[Decreased blink frequency in myotonic dystrophy]

We counted spontaneous eye blink rate in 11 myotonic dystrophy (MD) patients. Seven healthy subjects as well as 10 Parkinson disease (PD) and 7 facio-scapulo-humeral dystrophy (FSH) patients were used as controls. Blink frequency was significantly decreased in MD and PD patients (7.6 +/- 4.9/min in average and 11.0 +/- 7.5/min, respectively), compared with healthy subjects and FSH patients (17.5 +/- 4.3/min and 17.3 +/- 9.9/min, respectively). Normal blink frequency in FSH suggests that the facial muscle weakness is not responsible for decreased blink frequency in MD. This observation is compatible with prolonged R1 latency in blink reflex in MD, suggesting a dysfunction of central mechanism of blink control system as in the case of PD, although there remains a possibility that the myotonia in levator palpebrae muscles disturbs blinking.