Spinobulbar muscular atrophy (SBMA) (Kennedy et al., 1968) is a familial motor neuron disease with an XR inheritance pattern. It is caused by expansion of cytosine-adenine-guanine (CAG) repeats in the coding region of the androgen receptor gene mapped to Xq11-12 (La Spada et al., 1991). Symptoms usually manifest during middle age with atrophy of the tongue and proximal muscles associated with marked fasciculations of the tongue, limb, and facial muscles, often accompanied by hand tremor. Gynecomastia and a moderate degree of peripheral neuropathy may occur concomitantly.
Muscle biopsy shows extensive group atrophy usually accompanied by numerous extremely atrophic muscle fibers. Widespread fiber-type grouping with more than a hundred fibers of the same fiber type can be detected, sometimes mimicking fiber type predominance of either types I or II (Fig. 10).
In chronic neurogenic atrophy of muscle, as seen in this case of bulbospinal muscular atrophy, extremely atrophic fibers and hypertrophic fibers are observed (a, b). Fiber type grouping is also seen (c).