Repetitive nerve stimulation test

Repetitive nerve stimulation test

1. Repetitive nerve stimulation Dr Vaishal Shah SR Neurology GMC Kota

2. Introduction Variant of the Nerve conduction Study First Described by German Neurologist Jolly in 1895 Harvey and Masland (1941) reported electrical decremental muscle response on repetitive motor nerve stimulation. Electrical stimulation is delivered to a motor nerve repeatedly several times per second observing the change in the CMAP.

3. Introduction RNS is relatively insensitive,10-50% in ocular myastenia,75% in generalised MG. RNS is relatively specific (90%).

4. Introduction Quantum. - A quantum is the amount of Ach packaged in a single vesicle (5000-10000 molecules). End plate potential -EPP is the potential generated at the postsynaptic membrane following a nerve action potential and neuromuscular transmission.

5. Introduction Muscle action potential (MAP) – If EPP exceeds threshold generated MAP. CMAP – Sum of MAPs generated by no of fibres. Safety Factor –Amplitude of EPP above threshold needed to generate MAP.

6. Introduction Ach stores Primary or immediately available store - 1000 quanta- beneath presynaptic nerve terminal membrane. Secondary or mobilization store - 10,000 quanta- supplies the primary stores after few seconds. Tertiary or reserve store - More than 10,000 quanta –in the axon and cell body.

7. Physiological modelling of RNS m = pn . m=number of quanta released during each stimulation. P=probability of release ( effectively proportional to the concentration of calcium ). Typically 0.2 in normal subject. n=number of quanta in the immediately available store (at baseline, approximately 1000 in normal subjects )

8. Physiological modelling of RNS Low Rate RNS (<5 Hz) Progressive decline of Ach Quanta from Primary store. EPP falls in amplitude but it remains above the threshold to Generate muscle action potential with each stimulation. After 1-2 seconds, secondary mobilization store replaces depleted quanta .

9. Physiological modelling of RNS Rapid RNS ( 10 - 50 Hz ) It takes 100 msec for ca2+ to diffuse back out of the presynaptic terminals. With rapid RNS, the depletion of quanta is counterbalanced by (1) increased mobilization of quanta from the secondary to the primary store (2) calcium accumulation in the presynaptic terminal, which increases p, the probability of release.

10. Physiological modelling of RNS Rapid RNS In normal subject – Same as slow RNS In postsynaptic disorders – Decreased EPP is repaired with rapid RNS In presynaptic NMJ disorders - MFAP is generated where one had not been present previously.

11. Exercise testing During voluntary contraction of muscle at maximum force, motor unit fires at 30-50 hz . Thus it acts a rapid RNS. “Post exercise facilitation” occurs after brief period of exercise or rapid RNS for 10 seconds. “Post exercise exhaustion” is seen after prolonged exercise usually 1 minute. EPPs typically increase initially, as described earlier, but then subsequently decline over the next several minutes, usually falling below baseline

12. Exercise testing Exercise testing is painless, whereas rapid RNS is quite painful and often difficult to tolerate.

13. Technical factors 1) Immobilisation CMAP configuration may change with movement of recording electrode. Immobilisation is easy for distal nerve than proximal.

14. Technical factors 2) Stimuli must be supramaxiamal Submaximal stimulation can create problems including both artifactual CMAP decrements and increments.

15. Technical factors 3) Temperature must be controlled CMAP decrement may be diminished if the limb is cold. May be related to decreased functioning of the enzyme acetylcholinesterase when it is cold, effectively making more ACH available to bind at the ACHRs. 33 C

16. Technical factors 4) Acetylcholinesterase Inhibitors should be Withheld Prior to the Study Pyridostigmine – 4 hours before study

17. Nerve selection Yield of abnormality increases with the use of proximal nerves. But technical difficulties are more. Upper trapezius – spinal accessory nerve is favourable. 15-25 mA is supramaximal. Facial nerve – Nasalis, orbicularis oculi. But CMAP are small at baseline and muscle can’t be immobilised.

18. Number of Stimulations A train of 5-10 pulses are preferable. When the mobilization store begins to resupply the immediately available store, the decrement begins to improve. The result is a so-called U-shaped decrement , highly characteristic of true NMJ disorders

19. Decrement and Increment Calculation

20. Decrement and Increment Calculation Any decrement of >10% is defined as abnormal. It should be reproducible. In normal subjects, pseudofacilitation may cause an increment of up to 40%. Increments of >100% are often encountered in presynaptic NMJ disorders. Increments between 40 and 100% are best considered equivocal.

21. Other Disorders Show a Decrement on RNS Severe denervating disorders ( MND ). Myopathy – Myotonic dystrophy, metabolic myopathies ( McArdle’s disease )

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