CACNA1A and pathologic nystagmus: Familial hemiplegic migraine type 1 (FHM1, OMIM #141500), caused by gain‐of‐function variants in CACNA1A, is an extreme type of migraine with aura that typically presents in the first or second decade of life with episodes of headaches, sensory loss, visual disturbance, hemiparesis, and cerebellar signs such as nystagmus or ataxia (Byers et al., 2016; Jen, 2001).