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The prevalence varies: LD is seen worldwide but is more common in geographic isolates and areas with a high degree of inbreeding. In Western countries, prevalence is estimated to be below 1/1,000,000.

Onset occurs during adolescence, with generalised tonic-clonic or clonic-tonic-clonic seizures, action and resting myoclonus, negative myoclonus, and focal occipital seizures with transient amaurosis. The course is marked by prominent and rapid cognitive deterioration (the primary symptoms of which may precede the motor anomalies), and by the progressive increase in intensity of the seizures and myoclonus.

LD is genetically heterogeneous. Mutations/deletions of the EPM2A gene, localised in 1995 to 6q24 (product: laforin), are found in 80% of cases. The less common EPM2B variant is localised on 6p22 (product: malin). However, these two localisations do not account for all cases of LD.

The diagnosis of LD may be suspected on the basis of the family history, age at onset, typical appearance of symptoms, rapid worsening of cognitive function and detection of fairly typical electroencephalogram (EEG) features. It can easily be confirmed by axillary skin biopsy with detection of Lafora bodies (polyglucosan aggregates) in the sweat duct cells. Other biopsies, such as brain biopsy, are generally not necessary. Molecular biology is useful for diagnosis but the genetic heterogeneity does not allow LD to be excluded when none of the known mutations are detected.

Transmission is autosomal recessive. Genetic counselling and prenatal diagnosis are theoretically possible when the genetic anomaly has been documented in an affected member of the family.

The treatment of LD with antiepileptic and antimyoclonic drugs remains purely symptomatic. Drugs that may aggravate myoclonus must be avoided. Psychological and social management is of utmost importance in LD.

Death occurs 4 to 10 years after onset in typical forms.