Page 820 - Robbins Basic Pathology by Vinay Kumar, Abul K. Abbas, Jon C. Aster
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806 C H A P T E R 21 Peripheral Nerves and Muscles
AB C
Figure 21–6 Inflammatory myopathies. A, Polymyositis is characterized by endomysial inflammatory infiltrates and myofiber necrosis (arrow).
B, Dermatomyositis often shows prominent perifascicular and paraseptal atrophy. C, Inclusion body myositis, showing myofibers containing rimmed
vacuoles (arrows). Modified Gomori trichrome stain.
• Thyrotoxic myopathy may take the form of either acute or PERIPHERAL NERVE SHEATH TUMORS
chronic proximal muscle weakness, and can be the first
indication of thyrotoxicosis. Histologic findings include A number of different tumors arise from peripheral nerves.
myofiber necrosis and regeneration. Such tumors may manifest as soft tissue masses or with
pain or loss of function related to impingement on nerves
• Ethanol myopathy occurs after an episode of binge drink- or other surrounding structures. In most peripheral nerve
ing. The degree of rhabdomyolysis may be severe, some- tumors, the neoplastic cells show evidence of Schwann cell
times leading to acute renal failure secondary to differentiation. These tumors usually occur in adults and
myoglobinuria. Patients usually complain of acute include both benign and malignant variants. An important
muscle pain, which may be generalized or confined to a feature is their frequent association with the familial tumor
single muscle group. Microscopically, there is myocyte syndromes neurofibromatosis type 1 (NF1) and neurofibro-
swelling, necrosis, and regeneration. matosis type 2 (NF2). Tumors with skeletal muscle differ-
entiation also occur; these are discussed in Chapter 20,
• Drug myopathy can be produced by a variety of agents. along with other tumors of soft tissues.
Currently the most commonly implicated drugs are
those belonging to the statin family. The affected muscles Schwannomas and Neurofibromatosis Type 2
show evidence of myopathic injury, usually without an
inflammatory component. Schwannomas are benign encapsulated tumors that may
occur in soft tissues, internal organs, or spinal nerve roots.
S U M M A RY The most commonly affected cranial nerve is the vestibular
Disorders of Skeletal Muscle portion of the eighth nerve. Tumors arising in a nerve root
or the vestibular nerve may be associated with symptoms
• Skeletal muscle function can be impaired secondarily related to nerve root compression, which includes hearing
because of problems with muscle innervation or by a loss in the case of vestibular schwannomas.
primary myopathy that can be inherited or acquired.
Most schwannomas are sporadic, but about 10% are
• The genetic forms of myopathy fall into several fairly associated with familial neurofibromatosis type 2. NF2
distinct clinical phenotypes, including muscular dystrophy, patients are at risk of developing multiple schwannomas,
congenital myopathy, and congenital muscular dystrophy. meningiomas, and ependymomas (the latter are described
in Chapter 22). The presence of bilateral vestibular
• Dystrophinopathies are X-linked disorders caused by schwannomas is a hallmark of NF2. Affected patients carry
mutations in the dystrophin gene and disruption of the a dominant loss of function mutation of the merlin gene on
dystrophin-glycoprotein complex. Depending on the type chromosome 22. Merlin is a cytoskeletal protein that func-
of mutation the disease may be severe, such as Duchenne tions as a tumor suppressor by facilitating E-cadherin–
muscular dystrophy, or mild (e.g., Becker dystrophy). mediated contact inhibition (Chapter 5). Of note, merlin
expression is also disrupted in sporadic schwannomas.
• Acquired myopathies have diverse causes, including inflam- Despite the name of the syndrome, neurofibromas are not
mation and toxic exposures. a feature of NF2. Schwannomatosis is a familial condition
