Mononeuropathies

Abstract

Mononeuropathies, especially those seen with entrapment are among the most common conditions investigated with the use of electrodiagnostic (EDX) studies. These studies not only help to identify the single peripheral nerve involved, but also the site of the lesion, the type of fibers involved (sensory and/or motor), the character of the lesion (axonal vs. demyelinating), and the age/chronicity of the lesion. There is also the potential to obtain prognostic information, including features of ongoing reinnervation in axon loss lesions. The most distinctive localizing features occur with focal demyelination (as seen in many entrapment mononeuropathies). These include the presence of conduction block and/or focal conduction slowing, with or without concomitant axon loss features. However, these features may not always be found, and in some cases lesions are not able to be precisely localized with only findings of axonal loss on nerve conduction studies (NCS) and needle electromyography (EMG) in muscles innervated by the affected single nerve. However, knowledge of the anatomy of the course of individual peripheral nerves, and their branches which supply specific muscles allows the electromyographer to at least localize to a particular segment of the nerve in most cases.

In the upper extremity, the most common mononeuropathies include median neuropathy at the wrist (carpal tunnel syndrome), and ulnar neuropathy at the elbow. In the lower extremity, it is peroneal (fibular) neuropathy at the fibular head.

Causes of mononeuropathies include mechanical injury caused by compression or trauma like fractures or joint dislocations, repetitive activities like those seen in typists or carpenters, endocrine disorders like diabetes and hypothyroidism, local tumors like Schwannomas, pregnancy, prolonged limb posture during surgical procedures, among many others.

Most mononeuropathies involve sensory and motor fibers (potentially affecting corresponding sensory and motor responses on NCS), but some are purely motor including the anterior interosseus neuropathy, posterior interosseus neuropathy, long thoracic neuropathy and spinal accessory neuropathy. Others may be purely sensory, like lateral femoral cutaneous neuropathy (meralgia paresthetica), and superficial radial sensory neuropathy (cheiralgia paresthetica).

The EDX study is also very helpful to differentiate between mononeuropathies and other patterns of injury affecting the peripheral nervous system including radiculopathies, plexopathies, and polyneuropathies.