Background: Benign neonatal sleep myoclonus (BNSM) is characterised by myoclonic jerks that occur only during sleep and stop abruptly when the child is aroused. It is a benign condition that is frequently confused with epileptic seizures during infancy.
Aims: The purpose of this study was to examine the clinical characteristics of newborns and infants with BNSM.
Methods: In a retrospective study of 38 newborns with BNSM, the entire inpatient charts were reviewed and data on the clinical picture and results of investigations were studied. The data for analysis of the children's neurological development and the time at which myoclonic jerks ceased were provided from outpatient files and parental interviews.
Results: Myoclonic jerks started between day 1 and day 16 (median 3 days, 95% confidence interval 2-4 days); they were mostly bilateral synchronous and symmetrical in distribution and never involved the facial muscles. An episode of jerks lasted from a few seconds up to 20 min. In every instance, the jerks occurred only during sleep: when the child was drowsy or fell asleep in 20, when the child had been asleep for more than 20 min in 5, before awaking in 3 and in different stages of sleep in 5 infants. In the majority of cases (32), the jerks were reported to stop spontaneously, and in 6 on arousal. Neurological evaluation revealed optimal results in the majority of infants (22), hyperexcitability in 11 and mild abnormalities of muscle tone in 5. In two infants electroencephalography (EEG) performed during the attack of jerks showed no paroxysmal activity. In the remainder EEG performed between the episodes of jerks was normal, apart from mild abnormalities found in six infants. US revealed normal results in all except eight infants, in whom mild abnormalities were found. Myoclonic jerks remitted between the age of 2 weeks and 10 months (median 2 months, 95% confidence interval 1-2 months). Follow-up between 3 and 12 months of age revealed optimal results of neurological assessment in 30 infants and mildly abnormal neurological signs in 8.
Conclusion: In the studied group, myoclonic jerks occurred within the first 16 days (median 3 days) of life irrespective of other conditions. They were predominantly observed at the beginning of the sleep and disappeared spontaneously in the first 10 months (median 2 months) of life.