MT-TK and Cognitive impairment: caused by a single mtDNA mutation in the MT-TK gene in most cases, Presents in childhood with ataxia, sensorineural hearing loss and endocrine disorders. Myoclonus may be a late presentation. Other clinical features of MERRF include cognitive impairment, multiple lipomatosis, ptosis/PEO, myopathy, peripheral neuropathy and cardiomyopathy.