ElectrodiagnosisPrepared by: Floriza P. de Leon,

PTRP

Electrodiagnosis Concerned with the study of electrical

activity in motor units when stimulated by electrical pulses

Results maybe interpreted for diagnosis/prognosis

Rheobase & Chronaxie

Rheobase

Intensity of current necessary to produce a minimal perceptible and palpable contraction, using a prolonged pulse duration

Taken by:• Place cathode on the motor point, or use bipoloar

technique with the cathode on the distal end of the mm

• Use square pulses, 100-300 ms/1000 ms• Find intensity that will produce minimal

perceptible and palpable contraction (in mA/V)• (N) value = 2-18 mA; 5-35 V

Rheobase

Factors Affecting Rheobase• Skin Resistance and subcutaneous tissue

Palm/skin over lower leg -- ↑R; therefore ↑I After denervation, skin becomes dry and scaly – alters rheobase Each person has each own rheobase Obese - ↑R

• Edema and Inflammation Makes it difficult for current to pass through because the excess

fluid dissipates the current Therefore, ↑ intensity which is uncomfortable for many pxs

• Ischemia and underlying pain• Temperature

Heat - ↓rheobase Cold - ↑rheobase Therefore treat px with IRR before ES

Rheobase Diagnosis:

• Denervation ↓rheobase (around 59% of (N)) May also be found to ↑ -- due to other factors Falls below (N), 10-20 days after denervation and remains

low• Partial denervation – no change• Degeneration

↑rheobase 10-15 days after nerve lesion• Re-innervation

Sharp rise in rheobase (5-6x (N)) which then slowly falls After nerve repair, threshold increases abruptly when

nerve have reached mm, then returns to (N)

Chronaxie

Time to induce minimal visible contractions with a stimulus 2x the strength of the rheobase

(N) value = < 1 ms; 0.05-0.5 ms Birth – 10x (N) 3rdmos – lower than at birth 18th -20th mos – (N) Proximal mm - ↑ Distal mm - ↓ Facial mm – low ↑ chronaxie - ↓excitable of mm Factors affecting chronaxie

• Skin texture - Dry skin alters or makes it difficult to obtain chronaxie• Ischemia – decrease in blood blow; decrease mm excitability; 100%

increase in chronaxie• Edema – difficult to obtain chronaxie; fluid dissipates heat• Fatigue – 2x chronaxie, then goes back to (N)• Electrode positioning – when not on motor point, you get 10x (N)

chronaxie

Chronaxie Diagnosis

• Denervation if whole mm is affected – 50-200x increase (up to 25 ms) decrease to 15 ms by 30th-40th day after denervation

• partial denervation – little change• re-innervation

progressive decrease of chronaxie decreased chronaxie does not precede recovery and

does not give an indication of recovery chronaxie is the last criterion to reach (N); voluntary movement precede (N) chronaxie level

 

Accommodation Quotient

Accommodation Quotient

Formula (N) = 3-6 Denervated = below 3 No accommodation – 1 or below  

Pulse Ratio

Pulse Ratio Formula (N) = little or no difference (<2.2:1) Denervated = >2.5:1 Complete degeneration = no

response to 1 ms pulse  

Galvanic-tetanic ratio

Galvanic-tetanic ratio Formula (N) = 3.5 -6:1 Denervation – 1.5-1:1 Degeneration – 10:1 after 30 day,

then decreases until it reaches 1:1 Regeneration – 20:1, then decreases

to (N) value; voluntary contractions precede reaching (N) value

 

Nerve Excitability Test

Nerve Excitability Test Determines excitability and conduction of a nerve trunk Uses square pulse of 0.1 or 1 ms pulse duration, f= 1 Hz Threshold value to produce a minimal perceptible

contraction is determined Factors affecting nerve excitability test

• Heat - ↓values; cold - ↑values• Thickness of soft tissue – if thick, ↑R therefore ↑values• Electrode positioning• Movement and tension of mm• Note

Daily assessment is made from 3rd day after onset until 10th day; if changes are noticed continue until 14th day

(B) sides are assessed and the difference in values is noted Progressive increase in value in 6 days indicates

↑swelling around the nerve; indicates decompression by surgery

Nerve Excitability TestDifference In values

(mA) (V)

neuropraxia 1-2 2-4

Denervatio/axonostenosis

3-4 8-12

Denervation/axonotmesis

5-7 12-18

Neurotmesis/severe axonotmesis

nil Nil

Stimulation Point of Nerve Trunks:

Facial nn – anterior to the mastoid process Erb’s point – lower inner angle of the

supraclavicular fossa (results to contraction of deltoids, biceps, brachialis and brachioradialis – brachial plexus)

Ulnar nn – upper point (medial epicondyle); lower point – just above the wrist, ulnar border

Radial nn – halfway down the arm posteriorly Tibial nn – above center of popliteal crease Deep peroneal nn – just behind head of fibula Superficial peroneal nn – 1 cm below deep peroneal

nn  

Strength-Duration Curves (SDC)

Strength-Duration Curves (SDC) Curve obtain by joining pts that graphically represent the

threshold values X= duration; y = threshold value (intensity) How?

• Use pulse duration = 0.02-1000 ms (longest duration must be at least 100 ms)

• Intensity needed to produce minimal visible and palpable contraction is noted and plotted in the graph

• At least 6-10 pulses are taken Done 10-14 days after injury Individual mm is stimulated Factors affecting SDC

• Skin temperature – cold decreases threshold• Edema• Ischemia• Deeply placed mm – invalid results• Electrode positioning – cathode is on motor points

Practical uses of SDC in px• Presence or absence of excitable mm fiber

Complete denervation – only long pulse duration will produce response; increase intensity is needed for shorter pulse duration If with atrophy – no response to short duration pulses, no horizontal part

Partial denervation – revealed by presence of kinks – discontinuities; with an innervated and denervated components

• Signs of re-innervation 1st sign of recovery in completely denervated mm is presence of kinks May appear 3-4 mos before return of voluntary activity Also shown by movement of the kink to the left (good sign of

reiinervation)• Chronaxie• Progress of lesion• Utilization time – px at which the curve begins to flatten;

probable pulse duration suitable for electrical stimulation of mm

Use of progressive currents in SDC

• Factor of accommondation of mm and nerve is utilized

• Ensures stimulation of denervated mm only with long duration pulses Denervation – threshold drops (5-10x lower than (N)) during 1st 30 days (nerve easily responds to increasing intensity with progressive atrophy – threshold rises curve is displaced to the right and upwards

partial denervation – presence of kinks (noticeable); shift of utilization time to the right