Differential diagnosis of familial adult myoclonic epilepsy.

Abstract

Familial adult myoclonic epilepsy (FAME) is an under-recognized disorder characterized by cortical myoclonus, generalized tonic-clonic seizures and additional clinical symptoms, which vary depending on the FAME subtype. FAME is caused by pentanucleotide repeat expansions of intronic TTTCA and TTTTA in different genes. FAME should be distinguished from a range of differential diagnoses. The phenotypic features of FAME, including generalized tonic-clonic and myoclonic seizures, are also seen in other epilepsy syndromes, such as juvenile myoclonic epilepsy, with a resultant risk of misdiagnosis and lack of identification of the underlying cause. Cortical myoclonus may mimic essential tremor or drug-induced tremor. In younger individuals, the differential diagnosis includes progressive myoclonus epilepsies (PMEs), such as Unverricht-Lundborg disease; whereas, in adulthood, late-onset variants of PMEs, such as sialidoses, myoclonus epilepsy, and ataxia due to potassium channel pathogenic variants should be considered. PMEs may also be suggested by cognitive impairment, cerebellar signs, or psychiatric disorders. The EEG may show similarities to other idiopathic generalized epilepsies or PMEs, with generalized spike-wave activity. Signs of cortical hyperexcitability may be seen, such as an increased amplitude of somatosensory evoked potentials or enhanced cortical reflex to sensory stimuli, together with the neurophysiological pattern of the movement disorder. Recognition of FAME will inform prognostic and genetic counseling, and diagnosis of the insidious progression which may occur in older individuals who show mild cognitive deterioration. Distinguishing FAME from other disorders in individuals or families with this constellation of symptoms is essential to allow identification of the underlying aetiology.