Neurology

Diagnosing FHM and Ruling Out Look-Alikes

At a Glance

Familial hemiplegic migraine is diagnosed from repeated attacks of temporary one-sided weakness, other aura symptoms, and a compatible family history. CT or MRI, EEG, and sometimes a spinal tap help rule out stroke, seizures, or infection; a negative genetic test does not exclude FHM.

Diagnosing Familial Hemiplegic Migraine (FHM) is often described as a process of exclusion, though it is ultimately a clinical diagnosis based on ICHD-3 criteria combined with the targeted evaluation of dangerous alternatives. Because its primary symptom—temporary paralysis or weakness on one side of the body—is the hallmark of a medical emergency, doctors must first reliably assess that you are not having a stroke, a seizure, or a brain infection before attributing the event to migraine ^[1]^[2].

The Diagnostic Standard

The “gold standard” for an FHM diagnosis relies on a combination of your clinical history and your family tree. According to official guidelines, a diagnosis requires at least two attacks of one-sided weakness accompanied by other aura symptoms (like vision or speech changes) ^[3]. Crucially, you must also have at least one first-degree relative (parent, sibling, or child) or second-degree relative (grandparent, aunt, or uncle) who has experienced attacks meeting compatible hemiplegic-migraine criteria (they do not need to have “exactly the same” symptoms) ^[4]^[2].

Distinguishing FHM from “Look-Alikes”

Several serious conditions can mimic FHM. Your medical team will look for specific “clues” and perform tests to evaluate for them:

1. Ischemic Stroke or TIA

A stroke occurs when blood flow to the brain is blocked.

The “Migraine March”: In FHM, symptoms often build gradually. You might notice a shimmering in your vision, then tingling in your hand five minutes later, followed by weakness in your arm ten minutes after that ^[5]^[6]. This “march” of symptoms over 5 to 60 minutes is a classic sign of migraine ^[6].
The Stroke Pattern: A stroke is typically sudden, like a “bolt from the blue,” with all symptoms hitting at once ^[5]^[7]. However, because strokes can sometimes “stutter” or evolve, and FHM can sometimes be abrupt, doctors rely on imaging, not just symptom timing, to be sure ^[5]. Other vascular conditions like Transient Ischemic Attack (TIA), cerebral venous thrombosis, or arterial dissection must also be considered.

2. Epilepsy and Seizures

A focal seizure can cause temporary weakness (often called “Todd’s Paralysis”) after the seizure ends ^[2]. Because FHM and epilepsy share some of the same genetic triggers, they can sometimes happen at the same time ^[8]^[9].

3. Encephalitis or Meningitis

These are infections or inflammations of the brain and its lining. They can cause weakness, fever, and confusion—symptoms that also appear in severe “encephalopathic” FHM attacks ^[10]^[11].

Common Diagnostic Tests

When you arrive at a hospital with hemiplegic symptoms, you will likely undergo several targeted tests. Not every stable patient needs every test, but here is why they are ordered during an emergency:

Test	What It Looks For and Does Not Show	Why It Matters in FHM
CT Scan	Bleeding (hemorrhage) in the brain ^[12]. While it does not by itself reliably identify a major arterial blockage, it rapidly detects bleeding.	Used first because it is very fast; it helps rule out a “brain bleed” (hemorrhagic stroke). Vascular imaging (CTA/MRA) is used to assess blockages.
MRI (with Diffusion/Perfusion)	Microscopic signs of brain tissue death (infarction) ^[13].	An MRI can often see a stroke that a CT misses. In FHM, the MRI might show transient unusual blood flow patterns that cross multiple “territories” of the brain, but this is not a reliable diagnostic signature and can overlap with stroke ^[1]^[14].
EEG (Electroencephalogram)	Abnormal electrical activity in the brain ^[15].	Used selectively to see if your weakness is actually a “post-seizure” state. In FHM, the EEG may show general “slowing,” while between attacks, it is usually normal ^[15]^[16]. EEG is neither diagnostic for FHM nor sufficient to fully exclude epilepsy.
Lumbar Puncture (Spinal Tap)	Infection or inflammation in the spinal fluid ^[2].	If you have a high fever or severe confusion, this selective test evaluates for meningitis. Clinicians decide if it is indicated after a safety assessment, as raised pressure in the brain can make it unsafe ^[10]^[11].

The Role of Genetic Testing

While genetic testing can confirm which subtype of FHM you have (such as FHM1, 2, or 3), it is not required for a diagnosis. Because many FHM-triggering genes have not yet been discovered, a “negative” or “normal” genetic test result does not mean you don’t have the condition ^[2]^[17]. If you meet the clinical criteria and have the compatible family history, the diagnosis remains FHM regardless of the lab result ^[18].

Common questions in this guide

What is required for a familial hemiplegic migraine diagnosis?

Doctors generally look for at least two attacks involving temporary weakness on one side of the body along with other aura symptoms, such as visual or speech changes. A compatible hemiplegic-migraine history in at least one first- or second-degree relative supports the diagnosis, which is based on clinical criteria rather than a single test.

How do doctors distinguish FHM from a stroke?

Migraine symptoms often spread gradually over several minutes, while a stroke may cause sudden deficits, but timing alone cannot safely tell them apart. CT, vascular imaging, or MRI may be needed to look for bleeding, blocked blood flow, or injured brain tissue, so sudden one-sided weakness should be treated as an emergency.

Which tests may be used when hemiplegic migraine looks like something else?

A rapid CT scan checks for bleeding, and CT angiography or MRI can assess blood vessels and brain injury. An EEG may be used when a seizure is suspected, and a lumbar puncture may be considered for fever or severe confusion when infection is a concern; not every patient needs every test.

Can a negative genetic test rule out familial hemiplegic migraine?

No. Genetic testing can identify some FHM subtypes, but not all genes that cause FHM are known, so a negative result does not exclude the condition. Doctors can still diagnose FHM from compatible attacks and family history.

Could seizures be causing weakness that looks like FHM?

Yes. A focal seizure can be followed by temporary weakness called Todd paralysis, and some people can have both epilepsy and FHM. An EEG may help evaluate abnormal brain-wave activity, but a normal or nonspecific EEG does not by itself prove or rule out FHM or epilepsy.

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References (18)

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This page is for informational purposes only and does not constitute medical advice. It explains how clinicians evaluate familial hemiplegic migraine, but sudden weakness or other stroke-like symptoms require emergency medical care.

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