Late diagnoses of Dravet syndrome: How many individuals are we missing?

We report new genetic diagnoses of Dravet syndrome in a group of adults with complex epilepsy of unknown cause, under follow‐up at a tertiary epilepsy center. Individuals with epilepsy and other features of unknown cause from our unit underwent whole‐genome sequencing through the 100 000 Genomes Project. Virtual gene panels were applied to frequency‐filtered variants based on phenotype summary. Of 1078 individuals recruited, 8 (0.74%) were identified to have a pathogenic or likely pathogenic variant in SCN1A. Variant types were as follows: nonsense (stopgain) in five (62.5%) and missense in three (37.5%). Detailed review of childhood history confirmed a phenotype compatible with Dravet syndrome. Median age at genetic diagnosis was 44.5 years (range 28‐52 years). Tonic‐clonic seizures were ongoing in all despite polytherapy including valproate. All had a history of fever sensitivity and myoclonic seizures, which were ongoing in two (25%) and three (37.5%) individuals, respectively. Salient features of Dravet syndrome may be less apparent in adulthood, making clinical diagnosis difficult. Regardless of age, benefits of a genetic diagnosis include access to syndrome‐specific treatment options, avoidance of harmful drugs, and monitoring for common complications.