Phenotype of heterozygous variants of dehydrodolichol diphosphate synthase.

Aim: To further identify and broaden the phenotypic characteristics and genotype spectrum of the dehydrodolichol diphosphate synthase (DHDDS) gene. Method: Pathogenic variants of DHDDS were identified by whole‐exome sequencing; clinical data of 10 patients (six males, four females; age range 2–14y; mean age 5y 9mo, SD 3y 3mo) were collected and analysed. Results: All patients had seizures, and myoclonic seizures could be seen in eight patients, with myoclonic status epilepticus in three. The interictal electroencephalogram (EEG) in four patients at seizure onset showed generalized slow waves, slow wave mixed spikes, and spike and waves. Tremor, ataxia, and hypertonia was observed in six, five, and three patients respectively. The results of short‐latency somatosensory evoked potential in two patients were normal, and the symptom of tremor was captured on EEG without time‐locked discharges in one patient, suggesting that the tremor in both patients was a motor impairment rather than myoclonic seizures. Global developmental delay occurred in all patients, among whom nine showed severe intellectual disability and one moderate. Five DHDDS variants were identified, three of which have not been reported previously. Interpretation: Myoclonic seizure is the most common seizure type in heterozygous DHDDS variants, while myoclonic status epilepticus can also occur. The pattern of interictal EEG discharges is characterized by slow waves rather than spike and waves, and generalized discharges was prominent. [ABSTRACT FROM AUTHOR]