Chapter summary
Study guidelines
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A review of the electrical events described in Chapter 7 may be worthwhile. This chapter applies some of the basic principles described there.
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Remember that when nerves are stimulated electrically through the skin, whether for the study of motor or sensory conduction, the waves of depolarisation travel in both directions.
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Nerve conduction studies help to elucidate the nature and extent of neuropathies involving motor and/or sensory nerves. This is a quite complex area of investigation given the very large number of possible causes of peripheral neuropathies.
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Electromyography, whereby a recording electrode is passed into the interior of selected muscles, is essential for detecting spontaneously generated abnormal waveforms. It is used to help define the aetiology of muscle weakness and to monitor progress under therapy.
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Clinical Panel 12.2 may require consultation of the peripheral nerve section of a gross anatomy textbook.
The primary concerns of clinical neurophysiology laboratories are twofold: assessment of the functional state of the peripheral nervous system (PNS) and assessment of cerebral cortical function. PNS assessment entails the use of nerve conduction studies (NCS) by stimulation of selected peripheral nerves while recording the waveforms of their response, and the use of electromyography (EMG) by recording the waveforms generated by selected muscles during voluntary contraction. The combination of NCS and EMG is referred to as e lectrodiagnostic examination .
Nerve conduction studies
NCS are routinely employed as an extension of the clinical examination of suspected disorders of the PNS. Through stimulation of nerves allied to the recording of muscle fibre depolarisations, it is possible to determine whether the disorder involves the nerve, neuromuscular junction, or muscle. NCS can also determine whether the disorder is a focal or diffuse process involving sensory and/or motor axons and whether it is primarily affecting myelin or axons.
Nerve conduction in the upper limb
The main nerve to detect focal (as distinct from generalised) disorders within the peripheral neuromuscular system is the median nerve. The median nerve, a mixed motor and sensory nerve, has three key advantages for electrophysiologic studies of a general nature:
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It is readily accessible for stimulation and/or recording at the elbow and wrist.
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For motor NCS the abductor pollicis brevis, supplied by the median nerve, is readily available for either surface or needle EMG.
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For sensory NCS the skin of the index finger is ideal for recording action potentials travelling antidromically following median nerve stimulation at the elbow or wrist. (As noted in Chapter 11 , antidromic means ‘running against’ the normal [orthodromic] direction of impulse conduction.)
Motor nerve conduction
Stimulation
A typical stimulating electrode is one with an anode and a cathode in the form of two blunt prongs, which are applied to the skin surface overlying the nerve. In Figure 12.1 an electrode has been placed over the median nerve at the wrist (just lateral to the cordlike palmaris longus tendon). The cathode is placed nearer to the recording site than the anode to prevent any conduction block by the anode. When sufficient current is passed 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 motor units in the abductor pollicis brevis.
Recording
An active surface recorder, in the form of a disk in this situation, is placed over the midregion 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 magnify evoked motor responses is designed to record the potential differences between the 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 onscreen change in the tracing will be a small stimulus artefact on an otherwise flat tracing. As the current increases, small compound motor action potentials (CMAPs) appear. These are produced by activation of large myelinated axons close to the stimulator; the depolarisation wave travelling along each axon will in turn depolarise all of the muscle fibres in the territory of 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 two or three hundred 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 onscreen 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 the surface membrane depolarisation and hence lead to abnormalities of the waveform morphology.
Increasing the applied voltage activates additional motor units until all are activated by each pulse. The required stimulus is called maximal. For good measure the final stimulus is often supramaximal at 5 to 10% above maximal. The final waveform observed constitutes the CMAP. It is produced by summation of the individual muscle fibre potentials ( Figure 12.2 ).
Routine measurements of the final CMAP are shown in Figure 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. (The final, positive phase is produced by inward ion movement 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 ( Figure 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
It may be considered wise to perform a confirmatory MNCV on another nerve. 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 at 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 ( Figure 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 of the index finger, which is wearing an active recorder in the form of a ring.
The myelinated nerve fibres to be sampled by the ring recorder are those supplying the highly sensitive and discriminatory skin of the finger pad, described in Chapter 11 . The largest, serving Meissner and Pacinian corpuscles and Merkel cell–neurite complexes, are known to normally conduct at a speed of 60 to 100 m/s and the finest, serving mechanical nociceptors, at 10 to 30 m/s (these nerve fibres do not contribute to the routinely recorded sensory nerve responses). This variation is in marked contrast to that of the relatively uniform fibre size of the stem axons supplying the small motor units of the abductor 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 Figure 12.5 the asterisks are intended to highlight the difference in the shape of the distal versus proximal waveforms of the compound sensory action potentials (CSAPs). Two factors are involved:
- 1.
Physiologic temporal dispersion. As runners in a race become progressively separated over distance, the fastest impulse conductors take the lead and the slowest trail behind, with consequent elongation of the CSAP profile over the longer test distance, caused by this temporal dispersion (scattering over time).
- 2.
Phase cancellation. The later waveform is also flatter. This is explained in part by the phenomenon whereby positive and negative phases of adjacent waveforms tend to cancel each other out. It should be emphasised that this phase cancellation is not a physiologic event: the waves themselves are not affected by the ‘eavesdropper’ wrapped around the finger close to the finishing line. An additional factor is the diminishing amount of phase summation being recorded with increasing separation of action potentials. While a similar process of physiologic temporal dispersion resulting in phase cancellation of the recorded response occurs for MNCV, 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, 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 Figure 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 a ring recorder slipped onto the little finger.
Nerve conduction in the lower limb
Motor nerve conduction
The lower limb nerve most frequently sampled for MNCV is the deep peroneal nerve with recording from the extensor digitorum brevis on the dorsum of the foot ( Figure 12.6 ). The deep peroneal nerve is stimulated first in front of the ankle and then at the level of the neck of the fibula. At times, tibialis anterior is also sampled; in this case the common peroneal (fibular) nerve is stimulated first at the neck of the fibula and then at the lateral edge of the popliteal fossa next to the biceps femoris tendon.
A second choice for MNCV is the tibial nerve recording from the adductor hallucis, located on the medial side of the foot ( Figure 12.7A ).
Sensory nerve conduction
For SNCV assessment the sural nerve is the nerve of choice. It arises from the tibial nerve and receives a contribution from the common peroneal nerve; it supplies the skin along the lateral margin of the foot. The recorder is applied to the skin below the lateral malleolus, and the nerve is stimulated antidromically at the levels shown in Figure 12.7B .
Nerve root pathology
Nerve root pathology is known as radiculopathy ( L. radix, ‘root’). Radiculopathies are encountered:
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in the neck, where roots of spinal nerves C6 and C7 are especially prone to being pinched by osteophytes generated by cervical spondylosis (explained in Clinical Panel 12.1 );
Clinical Panel 12.1
Peripheral neuropathies
Peripheral neuropathies are amenable to several classifications, each with its own relevance:
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Histologic classification is touched upon in Figure 12.8 , where neuropathy originates in myelin sheaths of the top two nerve fibres and in the axon of the other three.
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Anatomic classification identifies nerve numbers and locations. Numbers include mononeuropathy, referring to a single spinal or cranial nerve (e.g. sciatica, trigeminal neuropathy, isolated peripheral nerve trauma), and mononeuropathy multiplex, referring to more than one affected nerve trunk (e.g. right median and radial, left sural, right peroneal). Location labels include plexopathy, referring to involvement of the cervical, brachial, or lumbosacral plexus in an individual case, and radiculopathy, referring to nerve root pathology, most frequently caused by compression of a nerve root in the region of an intervertebral foramen.
The terms primary and secondary are also anatomic. A primary neuropathy originates within nerve tissue (disease of myelin sheaths or of axons; if the process begins in the cell bodies within the dorsal root ganglion or AHCs then the term ‘neuronopathy’ is often used). A secondary neuropathy is typically caused by another medical illness that affects the peripheral nerves (e.g. diabetes mellitus).
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Aetiologic classification identifies causative agencies. Headings include toxins, including lead and arsenic; immune disorders, including effects of viruses; metabolic disorders, including diabetes; vitamin deficiencies such as B 12 in pernicious anaemia or thiamine deficiency associated with alcoholism; and genetic disorders such as Hereditary Motor and Sensory Neuropathy (HMSN or Charcot-Marie-Tooth disease).
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Time course classification may be condensed into acute/subacute, where the patient seeks help within days/weeks of onset, and chronic, where the patient may persevere for more than a year before seeking help.
One kind of acute polyneuropathy and two kinds of chronic polyneuropathy will now be described.
Guillain-Barré syndrome (GBS) is an acute, autoimmune, inflammatory neuropathy that occurs in all countries, affects men slightly more frequently than women, and affects all age groups but with an incidence that increases with age. In most cases GBS follows an infection, usually gastrointestinal or upper respiratory; other antecedent events may include immunisation, or a surgical procedure. The most common antecedent is infection with Campylobacter jejuni . Typical presentation is progressive, bilateral, and symmetric weakness accompanied by diminished or absent reflexes, commencing in the feet and hands and ascending to involve the muscles of the trunk, neck, and face and the respiratory muscles; autonomic dysfunction is often present. Rarely, progress may be so rapid as to cause death within a few days from respiratory and/or circulatory collapse; usually the peak of the illness occurs within 2 weeks (by 4 weeks in almost all cases). Aching pain and tenderness occur in affected muscles along with minimal cutaneous sensory loss. Reduced autonomic function may be demonstrated by fluctuating heart rate and blood pressure and/or retention of urine requiring catheterisation for a few days.
Electrodiagnostic examination reveals reduced conduction velocity, dispersion of motor responses, or conduction block in motor nerves, reflecting varying degrees of disruption of saltatory conduction. A lumbar puncture is performed in patients suspected of having GBS; diagnosis is supported by an elevated protein level and absence of leucocytes (albuminocytologic dissociation) .
Rapid recovery may be spontaneous in relatively mild cases but many patients require multidisciplinary care for respiratory failure, autonomic dysfunction, and prevention of the medical complications of prolonged immobility. Immunomodulatory therapy consisting of either immune globulin injections or plasma exchange is also provided because it can hasten recovery. Where axons have degenerated in the acute phase, recovery may take more than a year and is often incomplete with residual motor deficits.
Chronic polyneuropathy originating in myelin sheaths
This type of neuropathy is associated with chronic vitamin deficiency, longstanding diabetes, or chronic hypothyroidism. The myelin sheaths of the peripheral nerves degenerate while the axons remain relatively intact. Because potassium channels are largely obliterated when the sheaths are initially laid down and because saltatory conduction is lost along with the sheaths, sensory conduction is progressively impaired. As might be anticipated the longest nerves are the most affected, yielding ‘glove and stocking’ paraesthesia ( Figure 12.8 ). (The term paraesthesia refers to sensations of numbness, pins and needles, and/or tingling.)
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