Atrioventricular conduction defects including complete heart block, cardiomyopathy, short stature, elevated CSF protein, and ataxia are among the most frequent extraocular features seen. Pharyngeal, facial, and skeletal muscle weakness seem to be common features. Growth retardation, delayed sexual maturation, and mental deterioration occur in some patients. Older patients have a sensorineural hearing deficit as well.
EEG abnormalities are often present. CT scans reveals a diffuse leukoencephalopathy as well as a variety of CNS abnormalities in the cerebellum and brain stem. Muscle biopsies reveal 'ragged red' fibers.
This is a progressive disorder and many patients die in the third or fourth decades of life.