The EMG is quite helpful in diagnosis but has been normal in a small proportion of our patients, even when many muscles are sampled. A typical “myopathic pattern” is disclosed, that is, many abnormally brief action potentials of low voltage in addition to numerous fibrillation potentials, trains of positive sharp waves, occasional polyphasic units, and myotonic activity—all but the brief potentials possibly reflecting irritability of the muscle membranes (see Chap. 46). These findings are most apparent in weak muscles and are almost always seen when proximal weakness is well developed but they also may be observed in clinically unaffected areas. Indolent and chronic cases in which fibrosis of muscle and wasting have supervened may show polyphasic units that simulate denervation–reinnervation changes, juxtaposed with myopathic motor units. The EMG is also helpful in choosing a muscle for biopsy sampling but care must be taken not to obtain tissue from precisely the same site as a recent EMG needle insertion as a spurious histopathologic appearance of muscle damage may be obtained in this region (see the following text). Our approach has been to perform the needle EMG examination on one side of the body and biopsies on the other side.
As stated earlier, the ECG is abnormal in some cases and this finding may suggest the need for vigilance regarding cardiac symptoms and arrhythmias.
The results of magnetic resonance imaging (MRI) of muscle have been interesting and may aid the clinician in that abnormalities in T1, T2, and STIR signal intensity define regions of increased water content and inflammation and spectroscopic studies demonstrate regional deficits in energy production. Although MRI cannot at this time replace a biopsy for diagnosis, it can refine the distribution of lesions and aid in targeting the muscle biopsy, as well as provide a useful index of the efficacy of drug therapy. In some cases, MRI can distinguish IBM from either PM or metabolic muscle disease (see Lodi et al and also Dion et al).
Because of the scattered distribution of inflammatory lesions and destructive changes, only part (or none) of the complex of pathologic changes may be divulged in any single biopsy specimen. Because of this limitation, more than one site of biopsy or multiple samples through one incision is advisable.
In DM, there are several distinctive histopathologic changes. In contrast to the evident necrosis of single fibers of PM, DM is characterized by perifascicular muscle fiber atrophy (referring to changes at the periphery of a fascicle, for reasons noted below). Moreover, the inflammatory infiltrates in DM predominate in the perimysial connective tissue, whereas in PM they are scattered throughout the muscle and are most prominent in relation to the muscle fiber membrane and the endomysium. The muscle lesions in dermatomyositis of childhood are similar to those of the adult form, only greatly accentuated. In a biopsy sample, the diagnosis can be inferred from the perifascicular pattern of degeneration and atrophy of muscle fibers.
The principal changes in idiopathic PM consist of widespread destruction of segments of muscle fibers with an inflammatory reaction, that is, phagocytosis of muscle fibers by mononuclear cells and infiltration with a varying number of lymphocytes and lesser numbers of other mononuclear and plasma cells. Evidence of regenerative activity of muscle, mainly in the form of proliferating sarcolemmal nuclei, basophilic (RNA-rich) sarcoplasm, and new myofibrils, is evident in damaged regions. Many of the residual muscle fibers are small, with increased numbers of sarcolemmal nuclei. Some of the small fibers are found in clusters, the result of splitting of regenerating fibers. Either the degeneration of muscle fibers or an infiltration of inflammatory cells may predominate in any given biopsy specimen, although both types of changes are in evidence at autopsy.