This disorder, originally believed to be a type of glycogen storage disease, is actually a form of autophagic vacuolar myopathy. The characteristic vacuoles are found in muscle cytoplasm surrounded by sarcolemmal proteins and basal lamina. The primary extraocular disease occurs in the myocardium although skeletal muscle may also be involved. Intellectual disability is a variable and inconsistent feature.
Cardiac rhythm abnormalities are common and include AV nodal block, atrial fibrillation, and Wolff-Parkinson-White EKG findings. Hypertrophic cardiomyopathy (and sometimes dilated cardiomyopathy) with primary involvement of the left ventricle is common. Symptoms typically occur in males before the age of 20 years and somewhat later in females.
Some patients have muscular weakness and exercise intolerance. Diagnosis can be made when the characteristic vacuoles are present in a muscle biopsy but their absence does not rule out the diagnosis.