Pathophysiology
FHM1 (CACNA1A)
The first discovered FHM locus was the CACNA1A gene (originally named CACNL1A4), which encodes the P/Q-type calcium channel CaV2.1. Currently, 17  mutations in this channel are known (table 1), and these mutations are distributed throughout the channel. Some of these mutations result in patients with notable cerebellar degeneration or other dysfunction, including one mutation (S218L), which may be related to severe responses to mild concussion, up to and including delayed cerebral edema, coma, and death. Fifteen of these mutants have received at least some further analysis at the electrophysiological level to attempt to determine how they might lead to the FHM1 phenotype. Contradiction in the literature is increasing as to the end result of these mutations on channel kinetics and neuronal excitability.A good example of this contradiction can be seen in the literature regarding the R192Q mutation. The first investigation of this mutation, using the rabbit isoform of the channel expressed in oocytes, found that it did not alter any measured channel properties. A subsequent report, using human channels expressed in HEK293 cells, found a small, hyperpolarizing shift in the midpoint for activation, a result common among FHM1 mutants. This shift results in channels that open at more negative potentials, thus have a higher open probability than wild-type channels at most potentials. This report also found that the R192Q mutant produced almost twice as much whole-cell current compared to wild-type channels. This is not due to a change in single channel conductance, but to an equivalent increase in channel density.
Screening
Prenatal screening is not typically done for FHM, but it may be performed if requested. As penetrance is high, individuals found to carry mutations should be expected to develop signs of FHM at some point in life. Management
See the equivalent section in the main migraine article. People with FHM are encouraged to avoid activities that may trigger their attacks. Minor head trauma is a common attack precipitant, so FHM sufferers should avoid contact sports. Acetazolamide or standard drugs are often used to treat attacks, though those leading to vasoconstriction should be avoided due to the risk of stroke. Epidemiology
Migraine itself is a very common disorder, occurring in 15–20% of the population. Hemiplegic migraine, be it familial or spontaneous, is less prevalent, at 0.01% prevalence according to one report. Women are three times more likely to be affected than males. See also
Channelopathy
Childhood absence epilepsy
Hypokalemic periodic paralysis
Juvenile myoclonic epilepsy
Malignant hyperthermia
Timothy syndrome

References
External links
GeneReviews/NCBI/NIH/UW entry on Familial Hemiplegic Migraine
"The International Classification of Headache Disorders 2nd Edition". Cephalalgia. 24 (s1): 8–160. May 2004. ISSN 1468-2982.