Are migraines usually unilateral?

Migraines are Typically Unilateral, But Can Switch Sides

Yes, migraines are typically unilateral (one-sided), with unilaterality being a defining diagnostic characteristic in the International Classification of Headache Disorders (ICHD-3). 1, 2

Diagnostic Criteria for Migraine and Unilaterality

According to the ICHD-3 criteria, unilateral location is one of the four key characteristics used to diagnose migraine:

• For migraine without aura, the headache must have at least 2 of these 4 characteristics:

  1. Unilateral location
  2. Pulsating quality
  3. Moderate or severe pain intensity
  4. Aggravation by routine physical activity 1, 2

• For migraine with aura, at least one aura symptom is typically unilateral 1

Patterns of Unilaterality in Migraine

While migraines are characteristically unilateral, they display several important patterns:

• Side-switching is common: Unlike some other headache disorders, migraines rarely remain "side-locked" (consistently on the same side) 3
• In classic migraine, research shows:
  • 42% experience strictly unilateral pain
  • 42% experience pain that alternates between unilateral and bilateral
  • 84% experience unilaterality in some form 3
• Almost all patients with unilateral migraines experience side-shifting between attacks 3

Clinical Significance of Headache Laterality

Recent research suggests potential differences between left-sided and right-sided migraines:

• Patients with left-sided migraines may experience:

  • 3.6 fewer headache-free days
  • 2.4 more severe headache days in a 4-week period 4
  • Potentially worse quality of life, higher anxiety, and autonomic nervous system differences 5

• Patients with right-sided migraines may experience:

  • Different cognitive effects
  • Changes in autonomic function (blood pressure, skin temperature)
  • Different patterns on imaging studies 5

Unilateral Cranial Autonomic Symptoms

Some migraineurs experience unilateral cranial autonomic symptoms (UAS), which represent a distinct phenotype:

• These patients are more likely to have:
  • Strictly unilateral headaches (3.18 times more likely)
  • Longer attack duration (2.47 times more likely to have attacks >72 hours)
  • More severe headache pain
  • More frequent allodynia and photophobia 6

Clinical Implications

When evaluating a patient with suspected migraine:

1. Unilaterality is a key diagnostic feature but not required for diagnosis
2. Side-switching between attacks is typical for migraine
3. Persistent side-locked headache should raise suspicion for other headache disorders
4. The presence of unilateral cranial autonomic symptoms may indicate a more severe phenotype

Common Pitfalls

• Misdiagnosis: Assuming that bilateral headaches rule out migraine (they don't)
• Overlooking side-switching: Failing to ask if the headache switches sides between attacks
• Missing red flags: Side-locked headaches may indicate secondary causes requiring neuroimaging 2

The pathophysiology of migraine involves activation of the trigeminovascular system and release of calcitonin gene-related peptide (CGRP), which can produce the characteristic unilateral pain pattern 7. Understanding these mechanisms has led to targeted treatments like triptans and CGRP antagonists.