58 year-old male with 3-4 year history of leg muscle atrophy and progressive weakness. Patient has an uncle with a muscular dystrophy. CPK=175. EMG findings are suggestive of an inflammatory myopathy. Scroll Down For Diagnosis

H&E low power of paraffin section. Note multifocal endomysial (within the muscle fascicle) lymphocytic infiltrates, involving non-necrotic fibers. What is the differential diagnosis with this image alone?
H&E frozen section, high power. The myofiber in center has several rimmed vacuoles (holes lined by a crusty blue basophilic rim).
Immunohistochemistry with antibody SMI-31, phosphorylated neurofilaments Note dense positive cytoplasmic inclusion.

Diagnosis: Inclusion Body Myositis (IBM). the following is adapted from: Clinical features of IBM The weakness is of long duration (> 6 months) Age of onset > 30 years old Muscle weakness * Must affect proximal and distal muscles of arms and legs and * Patient must exhibit at least one of the following features: 1. Finger flexor weakness 2. Wrist flexor > wrist extensor weakness 3. Quadriceps muscle weakness ( = or < grade 4 MRC) The symptoms are usually unresponsive to steroid therapy Laboratory features Serum creatine kinase < 12 times normal Muscle biopsy 1. Inflammatory myopathy characterized by mononuclear cell invasion of nonnecrotic muscle fibers 2. Vacuolated muscle fibers 3. Either * Intracellular amyloid deposits (must use fluorescent method of identification before excluding the presence of amyloid) or * 15-18-nm tubulofilaments by electron microscopy or multiple SMI-31 positive cytoplasmic filamentous inclusions (corresponding to the tubulofilaments seen on EM) This page created on a Macintosh using PhotoPage by John A. Vink.