Reduced brain stem excitability in mitochondrial myopathy: evidence for early detection with blink reflex habituation studies.

Blink reflex (BR) was studied in 17 patients with histochemically and genetically confirmed mitochondrial myopathy (MM). Fourteen patients had chronic progressive external ophthalmoplegia (CPEO) associated with a mild to moderate craniosomatic myopathy without any symptoms or signs of central nervous system (CNS) involvement, 2 myoclonic epilepsy with ragged red fibers syndrome, and 1 Kearns-Sayre syndrome. The mean latencies of the early (R1) and late (R2) responses were prolonged (P < 0.01 and P < 0.001, respectively), and the corresponding amplitudes decreased (P < 0.001). Increased habituation of the reflex was clearly observed in 10 out of 14 patients tested (71.4%), 9 of whom presented CPEO. These findings suggest that the brain stem reticular network is in a state of basal inhibition which is presumably due to a subclinical impairment of the cerebral cellular metabolism. Multimodal evoked potentials revealed abnormalities suggestive of CNS involvement in 7 out of 17 patients (41.2%), 4 of whom had CPEO. These observations document the validity of BR in detecting clinically silent brain stem impairment in patients with apparently pure MM and provide important clues for a further understanding of the underlying pathophysiology.