What is sporadic inclusion body myositis?
• This is the most common IIM in patients aged >50 years.
• Bilateral, painless weakness develops insidiously and may be asymmetric. There is early involvement of distal muscles (foot extensor, finger flexors). Atrophy of forearms and quadriceps prominent which can lead to knee buckling with falls. Axial muscle weakness causes bent forward posture of spine. Dysphagia in 50%. No rash, ILD, or cancer association. May be associated with Sjögren’s syndrome.
• CPK can be high normal (20%–30%). Almost always <1000 U/L.
• Anti-cytosolic 5-nucleotide 1A, that is anti-cN1a or anti-NT5C1A, is an MSA found in 33% to 60% of IBM patients. Has been found less commonly in other IIM thus muscle biopsy is necessary to confirm a diagnosis of IBM.
• EMG shows both myopathic and neuropathic changes.
• Muscle pathology: CD8+ T cells invading healthy muscle fibers. Increased MHC class I expression on muscle cells. Ragged-red fibers (abnormal mitochondria), red-rimmed autophagocytic vacuoles (Gomori–Trichrome stain), and amyloid deposits next to vacuoles are seen.
• Slowly progressive and poorly responsive to immunosuppressive therapy.