The muscles showed severe myopathic changes. There were sparse, atrophied muscle fibers infiltrated with fat and collagen. There was syncytialization with clumps of nuclei irregularly distributed along the sarcolemma. In longitudinal sections, the nuclei were lined up in rows. Cross sections showed nuclei in the center of muscle fibers. Lymphocytes and few plasma cells were seen scattered around the fibers. The different sections of muscle varied in the degree of myopathy. The sternocleidomastoid was the most severely involved, with the majority of the tissue occupied by nerves, vascular structures, and connective tissue. The gastrocnemius was the least severely involved with larger fibers that were more densely packed with less fat and connective tissue between the fibers. There was evidence of myopathic changes in the heart with hypertrophied fibers having enlarged, box-shaped nuclei and irregularly shaped nuclei. There was little fibrosis between the muscle fibers. The SA and AV node also appeared to have myopathic changes with enlarged nuclei with moderate fibrosis and some vacuolated fibers.
Postmortem blood sent for genetic testing via Southern blot and PCR assays showed an abnormality of the DM-1 gene. One chromosome had an expanded CTG repeat sequence between approximately 530 and 700 repeats. The other chromosome had a normal CTG repeat of 21.