Definition and etiology of subarachnoid hemorrhage

David Goldemund M.D.
Updated on 29/04/2024, published on 29/03/2021

Definition, epidemiology

  • subarachnoid hemorrhage (SAH) is a clinical syndrome caused by bleeding into the subarachnoid space (located between the arachnoid membrane and the pia mater surrounding the brain)
  • traumatic SAH occurs due to traumatic brain injury (TBI)
  • non-traumatic SAH often results from the rupture of a cerebral aneurysm (~75-80% of cases) or, less frequently, from arteriovenous malformation (AVM)
    • aneurysms are acquired lesions related to hemodynamic stress on the arterial walls at bifurcations and bends
    • aneurysmal SAH accounts for 3-8% of all strokes; aneurysms measuring 5-15 mm are most likely to rupture
    • unruptured aneurysms are found in 0.3-5% of the population, with 15-20% of these individuals having multiple aneurysms
    • about 20% of patients with one aneurysm will have an additional aneurysm
  • the annual incidence of aneurysmal SAH varies by race, sex, and age
    • reported rates ~ 10-26 cases per 100,000 population
    • the incidence of SAH is higher in women than in men (ratio 3:2)
    • incidence increases with age 
      • peaking at 40-65 years (80% of SAH cases)
      • only 15% occuring in individuals aged 20-40 years and 5% in those < 20 years
      • SAH is rare in children
  • SAH may have an unfavorable prognosis (percentages vary in the literature):
    • 5-15% of patients die before reaching hospital
    • 30% present with severe initial deficits (Hunt-Hess 4-5)
    • about 2/3 have mild deficit initially; 50% deteriorate soon due to spasms, rebleeding, surgery, and other complications
    • 10-20% of hospitalized patients die within 24 hours due to rebleeding
  • risk factors
    • smoking, oral hormonal contraceptives, alcohol, hypertension, stimulants
    • genetic predisposition – the risk  increases with the number of family members involved or with a family history of adult polycystic kidney disease


  • there are three distinct SAH patterns, each with specific etiologies, treatments, and prognostic implications:
    • suprasellar cisterns with peripheral extension
    • perimesencephalic SAH (pmSAH)
    • convexal SAH (cSAH)

Traumatic SAH

  • history of trauma (fall, car accident, etc.)
  • commonly localized at the convexity between the gyri
  • exclude usual concomitant traumatic changes (e.g., contusions, subdural/epidrual hematoma, skull bone fissure, subcutaneous hematoma)   Subcutaneous hematoma
    • ! contusions may not be apparent for several hours ⇒ perform a control CT scan after a 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

Aneurysm rupture   Cerebal aneurysms

  • artery wall thinning and paucity of tunica media, diminished or absent elastic lamina
  • sometimes, there is a  familial occurrence or association of aneurysms with specific congenital diseases (e.g., coarctation of the aorta, Marfan syndrome, Ehlers-Danlos syndrome, fibromuscular dysplasia, polycystic kidney disease) 

→ Diagnosing a brain aneurysm

Perimesencephalic SAH   Perimesencephalic SAH on NCCT 
  • presumed venous source with negative angiography
  • mostly benign course with good prognosis
Arteriovenous malformations  → see here
  • AVMs typically present with ICH or IVH
  • SAH is less common; usually caused by an aneurysm in the feeding artery
  • risk of bleeding 2-4%/year,  rebleeding risk approx.  6-18%/year
  • risk factors:
    • previous bleeding
    • deep location and drainage
    • concomitant aneurysm in the feeding artery
Other causes of SAH (mainly DDx of non-traumatic cortical SAH)   Nontraumatic convexial SAH  ) → see here
~ 5%
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  • prognosis depends on the severity of the initial insult and our ability to prevent complications, specifically rebleeding (by early detection and treatment of the source)
  • overall mortality up to 50%
    • 10-15% of patients die before reaching the hospital
    • another 10-15% die within the first 24 hours
    • the most common causes of death are rebleeding or brainstem dysfunction due to a massive intraventricular hemorrhage or decompensated intracranial hypertension
  • 1/3 of 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
  • 1/3 return to their normal function
Factors influencing morbidity and mortality in SAH
  • severity of hemorrhage (⇒ Hunt-Hess score, Fisher scale)
  • presence and severity of cerebral vasospasms
  • occurrence of rebleeding
  • comorbidities and complications (e.g., infection, myocardial infarction, hydrocephalus, etc.)


  • SAH is a type of stroke characterized by bleeding into the subarachnoid space (the area between the brain and the tissues covering it)
  • this condition is often accompanied by a sudden, severe headache and can be life-threatening
  • the most common causes of SAH are traumatic brain injury and a ruptured cerebral aneurysm (~ 75-80%)
  • other causes include arteriovenous malformations (AVMs) and blood disorders; in some cases, the cause remains undetected
  • a cerebral aneurysm is a weak spot in a brain artery that bulges and fills with blood
  • when an aneurysm ruptures, it causes bleeding into the subarachnoid space, leading to SAH 
  • yes, genetic factors may play a role, especially in familial cases of brain aneurysms
  • conditions such as polycystic kidney disease and certain connective tissue disorders are also associated with a higher risk of aneurysm and SAH
  • yes, head injuries can cause SAH
  • traumatic SAH usually occurs at the site of impact or where the brain moves against the skull during the injury and is typically associated with contusion, subdural or epidural hematoma
  • yes, the use of certain drugs, particularly stimulants like cocaine and methamphetamine, can increase the risk of SAH due to their effects on blood pressure and arterial integrity
  • smoking is a major risk factor for the formation and rupture of cerebral aneurysms, thereby significantly increasing the risk of SAH
  • the risk is dose-dependent and decreases after smoking cessation
  • the prognosis of SAH depends on the cause, severity, and presence of complications
  • in severe cases, 50% die before reaching the hospital
  • of those who make it to the hospital:
    • 1/3 die in the hospital
    • 1/3 survive with a disability
    • 1/3 return to normal function

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Definition and etiology of subarachnoid hemorrhage