Juvenile myoclonic epilepsy (JME), also known as Janz syndrome, is a familial disorder that typically begins in the second decade of life and is characterized by mild myoclonic seizures, generalized tonic-clonic (GTC) or clonic-tonic-clonic seizures (a variation of GTC seizures in which there is an initial clonic phase), and occasionally absence seizures.
Janz described a familial disorder of myoclonic epilepsy associated with an excellent prognosis in patients who were otherwise mentally and neurologically normal. Janz initially called the syndrome "impulsive petit mal" to indicate that the myoclonic jerks are a type of minor seizure.34
The myoclonic seizures are usually mild to moderate in intensity and involve the neck, shoulders, and arms. The movements involve an entire extremity or body part rather than an isolated muscle contraction. They can occur either singularly or repetitively and may cause the patient to drop objects. They are generally bilateral, although sometimes asymmetric with changing left-right accentuation. Rarely the jerks may involve the legs and cause the patient to fall to the ground. More commonly, they are quite mild and the patient may attribute them to nervousness or clumsiness.35
Occasionally, the jerks become more severe and violent jerks occur in rapid succession, in a chorea-like episode. Myoclonic status can occur, in which the patient has myoclonic jerks every few seconds or salvos of three to five jerks. Despite preserved consciousness, the patient is often incapacitated by the continuous myoclonic jerks.
Usually the myoclonic seizures occur only during the first several hours after awakening from a night's sleep or a nap. In some patients they may continue all day, but this pattern is less frequent. In these patients the seizures may be less frequent in the middle of the day and increase again when the patient becomes fatigued. The seizures of JME typically are aggravated by sleep deprivation.
Nearly all patients with JME have tonic-clonic or clonic-tonic-clonic seizures. In one study of 43 patients, only 2 were free from these seizure types.36 In another study,37 10 of 12 patients had GTC seizures. In a large majority of cases, the myoclonic jerks precede the onset of the GTC seizures, but occasionally the GTC seizures occur before the onset of the myoclonic seizures, or the two types begin simultaneously. Like the myoclonic seizures, the tonic-clonic seizures often occur shortly after awakening or during early-morning sleep. At times patients will have a series of myoclonic seizures culminating in a generalized seizure. In addition, some patients will have several days of an increasing number of myoclonic seizures, followed by GTC seizures.
Absence seizures also occur in a substantial number of patients. In one report,36 40% of patients also had absences, usually in association with tonic-clonic or clonic-tonic-clonic seizures. As with other seizure types, these often occur shortly after awakening.
JME begins in childhood, usually in the second decade. In a study of 43 patients, the average age of seizure onset was 13.6 years, the range being 8 to 24 years.36 Findings from physical and neurologic examinations of these patients are usually normal and normal intelligence is the rule.
A positive family history of epilepsy is common. The mode of inheritance appears to be polygenic, with females having a lower threshold than males.36
The interictal EEG in this disorder is distinctive and easily distinguished from those of other forms of generalized epilepsy.36 Characteristic features of the EEG are the fast (3.5- to 6-Hz) spike-and-wave and multiple spike-and-wave complexes (See EEG). This pattern contrasts with the 3-Hz spike-and-wave complexes seen in classic absence and the slow (1.5- to 2.5-Hz) spike-and-wave complexes of the Lennox-Gastaut syndrome (LGS).
Example of a rapid spike-and-wave discharge in a patient with JME.
During myoclonic seizures, the ictal EEG consists of 10- to 16-Hz rapid spikes, followed by irregular slow waves. Photosensitivity may activate the epileptiform discharges.
If the diagnosis is suspected and the awake EEG is normal, a sleep-deprived EEG must be obtained, because this may be the only time the abnormality is present.