Inclusion Body Myositis

Pathogenesis and Pathophysiology. The specific hallmarks of inclusion body myositis (IBM) are inflammation, vacuolated muscle fibers, intracellular amyloid deposits, and 15 to 18 nm tubulofilamentous inclusions. Although there are familial forms of this disease, either autosomal dominant or recessive, inflammation is only rarely seen in those cases, Wi and, therefore, the familial forms are best referred to as inclusion body myopathies.

The inflammatory cells that are present in affected muscle are mostly CD8 + lymphocytes or macrophages expressing MHC II surface markers. These findings suggest a directed response against specific antigens and a role of immune-mediated cytotoxicity. In addition, there are changes in the mitochondria and the nuclei of the myofibers, amyloid protein, and abnormally present filaments that suggest a possible "neurodegenerative" pathogenic mechanism. It is also suggested that the expression of an abnormal gene product may lead to a cytodestructive process and help explain the inflammatory and neurodegenerative evidence. ^

Epidemiology and Risk Factors. IBM is the most common inflammatory muscle disease occurring over the age of 50.[125] Patients are greater than 30 years of age and usually greater than 50 at the time of onset of symptoms. Men are more often affected than women in contrast with other autoimmune and inflammatory diseases.^ There are no known risk factors other than genetic risk in those families with a history of IBM.

Clinical Features and Associated Disorders. The clinical weakness of IBM may resemble that of PM or DM, but more typically evolves over years and resembles a limb- girdle dystrophy. Distal weakness is common and dysphagia occurs in as many as 60 percent of patients. The weakness and atrophy may be asymmetrical and involve solitary muscles such as the quadriceps, iliopsoas, biceps, or triceps. Over years, there is more symmetry and weakness of the involved muscles. IBM is suspected when a patient with the diagnosis of PM does not respond to corticosteroid therapy, has early involvement of distal muscles such as

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long-finger flexors of the hand and wrist, and extensors of the foot, or has dysphagia. [125] Unlike other inflammatory myopathies, patients with IBM do not have an increased risk of malignancy.

Differential Diagnosis. As alluded to above, the other diagnoses to consider in these patients are PM, DM, or a myositis as the result of another systemic illness. With an earlier and gradually progressive onset, there may be features in common with limb-girdle muscular dystrophy. If there is focal atrophy or weakness, the possibility of a mononeuropathy or plexopathy should be considered. These can be differentiated with electrodiagnostic testing. The lack of respiratory involvement, EMG features, and slow course differentiate IBM from motor neuron disease.

Evaluation. As in DM and PM, the diagnosis of IBM relies on the assessment of CK, electromyography, and most important, muscle biopsy. Suggested diagnostic criteria are listed in Iab|elll50-7 . ^J CK levels are often mildly elevated but should be less than 12 times normal. ^ Electromyography may show fibrillation potentials, positive sharp waves, complex repetitive discharges, and a mixture of myopathic, normal, and neurogenic motor units. These findings are not specific to the diagnosis of IBM, though short- and long-duration polyphasic potentials may be more prominent in IBM than other myopathies.

_TABLE 50-7 -- CHARACTERISTIC FEATURES AND DIAGNOSTIC CRITERIA FOR INCLUSION BODY MYOSITIS_

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