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What are ragged red fibers?
Ragged red fibers are muscle fibers with an accumulation of subsarcolemmal and intermyofibrillar material that stains red with modified Gomori trichrome stain.
This red-stained material is actually mitochondria that are abnormal in number, size, and structure when viewed by electron microscopy.
If there is mitochondrial dysfunction, muscle fibers with ragged red fibers also show excessive SDH staining and reduced or absent COX staining.
Although ragged red fibers are typically seen in mitochondrial myopathies, they may occur in other conditions such as inclusion body myositis or could be normal.
Sources
Silvestri N and Wolfe GI: Asymptomatic/Pauci-symptomatic creatine kinase elevations (HyperCKemia). Muscle Nerve 47: 805–815, 2013.
