Myofibrillar

Myofibrillar is an adjective relating to myofibrils, the elongated, cylindrical subunits that make up most of the cytoplasm in striated muscle fibers. A myofibril contains repeating units called sarcomeres, with thick and thin filaments arranged in a hexagonal lattice, and it is the primary site of muscle contraction through actin–myosin cross-bridge cycling.

In medical terminology, the term is frequently encountered in reference to myofibrillar myopathy, a group of inherited muscular dystrophy–like disorders characterized by disintegration of myofibrils and abnormal protein accumulation at the Z-disks. Genetic forms involve mutations in DES, MYOT, CRYAB, LDB3 (ZASP), FLNC, BAG3, and PLEC, among others.

Clinically, patients typically present with proximal muscle weakness, scapuloperoneal or distal involvement, and sometimes cardiomyopathy; serum CK levels can be variably elevated. Muscle biopsy often shows myofibrillar disorganization with granulofilamentous material and Z-disk streaming.

Diagnosis relies on clinical evaluation supported by genetic testing, while management is primarily supportive, including physical therapy and monitoring of cardiac involvement when present. The term also appears in discussions of normal myofibril structure and function, which underlie the basic mechanism of voluntary muscle contraction.