A 32-year-old woman with focal epilepsy poorly controlled on levetiracetam is found to carry the SCN1A p.Arg1648Cys variant (gain-of-function mutation). The BEST pharmacological choice to add is:
- A Carbamazepine — first-line sodium channel blocker with SCN1A gain-of-function providing rationale
- B Phenytoin — reduces high-frequency firing by stabilizing inactivated state of Nav1.1 channels
- C Lamotrigine — blocks voltage-gated sodium channels in the inactive state, highly effective for SCN1A gain-of-function ✓
- D Vigabatrin — irreversibly inhibits GABA transaminase, increases GABA unrelated to sodium channel mechanism
Explanation
SCN1A encodes Nav1.1 channels. Gain-of-function mutations in SCN1A increase persistent sodium current and reduce seizure threshold. Sodium channel blockers (lamotrigine, phenytoin, carbamazepine) are rational therapeutics; however, in Dravet syndrome (loss-of-function SCN1A), sodium channel blockers worsen seizures by blocking residual Nav1.1 in inhibitory interneurons — making pharmacogenomic knowledge critical. For gain-of-function SCN1A variants causing focal epilepsy, lamotrigine is preferred because it blocks inactivated-state channels with high affinity, has a broad spectrum, and is better tolerated than carbamazepine; carbamazepine risks auto-induction. Vigabatrin does not address the primary pathomechanism.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.