• Inclusion body myositis is the most common inflammatory myopathy in patients older than 60 years of age. It is grouped with other forms of myositis, but it has yet to be determined whether inflammation is a cause or an effect in this disorder. The morphologic hallmark of inclusion body myositis is the presence of rimmed vacuoles (
Fig. 22.6C ) that contain aggregates of the same proteins that accumulate in the brains of patients with neurodegenerative diseases—hyperphosphorylated tau, amyloid derived from β-amyloid precursor protein, and TDP-43 (Chapter 23)—leading some to speculate that this is a degenerative disorder of aging. Other features typical of chronic inflammatory myopathies, including myopathic changes, mononuclear cell infiltrates, endomysial fibrosis, and fatty replacement, also are evident. The disease follows a chronic, progressive course and generally does not respond well to immunosuppressive agents, another feature suggesting that inflammation is a secondary event.
A number of insults can cause toxic muscle injury, including intrinsic factors (e.g., thyroxine) and extrinsic factors (e.g., acute alcohol intoxication, various drugs).
Thyrotoxic myopathy may take the form either of acute or chronic proximal muscle weakness, and it can be the first indication of thyrotoxicosis. Histologic findings include myofiber necrosis and regeneration.
Ethanol myopathy occurs after an episode of binge drinking. The degree of rhabdomyolysis may be severe, sometimes leading to acute renal failure secondary to myoglobinuria. Patients usually complain of acute muscle pain, which may be generalized or confined to a single muscle group. Microscopically, there is myocyte swelling, necrosis, and regeneration.
• Drug myopathy can be produced by a variety of agents. For example, myopathy is the most common complication of statins (e.g., atorvastatin, simvastatin, pravastatin), occurring in approximately 1.5% of users. Two forms of statin associated myopathy are recognized: (1) toxicity of the drug and (2) statin-induced HMG-CoA reductase autoantibodies causing an immune mediated myopathy.
Tumors of Skeletal Muscles
These are discussed in Chapter 21 along with other tumors of soft tissues.
Skeletalmusclefunctioncanbeimpairedbyaprimary(inheritedoracquired)myopathyorsecondarilybecauseofproblemswithmuscleinnervation.
Thegeneticformsofmyopathyfallintoseveralfairlydistinctclinicalphenotypes,includingmusculardystrophy,congenitalmyopathy,andcongenitalmusculardystrophy.
DystrophinopathiesareX-linkeddisorderscausedbymutationsinthedystrophingeneanddisruptionofthedystrophinglycoproteincomplex.Dependingonthetypeofmutation,thediseasemaybesevere,suchasDMD,ormild(e.g.,Beckerdystrophy).
Acquiredmyopathieshavediversecauses,includinginflammationandtoxicexposures.
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A number of different tumors arise from peripheral nerves. Such tumors may manifest as soft tissue masses, with pain or loss of function related to impingement on nerves or other surrounding structures. In most peripheral nerve tumors, the neoplastic cells show evidence of Schwann cell differentiation. These tumors usually occur in adults and include both benign and malignant variants. An important feature is their frequent association with the familial tumor syndromes neurofibromatosis type 1 (NF1) and neurofibromatosis type 2 (NF2).