Mitochondrial pseudomyasthenia.

The classic ocular motor presentation of mitochondrial disorders is chronic, symmetric, and diffuse weakness. We describe a man with 25 years of asymmetric ptosis, ophthalmoparesis, and facial weakness that partially responded to steroid therapy. Serologic and electrophysiological investigations for myasthenia gravis were negative, but muscle biopsy confirmed a mitochondrial myopathy. This case illustrates the potential of mitochondrial ophthalmoparesis to mimic the features of ocular myasthenia.