Nerve conduction studies

Nerve conduction studies determine the velocity of conduction and magnitude of evoked action potentials in peripheral nerves. Slowing of conduction across a selected segment of the nerve indicates compression or focal damage to the nerve at that segment. Decreased amplitude indicates loss of nerve fibers. Electromyography selectively tests alpha motor neurons. Loss of motor neurons is indicated by denervation fibrillation potentials in the muscles innervated, or by the presence of large and abnormal muscle action potentials.

Nerve conduction studies and electromyography are not tests of pain. They assess the integrity of large diameter sensory and motor fibers, and are useful for objectively establishing the presence of large fiber neuropathy. Any utility for the study of pain relies on the relationship between pain and impaired function in large diameter fibers. This relationship obtains in few conditions.

It is accepted that in patients with diabetic neuropathy, and other peripheral neuropathies, the pain experienced can be attributed to the neuropathy. Consequently, nerve conduction studies have a valid role to play in objectively establishing the presence and nature of the neuropathy.

Similarly, nerve conduction studies can objectively establish the presence of nerve compression in conditions such as carpal tunnel syndrome, tarsal tunnel syndrome, and ulnar nerve entrapment. Reciprocally, nerve conduction studies can exclude radial nerve entrapment as a differential diagnosis of lateral epicondylalgia of the elbow.50 In these conditions, although the mechanism by which pain is produced is uncertain, there seems to be general agreement that objective evidence of nerve compression is critical to making the diagnosis.51,52 5354 Caveats, however, apply.

Particularly with respect to the carpal tunnel, abnormal conduction velocities occur in asymptomatic indivi-duals.55,56 Consequently, nerve conduction studies carry a substantial and annoying false-positive rate. Although only a minority of normal individuals exhibit abnormal conduction velocities, these individuals outnumber patients with pain ostensibly due to carpal tunnel syndrome. Consequently, in patients with suspected carpal tunnel syndrome, investigators cannot be certain whether the abnormal conduction velocities they detect are due to disease or are an incidental (false-positive) finding. Investigators may choose to believe that the positive findings confirm their diagnosis, but this is a self-serving decision, not one based on epidemiological probity. For this reason, eminent authorities have challenged the validity of contemporary criteria for the diagnosis of carpal tunnel syndrome.57

Although commonly used in the assessment of patients with radicular pain, nerve conduction studies and elec-tromyography serve no useful purpose in this condition. They lack validity for any particular diagnosis. For the diagnosis of lumbar disk herniation, electromyography is confounded by too high a false-positive rate (Table 12.2).58,59,60 For the identification of the segmental level involved, electromyography is confounded by variations in the segmental innervation of muscles. It does not assist in pinpointing the diagnosis to any greater extent than clinical examination (Table 12.3).

Review articles have highlighted the lack of sensitivity and specificity of nerve conduction studies in the evaluation of radicular pain, and underscore their lack of utility in this condition.61,62,63,64 More elaborate studies, such as H reflexes and somatosensory-evoked potentials, do not improve the situation.61,6364 The only justifiable application of nerve conduction studies in patients with radicular pain is when the clinical picture is not clearly one of radiculopathy, and when the physician is genuinely concerned that the condition may be a peripheral neuropathy.

Table 12.2 The correlation between electromyography (EMG) findings and presence of a lumbar disc herniation identified at surgery.

Table 12.3 The correlation between electromyography (EMG) findings and the anatomical level of a lumbar disk herniation identified at surgery.

Table 12.2 The correlation between electromyography (EMG) findings and presence of a lumbar disc herniation identified at surgery.

EMG

Disk herniation

Sens.

Spec. LR

Present

Absent

Positive

126

14

0.78

0.30 1.1

Normal

35

6

Based on the data of Knutsson.

LR, likelihood ratio; Sens, sensitivity; Spec, specificity.

Table 12.3 The correlation between electromyography (EMG) findings and the anatomical level of a lumbar disk herniation identified at surgery.

EMG

Level affected

Sens.

Spec.

LR

Correct

Wrong

Positive

126

14

0.18

0.93

Normal

38

3

0.77

Based on the data of Knutsson.

LR, likelihood ratio; Sens, sensitivity; Spec, specificity.

Based on the data of Knutsson.

LR, likelihood ratio; Sens, sensitivity; Spec, specificity.

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