Type II Fiber Atrophy
Selective atrophy of fast-twitch muscle fibers, commonly seen in disuse, steroid myopathy, and aging, identifiable on ATPase staining.
Full Definition
Type II fiber atrophy refers to the selective atrophy of fast-twitch (type II) muscle fibers while slow-twitch (type I) fibers remain relatively preserved. This pattern is commonly observed in conditions such as corticosteroid myopathy, disuse atrophy, aging, cachexia, and critical illness myopathy. The diagnosis requires histochemical staining with myosin ATPase at different pH levels to distinguish fiber types. Type II fibers normally appear larger than type I fibers, so their selective atrophy can be subtle on routine H&E staining but becomes apparent with specialized techniques. This finding helps differentiate these conditions from neurogenic causes of muscle weakness.
Usage
Usage note: Use Roman numerals (II) rather than Arabic numerals (2). Hyphenate when used as compound modifier.
In Context
- "Type II fiber atrophy was prominent on ATPase staining." — Steroid myopathy diagnosis
- "The pattern of type II fiber atrophy suggested disuse rather than denervation." — Muscle biopsy interpretation