Electromyographynerve Conduction Studies

REPETITIVE STIMULATION

In the normal subject, repetitive stimulation of a motor nerve at a frequency of < 30/sec produces a muscle potential of constant form and amplitude. Increasing the stimulus frequency to >30/second results in fatigue manifest by a decline or 'decrement' in the amplitude. In patients with disorders of neuromuscular transmission, repetitive stimulation aids diagnosis:

Myasthenia gravis

A decrementing response occurs with a stimulus rate of 3-5/second.

Myasthenic (Eaton Lambert) syndrome With a stimulation rate of 20-50/second (i.e. rapid) a small amplitude response increases to normal amplitude -incrementing response.

SINGLE FIBRE ELECTROMYOGRAPHY

A standard concentric needle within muscle will record electrical activity 0.5-1 mm from its tip - sampling from up to 20 motor units. A 'single fibre' electromyography needle with a smaller recording surface detects electrical activity within 300 /im of its tip - sampling 1-3 muscle fibres from a single motor unit.

Parent axon from single anterior horn cell

Branch axons s* Muscle fibres

Parent axon from single anterior horn cell

Branch axons s* Muscle fibres

Recording needle

Action potentials recorded from two muscle fibres are not synchronous. The gap between each is variable and can be measured if the first recorded potential is 'locked' on the oscilloscope.

This variability is referred to as JITTER - normally 20-25 /is (2-5 ¿<s due to transmission in the branch axon -15-20 /is to variation in neuromuscular transmission).

Single fibre electromyography is occasionally helpful in the investigation of disorders of neuromuscular transmission. In ocular myasthenia, the affected muscles are not accessible and frontalis is sampled instead.

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