Much more puzzling have been cases of myopathy with the clinical features of idiopathic polymyositis and the presence of noncaseating granulomas in the muscle biopsy but with no evidence of sarcoidosis of the nervous system, lungs, bone, skin, or lymph nodes. Such cases call into question the validity of a muscle granuloma as a criterion of sarcoidosis, but the matter cannot be settled until we have a better definition and etiology for sarcoidosis. These cases are presently classified as granulomatous myositis and, if limited to one or a small group of muscles, localized nodular myositis (Cumming et al). In a syndrome described by Namba and colleagues, this type of myositis was combined with myasthenia gravis, myocarditis, and thyroiditis. The muscle process has, on a few occasions, also been associated with Crohn disease. Electron microscopy has disclosed muscle fiber invasion by lymphocytes, suggesting a cell-mediated immune reaction. Very rarely, a granulomatous myositis may complicate tuberculosis or syphilis.
The muscular dystrophies are a group of progressive hereditary degenerative diseases of skeletal muscles. The intensity of the degenerative changes in muscle and the cellular response and nature of the regenerative changes distinguish the dystrophies histologically from other diseases of muscle and also have implications regarding their pathogenesis. The category of more benign and relatively nonprogressive myopathies—each named from its special histopathologic appearance, such as central core, nemaline, mitochondrial, and centronuclear diseases—present greater difficulty in classification. Like the dystrophies, they are primarily diseases of muscle and are often heredofamilial in nature, but they are placed in a separate category because of a nonprogressive or slowly progressive course and their distinctive histochemical and ultrastructural features.
The current clinical classification of the muscular dystrophies is based mainly on the distribution of the dominant muscle weakness and the responsible mutation but several of the classical types have retained their eponymic designations: Duchenne, Becker, Emery-Dreifuss, Landouzy-Dejerine, Miyoshi, Welander, Fazio-Londe, and Bethlem are among the ones that still have utility in shorthand. To these are added myotonic dystrophy and a group of so-called congenital muscular dystrophies, usually severe in degree.
The extraordinary depth of information regarding the molecular nature of the dystrophies is one of the most gratifying developments of modern neuroscience. The majority of the dystrophies are caused by changes in structural elements of the muscle cell, mainly in its membrane, but other important mechanisms also are being identified. In keeping with the outlook expressed throughout the book, we adhere to a clinical orientation in describing the muscular dystrophies but make clear that treatment in the future could be determined based on understanding of molecular mechanisms. Each of the muscular dystrophies is described in accordance with this scheme.