When to image a headache?

From the Guidelines

Imaging for headache is recommended when there are concerning features suggesting a secondary cause, such as sudden-onset severe headache, new headache in patients over 40 years old, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache, or limited neck flexion on examination, as outlined in the Ottawa SAH Rule 1. When evaluating patients with headache, it is essential to consider the clinical presentation and risk factors for secondary causes, such as subarachnoid hemorrhage (SAH). The Ottawa SAH Rule provides a framework for identifying patients who require additional investigation for SAH, including those with severe nontraumatic headache reaching maximum intensity within 1 hour, age ≥40 years, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache, or limited neck flexion on examination. Key considerations for imaging include:

• CT scan is typically the first-line imaging modality for acute presentations, especially when SAH is suspected, due to its high sensitivity and specificity for detecting SAH within 6 hours of symptom onset 1.
• MRI provides better visualization of brain parenchyma, posterior fossa structures, and vascular abnormalities for non-emergent evaluations.
• Most primary headaches (migraines, tension-type, cluster) do not require imaging if the history and examination are typical. The rationale for selective imaging is that routine scanning of all headache patients has low diagnostic yield, exposes patients to unnecessary radiation (with CT), and increases healthcare costs without improving outcomes for most patients with benign primary headache disorders. However, in patients with concerning features or high-risk profiles, imaging is crucial for early detection and management of secondary causes, such as SAH, to improve morbidity, mortality, and quality of life outcomes 1.

From the Research

Imaging for Headaches

• The decision to image a headache depends on the presence of certain clinical features, known as "red flags", which can suggest life-threatening secondary etiologies 2.
• In patients with a severe and acute headache, computed tomography (CT) is recommended to evaluate for hemorrhage, either subarachnoid or intraparenchymal, as it is more sensitive to acute hemorrhage than magnetic resonance imaging (MRI) 3.
• For patients with chronic headaches and a normal neurological examination, the pre-test probability of significant intracranial abnormalities is low, around 0.9% 4.
• Clinical variables such as abnormal neurological exam, undefined headache, headache aggravated by exertion or valsalva, and headache with vomiting have statistically significant positive likelihood ratios for detecting significant intracranial abnormalities 4.
• The choice of imaging modality, CT or MRI, depends on the suspected underlying cause of the headache, with CT being more sensitive for acute hemorrhage and MRI providing better characterization of certain lesions 3, 5.
• In cases where subarachnoid hemorrhage is suspected, CT followed by lumbar puncture is a common diagnostic strategy, but alternative approaches using MRI with magnetic resonance angiography or CT with computed tomography angiography may be considered 6.

Specific Imaging Modalities

• CT is recommended for:
  • Acute and severe headaches to evaluate for hemorrhage 3
  • Suspected subarachnoid hemorrhage, possibly followed by lumbar puncture 6
• MRI is recommended for:
  • Characterization of certain lesions, such as vascular malformations and parenchymal lesions 3
  • Evaluation of chronic headaches with a normal neurological examination, to provide increased contrast between soft tissues 4, 5
  • Suspected subarachnoid hemorrhage, especially in late presentations or when CT is inconclusive 6