Subarachnoid hemorrhage

Differential diagnosis
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By Tania Rebeiz MD and James R Brorson MD

Headache, the primary symptom of subarachnoid hemorrhage, is a common presenting complaint in office and emergency room settings, but only a small fraction of patients with headache have a subarachnoid hemorrhage, creating a diagnostic challenge. The headache of subarachnoid hemorrhage characteristically is severe, very rapid in onset, and holocephalic or posterior, but atypical presentations occur. Sudden onset severe headaches are often described as “thunderclap headaches,” and have a wide differential diagnosis that must be carefully considered with query for characteristic clinical features and performance of appropriate diagnostic testing (Table 6). The difficult challenge in the emergency setting is how to recognize which headache cases warrant further diagnostic testing, with brain imaging and lumbar puncture in particular, among all the patients presenting and seeking relief from a severe headache. In a multicenter study of 10 university-affiliated emergency departments, among alert and oriented patients with nontraumatic headache peaking in intensity within 1 hour of onset, exclusion of patients with 3 or more similar previous recurrent headaches as well as those with other known causes for headache resulted in a 6.2% rate of confirmed subarachnoid hemorrhage among remaining patients (Perry et al 2013). A selection rule was applied to this population, calling for diagnostic investigation in patients with new severe headache and any of the following high-risk variables:

  • Age of 40 years or more
  • Neck pain or stiffness
  • Witnessed loss of consciousness
  • Onset during exertion
  • Thunderclap headache (defined as instantly peaking pain)
  • Limited neck flexion on examination

This rule, termed the “Ottawa subarachnoid hemorrhage rule,” resulted in a very high sensitivity (100%) and modest specificity (15.3%) in this selected population of headache patients (Perry et al 2013). The low specificity points to the common clinical features shared by a number of other conditions that can mimic subarachnoid hemorrhage.

Table 6. Differential Diagnosis of Severe Sudden Headache (“Thunderclap Headache”)


Clinical features (in addition to headache)

Key diagnostic test

Subarachnoid hemorrhage

Meningismus, altered mentation, seizures, sometimes focal signs

CT followed by lumbar puncture

Cerebral venous sinus thrombosis

Focal or bilateral deficits, seizures, papilledema

MR or CT venography

Reversible cerebral vasoconstriction syndrome

Focal deficits

Vasoconstriction on angiography (MRA, CTA, or DSA)

Craniocervical arterial dissection

Posterior headache and/or neck pain, focal deficits, Horner syndrome

Angiography (MRA, CTA, or DSA), MR vessel wall imaging

Pituitary apoplexy

Visual field deficits and hypopituitarism

MRI brain

Intraparenchymal hemorrhage

Progressive focal deficits, seizures, signs of intracranial hypertension

CT brain

Subdural or epidural hematoma

Progressive focal deficits, signs of midline shift or herniation

CT brain

Infectious or aseptic meningitis

Fever, leukocytosis, meningismus, altered mentation

Lumbar puncture

Sentinel hemorrhage


Lumbar puncture

Benign coital headache

Historical setting, nonfocal exam

Clinical diagnosis


Clinical features (aura, nausea, photo/phonophobia)

Clinical diagnosis

Trigeminal neuralgia

Characteristic distribution and time course, trigger points

Clinical diagnosis

Idiopathic intracranial hypertension

Global headache, postural changes, papilledema, transient visual obscurations, visual loss

Lumbar puncture with measurement of opening pressure

Cerebral vasculitis

Premonitory symptoms, confusion, psychosis, depression, focal signs, hemorrhage

Angiography (MRA, CTA, or DSA) and vessel wall imaging

When brain CT imaging shows features suggesting subarachnoid hemorrhage, in addition to aneurysmal subarachnoid hemorrhage, other diagnoses must be considered:

Traumatic subarachnoid hemorrhage.Traumatic subarachnoid hemorrhage is usually recognized with a clear history of trauma. It typically affects the cerebral convexities and is often accompanied by other signs of injury such as orbital frontal contusions, skull fracture, or external scalp trauma.

Pseudo-subarachnoid hemorrhage.Pseudo-subarachnoid hemorrhage is defined as increased density of the basal cisterns and subarachnoid spaces on computed tomography, not due to blood products. It is due to radiographic mimics of subarachnoid hemorrhage, which could be due to pyogenic leptomeningitis, intrathecal administration of contrast material, high-dose intravenous contrast, and diffuse cerebral edema (Given et al 2003).

In This Article

Historical note and nomenclature
Clinical manifestations
Clinical vignette
Pathogenesis and pathophysiology
Differential diagnosis
Diagnostic workup
References cited