How should a unilateral headache be evaluated and managed?

Unilateral Headache and Subarachnoid Hemorrhage (SAH) Evaluation

A unilateral headache requires immediate exclusion of subarachnoid hemorrhage if it presents with sudden ("thunderclap") onset, maximal intensity at onset, or is the "worst headache of life," warranting urgent neuroimaging (CT head without contrast) and lumbar puncture if imaging is negative but clinical suspicion remains high.

Red Flags Requiring Urgent Evaluation

When evaluating unilateral headache with concern for SAH, specific warning signs demand immediate action 1:

• Abrupt onset of severe headache (reaches maximum intensity within seconds to minutes)
• "Worst headache of life" quality
• Headache awakening patient from sleep
• Rapidly increasing frequency of headaches
• Focal neurologic signs or symptoms (uncoordination, weakness, sensory changes)
• Persistent headache following head trauma
• Marked change in established headache pattern

Immediate Diagnostic Approach

For suspected SAH:

1. Non-contrast CT head immediately - highest sensitivity within first 6 hours
2. Lumbar puncture if CT negative but clinical suspicion remains - look for xanthochromia and elevated red blood cells
3. CT angiography or conventional angiography if SAH confirmed to identify source

Distinguishing Primary Unilateral Headaches from SAH

Migraine Characteristics (Most Common Primary Unilateral Headache)

Migraine presents with gradual onset over minutes to hours, contrasting sharply with SAH's instantaneous peak 2, 3:

Diagnostic criteria require at least 2 of the following:

• Unilateral location (but can alternate sides)
• Pulsating quality
• Moderate to severe intensity
• Aggravation by routine physical activity

Plus at least 1 of:

• Nausea and/or vomiting
• Photophobia AND phonophobia

Duration: 4-72 hours when untreated 2, 3

Critical Temporal Distinction

Migraine aura symptoms spread gradually over ≥5 minutes and occur in succession, whereas vascular events like SAH or TIA have sudden, simultaneous onset 2. This temporal pattern is diagnostically crucial.

Neuroimaging Indications

Neuroimaging IS indicated when 1:

• Unexplained abnormal findings on neurologic examination
• Headache worsened by Valsalva maneuver
• New onset in older person (>50 years)
• Progressively worsening pattern
• Any red flags listed above present

Neuroimaging is NOT routinely indicated when 1:

• Normal neurologic examination
• Typical migraine features meeting ICHD criteria
• Established pattern without change
• No atypical features

Common Pitfall

Do not conflate aggravating factors (physical activity worsening migraine pain) with warning signs. Migraine characteristically worsens with routine activity, but this develops over the course of the headache, not instantaneously 2.

Other Unilateral Headache Considerations

Cluster Headache

• Severe unilateral orbital/temporal pain lasting 15-180 minutes
• Ipsilateral autonomic features: lacrimation, nasal congestion, ptosis, miosis, eyelid edema
• Frequency: 1-8 attacks per day during cluster periods 1

Cervicogenic Headache

• Fixed unilateral headache with cervical onset
• Reduced neck mobility and flexion strength
• History of whiplash injury common
• Pain radiates from neck to trigeminal distribution 4, 5

Paroxysmal Hemicrania and SUNCT

• Very brief attacks (seconds to minutes)
• Dramatic response to indomethacin (paroxysmal hemicrania)
• Require specialist evaluation 6, 7

Management Algorithm

Step 1: Rule out SAH and other emergencies

• If thunderclap onset or red flags → immediate CT head ± LP

Step 2: If benign primary headache confirmed

• Migraine without aura (most common unilateral primary headache):
  • First-line acute: NSAIDs (ibuprofen 400-800mg, aspirin 900-1000mg) 2, 8
  • Second-line acute: Triptans (most effective when taken early while pain still mild) 2, 9
  • Newer options: CGRP inhibitors (gepants) for acute treatment 8

Step 3: Consider preventive therapy if:

• ≥2 attacks per month producing ≥3 days disability
• Using acute medication >2 times per week
• Acute treatments contraindicated or ineffective 9

Preventive options include:

• Topiramate, valproate, propranolol, amitriptyline
• CGRP monoclonal antibodies
• Atogepant 8

Critical Warning About Medication Overuse

Avoid medication-overuse headache by limiting acute medication use:

• Non-opioid analgesics: <15 days/month
• Triptans/combination analgesics: <10 days/month
• Regular overuse >3 months transforms episodic to chronic headache 2, 9

The key distinction remains temporal: SAH is instantaneous and catastrophic; migraine builds gradually over minutes to hours. When in doubt about sudden-onset severe unilateral headache, image immediately.