Myoclonic Epilepsy in Lysosomal Storage Disorders

Abstract

Progressive myoclonic epilepsy (PME) constitutes an heterogeneous group of diseases, usually of genetic origin, which begins in childhood and adolescence and presents a variable evolution, ranging from slowly to rapidly progressive forms with refractory seizures and dead within few years (Marseille Consensus Group, 1990). Despite its broad spectrum of manifestations, patients affected with PME share some common specific clinical and electrophysiological features, such as: myoclonus, multiple type of seizures, delay or regression of psychomotor development, cerebellar ataxia, slow background activity on electroencephalogram (EEG), spikes and waves induced by intermittent photo-stimulation and sensory evoked giant potentials (Marseille Consensus Group, 1990). From the genetic point of view, PME occurs in disorders presenting different genetic inheritance, including: the dentatorubralpallidolusyian atrophy (DRPLA), a disease of trinucleotide repeats, the myoclonic epilepsy with ragged red fibers (MERRF), a mitochondrial disease and autosomal recessive disorders, which may be divided in two main categories: non-lysosomal-related diseases such as Lafora disease and lysosomalrelated-disease such as lysosomal storage disorders (LSDs). Lysosomal storage disorders are severe genetic diseases caused by the defective activity of lysosomal proteins, cofactors or integral membrane proteins, which result in the intralysosomal accumulation of undegraded metabolites such as sphingolipids, cholesterol, glycoproteins, mucopolysaccharides or glycogen. Even if they are individually rare, the combined frequency of LSDs is estimated to be approximately 1 in 8000 live births (Meilke et al., 1999; Poorthuis et al., 1999; Applegarth et al., 2000; Dionisi-Vici et al., 2002; Pinto et al., 2004; Poupetova et al., 2010). More than 50 LSDs have been described to date (Staretz-Chacham et al., 2009). Although they are characterized by a wide spectrum of clinical phenotypes, many of these disorders present with severe progressive neurological impairment. Among the neurological symptoms, the presence of PME has been reported in different LSDs, including Gaucher disease, action myoclonus-renal failure syndrome, neuronal ceroid lipofuscinoses, sialidosis, Niemann Pick type C disease, and GM2 gangliosidosis. Each of these LSDs is characterized by a series of specific sings and symptoms. However, many of them share some clinical and biochemical features, such as the presence of signs of neurological impairments other than PME or organ disorders, which may be useful in the diagnosis of patients presenting with PME due to LSD.