Entrapment Neuropathies

Peripheral neuropathy is defined as deranged function and structure of peripheral, motor, sensory, and autonomic neurons, involving either the entire neuron or selected levels. The major categories of peripheral neuropathies are seen in Table 28-1. Because this chapter is concerned with nerve problems seen in the foot that are most amenable to local treatment, only the last four categories are considered. A true neuroma consists of an unorganized mass of ensheathed nerve fibers embedded in scar tissue that originate from the proximal end of a transected peripheral nerve. Neuromas are always the result of trauma. When the injury is incomplete (partial laceration, traction) or the result of blunt trauma the lesion will form within the epineurium and produce a fusiform or eccentric nodular swelling termed "neuromain-continuity." In either case, the axonal elements are disrupted such that they are arranged in a somewhat haphazard fashion. Morton's neuroma, the interdigital or intermetatarsal lesion accurately described initially by the English chiropodist Louis Durlacher, is actually a misnomer. It is neither a true neuroma nor a neoplasm. Rather, it is best defined as a mechanical neuropathy with compression, stretching, and entrapment components in its etiology. Pathologically, this lesion is a progressive degenerative, and at times regenerative, process in which early and late changes may be found. Characteristic histologic findings support this etiology (Table 28-2). As a result, Morton's neuroma might be more accurately termed a perineural fibroma. 6,7 Mechanical peripheral neuropathies are caused by local or extrinsic compression phenomena or impingement by an anatomic neighbor causing a localized entrapment. Entrapment may also be caused by scarring or fibrosis from local trauma, bleeding, or traction that tends to bind the nerve down, thus restricting normal mobility within the tissues. Traumatic neuropathies are the result of either closed injuries or open injuries to peripheral nerves. Early treatment usually involves prophylaxis and repair, while later attention is directed toward the painful neuromas or nerve entrapments that result from the body's healing processes. Nerve sheath tumors are named according to their structure derivation. They can be benign or malignant. Nerve sheath tumors fall under another general category known as parenchymatous disorders because they can involve excessive growth of specific neural elements: neuron or axon, Schwann cell, perineurial cell, and endoneurial fibroblast. This is in contrast to the lesions described previously, termed interstitial disorders, in which external factors cause the derangements.