Some cases of minor head trauma in patients with hemiplegic migraine can develop into delayed cerebral edema, a life-threatening medical emergency.

FHM1, which accounts for about 50% of FHM patients, is caused by mutations in a gene coding for the P/Q-type calcium channel α subunit, CACNA1A.

In short, FHM is typified by migraine with aura associated with hemiparesis, and in FHM1, cerebellar degeneration, which can result in episodic or progressive ataxia.

Other symptoms are altered consciousness (in fact, some cases seem related to head trauma), gaze-evoked nystagmus, and coma.

Aura symptoms, such as numbness and blurring of vision, typically persist for 30–60 minutes, but can last for weeks to months.

[citation needed] The first discovered FHM locus was the CACNA1A gene (originally named CACNL1A4), which encodes the P/Q-type calcium channel CaV2.1.

[4] 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.

[7] They confirmed that the R192Q mutant activates at more negative potentials and that neurons producing these channels have much larger whole-cell current.

[citation needed] The second subtype of familial hemiplegic migraine, FHM2, is caused by mutations in the gene ATP1A2 that encodes a Na+/K+-ATPase.

Twenty-nine known mutations in this gene are associated with FHM2 (table 2), many clustering in the large intracellular loop between membrane-spanning segments 4 and 5 (figure 1).

Astrocytes expressing these mutant ion pumps will have much higher resting potentials and are believed to lead to disease through a poorly understood mechanism.

Brain-imaging techniques, such as MRI, CAT scan, and SPECT,[46] are used to look for signs of other familial conditions such as CADASIL or mitochondrial disease, and for evidence of cerebellar degeneration.

As penetrance is high, individuals found to carry mutations should be expected to develop signs of FHM at some point in life.

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.