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Prevalence is unknown but the disorder is principally reported in Northern Finland.

The disorder is characterized by generalized tonic-clonic seizures with onset at 5 to 10 years of age and subsequent slowly progressive mental deterioration. The seizures increase in frequency until puberty after which the epileptic activity starts to decline. The intellectual deficit is severe, develops 2-5 years after the onset of seizures and progresses continuously through adulthood. Visual problems are not a prominent feature of this disorder; if present they may be mild and go unrecognized.

Progressive epilepsy-intellectual deficit, Finnish type is caused by mutations in the CLN8 gene (8p23.3) encoding a putative transmembrane protein of unknown function.

Diagnosis is based on the clinical picture, family history and electron microscopy studies of tissue specimens showing intracellular accumulation of storage material. The diagnosis can be confirmed by molecular analysis.

The differential diagnosis should include other causes of dementia and seizures starting at school age, including atypical forms of other NCL disorders (typically juvenile NCL (see this term) caused by mutations in PPT1 (CLN1 disease), TPP1 (CLN2 disease), or CTSD (CLN10 disease)), as well as mitochondrial disorders and chronic types of encephalitis (see these terms).

Prenatal diagnosis is feasible for families in which the disease-causing mutation has already been identified and genetic counseling should be provided.

Progressive epilepsy-intellectual deficit, Finnish type is transmitted in an autosomal recessive manner

Treatment is supportive only with palliative care including administration of anticonvulsants, as well as educational, psychological, and psychiatric management.

Although patients suffer from slow cognitive decline, life expectancy is less severely reduced than in other forms of NCL with juvenile onset and patients may reach 50 or 60 years of age.