrenomedullary hormonal system evoke sweating.One can also measure sweating from humidity in acapsule applied to the skin.

Advantages of sweat tests are that they are generally safe,simple, and quick. A disadvantage is that they onlymeasure physiological changes as a result of release ofacetylcholine from sympathetic nerve terminals orepinephrine from the adrenal gland. There aredysautonomias where the patient has normal sweating.

Another disadvantage is that sweat tests are onlyindirectly and complexly related to activity of thesympathetic nervous system, and they provide littleinformation about the exact mechanism of thedysautonomia.

The Cold Pressor TestIn the cold pressor test, the patient dunks a hand into ice-cold water. This rapidly increases the blood pressure, byincreasing activity of the sympathetic nervous system.Since the test involves not only cold but also pain, thecold pressor test can only be done for a minute or two. Asimilar limitation applies for isometric handgrip exercise.

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NeuropharmacologicTests

QSART“QSART” stands for “Quantitative Sudomotor AxonReflex Test.”

The QSART is a special form of sweat test.

This test is a form of sweat test. Sweating in response toaltered environmental temperature results from theeffects of the chemical messenger, acetylcholine,released from sympathetic nerve terminals near sweatglands in the skin. This arrangement is different from thatfor alterations in the pulse rate and blood pressure thatresult from effects of norepinephrine released fromsympathetic nerve terminals in the heart and blood vesselwalls. The QSART is a test of the ability of sympatheticnerve terminals in the skin to release acetylcholine andincrease sweat production.

As in some other sweat tests, in the QSART procedure,dried nitrogen (or dried air, or air with a known amountof humidity) is pumped at a controlled rate through asmall plastic, dome-like capsule placed on the skin.When the person sweats, the humidity in the chamberincreases, as the sweat droplets evaporate. This providesa rapid measure of sweat production. For QSART testing,

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From Spinal Cord

ACh

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Ganglion

Diagram of the QSART test.

a drug that stimulates acetylcholine receptors (forinstance acetylcholine itself) is applied to a nearby patchof skin, by a special procedure called iontophoresis. Thelocally applied acetylcholine evokes sweating at the sitewhere it is given, but in addition, by way of a type ofreflex called an axon reflex, sympathetic nerve terminalsunder the nearby plastic capsule release the body’s own

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acetylcholine, resulting in sweat production measured byincreased humidity in the capsule.

If a person had a loss of sympathetic nerve terminals thatrelease acetylcholine (loss of cholinergic terminals), thenapplying acetylcholine to the patch of skin near the testcapsule would not lead to increased sweating orincreased humidity in the test capsule. On the other hand,if the person had intact sympathetic cholinergic nerveterminals, then applying acetylcholine to a patch of skinnear the test capsule would increase the humidity in thecapsule. If the person had a brain disease that preventedincreases in sympathetic nerve traffic during exposure toincreased environmental temperature, then the personwould not be able to increase the humidity in the capsulein response to an increase in the room temperature, andyet the person would have a normal QSART response.

By this sort of neuropharmacologic test, doctors candistinguish sympathetic cholinergic failure due to loss ofcholinergic terminals from failure due to abnormalregulation of sympathetic nerve traffic to intactcholinergic terminals.

Advantages of the QSART are that it is generally safe,quick, quantitative, and easy for a technician to perform.There are also several disadvantages. The equipmentrequired is fairly expensive, and relatively few centershave QSART testing available, so that availability of thetest is an issue. As in other tests where the key factorbeing measured is physiological (in this case, sweat

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production), the results are indirect . For instance, aproblem with the ability to make acetylcholine in thenerve terminals or with the ability of acetylcholine tobind to its receptors in the sweat glands would result inthe same abnormal QSART responses as if thesympathetic cholinergic terminals were lost. Finally,QSART results may or may not identify problems inregulation of the heart and blood vessels by other parts ofthe autonomic nervous system. In other words, theQSART results might not be representative.

Trimethaphan Infusion TestTrimethaphan is a type of drug called a ganglion blocker.

In the autonomic nervous system, control signals from thebrain and spinal cord are relayed through the ganglia,and nerves from the ganglia, postganglionic nerves,deliver those signals to the nerve terminals near or in thetarget tissues. The control signals are relayed in theganglia by release of the chemical messenger,acetylcholine, which binds to specific receptors on thepostganglionic cells, called nicotinic receptors.

Stimulation of the nicotinic receptors, such as by nicotineitself, increases postganglionic nerve traffic in both theparasympathetic nervous system and the sympatheticnervous system.

Trimethaphan does just the opposite. It blocks nicotinicreceptors in the ganglia. By blocking the receptors,

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trimethaphan blocks the transmission of nerve impulsesin the ganglia to the postganglionic nerves of thesympathetic nervous system and parasympathetic nervoussystem. The rates of sympathetic nerve traffic andparasympathetic nerve traffic fall to virtually zero.

Because of the blockade of transmission of nerveimpulses in ganglia, trimethaphan normally producesclear effects on a variety of body functions. When aperson stands up, the ability to maintain blood pressuredepends importantly on reflexes that tighten bloodvessels, by way of increased sympathetic nerve traffic.Trimethaphan therefore always produces a fall in bloodpressure when the person stands up, called orthostatichypotension. If the person is lying down at the time, thentrimethaphan produces a relatively small decrease inblood pressure. Probably the most noticeable effect oftrimethaphan in someone who is lying down is a drymouth. This is because of blockade of theparasympathetic nervous system, which is responsible forproduction of watery saliva.

In the trimethaphan infusion test, the drug is given byvein at a dose calculated so as not to decrease the bloodpressure excessively. The blood pressure and pulse rateare monitored frequently or continuously, and blood maybe sampled from an indwelling catheter in an arm vein,for measurements of plasma norepinephrine levels orlevels of other neurochemicals.

If a patient had autonomic failure due to a loss ofsympathetic nerve terminals, such as in Parkinson’s

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disease with orthostatic hypotension, there would be norelease of norepinephrine from the nerve terminals,because of the absence of the terminals. Trimethaphan insuch a patient would not affect the blood pressure. But ifa patient had autonomic failure due to a brain disease,such as the Shy-Drager syndrome (multiple systematrophy with sympathetic neurocirculatory failure),where there was an inability to regulate sympatheticnerve traffic to intact terminals, there might be ongoing,unregulated release of norepinephrine from the nerveterminals. Trimethaphan in such a patient would decreasethe blood pressure.

The trimethaphan infusion test therefore can provideinformation about whether autonomic failure isassociated with a loss of sympathetic nerve terminals orfrom failure of the brain to regulate sympathetic nervetraffic appropriately.

In some patients with long-term high blood pressure(hypertension), the hypertension seems to reflect anoverall increase in the rate of nerve traffic in thesympathetic nervous system. This increases delivery ofnorepinephrine to its receptors in the heart and bloodvessels, causing an increase in the output of blood by the

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Tr imethaphan i . v .

P a r k i n s o n ' s D i s e a s e w i t hO r t h o s t a t i c H y p o t e n s i o n

S h y - D r a g e r S y n d r o m e

The trimethaphan infusion test can help toidentify different causes of autonomicfailure.

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heart (cardiac output) and tightening of blood vessels(vasoconstriction). By either or both mechanisms, theblood pressure would be high because of the high rate ofdelivery of norepinephrine to its receptors. Someinvestigators have called this hypernoradrenergichypertension. In a patient with hypernoradrenergichypertension, infusion of trimethaphan would beexpected to decrease the rate of norepinephrine releasefrom the sympathetic nerve terminals, and the extent ofthe fall in the plasma norepinephrine level would berelated to the extent of the fall in blood pressure. In apatient with an equal amount of hypertension, but with anormal rate of nerve traffic in the sympathetic nervoussystem, trimethaphan would not be expected to decreasethe blood pressure as much.

Because trimethaphan is a potent blocker of thesympathetic nervous system and the parasympatheticnervous system, the drug must be given at a carefullycontrolled rate, by personnel who are well acquaintedwith its effects. If the dose is too high, then the bloodpressure (especially the systolic blood pressure) can falltoo much. The effects of trimethaphan wear off quicklyafter the infusion is stopped, and so if too much drug isbeing given, decreasing the infusion rate or stopping theinfusion will eliminate the side effects within minutes.An antidote drug should be available that directlystimulates norepinephrine receptors. Sometimestrimethaphan can evoke release of histamine, which canproduce itching, wheezing, or decreased blood bloodpressure. An anti-histamine drug should also beavailable.

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Yohimbine Challenge TestYohimbine is a type of drug called an alpha-2adrenoceptor blocker. Alpha-2 adrenoceptors arereceptors for norepinephrine that exist at highconcentrations in certain parts of the brain, onsympathetic nerve terminals, and in blood vessel walls.

When alpha-2 adrenoceptors in the brain are blocked,this increases sympathetic nerve traffic. Alpha-2adrenoceptors on sympathetic nerve terminals act like abrake on norepinephrine release from the terminals.When alpha-2 adrenoceptors on sympathetic nerveterminals are blocked, this increases the amount ofnorepinephrine release for a given amount of sympatheticnerve traffic. Yohimbine, by blocking alpha-2adrenoceptors in the brain and on sympathetic nerveterminals, therefore releases norepinephrine from theterminals. The released norepinephrine binds to alpha-1adrenoceptors in blood vessel walls, causing an increasein blood pressure.

Because of the blockade of alpha-2 adrenoceptors in thebrain, yohimbine can produce any of several behavioralor emotional effects, which vary from person to person.Yohimbine can cause an increase in alertness or feelingssuch as anxiety or sadness, or, on the other hand,happiness or a sense of energy. Rarely, yohimbine cancause a panic attack.

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Yohimbine usually causes trembling,which sometimes isso severe that the teeth chatter, and it sometimes alsocauses paleness of the skin, goosebumps, and hairstanding out, as if the person were either very cold ordistressed. Actually, the body temperature does not fall atall, and the person does not feel cold. Another sometimesnoticeable effect of yohimbine is an increase insalivation. This is probably because of an increase in therate of sympathetic nerve traffic to the salivary glands.

In the yohimbine challenge test, the drug is given by veinfor several minutes or given by mouth as a single dose.Yohimbine given by vein is currently an investigationaldrug. The blood pressure and pulse rate are monitoredfrequently or continuously, and blood often is sampledfrom an indwelling catheter in an arm vein, formeasurements of plasma norepinephrine levels or levelsof other neurochemicals.

If a patient had autonomic failure due to a loss ofsympathetic nerve terminals, such as in Parkinson’sdisease with orthostatic hypotension, there would be norelease of norepinephrine from the nerve terminals,regardless of the nerve traffic, because of the absence ofthe terminals. Yohimbine in such a patient would notaffect the blood pressure. But if a patient had autonomicfailure due to a brain disease, such as the Shy-Dragersyndrome (multiple system atrophy with sympatheticneurocirculatory failure), where there was an inability toregulate sympathetic nerve traffic to intact terminals,yohimbine would increase the blood pressure, and

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Yoh imbine i . v .

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The yohimbine challenge test can help toidentify different causes of autonomicfailure.

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because of the inability to regulate sympathetic nervetraffic, the brain would not reflexively decrease thesympathetic nerve traffic to compensate for the increasedblood pressure. This means that infusion of yohimbineinto such a patient might produce a large increase inblood pressure. If the patient already had high bloodpressure, or if the doctor already strongly suspected adisease such as the Shy-Drager syndrome, then the dosewould be decreased, or the doctor might decide thatcarrying out the test would not be worth the risk.

In some patients with long-term high blood pressure(hypertension), the hypertension seems to reflect anoverall increase in the rate of nerve traffic in thesympathetic nervous system. This increases delivery ofnorepinephrine to its receptors in the heart and bloodvessels, causing an increase in the output of blood by theheart (cardiac output) and tightening of blood vessels(vasoconstriction). By either or both mechanisms, theblood pressure is high because of the high rate ofdelivery of norepinephrine to its receptors. Someinvestigators have called this hypernoradrenergichypertension.

In patients with hypernoradrenergic hypertension,some of the released norepinephrine binds to the alpha-2adrenoceptors on the sympathetic nerve terminals, andthis puts a brake on the norepinephrine release. Infusionof yohimbine by vein into such patients increases bothblood pressure and the plasma norepinephrine level, byblocking this restraint. The finding of a large increase inblood pressure coupled with a large increase in the

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plasma norepinephrine level provides support for thediagnosis of hypernoradrenergic hypertension.

In patients who have decreased activity of the cellmembrane norepinephrine transporter, or NET, whenyohimbine releases norepinephrine from the sympatheticnerve terminals, the released norepinephrine is notinactivated by “recycling” of the norepinephrine backinto the nerve terminals. This results in excessivedelivery of norepinephrine to its receptors, both in thebrain and outside the brain. In patients with NETdeficiency, yohimbine therefore produces a large increasein the plasma norepinephrine level and large increases inthe pulse rate and blood pressure. Yohimbine can alsoevoke panic or chest pain or pressure that can mimic thechest pain or pressure in coronary artery disease.

The yohimbine challenge test therefore can provideinformation about whether autonomic failure isassociated with a loss of sympathetic nerve terminals orfrom failure of the brain to regulate sympathetic nervetraffic appropriately. The test can also be used to identifyhypernoradrenergic hypertension or NET deficiency.The effects of yohimbine wear off over several minutes,once the infusion ends. If the blood pressure increasewere excessive, the quickest way to bring the pressuredown would be to stop the infusion and, if the patient hadorthostatic hypotension, have the patient stand up. Veryrarely, an antidote drug that stimulates alpha-2adrenoceptors, such as clonidine, has to be given.

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Because yohimbine given by vein is an investigationaldrug that produces important effects on functions of thesympathetic nervous system, the yohimbine challenge testshould be done only by personnel who are wellacquainted with its effects.

Clonidine Suppression TestClonidine stimulates alpha-2 adrenoceptors in the brain,on sympathetic nerve terminals, and in blood vesselwalls. When alpha-2 adrenoceptors in the brain arestimulated, this decreases sympathetic nerve traffic, andwhen alpha-2 adrenoceptors on sympathetic nerveterminals are stimulated, this decreases the amount ofnorepinephrine release for a given amount of sympatheticnerve traffic.

Released norepinephrine binds to both alpha-2adrenoceptors and alpha-1 adrenoceptors in blood vesselwalls. Even though clonidine stimulates alpha-2adrenoceptors, which would constrict blood vessels, thedrug is so powerful in decreasing release ofnorepinephrine that normally after a dose of clonidinethe blood pressure falls. Clonidine is an approved drugfor the treatment of long-term high blood pressure(hypertension).

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a1

a2

NE

NE

SNS

Brain

a2

Bloodstream

CLONIDINE NE

Smooth Muscle

The clonidine suppression test can behelpful in identifying causes of abnormallevels of norepinephrine (NE), the chemicalmessenger of the sympathetic nervoussystem (SNS).

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By stimulating alpha-2 adrenoceptors in the brain,clonidine usually produces some sedation. It can cause adecrease in alertness or a decrease in the sense of energy.Clonidine is effective in relieving symptoms ofwithdrawal from alcohol or opiate-type narcotics.

Clonidine can produce a dry mouth and a small decreasein the pulse rate. This is probably because of a decreasein the rate of sympathetic nerve traffic to the salivaryglands and heart.

In the clonidine suppression test, the drug is given bymouth, usually at a dose of 300 micrograms, whichwould be the total amount of the drug given in divideddoses in a day. The blood pressure and pulse rate aremonitored over the course of a few hours, and blood issampled from an indwelling catheter in an arm vein, formeasurements of plasma norepinephrine levels.

If a patient had autonomic failure due to a loss ofsympathetic nerve terminals, such as in Parkinson’sdisease with orthostatic hypotension, there would be norelease of norepinephrine from the nerve terminals,regardless of the nerve traffic, because of the absence ofthe terminals. Clonidine in such a patient would notaffect the blood pressure, or it might actually increase theblood pressure, due to stimulation of alpha-2adrenoceptors in the blood vessel walls.But if a patient had autonomic failure due to a braindisease, such as the Shy-Drager syndrome (multiplesystem atrophy with sympathetic neurocirculatoryfailure), where there was an inability to regulate

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sympathetic nerve traffic to intact terminals, there wouldbe norepinephrine in the terminals, and clonidine wouldinhibit its release. Clonidine in such a patient woulddecrease the blood pressure, and because of the inabilityto regulate sympathetic nerve traffic, the brain would notreflexively increase the sympathetic nerve traffic tocompensate for the decreased blood pressure. This meansthat clonidine might produce a large decrease in bloodpressure.

In some patients with long-term high blood pressure(hypertension), the hypertension is associated with anoverall increase in the rate of nerve traffic in thesympathetic nervous system. The blood pressure wouldbe high because of the high rate of delivery ofnorepinephrine to its receptors—hypernoradrenergichypertension. Clonidine binds to the alpha-2adrenoceptors on the sympathetic nerve terminals, andthis puts a brake on the norepinephrine release. Clonidinegiven to such patients decreases both blood pressure andthe plasma norepinephrine level.The finding of a largedecrease in blood pressure coupled with a large decreasein the plasma norepinephrine level provides support forthe diagnosis of hypernoradrenergic hypertension.

Rarely, hypernoradrenergic hypertension results from atumor that produces catecholamines such asnorepinephrine and epinephrine. The tumor is called apheochromocytoma. The clonidine suppression test is anaccepted diagnostic test for pheochromocytoma. If thehypernoradrenergic hypertension resulted from a highrate of sympathetic nerve traffic, then clonidine would

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decrease the elevated plasma norepinephrine level. But ifthe patient had a “pheo,” which would producecatecholamines independently of the rate of sympatheticnerve traffic, clonidine would fail to decrease the plasmanorepinephrine level. In other words, in a positiveclonidine suppression test for pheochromocytoma, theplasma norepinephrine level fails to decrease after a doseof clonidine, despite the presence of hypernoradrenergichypertension.

The effects of clonidine wear off over several hours.Patients can feel sedated even up to the next day.Therefore, patients should not drive or operate heavymachinery for at least 24 hours after having a clonidinesuppression test. The drug rarely if ever produces adangerous fall in blood pressure.

Isoproterenol Infusion TestIsoproterenol (brand name Isuprel™) stimulates beta-adrenoceptors. Beta-adrenoceptor stimulation hasseveral important effects in the body. Stimulation ofbeta-adrenoceptors in the heart increases the rate andforce of the heartbeat and therefore increases the outputof blood by the heart per minute (cardiac output).Stimulation of beta-adrenoceptors in the bronchioles, thesmall airway tubes in the lungs, opens them andtherefore can

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a1

b2

NE

SNS

Bloodstream

ISOPROTERENOL NE

Smooth Muscle

b1

b2

The isoproterenol infusion test can helpidentify causes of abnormal heart rate orinability to tolerate prolonged standing.

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reverse acute asthma attacks. Stimulation of beta-adrenoceptors in the liver converts stored energy in theform of glycogen to immediately available energy in theform of glucose. Stimulation of beta-adrenoceptors inblood vessel walls of skeletal muscle relaxes the bloodvessels, decreasing the resistance to blood flow in thebody as a whole (total peripheral resistance).Stimulation of beta-adrenoceptors on sympathetic nerveterminals increases the release of norepinephrine.

Isoproterenol is infused by vein as part of diagnostictesting for a few types of dysautonomias, which appear tooverlap and may be different forms of the same problem.In the hyperdynamic circulation syndrome, the patienthas a relatively fast pulse rate, high cardiac output, avariable blood pressure that tends to be increased, atendency towards panic or anxiety attacks, excessiveincreases in pulse rate in response to isoproterenol givenby vein, and improvement by treatment with the beta-adrenoceptor blocker, propranolol. The same holds truefor many relatively young patients with early, borderlinehypertension. Patients with the postural tachycardiasyndrome (POTS) also can have a fast pulse rate, evenwhen lying down, excessive increases in pulse rateduring isoproterenol infusion, and sometimes panicevoked by the infusion.

Isoproterenol infusion is also done as part of tilt-tabletesting in patients with chronic fatigue syndrome orchronic orthostatic intolerance. After prolonged uprighttilting, infusion of isoproterenol can bring on a rapid fallin blood pressure or loss of consciousness, converting a

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negative tilt-table test to a positive tilt-table test. Thismight arise from stimulation of the heart by isoproterenolor from relaxation of blood vessels in skeletal muscle,which would shunt blood away from the brain andtowards the limbs.

Of course, this brings up the issue of how frequently ahealthy person might have one of these reactions inresponse to isoproterenol infusion in the setting ofprolonged tilting, which would be a false-positive result.

The effects of isoproterenol wear off rapidly withinminutes of stopping the infusion. The drug does not enterthe brain well, and so there are usually few if anybehavioral or emotional responses. Isoproterenol canincrease the rate or depth of respiration, producetrembling, or bring on abnormal heart rhythms orabnormal heartbeats. The risk of these side effectsdisappears as soon as the drug wears off. Because of theincrease in pulse rate and the force of the heartbeat,isoproterenol infusion increases the work of the heart,and this might cause problems such as chest pressure inpatients with coronary artery disease.

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NeurochemicalTestsNeurochemical tests of autonomic nervoussystem function mainly involve thesympathetic nervous system oradrenomedullary hormonal system. This isbecause the main chemical messengers ofthese systems, norepinephrine andepinephrine (adrenaline), are relativelystable and can be measured in the plasma,while the main chemical messenger of theparasympathetic nervous system,acetylcholine, undergoes rapid breakdownand cannot be measured in the plasma.

Plasma NorepinephrineLevelsSince norepinephrine is the main chemical messenger ofthe sympathetic nervous system, doctors have often usedthe plasma norepinephrine level as an index ofsympathetic nervous system “activity” in the body as awhole.

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Plasma norepinephrine is used to test thesympathetic nervous system.

There is some validity in doing this, but the relationshipbetween the rate of sympathetic nerve traffic and theconcentration of norepinephrine in the plasma is complexand indirect and is influenced by many factors such ascommonly used drugs and activities of daily life. Theblood sample therefore should be obtained undercarefully controlled or monitored conditions, and theplasma norepinephrine level should be interpreted by anexpert.

Here is a brief description of some of the complexitiesinvolved:

First, only a small percent of the norepinephrine releasedfrom sympathetic nerve terminals actually makes its wayinto the bloodstream. Most is “recycled” back into thenerve terminals, by a process called “Uptake-1,” using aspecial transporter called the “cell membranenorepinephrine transporter,” or “NET.” This means thata person might have a high plasma norepinephrine level,despite a normal rate of sympathetic nerve traffic, if theNET were blocked by a drug or weren’t working right.

Second, the plasma norepinephrine level is determinednot only by the rate of entry of norepinephrine into theplasma but also by the rate of removal of norepinephrinefrom the plasma. It happens that norepinephrine iscleared from the plasma extremely rapidly. This means

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that a person might have a high plasma norepinephrinelevel because of a problem with the ability to removenorepinephrine from the plasma, such as in kidneyfailure.

Third, norepinephrine is produced in sympathetic nerveterminals by the action of three enzymes (tyrosinehydroxylase, or TH, DOPA decarboxylase, or DDC, anddopamine-beta-hydroxylase, or DBH), along with otherrequired chemicals such as vitamin C, vitamin B6, andoxygen. In addition, norepinephrine is produced in,stored in, and released from tiny bubble-like “vesicles”in sympathetic nerve terminals. For norepinephrine to beproduced in the vesicles requires another transporter,called the “vesicular monoamine transporter,” or“VMAT.” A problem with any of these enzymes, co-factors, or the VMAT can result in decreasednorepinephrine production and therefore low plasmanorepinephrine levels, regardless of the rate ofsympathetic nerve traffic.

Fourth, the plasma norepinephrine level usually ismeasured in a blood sample drawn from a vein in thearm. Because the skin and skeletal muscle in the forearmand hand contain many sympathetic nerve terminals, theplasma norepinephrine level in blood from an arm vein isdetermined not only by the amount of norepinephrinerelease from sympathetic nerve terminals in the body as awhole but also by the amount of release locally in theforearm and hand.

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Fifth, the plasma norepinephrine level varies dependingon the posture of the person at the time of blood sampling(the level doubles within minutes of standing up fromlying down), the time of day (highest in the morning),whether the person has been fasting, the temperature ofthe room, dietary factors such as salt intake, and any of alarge number of commonly used over-the-counter andprescription drugs or herbal remedies.

Plasma Epinephrine(Adrenaline) LevelsCompared to the plasma norepinephrine level, which iscomplexly and indirectly related to sympathetic nervoussystem “activity” in the body as a whole, the plasmaepinephrine (adrenaline) level is a fairly direct indicatorof activity of the adrenomedullary hormonal system.

Plasma epinephrine (adrenaline) is used totest the adrenomedullary hormonal system.

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SYMPATHETICNERVE

ENDING

NE

DHPGDA

DOPA

BLOODSTREAM

[NE]

Uptake-1via the NET

MAONE

TH

TYR

DBH

DDC

Several factors influence plasmanorepinephrine levels.

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Nevertheless, some of the same factors that makeinterpreting plasma norepinephrine levels difficult cancomplicate interpreting plasma epinephrine levels. Alarge number of common and difficult to control lifeexperiences that influence activity of theadrenomedullary hormonal system. These include drugs,alterations in blood glucose levels (such as after a meal),body temperature, posture, and emotional distress.

An additional problem is technical. Epinephrine is a verypowerful hormone. Not surprisingly, the plasmaepinephrine level normally is very low. The level can beso low that it is about at or below the limit of sensitivityof the measurement technique, which would invalidatethe result. Other chemicals besides epinephrine caninterfere with the measurement. This can especially be aproblem in people who drink a lot of coffee, even if it isdecaffeinated, because of high plasma levels of achemical called dihydrocaffeic acid, which can mimicepinephrine in some assay procedures.Because of these issues, it is important that bloodsampling and chemical assays for plasma epinephrinelevels be carried out by experienced and expertpersonnel.

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NeuroimagingTestsCompared to other types of testing for dysautonomias,testing using neuroimaging is new.

Neuroimaging is a way to actually see nervous systemtissue, such as in the brain. In testing for dysautonomias,the neuroimaging involves seeing the sympathetic nervesin an organ outside the brain, such as in the heart.

Sympathetic nerves in the heart travel with the coronaryarteries that deliver blood to the heart muscle. Thenerves then dive into the muscle and form mesh-likenetworks that surround the heart muscle cells. Becauseneuroimaging tests have a limit of resolution of a fewmillimeters, the imaging does not show individual nervesbut gives a general picture, and because the nerves arefound throughout the heart muscle, the picture looks verymuch like a scan of the heart muscle.

The radioactive drugs used for imaging the sympatheticnerves in the heart are given by vein, and they aredelivered to the heart muscle by way of the coronaryarteries. This means that one must be able to distinguisha local loss of radioactivity in the scan that is due to lossof sympathetic nerves from a local loss that is due to aplace where the coronary artery is blocked, because

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either nerve loss or coronary blockage could lead to thesame lack of radioactivity in the heart muscle.Centersthat carry out sympathetic neuroimaging therefore oftendo two scans in the same test, one scan to see where theblood is going and one to see where the sympatheticnerves are.

MIBG Scanning

In the United States, sympathetic neuroimaging isavailable in few centers but is available fairly widely inEuropean countries and Japan. Worldwide, probably themost commonly used sympathetic neuroimaging agent is123I-metaiodobenzylguanidine, or 123I-MIBG. 123I-MIBGis a radioactive form of a drug that is taken up bysympathetic nerve terminals, making them visible on anuclear medicine scanner.

Fluorodopamine PETScanning

At the National Institutes of Health’s Clinical Center, inBethesda, Maryland, another sympathetic neuroimagingagent has been developed, which is 6-[ 18F]fluorodopamine. This is a radioactive form of thecatecholamine, dopamine. After injection of 6-

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[ 18F]fluorodopamine, by vein, the drug is taken up bysympathetic nerve terminals, and the radioactivity isdetected by a special type of scanning procedure calledpositron emission tomographic scanning, or “PETscanning.”

Imagine you had a radioactive object in a box. You coulddetermine if there were something radioactive inside byusing a detector, such as a Geiger counter. Now imaginethat you had many little Geiger counters all around thebox. Each counter would detect a different amount ofradioactivity, depending on the shape of the object andthe distance of the counter from the object. If you had away to construct a picture, such as in a newspaper photo,where the size and intensity of each dot depended on theamount of radioactivity, then you could construct animage of the object inside the box. Tomographic scansare two-dimensional images, or slices. Tomographicslices would allow you to see what was inside the box atany level. If the object were small, most of the sliceswould be empty. Eventually, at the level of the object,you would see an image of the object in the slice.

A positron emitter is a type of radioactive substance thatreleases a short-lived form of radiation that can penetratethe body and reach detectors outside it, enablingconstruction of a PET scan. Other scans in nuclearmedicine use a somewhat different source ofradioactivity, but the idea is about the same.

Fluorodopamine is structurally similar to thebiochemicals of the sympathetic nervous system,

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norepinephrine (noradrenaline) and epinephrine(adrenaline). Just as some radioactive chemicals gettaken up by bone, producing a bone scan, or get taken upby the brain, producing a brain scan, fluorodopamine getstaken up by sympathetic nerve endings, and the result is ascan of the sympathetic nervous system. For instance, weknow that fluorodopamine gets taken up very readily inthe heart walls, since there are so many sympatheticnerve endings there. Because there are so manysympathetic nerve endings in the heart, PET scans of theheart after injection of fluorodopamine basically look likeimages of the heart itself. One can easily make out themain pumping muscle (left ventricular myocardium), theseptum between the left and right ventricles, and the leftand right ventricular chambers that contain the blood theheart pumps.

Different forms of dysautonomia result in remarkablydifferent pictures of the sympathetic nerves in the heartby fluorodopamine PET scanning. Probably the moststriking pictures occur in diseases where there is a loss ofsympathetic nerve terminals, such as in pure autonomicfailure and in Parkinson’s disease, because even whenthe blood flow to the heart muscle is normal, there is noheart visible in the PET scan!

A much more difficult issue is whether analysis of theamount of radioactivity in the heart can provideinformation about how the sympathetic nerve terminalsare functioning. This is a matter of research interest now.

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Interventricular Septum

LeftVentricularChamber

Left Ventricular Free Wall

RightVentricularChamber

Fluorodopamine PET scanning can showthe sympathetic nerves in the heart muscle.

Documents

Tests for Dysautonomias