Electrodiagnostic Examination

Motor Nerve Conduction

Stimulation . A typical stimulating device is one with a cathode and an anode in the form of two blunt prongs and applied to the skin surface overlying the nerve. In Fig. 12.1 it was placed over the median nerve at the wrist (just lateral to the cordlike palmaris longus tendon). The cathode is positioned nearer to the recording site than the anode. When sufficient current passes from cathode to anode, transmembrane ionic movements initiate impulse propagation in both directions along the nerve. Large myelinated nerve fibres lying nearest to the cathode are the first to become depolarised; these include the Aα diameter axons of anterior horn motor neurons. A pulse of 20 to 40 mA with a duration of 0.1 ms is usually sufficient to activate all the motor axons that innervate the abductor pollicis brevis.

Recording. The active electrode is placed over the mid-region of the muscle where the motor end plates are concentrated, the motor point. A second reference electrode is placed over a ‘neutral’ site a short distance away. The amplifier used to record these evoked motor responses is designed to record the potential differences between these two sites. The setup is arranged so that if the active electrode records a more negative response, this will take the form of an upward deflection on the monitor.

At a low level of stimulation, the only on-screen change in the tracing will be a small stimulus artefact on an otherwise flat tracing. As the current increases a small motor action potential appears. This results from activation of large myelinated motor axons close to the stimulator; the depolarisation wave travelling along each axon will in turn depolarise all of the muscle fibres innervated by that axon. In the case of the intrinsic muscles of the hand, including the abductor pollicis brevis, each motor unit has an innervation ratio of 200 or 300 muscle fibres per motor neuron. In large muscles not specialised for fine movements (e.g., deltoid, gastrocnemius), the minimum deflection on the monitor will be several times larger for two reasons: their motor innervation ratio is 1/1000 or more, and their larger muscle fibres generate action potentials of greater amplitude.

It should be emphasised that the on-screen waveform is not produced by the contraction process itself but by the extracellular potentials generated by depolarisation of the muscle membranes and filtered through the tissues and skin. However, while this distinction needs to be remembered, most disorders of muscle will also affect surface membrane depolarisation and hence lead to abnormalities of the waveform morphology.

Increasing the applied current activates additional axons until all the axons and their motor units are activated by each pulse. The required stimulus is called maximal but, for good measure, the final stimulus is supramaximal at 5% to 10% above maximal to ensure that all axons have been activated. The final waveform observed constitutes the compound motor action potential (CMAP) . It represents the summation of the individual muscle fibre potentials of all the motor units of all the motor axons of that nerve ( Fig. 12.2 ).

Routine measurements of the final CMAP are shown in Fig. 12.3 . They include the latency (time interval) between stimulus and depolarisation onset, and the amplitude and duration of the negative phase of the waveform. (Inward ion movement (K+ ions) produces the final, positive phase during collective repolarisation of the muscle fibres.)

Motor nerve conduction velocity . The setup required to determine motor nerve conduction velocity (MNCV) for the median nerve is straightforward ( Fig. 12.4 ). Here the nerve has first been activated at the wrist (S1) to generate and store a ‘wrist-to-muscle’ velocity record. The stimulator has then been placed over the median nerve at the elbow (S2) to provide an ‘elbow-to-muscle’ record. Speed being the product of distance over time, the elbow-to-wrist conduction velocity is given by subtracting one value from the other, as illustrated by the case example.

Second choice . Normally a confirmatory MNCV is performed on another nerve, and in the upper limb the ulnar nerve is usually the best second choice. The ulnar is the standard second choice: S1 is performed over the nerve at the wrist just lateral to flexor carpi ulnaris, and S2 is performed where the nerve emerges from behind the medial epicondyle. The active recorder is applied over the hypothenar muscles on the medial margin of the palm.

Sensory Nerve Conduction

For studies of sensory nerve conduction velocity (SNCV) , the median is again the nerve of choice ( Fig. 12.5 ). Again, it is large myelinated nerve fibres that will be stimulated, and the site and manner of stimulation at the elbow and wrist will be the same. On this occasion, however, we are selectively recording antidromic stimulation of cutaneous sensory fibres – specifically, of the digital branches of the median nerve to the skin over the index finger, using ring electrodes.

The myelinated nerve fibres to be sampled by the ring electrodes are those supplying the highly sensitive and discriminatory skin of the finger pad (described in Chapter 11 ). The largest fibres, serving the Meissner and the Pacinian corpuscles and the Merkel cell–neurite complexes, are known to normally conduct at a speed of 60 to 100 m/s and the finest fibres, serving mechanical nociceptors, at 10 to 30 m/s (these nerve fibres do not contribute to these routinely recorded sensory nerve responses). This variation is in marked contrast to that of the relatively uniform fibre size of the axons supplying the small motor units of the abductor pollicis brevis muscle and conducting at 45 to 55 m/s. One consequence is that when stimulating sensory nerves at increasing distances from the recording site, a change in the waveform shape is normally noted. In Fig. 12.5 the asterisks are intended to highlight the difference in the shape of the distal versus proximal waveforms of the compound sensory nerve action potential (CSNAP) . Two important factors involved in the CSNAP are:

• 1.

  Physiologic temporal dispersion . As runners in a race become progressively separated over distance, the fastest takes the lead and the slowest trails behind. The variable conduction velocities of the sensory axons that contributed to the CSNAP exhibit a similar phenomenon, with consequent elongation of the CSNAP profile over the longer test distance, resulting in temporal dispersion (scattering over time).

• 2.

  Phase cancellation . The later CSNAP waveform is also flatter. This is explained in part by the phenomenon where the recorded positive and negative phases of individual sensory axon waveforms tend to ‘cancel’ each other out. It should be emphasised that this phase cancellation is not a physiologic event and the individual axonal waveforms themselves are not affected by the ‘eavesdropper’ wrapped around the finger close to the finishing line. The diminishing amount of phase summation of the later CSNAP is the result of this increasing separation of action potentials.

While a similar process of physiologic temporal dispersion results in phase cancellation of the recorded response of the CMAP, it is normally not as evident. This is a result of less variation in conduction velocity of individual axons and characteristics of the motor waveform (duration and amplitude) itself. When temporal dispersion is detected in a CMAP, it is a pathologic sign and indicates demyelination.

Sensory nerve conduction velocity . The basic modes of operation and calculation are the same as shown for the MNCV study. A case example is included in Fig. 12.5 , which demonstrates phase cancellation from physiologic temporal dispersion.

Second choice . The ulnar nerve is the standard second choice. Ulnar nerve stimulation is performed at the wrist and elbow as before, with the CMAP recorded over the abductor digiti minimi muscle and a ring electrode slipped onto the little finger to record from the ulnar supplied digital nerves to the finger.

Motor Nerve Conduction

The lower limb nerve most frequently sampled for MNCV is the deep peroneal (fibular) nerve with recordings from the extensor digitorum brevis on the dorsum of the foot ( Fig. 12.6 ). The deep peroneal (fibular) nerve is first stimulated in front of the ankle and then at the level of the neck of the fibula.

A second MNCV is performed on the tibial nerve with recordings from the adductor hallucis, located on the medial side of the foot ( Fig. 12.7A ).