Polysomnographic Study in Patients with Severe Myoclonic Epilepsy During Infancy
Authors: Sugimoto Junko , Itoh Masahiro , Hasegawa Takeshi , Shimohira Masayuki , Kohyama Jun , Iwakawa Yoshihide 
Published: August 2009

Purpose: Patients with severe myoclonic epilepsy in infancy (SMEI) exhibit frequent febrile seizures before age 2 years when they show no neurologic deficits. They subsequently develop intractable nonfebrile myoclonic and generalized tonic‐clonic seizures with the appearance of ataxia, developmental delay, and mental retardation. Although its cause remains obscure, the progressive involvement of the cortical as well as subcortical structures has been described. Recently we introduced two indices (tonic inhibition index, TII; and phasic inhibition index, PII) that quantify outputs implicated in motor suppression. These indices were calculated through the standard sleep recording by making use of phasic chin‐muscle activity (PCMA) during rapid‐eye‐movement sleep (REMS). To assess the brainstem involvement in SMEI, these indices were examined in patients with SMEI.

Methods: Five polysomnograms obtained from three patients aged from 9 to 25 months were examined. Each polygram consisted of electroencephalography, bipolar horizontal electrooculography, and surface electromyography including the chin muscle. In addition to the proportion of sleep stages in the total sleep time, the TII and PII were calculated. These parameters were compared with those obtained from the age‐matched controls (n = 12). PCMA was defined as a phasic chin‐muscle activity that lasted ≥ 2 s, with a peak amplitude of ≤ 50% above the baseline. TII is the rate of a short PCMA during REMS among the total numbers of PCMA during REMS. The short and long PCMA were separated at a duration of 0.5 s. TII expresses the degree of shortness of PCMA during REMS. PII is a geometric means of the following two values in REMS: (a) the percentage of PCMA that occurs with bursts of rapid eye movements (RBs) in relation to the total number of PCMA, and (b) the percentage of RBs that appear with PCMA in relation to the total number of RBs. As long as rapid eye movement‐related phasic inhibition acts on the chin muscle, PCMA is unlikely to appear in association with RBs. PII expresses the rate of the simultaneous occurrence of PCMA and RBs.

Results: With the progression of age, the proportion of REMS and of slow‐wave sleep stages in patients gradually showed values lower than those in controls. The mean TII value in patients (0.66; SD, 0.08) did not differ significantly from that in controls (0.63; SD, 0.05). After 100 weeks of postconceptional age, the mean PII value in patients (7.83; SD, 2.11) was significantly higher than that in controls (4.52; SD, 2.32), whereas there was no difference between the patients (5.8) and controls (5.30; SD, 2.75) at earlier ages. Moreover, in contrast to the controls, PII increased with age in the patients.

Conclusions: Although the decrease in the amount of REMS and slow‐wave sleep stages might reflect subcortical impairments, the responsible regions for these findings are uncertain. Because PII decreases during infancy in controls, an increase of PII with age in patients with SMEI suggests a progressive involvement of the neuronal systems. The mesopontine tegmentum plays key roles in the occurrence of rapid eye movement‐related phasic motor reduction, which is responsible for determining the PII value. The pontine as well as rostra1 and caudal medullary atonia zones are crucial for determining the TII value; however, an impairment of these brainstem atonia zones might also affect the PII value. Because TII values remain unaffected in our patients, we conclude that the mesopontine tegmentum is progressively disturbed in patients with SMEI.