Epileptic and electroencephalographic manifestations of guanidinoacetate‐methyltransferase deficiency

ABSTRACT Aim. Describe the seizure‐related manifestations of guanidinoacetate methyltransferase (GAMT) deficiency in two new cases and compare these to the related literature. Methods. We reviewed the clinical and electroencephalographic manifestations of two siblings with GAMT deficiency. We also performed a thorough literature review of all cases of GAMT deficiency, using the PubMed database, and compared our findings to those previously reported. Results. One sibling presented with Lennox‐Gastaut syndrome while the second had manifestations of late‐onset West syndrome. Based on a literature search, we found that the clinical picture of GAMT deficiency has been described in a total of 58 cases, including our two patients, 45 of whom had at least some description of EEG and/or seizure manifestation. Epilepsy was present in 81%, with age at onset usually between 10 months and 3 years. Drug resistance was observed in approximately 45%. Initial seizures were febrile, tonic, or tonic‐clonic. Drop attacks and generalised seizures were the most frequent seizure type. Absence and febrile seizures also occurred. Less frequently, focal seizures and late‐onset infantile spasms (one prior case) were observed. Multifocal spikes and generalised <3‐Hz‐spike slow waves were common while only one prior single case report of hypsarrhythmia was described. Lennox‐Gastaut syndrome was common, while progressive myoclonic epilepsy was also, less frequently, reported. Conclusions. To our knowledge, this is the second report of the occurrence of West syndrome in GAMT deficiency. The majority of patients with GAMT deficiency have seizures and approximately half are drug‐resistant. Late‐onset of hypsarrhythmia and/or epileptic spasms could potentially prove to be a distinctive, albeit infrequent, feature of this treatable metabolic disorder.