Encyclopedia/Central nervous systemVascular

Subarachnoid hemorrhage

Synonyms: SAH, Subarachnoid haemorrhage

Bleeding into the subarachnoid space, most often from trauma or, in spontaneous cases, a ruptured intracranial aneurysm.

SystemsCentral nervous system, Vascular
Cases1
Last revisedNov 5, 2026

Subarachnoid hemorrhage (SAH) is blood in the cerebrospinal fluid spaces around the brain. Trauma is the commonest cause overall. About 85% of spontaneous cases are due to a ruptured saccular aneurysm.

Clinical presentation

A sudden, severe thunderclap headache reaching maximum intensity within a minute, often with vomiting, neck stiffness, photophobia, seizures or reduced consciousness.

Radiographic features

Non-contrast CT

  • Hyperdense blood in the basal cisterns, Sylvian fissures and sulci
  • The distribution hints at the aneurysm site — for example the interhemispheric fissure with anterior communicating artery aneurysms
  • Associated hydrocephalus and intraventricular blood
  • Sensitivity is very high within six hours of onset and falls over the following days

CT angiography

Used to find and characterise the aneurysm; catheter angiography remains the reference standard.

Treatment and prognosis

Aneurysms are secured by endovascular coiling or surgical clipping. Nimodipine reduces the risk of delayed cerebral ischaemia. Rebleeding, vasospasm and hydrocephalus are the main complications.

Differential diagnosis

  • Pseudo-subarachnoid hemorrhage in diffuse cerebral oedema
  • Meningitis
  • Intrathecal contrast

Cases and figures

Subarachnoid hemorrhage — imaging
CT
2
Case 1010Adult

Subarachnoid hemorrhage

Sudden “worst ever” headache reaching peak intensity within a minute, with vomiting.

References

  1. Perry JJ, Stiell IG, Sivilotti ML, et al. Sensitivity of computed tomography performed within six hours of onset of headache for diagnosis of subarachnoid haemorrhage: prospective cohort study. BMJ. 2011;343:d4277.
  2. Connolly ES Jr, Rabinstein AA, Carhuapoma JR, et al. Guidelines for the management of aneurysmal subarachnoid hemorrhage. Stroke. 2012;43(6):1711–1737.

Cite this entry

Subarachnoid hemorrhage. Radflow Encyclopedia. Last revised 5 November 2026. https://radflow.cc/encyclopedia/subarachnoid-hemorrhage (accessed 15 November 2026).

Related entries

Extradural hematomaArterial bleeding between the skull and the dura, typically after a temporal bone fracture, forming a biconvex collection that does not cross sutures.

Educational reference for healthcare professionals and students. It is not medical advice and does not replace clinical judgement or local guidelines.