When is a Magnetic Resonance Imaging (MRI) indicated for hypnic headache?

MRI for Hypnic Headache: When to Image

MRI is indicated for hypnic headache when "red flags" are present, such as abnormal neurological examination findings, sudden severe onset, or atypical features that suggest a possible secondary cause.

Red Flags Requiring Neuroimaging

Hypnic headache is generally considered a primary headache disorder, but certain clinical features warrant neuroimaging to rule out secondary causes:

• Abnormal neurological examination findings 1
• Sudden severe (thunderclap) headache onset 1, 2
• Progressive or worsening pattern of headache 2
• Headache that awakens the patient from sleep (note: while this is characteristic of hypnic headache, it's also a red flag for secondary headaches) 1
• New onset in patients over 50 years of age 2
• Positional headache 2
• Persistent vomiting 2
• Immunocompromised state 2
• History of cancer 2

Imaging Modality of Choice

When imaging is indicated for hypnic headache evaluation:

• MRI without contrast is the preferred initial imaging modality due to:

  • Superior soft tissue resolution 2
  • Better detection of posterior fossa and brainstem abnormalities 2
  • No radiation exposure 2
  • Higher sensitivity for detecting parenchymal abnormalities 1, 2

• Add contrast to MRI only if:

  • Abnormalities are found on non-contrast scan 2
  • Suspicion of infection 2
  • Concern for mass lesion 2
  • Inflammatory conditions are suspected 2

Evidence for Secondary Causes

While hypnic headache is typically considered primary, case reports have documented secondary causes that may be detected on MRI:

• Basilar artery dolichoectasia 3
• Posterior fossa lesions 3
• Brain stem abnormalities 3
• Pituitary lesions 3

When Neuroimaging is Not Indicated

Neuroimaging is not routinely warranted in patients with:

• Typical clinical presentation of hypnic headache 1
• Normal neurological examination 1
• No red flags in history or examination 1, 2

Potential Pitfalls

• Overimaging: The yield of neuroimaging in primary headache disorders without red flags is extremely low 2, 4
• Incidental findings: MRI may reveal clinically insignificant abnormalities that can cause unnecessary patient anxiety and lead to further unnecessary testing 1
• Misdiagnosis: Some secondary headache disorders can mimic primary headache patterns 1, 3

Special Considerations

• For suspected vascular abnormalities (based on clinical features), consider adding MR Angiography (MRA) 1, 2
• For suspected venous sinus thrombosis, MR Venography (MRV) may be appropriate 1, 2
• In emergency settings where rapid assessment is needed, CT may be appropriate as an initial test, but MRI remains superior for comprehensive evaluation 2

Remember that while hypnic headache is rare and typically affects older adults, cases have been reported in children and adults under 50 years of age 5, 6, which may require adjusting the threshold for imaging based on the atypical age presentation.