Anaesthesia for spinal surgery

Anaesthesia for spinal surgery

Ian Calder MBChB, FRCA

The National Hospital for Neurology and Neurosurgery, Queen Square, London WC1N 3BG, United KingdomThe Royal Free Hospital, Pond Street, London NW3 2QG, United Kingdom

Patients with and without spinal disease are at risk of neurological damage during anaesthesia,even when the surgery is not directed at the spine. Certain types of spinal surgery carry asubstantial risk. Reports of spinal cord damage due to direct laryngoscopy are unconvincing.Evoked potential spinal cord monitoring is used increasingly during anaesthesia, and successfulrecording of potentials requires the co-operation of the anaesthetist. Airway problems arecommon in cervical spine disease, particularly if the disease a�ects the upper three vertebrae.Anterior surgical approaches to the craniocervical junction involve extensive surgery, withimplications for airway management and nutrition.

Key words: anaesthesia; spinal cord.

PATHOLOGY

Spondylosis

This term is commonly used to denote a degenerative arthritic condition a�ecting thespine, which is pathologically similar to osteoarthritis. Osteophytic protrusions occurat intervertebral joints, which may produce radiculopathies (disease of nerve root) ormyelopathies (disease of spinal cord) or both. A variant of these disease known asossi®cation of the posterior longitudinal ligament is seen in the cervical spine and canproduce cord compression at several levels.

Herniation of intervertebral discs

Most disc protrusions are postero-lateral and cause radiculopathies; occasionally theprotrusion is posterior, which is known as a `central' disc. Central disc protrusions cancause myelopathies or the cauda equina syndrome and constitute a surgical emerg-ency. It is rare for a disc to protrude anteriorly. Disc protrusions present most oftenfrom the lower lumbar intervertebral joints. Thoracic disc protrusions are uncommonbut often produce myelopathies, owing to the relatively narrow con®nes of thethoracic spinal canal.

Spinal stenosis

The descriptive term spinal stenosis is applied when the cross-sectional area of thespinal canal is reduced. A normal-sized spinal canal can become stenosed owing to

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BaillieÁ re's Clinical AnaesthesiologyVol. 13, No. 4, pp. 629±642, 1999

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degenerative changes such as osteophytes or disc protrusions, but some patients have acongenitally narrow canal, which is compromised by minor degenerative changes.

Spondylolisthesis

Degenerative changes in the spine can lead to slippage of one vertebra on the onebelow. The direction of slip is anterior in most cases. The commonest level is L4. Thecause is unknown.

Tumours

Nervous tissue tumours in the spinal canal account for about 15% of primarycentral nervous system neoplasms. Secondary tumours are commoner than thosearising from nervous tissue.

Syringomyelia

Syrinxes occur in the cervical cord in association with an Arnold±Chiari malformation.

Infections

Tubercle and staphylococcal infections occur most often in immune-suppressedpatients but can occur in normal patients. Bacterial spinal infections can occur aftertonsillectomy.1

Rheumatoid arthritis

Spinal involvement in rheumatoid arthritis is common, with the upper cervical jointsbeing a�ected most often. The disease destroys synovial joints, ligaments and bone.The stability of the vertebral joints may be impaired, with three main patterns ofinvolvement being seen in the cervical spine: atlanto-axial subluxation, vertical sub-luxation and subaxial subluxation. All three types may be present.

Atlanto-axial subluxation is the commonest abnormality (65% of subluxations2). Thedirection of subluxation is nearly always anterior. About 20% of subluxations are of the`vertical' type where erosion of the lateral masses of the axis, and loss of joint spacebetween the occiput, atlas and axis, results in the odontoid peg approaching the skullbase, or even entering the foramen magnum. The phenomenon is often described ascranio-vertebral settling. Some 15% of subluxations occur at levels below the axis. Theamount of atlanto-axial subluxation has been assessed by measuring the distance fromthe anterior arch of the atlas to the odontoid peg on lateral radiographs, the distancebeing known as the atlanto-dental interval (ADI). However, the size of the ADI doesnot correlate well with neurological symptoms and signs2 or cord compression seen onmagnetic resonance (MR) scans.3 The distance between the odontoid peg and theposterior arch of the atlas, the posterior atlanto-dental interval, has been suggested tobe of greater prognostic value than the ADI, although the correlation betweenmeasurement and neurology is still not good.

The clinical presentation of cervical rheumatoid disease is rarely acute. An insidiousprogression of neck pain, occipital headache, sti�ness and crepitus occurs. Lhermitte'sphenomenon of an electric shock sensation up and down the spine can occur. Thedetection of myelopathy in rheumatoid patients is particularly di�cult because they

630 I. Calder

usually have muscle and joint disease in the limbs. The onset of urinary dysfunction(frequency or retention) is said to be an important warning sign.2 Temporomandibularjoint and arytenoid joint involvement is common.

Surgical treatment of cervical rheumatoid disease may involve stabilization pro-cedures, such as transarticular atlanto-axial screws or occipito-cervical ®xation, ordecompressive procedures such as transoral odontoidectomy and pannus excision. Allsuch procedures are hazardous and little bene®t has been observed in severelymyelopathic patients.4

Because rheumatoid cervical disease may be symptomless5, the question arises as towhether patients should have ¯exion±extension lateral radiographs prior to anaesthesiafor procedures unrelated to the cervical spine. Speci®c enquiry for the symptomsmentioned above should be made and if suspicion of symptomatic cervical involvementis aroused the patient should be investigated before undergoing general anaesthesia.This will involve an MR scan in most cases. It is not mandatory to obtain pre-operativeradiographs in symptomless patients, provided that the patient is regarded as àt risk'and handled accordingly. In most cases it is sensible to ask for radiographs because it isin the patient's interest to have the study at some time, and the request demonstratesthe anaesthetist's concern and appreciation of the possibilities.

Ankylosing spondylitis

Surgery to correct severe deformity is occasionally performed. Patients withankylosing spondylitis may su�er cervical fractures after minor trauma.6

Spinal curvature

Scoliosis

This a ®xed lateral curvature of the spine. Some rotation is usually present. Eighty-®veper cent of curves are idiopathic and the onset is in late childhood. Signi®cantpulmonary compromise is unusual. Vedamten and Crawford7 have questioned theneed for pulmonary function tests before anaesthesia for idiopathic scoliosis after®nding a post-operative pulmonary complication rate of only 2.3%. Correction of thecurve has not been shown to improve pulmonary function.8

Kyphosis

This is a ®xed increase of the thoracic curve. Curves greater than 908 may requiresurgery, but the risk of neurological damage is considerable.

SURGICAL APPROACHES AND POSITIONING

The prone position allows surgical access to the majority of lesions in the spine andspinal cord. However, lesions anterior to the cord cannot be approached posteriorlybecause cord retraction is hazardous.

Surgical access to anterior lesions in the cervical spine, from C6 to C3, can beobtained through an anterior neck incision with the patient supine. Anterior lesionsabove C3 can be approached by a transoral, or lateral, approach, provided that they arenot extending more than about one level up or down. Anterior lesions, which involve

Anaesthesia for spinal surgery 631

the clivus, require a maxillotomy to obtain access. Lesions which extend above andbelow C3 may need a mandibular split with midline tongue division. Posterior cervicallesions can be approached with the patient in the sitting position. This technique isused infrequently because of the risk of quadraplegia due to poor cord perfusion.9 Thethoracic region clearly involves the greatest di�culties when approaching anteriorlesions such as discs. Most anterior thoracic spine procedures require a thoracotomywith lung de¯ation, or costotransversectomy.

Problems with the prone position

Abdominal compression

Care must be taken to ensure that the abdomen is not compressed so that venousreturn and ventilation are not compromised. Wadsworth et al10 found that cardiacindex was reduced most when patients were placed in the knee±chest position, lesswhen placed in a Relton±Hall frame and least when the patient was supported on anevacuatable mattress or on pillows

Head and neck alignment

It is not easy to position the head and neck so that venous drainage is unimpaired andthe eyes and facial tissues are not compressed. Cases of severe tongue swelling havebeen reported, which are thought to have been due to impaired lymphatic and venousdrainage.11

Ocular complications

Corneal abrasions. Gild et al12 have pointed out that only 20% of àbrasions' are causedby direct contact or chemical injury, 80% being due to hypoxic corneal damage. Thecornea is at risk of hypoxic damage because it relies on an intact tear ®lm to allowdi�usion of oxygen. The tear ®lm is jeopardized during anaesthesia because of reducedtear production, lagophthalmos (tendency for the eye to shut incompletely) and theinhibition of Bell's phenomenon (the re¯ex in which the eye turns upwards duringsleep). General anaesthesia and surgery for head and neck complaints, surgery in theprone position and surgery lasting more than 90 min have been identi®ed as riskfactors for corneal abrasions.13 The best protection is to ensure that the eyes are tapedshut.

Ischaemic optic neuropathy. This condition is believed to result from poor perfusion ofthe optic nerve and has been reported after cardiac arrest, haemodialysis and normalsleep.14 Myers et al15 reviewed 37 cases of visual loss associated with spinal surgery. In97% of cases the patients had been in the prone position. The cause of visual loss wasbelieved to be ischaemic optic neuropathy (ION) in 22 of the cases, retinal arterythrombosis in nine cases and cortical ischaemia in three, with three cases unclassi®ed.Risk factors for ION, other than the prone position, were large blood loss and greaterduration of surgery compared with controls. Prevention of ION should be sought byavoiding pressure on the eye (increased ocular pressure will reduce perfusionpressure), hypotension and low haematocrits.14

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Retinal artery occlusion. Pressure on the eye is believed to be a risk factor. The durationand amount of pressure required are not known but may be short, because a case ofblind eye has been reported after a patient complained to the anesthesiologist that hewas pressing on his eye as he went to sleep.16 The eyes are at great risk of compressiondamage if a horseshoe-type support is used.17

The May®eld2 pins

In order to ensure that the head stays completely still during surgery on the cervicalspine, and to avoid pressure on the eyes, it is usual to use skull pins to ®x its position.Insertion of the pins is very painful and large rises in blood pressure are seen unlesssteps are taken to obtund the response. Little other stimulation is occurring at thispoint so that the patient is usually borderline hypotensive and the addition of powerfulanalgesics causes actual hypotension. Jamali et al18 found that it was necessary to giveinotropic agents to nearly all their patients to correct hypotension.

Major vascular damage

Damage to the great vessels during spinal surgery continues to occur.19 Surgeons areusually unaware that they have perforated the anterior longitudinal ligament and aortaor inferior vena cava. A vital diagnostic point is that bleeding into the surgical woundoccurs in only a minority of cases. The presentation is with acute hypotension orcardiac arrest with electromechanical dissociation, and mortality is high.20 Thepresentation of patients with iliac vessel damage is usually less acute and interventionmay be delayed if the diagnosis is not considered.

RISK OF NEUROLOGICAL DETERIORATION DURING SURGERYAND ANAESTHESIA

Neurological damage can occur in patients with normal spines if they are placed in aposition which is abnormal enough and/or the perfusion pressure is low enough, forlong enough. Quadraparesis has been reported in a normal patient who had been tiedup with his cervical spine acutely ¯exed for 10 hours.21 Bromage22 described threecases of paraplegia after general anaesthesia, during which the normal lumbar lordoticcurve was obliterated. Amoiridis et al23 described a similar case after an operationlasting 10 hours. Dinsmore et al24 have shown that spinal posture can alter cere-brospinal ¯uid pressure (and hence, perfusion pressure) by more than 20 mmHg. Singhet al25 reported four cases of quadraparesis caused by induced hypotension in patientswithout spinal diseases.

Patients with spinal disease have su�ered neurological damage during anaesthesia andsurgery unconnected with their spinal disease. Ehni26 reported four cases of caudaequina syndrome in patients with lumbar spinal stenosis, after surgery remote from thespine. Linstedt et al27 reported a case in which a child with congenital spinal disease hadprolonged extraspinal surgery. Spinal cord monitoring with sensory evoked potentials(SEPs) was in place and evoked potentials deteriorated after 3 hours. The child wasrepositioned with the head and neck in a di�erent alignment and the potentialsrecovered.Deemet al28 reported the case of a patientwith cervical spondylosiswhohadsurgery on his lumbar spine. The patient was intubated and positioned for surgeryawake but was found to have sustained a central cord syndrome (a pattern of weakness


where the legs are less a�ected than the arms) during the operation. Therefore, aposition which is initially tolerable may eventually be damaging.

May et al29 have identi®ed some risk factors associated with neurological deteriora-tion during surgery on the cervical spine: (1) poor pre-operative neurological function;(2) use of instrumentation; (3) upper cervical and clival surgery; (4) multisegmentalsurgery.

The risk of damage to the cord or cauda equina during anaesthesia depends on theinteraction of

1. spinal pathology and the surgery required,2. deformation of, or pressure on, neural tissue during the procedure,3. the perfusion pressure,4. the duration of the procedure.

Is direct laryngoscopy for tracheal intubation neurologically hazardous?

There is evidence that direct laryngoscopy is hazardous to the spinal cord in patientswith cervical spine disease.30 Magnaes30 studied patients with cervical spinal stenosisdue to spondylosis. He demonstrated that the cerebrospinal ¯uid (CSF) pressure(CSFP) generated by injecting saline (and hence, the perfusion pressure of the cord)was clearly a�ected by varying head position during simulated laryngoscopy. However,there is still no convincing case of spinal cord damage due to laryngoscopy in theliterature.31 It may be that direct laryngoscopy does not cause damage because (1) itproduces hypertension, which increases cord perfusion pressure, (2) it is of shortduration and (3) it produces little movement of the cervical spine below C3.32 Thereare ®ve case reports which implicate direct laryngoscopy.33±37 The relationshipbetween laryngoscopy and neurological deterioration is not clear enough to provecausation, as opposed to association, in any of the reports. In all of the reports thereare other factors which could have caused the damage. Patients with spinal disease areat risk throughout anaesthesia, not just at intubation.

MINIMIZING THE RISK OF NEUROLOGICAL DAMAGE DURINGANAESTHESIA AND SURGERY

It is probable that a relatively small increase in the number of surviving axons after aspinal cord injury can have a dramatic e�ect on function. The spinal cord of the catcontains about 500 000 axons. In a traumatic damage model it was found that the meanpopulation of axons in the cats that recovered locomotion was 60 000, compared with20 000 in those that did not.38

Positioning

It is not possible to be certain that a position is safe, unless continuous spinal cordmonitoring (see below) is performed. Performing awake intubation and positioningdoes not guarantee safety28, because it may take some hours for the neurologicaldamage to occur. Àwake' intubation will involve the use of psychoactive drugs, so thatthe patient's opinion may be unreliable.39

634 I. Calder

Maintaining spinal cord perfusion pressure

The thoracic portion of the cord is believed to be less well perfused at critical bloodpressures than the cervical cord.25 Similarly to the brain, the cord is suspended in CSF,so that SCPP � MAP ÿ CSFP (where SCPP is spinal cord perfusion pressure and MAPis mean arterial pressure). The normal CSFP is about 10±15 mmHg, so that an increasein SCPP can be obtained by drainage of CSF. CSF drainage is current practice invascular surgery on the thoracic aorta, during which perfusion pressure of the cordmay be low. Marini et al40 demonstrated that CSF drainage had a protective e�ectduring aortic surgery. CSF drainage has not been studied during spinal surgery.

There is uncertainty as to what should be considered an ideal SCPP in patients atrisk of cord damage or in whom cord damage might have occurred. It is generallybelieved that hypotension should be avoided, but in extensive spinal surgery a degreeof induced hypotension can signi®cantly reduce blood loss.41 A similar lack of certaintyexists as to the ideal perfusion pressure in cranial neurosurgical practice.42 It has beensuggested that the use of vasopressors to raise the blood pressure may increase thepossibility of haemorrhage into damaged spinal cords.43,44 The current position seemsto be that hypotension (590 mmHg systolic) should be avoided and induced hyper-tension considered carefully.

Use of drugs

Steroids

Two trials in the United States have shown an improvement in outcome after acutespinal cord injury (ASCI) if the patients were treated with methylprednisolone within8 hours.45 The signi®cance of these results has been debated, because the improvementdid not greatly a�ect the patient's functional ability, and there was an increasedincidence of sepsis in the treatment groups. Problems with steroid psychosis have alsobeen reported.46 Bracken et al45 recommend that patients receive a bolus of 30 mg/kgof methylprednisolone and then 5.4 mg/kg for 24 hours if the treatment is begunwithin 3 hours or for 48 hours if treatment commences 3±8 hours after injury.

GM-1 ganglioside

The gangliosides are complex glycolipids which form a major part of cell membranes.GM-1 ganglioside has been reported as bene®cial in ASCI, and a trial is in progress.47

Excitatory amino acid blockage

It is believed that excitotoxic damage occurs after neurotrauma or ischaemia owing tohyperstimulation of the N-methyl-D-aspartate (NMDA) receptor site by glutamine.NMDA receptor blockers such as magnesium, MK 801 and ketamine have shownpromise in animal studies.48

Hypothermia

Inducing local hypothermia for spinal cord protection has great practical di�culties.One group has claimed worthwhile results in patients undergoing major vascularsurgery, but they pointed out that CSFP was raised during perfusion.49


Avoiding post-operative spinal haematomas

If the spinal canal is not decompressed within 8±12 hours of the onset of symptoms noimprovement is seen as a result of surgical drainage.50 Pain in the general area or in aradicular distribution is an important symptom but is not always present. Muscleweakness, sensory loss and bladder pareses occur.

There is no consensus on the place of heparin prophylaxis in spinal surgery.51 Mostwould agree that heparin should be avoided until after surgery. The situationregarding aspirin and non-steroidal drugs is also uncertain. Aspirin has been shown tobe a major risk factor for post-operative haematoma formation after intracranialsurgery.52 The policy in the author's unit is to stop aspirin for 10 days prior to electivespinal surgery.

MONITORING NEUROLOGICAL FUNCTION DURINGSPINAL SURGERY

Use of local anaesthesia

Reports of surgery on the spine under local anaesthesia exist. Nygaard et al53 used0.5% lidocaine in posterior cervical surgery, which included sublaminar wiring in somecases. The volume of lidocaine used ranged between 60 and 210 ml.

Avoiding muscle relaxants

Surgical stimulation of a root, or the cord, will usually produce movement of a limb inthe anaesthetized, but not paralysed patient. This is a valuable indication of surgicaltrespass. The mechanism of immobility due to anaesthesia is not well understood.Re¯ex suppression in the spinal cord is probably important.54 Rampil55 has shown thatminimal alveolar concentration (MAC) values are not altered by decerebration.

Sensory evoked potentials

SEPs are the most frequently used method of cord monitoring during spinal surgeryunder general anaesthesia. The use of intraoperative SEPs is believed to reduce theincidence of neurological damage.56

Motor evoked potentials

The motor cortex can be stimulated with a transcranial electrical or magneticdischarge, but motor evoked potentials are very sensitive to volatile anaestheticagents.

ANAESTHESIA TECHNIQUES FOR SPINAL SURGERY

Preparation for anaesthesia and surgery

Poor pre-operative nutrition has been shown to be a major risk factor for thedevelopment of post-operative infections.57 Patients who are likely to be unable toswallow for a period after surgery should have a percutaneous gastrostomy. Patientsundergoing maxillotomy or mandibular split approaches often have swallowing

636 I. Calder

di�culties for more than a month. A nasogastric tube can be used if the period ofswallowing di�culty is likely to be short. It is useful to get the patient to swallow thisbefore the induction of anaesthesia, if intubation of the trachea is likely to be di�cult.

Local anaesthesia

The attractions of the technique are obvious and success has been reported.53 Thedisadvantages are equally obvious.

Regional techniques

Ramirez and Thisted58 reported that 7% of lumbar discectomies were performedunder spinal anaesthesia in their institutions, without complication. Tesla� et al59 foundthat 3 ml of plain 0.5% bupivacaine resulted in fewer episodes of hypotension andbetter conditions than 2 ml of 0.75% bupivacaine in 8.25% glucose. Laakso et al60 notedthat the intensity of the block diminished rapidly when a CSF leak occurred duringlumbar discectomy but was restored when the leak was repaired.

Epidural techniques can be used, but it may be unwise to insert a catheterpercutaneously in patients with spinal stenosis.

General anaesthesia

Airway problems

Airway problems encountered in anaesthesia for spinal surgery fall into three groups.

Patients in actual or imminent respiratory failure due tomyelopathy or spinal deformity.Thesepatients are likely to have a di�cult post-operative period and may require prolongedventilatory support. Respiratory complications are the major cause of death aftersurgery in patients with severe myelopathy.4 As a general rule patients with severemyelopathy undergoing major surgery are better managed with a tracheostomy.

Patients in whom itmay be di�cult to intubate the trachea.The causes of di�cult intubationare sti�ness, deformity or swelling. Di�cult direct laryngoscopy ismore commonwhenthere is cervical disease, particularly cervical disease above C3, a cranio-cervical ®xationdevice, a ¯exion deformity, or associated temporo-mandibular joint dysfunction.61

Rheumatoid arthritis is the disease process associated most commonly with di�cultlaryngoscopy and when surgery is required for cervical rheumatoid disease theprevalence of di�culty is about 40%.61 Rheumatoid arthritis also a�ects the glottictissues, producing swelling and sti�ness of the arytenoid joints. Patients often admit toperiodic hoarseness or even stridor. Airway obstruction can occur post-operatively.Wattenmaker et al62 suggested that reduced glottic trauma at intubation accounted fortheir observation that fewer post-operative glottic problems occurred in rheumatoidpatients who had been intubated ®bre-optically than after intubation with directlaryngoscopy.

Identi®cation of the patients who will be di�cult to intubate remains an inexactscience, with many false positives and false negatives. The two important consider-ations are the amount of cranio-cervical extension and mandibular protrusion present.The Mallampati examination is probably as good a test for cranio-cervical extension asany, imperfect as it is.61 Mandibular protrusion can be assessed as class A, B or C


where C is the inability to advance the mandible to allow the incisors to touch.61 ClassC protrusion is the only sign invariably associated with di�cult laryngoscopy in theauthor's experience.

Patients in whom di�cult laryngoscopy is essentially due to sti�ness, rather thandeformity or swelling, are generally easily intubated with the ¯exible ®bre-opticlaryngoscope. Flexible ®bre-optic intubation can be technically di�cult in patients withdeformities of the cervical spine which cause the glottis to be displaced.63 Patients inwhom the problem is largely due to swelling, such as those with cervical infections, areoften di�cult to intubate with a ®bre-optic laryngoscope, because of secretions anddiminished airspace.

The best conditions for ¯exible ®bre-optic intubation occur when the patient issedated and topical anaesthesia is used.64 Fibre-optic intubation under topical anae-sthesia has been reported as having a stable cardiovascular pro®le64, but hypotension isnot infrequently seen after induction of anaesthesia, possibly because of absorption oflidocaine (Mason RA, personal communication).

In cases where di�cult laryngoscopy was not expected the ®bre-optic laryngoscopecan be used instead of a rigid blade. The technique of ®bre-optic intubation through thelaryngeal mask airway (LMA) is useful. The only tube that is suitable is a size 6Mallinkrodt Safety-Flex2.65 It is vital that the tube is rotated many times as it isadvanced.65 If a larger size of tube is required or the LMA must be removed duringsurgery, the tube can be substituted by introducing a gum-elastic bougie. The McCoylaryngoscopewill improve theglottic viewatdirect laryngoscopy in about40%of cases.66

Anterior cervical haematoma is a regular cause of airway obstruction after anteriorcervical surgery. The patients complain of not being able to breathe. SpO2 is a poorguide to airway patency; a small haematoma can obstruct the airway; a collar willobscure it; patients can deteriorate rapidly, and stridor may not occur. Patients ®nd itdi�cult to swallow and talk and are reluctant to lie ¯at. The wound should be openedas soon as possible and explored with a ®nger. Large bleeding vessels are rarely seen.Consider awake intubation in sitting position, which is unlikely to be easy (secretions,oedematous tissues). An inhalational induction with sevo¯urane or halothane isgenerally regarded as the wisest option. Do not extubate for at least 24 hours.

Surgical intrusion on the airway. High cervical surgery undertaken through maxilloto-mies or mandibular splits may result in signi®cant swelling of the soft tissues for somedays post-operatively. An elective tracheostomy is probably the best way to secure theairway.

Choice of agents for general anaesthesia

Spinal surgery is often painful, blood pressure must be maintained, but blood lossminimized, evoked potential monitoring must be facilitated and muscle relaxants oftenavoided. The author has found remifentanil to be a useful agent.

Blood loss

The bene®ts of normotension for spinal cord perfusion have to be weighed against thebene®ts of reduced blood loss in major spinal surgery. It is useful to have spinal cordmonitoring in place when blood pressure is to be reduced.

Desmopressin has been used to reduce blood loss by an e�ect on platelet activity.Worthwhile reductions in loss have been observed, but the e�ect is unpredictable.67

638 I. Calder

Hypothermia should be avoided, because blood clotting is impaired68 and theincidence of wound infection is increased.69

Extubation

Extubation should not be considered at the conclusion of major spinal surgery untilthe patient is nearly normothermic and has normal acid±base balance, but prolongedsedation and ventilation, which may encourage hypotension and mask the symptomsand signs of neurological deterioration, is not in the patient's interests.

Patients who have been di�cult to intubate should be extubated awake, and withinnormal hours whenever possible. Tracheal tube exchange catheters are useful if thereis a possibility that extubation may prove premature and are surprisingly well toler-ated.70 Patients that will be di�cult to intubate in the future should be advised to joinMedic-Alert.

Post-operative analgesia

Most spinal surgery involves excision of bone and is painful.Epidural catheters can be placed under direct vision at the time of surgery in some

procedures and allow excellent post-operative analgesia.71 Low concentrations ofbupivacaine and opiates should be used and the infusion adjusted to ensure that thesigns of a spinal haematoma are not obtunded.

FUTURE DEVELOPMENTS OF SPINAL SURGERY ANDANAESTHESIA

Methods of repair of central nervous tissue remain experimental. Recovery of hindlimb function after transplantation of intercostal nerve into the spinal cords of rats hasbeen reported.72 Percutaneous endoscopic techniques may prove useful.73 Framelessstereotactic techniques have been adopted with enthusiasm in cranial neurosurgery,and systems which will function in spinal surgery are being developed.74

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Anaesthesia for spinal surgery