Genetics and Subtypes of Familial Hemiplegic Migraine

While clinical symptoms define a hemiplegic migraine, genetics provide the “blueprint” for why they happen. Familial Hemiplegic Migraine (FHM) is categorized into subtypes based on which specific gene is affected. Identifying your subtype can help your care team understand your personal risk for certain symptoms and how the condition might manifest in your family ^[1][2]. However, it is important to remember that genetics in FHM are complex; a subtype guides monitoring, but cannot perfectly predict an individual patient’s severity.

The Three Primary Subtypes

Most confirmed cases of FHM involve one of three well-studied genes. Each gene is responsible for a different type of “gate” or pump that regulates electrical activity in your brain ^[3].

• FHM1 (CACNA1A Gene): This is the most common form. The CACNA1A gene controls calcium channels in the brain ^[4].
  • Distinct Features: Attacks are sometimes triggered by minor head trauma ^[2].
  • Associated Conditions: Some variants in this gene are associated with ataxia—a condition where the part of the brain that controls balance (the cerebellum) begins to function less effectively, leading to a shaky gait. This ataxia may be episodic, static, or progressive, and is highly variant- and family-dependent ^[2][4].
• FHM2 (ATP1A2 Gene): This gene controls a pump that clears out excess chemicals between brain cells ^[5].
  • Distinct Features: FHM2 is sometimes associated with more severe “encephalopathic” attacks, which may include high fever, seizures, or prolonged confusion ^[6][7].
  • Associated Conditions: Some selected families with FHM2 also report higher rates of childhood “febrile” (fever-triggered) seizures or developmental delays. These are reported associations, not universal expectations for everyone with FHM2 ^[8][5].
• FHM3 (SCN1A Gene): This gene manages sodium channels that help neurons fire ^[9].
  • Distinct Features: FHM3 often begins early in childhood, though this is not obligatory ^[10].
  • Associated Conditions: While loss-of-function SCN1A variants are famously linked to severe epilepsy syndromes like Dravet Syndrome, the specific gain-of-function mutations that cause FHM3 usually lead to migraines rather than the severe epilepsy seen in other SCN1A conditions ^[9][10].

Other Genetic Links

New research has identified additional genes, such as PRRT2. Mutations in PRRT2 can cause hemiplegic migraine, but they are also linked to “paroxysmal” movement disorders (sudden, brief involuntary movements) and certain types of childhood epilepsy ^[11]. In families with a PRRT2 mutation, one relative might have movement issues while another has hemiplegic migraines ^[11].

Understanding Your Genetic Test Results

Genetic testing is a powerful tool for confirming a diagnosis, but it is not a “yes or no” test for the condition itself. Interpreting these results ideally involves a genetic counselor.

• What a Positive Result Means: Finding a known mutation supports a molecular diagnosis only when the variant is classified as “pathogenic” or “likely pathogenic.” A “variant of uncertain significance” (VUS) does not confirm FHM. However, even in the same family, the same confirmed mutation can cause a severe attack in one person and only mild symptoms in another—a concept called variable expressivity and incomplete penetrance ^[12][13].
• What a Negative Result Means: A negative genetic test does not rule out FHM ^[1]. In fact, current testing only finds a known mutation in a minority of clinically diagnosed cases (about 20% to 25% depending on the panel) ^[14][15]. This is because scientists believe many FHM genes have not yet been discovered ^[2].
• Clinical vs. Genetic Diagnosis: If you have the characteristic one-sided weakness and a compatible family history of the same, you still have FHM according to the official ICHD-3 medical guidelines, regardless of what the genetic test shows ^[1][16].

How FHM is Inherited

FHM follows an autosomal dominant inheritance pattern. This means you only need to inherit one copy of the mutated gene from one parent to have the condition. Each child of a parent carrying a confirmed pathogenic variant has a 50% chance of inheriting the mutation ^[17]. However, this 50% transmission risk applies when a parent carries a confirmed pathogenic variant; it does not automatically apply to every person with a clinical FHM label who tests negative. Relatives should not undergo broad testing solely because they have ordinary migraines unless a genetics professional recommends targeted cascade testing ^[17][12].