Chapter 14: Severe Myoclonic Epilepsy of Infancy (Dravet Syndrome)

OVERVIEW

Recognized as a syndrome by the International League Against Epilepsy in the last classification,¹ severe myoclonic epilepsy of infancy (SMEI) has been placed among the "epileptic encephalopathies", defined as conditions in which the epileptiform abnormalities are believed to contribute to a progressive disturbance in cerebral function, whether seizures are generalized, localized, symptomatic, idiopathic or cryptogenic.² Since 2001³ its etiology is regarded to be genetic as the majority of affected patients carry a mutation in the sodium-channel gene SCN1A. However, it remains unproven whether the cognitive decline observed in the first stages of the disease is a consequence of the epilepsy. In this new scheme, SMEI is renamed "Dravet syndrome" because of the lack of myoclonic seizures in many patients.

EPIDEMIOLOGY

The frequency in the general population is not well known but has been estimated at 1/40,000 births.⁴ Since 1990, knowledge of the disease has grown considerably and it is likely its frequency is higher, but there is no other epidemiological data.

CLINICAL DESCRIPTION

The typical features of the core syndrome are present in the majority of the patients, although variations exist in semiology and outcome.

ONSET

Seizures typically begin in the first year of life in a normal infant without pathological antecedents. A family history of epilepsy and febrile seizures is frequent. Seizures begin between ages 4 and 9 months, accompanied by mild hyperthermia, and last longer than the simple febrile seizures (10 minutes or more). They are clonic, generalized, unilateral or predominating on one side of the body, and change from one seizure to the next. They may have localized onset. In some patients, isolated episodes of focal myoclonia are observed before the appearance of the first convulsive seizure. Purely focal complex seizures are exceptional at the onset.

The first convulsive seizure can be afebrile, after a vaccination, a bath, or during a cold. Usually, afebrile seizures are quickly associated with febrile seizures. This first seizure is considered as an occasional seizure that does not require a treatment. But, shortly thereafter, other seizures occur, with or without fever, and persist in spite of the institution of a chronic treatment.

STEADY STATE

Seizure frequency increases during the second year of life. Affected patients have many upper respiratory tract infections and are repeatedly hospitalized. Some seizures are resistant to acute treatment and evolve to status epilepticus (SE) requiring intensive care. Other seizure types appear and psychomotor development slows.

Ictal Semiology

Convulsive seizures are reported to be generalized by the parents but have variable features. They may be truly generalized, clonic or tonic clonic. Others are "falsely generalized" or "unstable" with some focal elements, clinical or EEG, and localize to different areas during the course ...