Diagnosis of subarachnoid hemorrhage

David Goldemund M.D.
Updated on 29/04/2024, published on 30/03/2021
  • the diagnosis of SAH should be considered in any patient presenting with a severe, sudden-onset, or rapidly worsening headache
  • deciding which patients require evaluation for SAH is often challenging in emergency care
  • imaging methods can confirm the diagnosis of subarachnoid hemorrhage (SAH)
    • the standard imaging method is computed tomography (CT), followed by CT angiography (CTA) to detect or exclude a source of bleeding
    • if CT scan is negative, a lumbar puncture (LP) with proper cerebrospinal fluid (CSF) analysis is performed

Computed tomography

  • SAH is manifested by increased density in the subarachnoid (SA) space  → Fisher scale grading
  • CT scan sensitivity for SAH is 90-95% within the first 24 hours, depending on the amount of blood
    • CT scan sensitivity decreases over time – to 70% by day 3 and to < 50% a week later
    • presence of a significant amount of blood on a CT scan after one week strongly suggests rebleeding
Aneurysmal subarachnoid hemorrhage

SAH caused by rupture of top basilar artery aneurysm

ACA aneurysm bleeding

SAH location
Aneurysm location
Sylvian fossa MCA
interhemispheric fissure
suprasellar cistern
distal ICA
perimesencephalic cistern top basilar artery
4th ventricle PICA
SAH caused by rupture of PComA aneurysm
SAH caused by rupture of basilar apex aneurysm
SAH caused by ACA aneurysm rupture
SAH caused by MCA aneurysm rupture
Saccular A1 aneurysm

Combination of SAH and intraparenchymal bleeding

  • SAH may be accompanied by intraparenchymal hematoma (especially in MCA aneurysms) or by intraventricular bleeding  Intracerebral hemorrhage from MCA aneurysm  Intraventricular bleeding from an ICA aneurysm
  • conversely, a primary ICH from AVM or in basal ganglia can secondarily extend into the subarachnoid space resulting in secondary SAH  Hypertonic hemorrhage in basal ganglia with secondary SAH

Convexial SAH

  • traumatic
    • other signs of trauma may be present, such as SDH/EDH, ICH, contusion, subcutaneous hematoma, or  a skull bone fissure (inspect the bone window) Subcutaneous hematoma Traumatic SAH and epidrual hematoma on NCCT Traumaic SAH with skull fissure
  • bleeding from a dural fistula
  • rarely, intracranial venous thrombosis may initially manifest as convexal SAH  [Sharma, 2010] [Oppenheim, 2005]
    • rupture of a dilated feeding vein due to retrograde intravenous hypertension
    • secondary rupture of a hemorrhagic infarction into the subarachnoid space

→ nontraumatic convexal SAH

Perimesencephalic SAH (PMSAH)

  • blood is present only in the (pre)pontine and interpeduncular subarachnoid cisterns   Perimesencephalic SAH
    • it may extend into the suprasellar cistern or to the medial (not lateral) Sylvian fissure and proximal interhemispheric fissure
  • accounts for ~ 5-10% of all SAHs
  • mainly low-pressure bleeding, presumably caused by rupture of small perimesencephalic and/or prepontine veins → normal CTA/DSA
    • favorable clinical course (HH usually I-II), low recurrence risk (~1%)
  • the minority of MPSAHs are caused by vertebrobasilar aneurysms or AVM rupture
  • if NCCT criteria are met and CTA is negative, DSA is not required
Perimesencephalic subarachnoid hemorrhage (PMSAH)

Perimesencephalic subarachnoid hemorrhage (PMSAH)

Perimesencephalic subarachnoid hemorrhage (PMSAH)

Magnetic resonance imaging (MRI)

  • MRI is sensitive to subarachnoid blood and can visualize it within the first 12 hours
    • T2, FLAIR – hyperintense
    • T1 – isointense to hypointense
    • GRE/SWI – hypointense
      • supersensitive to blood products
  • MRI can also detect thrombosed aneurysms, sometimes not visible on angiography
  • MR angiography and MR venography can detect an aneurysm or other sources of bleeding
  • DWI may show potential ischemic changes
    • both early and delayed (DID associated with vasospasm)
DDx of non-traumatic convexial SAH (cSAH)   Nontraumatic convexial SAH (FLAIR and GRE)
stenosis with collateral flow
septic aneurysm
septic emboli
cavernous malformation
Subarachnoid hemorrhage on MRI
Subarachnoid hemorrhage caused by rupture of na ICA aneurysm (GRE)

SAH in patient with reperfusion syndrom after carotid endarterectomy

Cerebrospinal fluid analysis

  • it is recommended to wait ≥ 12 hours from the onset of presenting symptoms before performing a lumbar puncture (LP) [Beetham, 2003]
    • if performed earlier, the CSF can appear “artificially normal”
  • rapid analysis of collected CSF is required
  • bedside multiple-tubes test is useful  Multiple tubes test - no clearing of blood points toward SAH diagnosis
    • if there is bleeding due to the puncture, the CSF should clear when collected in multiple tubes
  • xanthochromia is evidence of previous bleeding
  • risk of early SAH recurrence as a complication of lumbar puncture is negligible
  • NCCT may miss some SAH cases; cerebrospinal fluid (CSF) spectrophotometry should be performed in those patients with negative CT scans or those who present several days after suspected hemorrhage
  • spectrophotometry provides information about the presence of hematogenous pigments in the CSF and helps differentiate SAH from artificial bleeding (sample must be delivered and processed immediately and protected from light)
  • under physiological conditions, the spectrophotometric curve of CSF is flat or slightly elevated
  • CSF xanthochromia (yellowish discoloration) can be caused by oxyhemoglobin or bilirubin
  • hemolysis occurs 2 hours after erythrocytes enter the CSF ⇒ oxyhemoglobin – oxyHb-type curve (wavelength 410-418nm) can be detected
    • the presence of oxyhemoglobin alone is not proof of SAH; it may be increased because of in vitro hemolysis of red cells introduced into the CSF during lumbar puncture
  • bilirubin can be detected after ~9-15 hours (wavelength 450-460nm) [Beetham, 2003]
    • the presence of bilirubin in CSF is macroscopically manifested by a distinctly yellow or at least yellowish color (xanthochromia)
    • bilirubin is formed only in vivo and rules out a bloody tap
      long bilirubin (460 nm) – SAH, hemolytic anemia
    • short bilirubin (430 nm) – SAH, obstructive icterus
  • initially, the oxyHb peak dominates (oxyHb-type curve), bilirubin gradually increases (oxyHb-Bi type curve), and later bilirubin dominates (Bi-type curve)
  • the CSF may contain bilirubin for up to 2 weeks – in a patient with a severe headache of several days’ duration and no blood on a CT scan, the detection of xanthochromia/bilirubin is significant
  • it is important to rule out passive transfer of bilirubin from the plasma
    • therefore, a serum/plasma sample from the patient is also required
CSF spectrophotometry
  • cytology is useful only when monocyte activation and erythrocyte phagocytosis have begun
  • aseptic meningitis is usually present at later stages of SAH
  • cytology helps to distinguish repeated bleeding
    • this is indicated by a disruption in the sequence of erythrocytes phagocytosis and the gradual appearance of hematogenous pigments in macrophages
    • simultaneous phagocytosis of intact erythrocytes with hemosiderin granules or simultaneous appearance of hematoid crystals with erythrocyte digestion may be observed

Erythrocyte phagocytosis   Erythrophage
recent bleeding, erythrophages appear  (≥ 6 h from bleeding)

Digestion of phagocytosed erythrocytes
erythrocytes decolorization and disintegration, optically empty vacuoles are present

Hemosiderin  Siderophage
iron-containing hematogenous pigment, present in siderophages (from day 4-5)

Hematoidin crystals
hematogenic pigment forming rhomboid crystals in macrophages
may be present extracellularly (after macrophage death) (~ from day 13)

CSF cytology in SAH


  • the diagnosis usually involves a CT scan of the head, which is effective in detecting blood in the subarachnoid space
  • if the CT scan is inconclusive, a lumbar puncture (spinal tap) may be performed to look for blood in the cerebrospinal fluid
  • a CT scan should be performed as soon as possible, ideally within the first few hours after symptom onset, for the highest sensitivity in detecting a subarachnoid hemorrhage
  • MRI, particularly fluid-attenuated inversion recovery (FLAIR) and gradient echo (GRE) sequences, can be used to diagnose SAH
  • MRI is typically more sensitive than CT for subacute or chronic hemorrhage
  • a lumbar puncture is particularly useful when the CT scan is negative but clinical suspicion remains high. It can detect blood in the cerebrospinal fluid that might not be visible on a CT scan, especially in cases of minor hemorrhages.
  • yes, SAH can be misdiagnosed, especially if symptoms are mild or atypical. Misdiagnosis can occur in emergency settings if the symptoms are attributed to less serious conditions like migraine or tension headache
  • blood tests are not available to diagnose subarachnoid hemorrhage

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Diagnosis of subarachnoid hemorrhage