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IBM remains a poorly understood form of idiopathic inflammatory myopathy, although great progress in the areas of clinical recognition and pathophysiology have been made recently. The question of whether therapy can favorably influence short- and/or long-term outcome is still unanswered. Several recent reports suggest some possibility of at least slowing progression with immunosuppressive therapy. Long-term therapeutic trials with goals that include stabilization, rather than improvements in strength, are urgently needed.

IBM has emerged as a clinicopathological entity during the past 25 years but with increasing complexity. It occurs primarily in elderly persons (over the sixth decade of life, with 3:1 male preponderance), but young adults or children may also be affected in some families. FIBM is by and large non-inflammatory though some autosomal dominant FIBM cases have inflammatory cell infiltrates. In IBM, slowly progressive weakness of proximal as well as distal muscle groups occurs and is usually not associated with skin rash or malignancy. The incidence of associated collagen-vascular disease is thought to be lower than in DM or PM but is reported to be as high as 15%. It is generally refractory to treatment with corticosteroids or other immunosuppressants. Muscle biopsy and electromyography may suggest a neurogenic process mixed with myopathic features. None of the histopathological features is specific enough to be a diagnostic criterion. The diagnostic criteria have to be collective, encompassing both clinical and pathological criteria in different combinations. The presence of eosinophilic intranuclear or cytoplasmic inclusions immunoreactive for both beta-amyloid and ubiquitin in affected myofibres may facilitate the diagnosis of IBM. The diagnosis no longer depends on the ultrastructural demonstration of characteristic microtubular filaments as previously thought. The identification of both beta-amyloid and ubiquitin may provide a new concept for the disease process in IBM. A chronic persistent intracytoplasmic synthesis of abnormal amyloid protein in IBM is suspected to be similar to that in Alzheimer's disease. IBM is considered to be intimately related to a heterogenous group of non-inflammatory IBMD, including DMY, OPMD, and both autosomal recessive and dominant FIBM. An inflammatory response has been seen, however, in muscles of both OPMD and autosomal dominant FIBM. The pathogenesis in IBM and in IBMD may not be the same. Unlike IBM, there is no abnormal sarcolemmal expression of MHC-I antigen in IBMD as a sign of T-cell-mediated cytotoxicity causing myofibre destruction. The prion theory derived from identification of amyloidogenic protein in the filament inclusions in the rimmed vacuoles is provocative. If one believes in the contention that the amyloidogenic filaments are the primary pathogen of either IBM or IBMD, one must account for the fact that these filaments are originally derived from sarcolemmal nuclei and not from autophagic vacuoles. Until this is clarified, the possibility that the filaments represent either abnormal or defective 'slow' virus nucleocapsids cannot be completely ruled out.

Clinical and electromyographic findings do not clearly distinguish inclusion body myositis (IBM) from chronic polymyositis (PM). The rimmed vacuoles and filamentous nuclear and cytoplasmic inclusions that characterize IBM are often sparse and may be overlooked; conversely, these features may occasionally be seen in other diseases. Preliminary studies suggested that muscle fiber hypertrophy occurred more frequently in IBM than in PM. To investigate whether fiber hypertrophy can be used to improve the ability to separate IBM from PM, we report a morphometric analysis of 28 IBM cases, 22 PM and 22 dermatomyositis (DM) cases. The analysis, using a computer automated system, included proportion of hypertrophied fibers and also fiber type proportions, average fiber diameter, proportion of atrophic and angulated fibers, and the co-dispersion index (CDI). The proportion of hypertrophied fibers was greater in IBM than the other two conditions (IBM (mean +/- SEM) 31.0 +/- 4.7% and 12.2 +/- 2.4% for type 1 and type 2 fibers, respectively, compared to 9.8 +/- 3.0% and 3.3 +/- 1.7% in PM, and 7.7 +/- 2.7% and 3.9 +/- 1.9% in DM). These differences were statistically significant (P < 0.05) in both sexes for type 1 fibers and in women for type 2 fibers. Also, the average fiber size and hypertrophy factors for type 1 and type 2 fibers were increased in IBM compared to PM and DM. This study confirms that the presence of muscle fiber hypertrophy in biopsies from IBM patients may help differentiate them from other clinically similar inflammatory myopathies.

Inclusion body myositis is characterized by an insidious onset, progressive indolent course, and is generally felt to be refractory to standard therapy for myositis. We reviewed the charts of 32 patients with muscle biopsy findings suggestive of inclusion body myositis. The average time from symptom onset to diagnosis was 37 months, but initially 40% were incorrectly diagnosed. Twenty-eight patients (88%) were classified as definite or probable inclusion body myositis and were treated with various combinations of prednisone and immunosuppressive agents. Sixty-eight percent of those treated experienced a decrement in function and muscle strength. Three patients exhibited longterm improvement while 12 patients experienced delayed progression, defined by short term improvement in strength or a stable functional class, All of these patients received therapy, 5 in the form of methotrexate and prednisone. All untreated patients deteriorated clinically. In summary, (1) inclusion body myositis is a clinically distinct entity which is frequently misdiagnosed initially. (2) While clinical improvement with therapy is rare, our observations support recent reports that therapy may be associated with a slower rate of clinical progression. (3) Optimal therapy remains uncertain, but the use of low dose methotrexate and prednisone may warrant further study.

Myositis induced by Coxsackie A4 and A9 viruses was investigated in the gastrocnemius muscles of suckling mice and adult mice with denervation. Denervation markedly increased the susceptibility of adult mice to Coxsackie A virus infection, and this effect was initiated as early as 1 day after denervation. Light microscopy demonstrated inflammation and necrosis in the denervated gastrocnemius muscle of adult mice, whereas muscle from the contralateral leg showed only infrequent, mild, focal myositis. Ultrastructually, crystalline arrays of virus particles were seen in the infected muscle fibers and in the phagocytes of both suckling and adult mice. Nuclear alterations, especially in the myotubes, and a characteristic compound membrane-vesicle complex (CMVC) in the sarcoplasm, developed simultaneously. Replicating and fusing myoblasts, activated as part of the regenerative process after denervation, appeared to be closely associated with the susceptibility of muscle to Coxsackie A virus infection.

Among the generalized chronic idiopathic inflammatory myopathies, inclusion body myositis (IBM) has emerged as a clinico-pathologic variant during the past two decades. It occurs primarily in elderly persons (in approximately the sixth decade of life), but young adults (in approximately the second decade of life) may also be affected. Slowly progressive weakness of distal as well as proximal muscle groups in IBM is usually not associated with skin rash, malignancy or collagen-vascular disease, and is refractory to treatment with steroids or other immunosuppressants. Exceptions to each of these general rules have been found. Muscle biopsy and electromyography may suggest a neurogenic process mixed with myopathic features. Rimmed vacuoles with basophilic granules in cryostat sections stained with hematoxylin-eosin are strongly suggestive of IBM if accompanied by the histopathologic triad of polymyositis. The presence of eosinophilic intranuclear or cytoplasmic inclusions in affected myofibers is further suggestive of IBM. The ultimate diagnosis, however, depends on ultrastructural demonstration of characteristic microtubular filaments resembling the nucleocapsids of the paramyxovirus group. Recent reports of immunostaining of the inclusions for mumps virus antigen strongly suggest a chronic persistent mumps virus infection as the cause of IBM. IBM is considered to be pathologically related to both distal myopathy (DM) and oculopharyngeal muscular dystrophy (OPMD).

Abnormal transverse or T tubules spatially continuous with sarcoplasmic reticulum (SR), as visualized with a lanthanum (La) tracer, were found in biopsy specimens from nine patients with polymyositis whose serum enzyme (creatine phosphokinase [CPK]) levels were elevated before the time of biopsy. On comparison with the T system in more than 50 control subjects without CPK elevation, it became apparent that presumed anastomoses of T tubules with SR were the most consistent anatomical correlates of the enzyme leakage. Two types of anastomoses were seen on the basis of passage of La: first, early segmental dilation of T tubules and perpendicular continuations with SR; and second, focal proliferation of T tubules forming "T tubule networks" and then anastomosing with SR at the periphery of the networks. The latter is considered to be a concurrent repair process for the injured triads. Lanthanum did not penetrate freely through the plasma membrane of necrotized muscle fibers. It is concluded that junctional sites between T tubules and SR may be the primary sites of leakage of sarcoplasmic enzymes in polymyositis.

Inclusion body myositis (IBM) has been viewed as a distinct and rare form of inflammatory myopathy. Previously reported findings from series of IBM patients have suggested that clinical and pathologic features are present which readily distinguish it from idiopathic polymyositis. We report 4 cases of IBM presenting clinically and pathologically as polymyositis, each of which was refractory to therapy. Our data suggest that IBM may be a more common and heterogeneous form of inflammatory myopathy than has been previously suggested. Furthermore, IBM may be clinically and electrophysiologically indistinguishable from polymyositis. Reasons for failing to recognize IBM by pathologic studies appear to include: the skip lesion nature of the pathologic findings, failure to examine tissues by electron microscopy, and a low level of suspicion or lack of recognition. Because of its insidious clinical course and its failure to respond to immunosuppressive therapy, IBM may be an important variant of treatment-resistant polymyositis.

Intranuclear and intracytoplasmic aggregates of filaments with tubular structures and transverse striations occurred in muscle tissues biopsied from a patient with chronic polymyositis. The filamentous tubules bear a close resemblance to the incomplete form of myxovirus in which the envelope is missing. Three biopsies from the same patient, taken during a period of 1(1/2) years, all revealed these structures. This finding provides presumptive evidence that a chronic persistent viral infection may be involved in the pathogenesis of chronic polymyositis.