SEGMENTAL DEMYELINATION

Segmental demyelination occurs when there is dysfunction of the Schwann cell (as in Guillain-Barré Syndrome) or damage to the myelin sheath (e.g., in hereditary motor and sensory

neuropathy); there is no primary abnormality of the axon. The process affects some Schwann cells and their corresponding internodes while sparing others (see Fig. 27-1 ). The

disintegrating myelin is engulfed initially by Schwann cells

Figure 27-4 A, ATPase histochemical staining, at pH 9.4, of normal muscle showing checkerboard distribution of intermingled type 1 (light) and type 2 (dark) fibers. B, In contrast,

fibers of either histochemical type are grouped together after reinnervation of muscle. C, A cluster of atrophic fibers (group atrophy) in the center (arrow).

Figure 27-5Electron micrograph of a single, thinly myelinated axon (arrow) surrounded by concentrically arranged Schwann cells, forming an onion bulb. (Courtesy of G. Richard

Dickersin, MD, from Diagnostic Electron Microscopy: A Text-Atlas. New York, Igaku-Shoin Medical Publishers, 2000, p. 984.) Inset, Light microscopic appearance of an onion bulb

neuropathy, characterized by "onion bulbs" surrounding axons.

Figure 27-6Electron micrograph of a degenerating axon (arrow) adjacent to several intact unmyelinated fibers (arrow-heads). The axon is markedly distended and contains numerous

degenerating organelles and dense bodies.

TABLE 27-2-- Hereditary Sensory and Autonomic Neuropathies

Disease and Inheritance Gene and Locus Clinical and Pathologic Findings

HSAN I; autosomal-dominant Serine palmitoyl transferase, long-chain base, subunit 1

(SPTLC1) gene; 9q22.1-q22.3

Predominantly sensory neuropathy, presenting in young adults; axonal

degeneration (mostly myelinated fibers)

HSAN II; autosomal-recessive (some cases

are sporadic)

Unknown Predominantly sensory neuropathy, presenting in infancy; axonal

degeneration (mostly myelinated fibers)

HSAN III (Riley-Day syndrome; familial

dysautonomia; most often in Jewish children);

autosomal-recessive

Inhibitor of kappa light polypeptide gene enhancer in B

cells, kinase complex-associated protein (IKBKAP or

IKAP) gene; 9q31-q33

Predominantly autonomic neuropathy, presenting in infancy; axonal

degeneration (mostly unmyelinated fibers); atrophy and loss of sensory

and autonomic ganglion cells

The pathologic findings of many of the hereditary neuropathies are those of an axonal neuropathy. Fiber loss is the most prominent finding.

Hereditary Motor and Sensory Neuropathy Type I

The most common hereditary peripheral neuropathy, Charcot-Marie-Tooth (CMT) disease, hypertrophic form (HMSN I), usually presents in childhood or early adulthood. A

characteristic progressive muscular atrophy of the calf seen in these patients gives rise to the common clinical term peroneal muscular atrophy. Patients may be asymptomatic, but when

they present, it is often with symptoms such as distal muscle weakness, atrophy of the calf, or secondary orthopedic problems of the foot (such as pes cavus).

Date: 2016-04-22; view: 811