Muscle pathology and diseases are a high-yield topic for neurology examinations as many of these disorders have characteristic or pathognomonic findings that allow for easy test question making! For this topic, pathology slide interpretation will be a great skill to have! Luckily this chapter has dozens of high-quality images with which to practice.
Author: Brian Hanrahan MD
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Table of Contents
Acquired Myopathies
Idiopathic inflammatory myopathies
Polymyositis
- A female-predominant disease seen in one’s adult years that presents with subacute symmetric proximal weakness and pain.
- Can be associated with rheumatoid arthritis, HIV, or underlying malignancy.
- Muscle biopsy shows endomysial and perivascular monocytic inflammation and necrosis with regeneration.
Dermatomyositis
- A female-predominant disease which presents with subacute proximal weakness and pain.
- Dermatologic manifestations include a heliotrope rash on eyelids and an erythematous rash of the face or neck.
- Can be associated with connective tissue disease, malignancy, and interstitial lung disease.
- Muscle biopsy shows perifascicular inflammation and atrophy with sparing of the central fascicle.
- Anti-TIF1y positive patients have the highest rate of associated cancer.
Inclusion body myositis (IBM)
A male-predominant slowly progressive idiopathic inflammatory condition of patients over the age of 50.
Clinical features include asymmetric weakness of the finger flexors and the quadriceps muscles.
Muscle biopsy shows rimmed vacuoles, filamentous nuclear and cytoplasmic inclusions, and scant lymphoid infiltrates.
May be associated (70% sensitivity, 92% specificity) with autoantibodies to cytoplasmic 5′-nucleotidase 1A (cN1A; NT5C1A).
Immune-mediated necrotizing myopathy
- Presents with proximal weakness and myalgia, and elevated CK.
- Pathology with necrosis and dystrophic changes with fibrosis, but can also have capillary changes.
- Patients often have concurrent connective tissue disease or malignancy.
- Associated with some antibodies including Anti-SRP (signal recognition particle), or Anti-HMG-CoA reductase antibodies
- Often associated with a history of statin use.
- Responds to steroids, IVIG, and other immunosuppressants.
- EMG findings:
- Membrane irritability will lead to positive waves and fibrillations.
- Low amplitude, decreased duration, polyphasic MUAPs with early recruitment.
- Note that if symptoms are chronic, large, long MUAPs may be present!
Treatment
- Polymyositis and dermatomyositis are responsive to immunosuppressive therapies (steroids, methotrexate, azathioprine, mycophenolate, etc.) while IBM is not.
