Muscle pathology in patients with autosomal dominant MYH2 was highly variable, also in different muscles of the same individual. A consistent finding was a predominant involvement of type 2A muscle fibers. In children and mildly affected adults the type 2A fibers were reduced in number, frequently smaller than normal and often demonstrating disorganization of myofibrils, similar to what is observed in multi-mini core disease. Adults with progressive course and increased s-CK levels demonstrated dystrophic muscle pathology with increased interstitial fat and connective tissue. The presence of rimmed vacuoles in many fibers in the more advanced cases was the reason at that time to consider this myopathy as a variant of hereditary inclusion body myopathy (hIBM3). The number of rimmed vacuoles was variable also in the severely affected cases.
Muscle biopsy in patients with recessive MYH2 mutation demonstrated in addition to complete loss of MyHC IIa protein, unspecific myopathic changes with fiber variability, internalized nuclei and interstitial fatty infiltration. Some muscle biopsy samples demonstrated type 1 fiber uniformity. Unlike the dominant MyHC IIa myopathy, no rimmed vacuoles or protein aggregates have been identified.
MYH2