Benign Neonatal Sleep Myoclonus Clinical Presentation

Although children are sometimes identified with abnormal movements within the first several hours of birth while still in the hospital, parents are often the first to witness the movements in children who were discharged early. These movements are often characterized as jerking of a limb during sleep. This may be repetitive and rhythmic and, thus, may prompt concerns regarding seizure. Unless the movements are previously videotaped or witnessed in the outpatient setting, patients are generally admitted for observation and workup, depending on the clinical concern for seizures. Caretakers should be aware of the clinical characteristics of benign neonatal sleep myoclonus (BNSM), which are delineated in the International Classification of Sleep Disorders, revised: Diagnostic and Coding Manual (2nd Edition), as follows:^[21]

• Repetitive myoclonic jerks that involve the whole body, trunk, or limbs
• Movements that occur in early infancy, typically from birth to age 6 months
• Movements that occur only during sleep
• Movements that stop abruptly and consistently when the child is aroused
• A disorder that is not better explained by another sleep disorder, by a medical or neurologic disorder, or by medication use

An association with sleep is important because clinically evident seizures are often associated with eye opening. Gentle restraint has been reported to possibly worsen the manifestations. Provocative maneuvers include sound stimulus and, in one report, repetitive head-to-toe rocking of the infant.^[19] In this report, increased rocking frequency seemed to be associated with increased clinical manifestations. Passive restraint of the child did not ameliorate the signs.

The most important maneuver is waking the child, which should entirely eliminate the symptoms. Movements are often superimposed on normal, purposeless movements of the infant and do not appear to occur in isolation, as is the case in the clonic movements of a seizure. One study reported an infant with benign neonatal sleep myoclonus who developed a pathologic form of myoclonus (ie, myoclonic-astatic epilepsy).^[22] This association is likely incidental, and no clear evidence suggests that benign neonatal sleep myoclonus occurs in a continuum with other, more consequential forms of myoclonus.

Physical examination findings of benign neonatal sleep myoclonus are normal, except for the movements themselves. Children are generally otherwise well; however, in one report, neurologic findings were reported.^[11] These were described as mild and included hyperirritability and hypoxia. The authors believed these findings were incidental and not causative; long-term follow-up of these same children indicated only tonal abnormalities. Whether these children had presenting neurologic abnormalities and the degree to which their tone was abnormal is unclear.

Most other reports emphasize the normal aspects of the physical examination findings. In the author's experience, children have normal examination findings and no long-term residua. In fact, a paucity of neurologic findings is, in itself, an aspect of the diagnostic criteria. Additional neurologic findings should prompt more extensive diagnostic testing for possible causes of pathologic myoclonus in infants.

The cause of benign neonatal sleep myoclonus is unknown. However, 2 reports indicate a probable genetic contribution, with several individuals affected within 2 pedigrees.^{[14, 15]}