Benign Myoclonic Epilepsy in Infancy (BMEI)
Incidence
Benign myoclonic epilepsy in infancy (BMEI) is a rare syndrome of idiopathic generalized epilepsies with onset below 3 years of age, with just over 100 cases reported. The prevalence is ~1% to 2% of epilepsies that start before the age of 3 years. Males (66%) predominate. Probably genetic. It is the earliest form of idiopathic generalized epilepsy (IGE).
Clinical Characteristics
Benign myoclonic epilepsy is characterized by brief myoclonic attacks in normal infants between the ages of 6 months and 3 years. An earlier onset is uncommon. A family history of epilepsy or febrile convulsions is present in 30% of cases. As a rule, patients do not have any pathological history prior to the onset of the myoclonic fits. However, the occurrence of febrile convulsion has been reported in 20% of patients. They were always rare, simple febrile convulsions, usually preceding the onset of myoclonias. The myoclonic attacks involve the upper limbs and the head, though rarely the lower limbs. Their intensity is variable, rarely inducing a fall. In babies they are not easy to describe; parents report a head nod. They occur several times a day at irregular and unpredictable times. They are not favored by awakening, but they can be triggered by a sudden noise or sudden contact. The state of consciousness is difficult to assess, but the isolated seizures do not interrupt activity. Only when they are grouped in clusters of 2 or 3 pseudorhythmically repeated elements, lasting up to 5 to 10 seconds, is there a slight impairment of consciousness. They are more or less massive, involving the axis of the body and the limbs, provoking a head drop and an upwards-outwards movement of the upper limbs, with flexion of the lower limbs, and sometimes a rolling of the eyeballs. It is considered an epileptic syndrome. Initially, the development continues normally, and parents and pediatricians tend not to consider these movements as pathological events.
When an EEG is performed, it can be normal if no myoclonic fit is recorded. But myoclonias are always associated with fast generalized spike and waves and polyspike and waves at more than 3 Hz, that are more or less regular, lasting 1 to 3 seconds localized in the anterior regions. Myoclonias may be followed by a brief atonia. Sometimes, after the attack, there is a voluntary movement that is visible as a normal muscular contraction. During drowsiness, there is an enhancement of the myoclonias that usually, but not always, disappear during sleep. Intermittent photic stimulation can also provoke myoclonic fits. The interictal EEG is normal for the child’s age. Spontaneous spike and wave discharges are rare; some slow wave may be found over the central areas. Intermittent photic stimulation does not provoke spike and wave without concomitant myoclonia. Nap sleep recordings have shown a normal organization of the sleep; generalized spike and wave discharges may occur during REM sleep.
No other type of seizure is observed in children with benign myoclonic epilepsy, even if they are left untreated (for up to 8.5 years in one patient), particularly, no absence or tonic seizures. Clinical examination is normal. Many patients have not been investigated, but when CT and MRI were performed, they were normal.
The outcome seems to depend on an early diagnosis and treatment. If left untreated, the patient continues to experience myoclonic attacks, and this may lead to impaired psychomotor development and behavioral disturbances. Myoclonias are easily controlled by valproate alone and the child may then develop regularly, according to normal milestones. The attacks provoked by noise or contact were more easily controlled than the spontaneous ones. Conversely, the photosensitivity was more difficult to control and persisted several years after the arrest of seizures. Remission usually occurs within 1 year (6 months to 5 years) from onset. Approximately 10% to 20% develop infrequent generalized tonic-clonic seizures in their early teens. Also, 10% to 20%, if untreated, may develop mild cognitive, behavioral, or motor deficits.
The psychological outcome is more variable. On the whole it is rather good. The isolated cases published continue to have normal psychomotor development. However, in a series with a long-term follow-up, there were some who presented moderate mental retardation, personality disturbances, or slight behavioral impairment. No patient had been institutionalized. This psychological outcome partly depends on an early diagnosis, allowing appropriate treatment and family reassurance.
Precipitants
Intermittent photic stimulation can provoke myoclonic fits. Also photosensitivity (20%) or unexpected acoustic or tactile stimuli (10%).
Provocation Tests
Intermittent photic stimulation on EEG can provoke myoclonic fits.
Diagnostic Procedures
It is a clinical-EEG diagnosis.