(Ishii, A. et. al. 2017). In studying the mutations in a large group of 285 Japanese patients with Dravet syndrome, the authors found that missense mutations were found more often in a few specific, highly functional areas of the sodium ion channel. Truncation mutations were associated with a more rapid rate of cognitive decline, regardless of age at seizure onset, while the age at seizure onset was a predictor for the rate of cognitive decline for missense mutations.

Ishii A, Watkins JC, Chen D, Hirose S, Hammer, MF. (2016). Clinical implications of SCN1A missense and truncation variants in a large Japanese cohort with Dravet syndrome. Epilepsia Dec 24. doi: 10.1111/epi.13639 https://www.ncbi.nlm.nih.gov/pubmed/28012175