Canine distemper myoclonus and sleep: observation of a case.

Evolution of myoclonus was analysed electromyographically throughout quiet wakefulness, NREM sleep and REM sleep in a dog with canine distemper myoclonus. Compared with quiet wakefulness, the frequency of myoclonus was decreased and the intensity of discharges in individual myoclonic bursts was also lowered during NREM sleep. When NREM sleep shifted to REM sleep, neither of these parameters was noticeably altered. However, as REM sleep continued, the former was increased markedly and the latter was further attenuated. In general, violent motor activity was concurrent in the limbs, trunk and/or head. Thus, the increase in the frequency of myoclonus seemed to be similar in nature to the phasic event during REM sleep. This indicated that lower motoneurons producing myoclonic discharges responded well to supraspinal influences. In successive myoclonic bursts, a significant and consistent positive correlation existed between the silent period and the subsequent discharge period throughout the three different levels of consciousness, that is, the longer the silent period, the longer the subsequent episode of myoclonus. Therefore, this feature may be intrinsic to myoclonic firing of lower motoneurons in canine distemper myoclonus. These findings may support the idea that hyperexcitability of the lower motoneurons is primarily responsible for the genesis of canine distemper myoclonus.