Definition and etiology of subarachnoid hemorrhage

Created 29/03/2021, last revision 21/04/2023

Definition, epidemiology

  • subarachnoid hemorrhage (SAH) is a clinical syndrome resulting from bleeding into the subarachnoid space (the area between the arachnoid membrane and the pia mater surrounding the brain)
  • the most common cause of SAH is head trauma (traumatic SAH)
  • non-traumatic SAH is often caused by rupture of a cerebral aneurysm (~80%) or, less frequently, due to arteriovenous malformation (AVM)
    • aneurysms are acquired lesions related to hemodynamic stress on the arterial walls at bifurcations and bends
    • aneurysmal SAH is a cause of 3-8% of all strokes; aneurysms 5-15 mm in size are the most common aneurysm to rupture
    • unruptured aneurysms are found in 0.3-5% of the population, multiple in 15-20% of cases
  • the annual incidence of aneurysmal SAH is race-, sex-, and age-related
    • reported rates vary between 10 and 26 cases per 100,000 population
    • the incidence of SAH in women is higher than in men (ratio of 3 to 2)
    • incidence increases with age 
      • the peak at the age of 40-65 years (80% of SAH)
      • only 15% occur in people aged 20-40 years and  5%  in people < 20 years of age
      • SAH is rare in children
  • SAH may have an unfavorable prognosis (reported percentages vary in the literature):
    • 5-15% of patients die before arriving at the hospital
    • 30% have initially severe deficits  (Hunt-Hess 4-5)
    • about 2/3 have a mild deficit initially; 50% deteriorate soon  (spasms, rebleeding, surgery, and other complications)
    • 10-20% of those admitted to hospital die within 24 hrs due to rebleeding
  • risk factors
    • smoking, hormonal oral contraception, alcohol, hypertension, stimulants
    • genetic predisposition – family history of a ruptured aneurysm increases risk 3-7 times


  • there are three distinct SAH patterns; for each pattern, specific etiologies, treatment, and prognostic implications were described
    • suprasellar cisterns with peripheral extension
    • perimesencephalic SAH (pmSAH)
    • convexial SAH (cSAH)

Traumatic SAH

  • history of trauma
  • localization at the convexity between gyri is common
  • compared to cSAH, more significant bleeding is usual
  • exclude concomitant traumatic changes (e.g., hemorrhagic contusion, subdural hematoma, skull bone fissure, subcutaneous hematoma)   Subcutaneous hematoma
    • ! contusions may become apparent after a few hours ⇒ perform a control CT scan with sufficient delay
Traumatic SAH and epidural hematoma on NCCT
Traumatic SAH
Traumatic SAH - initila NCCT (A), CT control in 24hrs (B), calval fissura in bone window (C)

Spontaneous (non-traumatic) SAH

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  • prognosis depends on the severity of the initial insult and our ability to prevent complications, specifically rebleeding (by early detecting and treating the source)
  • overall mortality up to 50%
    • 10-15% of patients die before arriving at the hospital
    • another 10-12% die within 24 h
    • rebleeding or brainstem dysfunction due to a massive intraventricular hemorrhage or decompensated intracranial hypertension are the usual causes of death
  • 1/3-1/2 of the survivors have significant functional deficits (motor and/or cognitive)
    • cognitive deficits are present in numerous patients considered to have a good outcome
    • SAH patients commonly experience deficits in memory, executive function, and language; these symptoms are accompanied by depression, anxiety, fatigue, and sleep disturbances
Factors affecting morbidity and mortality in SAH
  • the severity of hemorrhage (⇒ Hunt-Hess score, Fisher scale)
  • presence and severity of cerebral vasospasms
  • rebleeding occurrence
  • comorbidities and complications (e.g., infections, myocardial infarction, hydrocephalus, etc.)
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Definition and etiology of subarachnoid hemorrhage