A method for the diagnosis of SMEI in a patient comprising: (1) detecting an alteration in the SCN1A gene, including in a regulatory region of the gene, in a patient sample; (2) ascertaining whether the alteration is known to be SMEI associated or non-SMEI associated; and (3) (a) establishing a diagnosis of a high probability of SMEI where the alteration is known to be SMEI associated; or (b) establishing a diagnosis of a low probability of SMEI where the alteration is non-SMEI associated; or (e) or, if not known to be either, (i) considering genetic data for parents and/or relatives; (ii) establishing whether the alteration has arisen de novo or is inherited; and (iii) establishing a diagnosis of a low probability of SMEI where the alteration is inherited but a diagnosis of a high probability of SMEI if the alteration is de novo.