76 years old female found unconscious

History
76 years old female found unconscious at home.

Imaging findings
Unenhanced head scans (A, C) show subarachnoid hemorrhage with symmetric clot deposition predominantly within the basal cisterns. Blood is also found in the ventricle system. Volume rendering technique (B) and maximum intensity projection (D) of a CT angiogram depict a small focal outpouching of the anterior communicating artery (red circle).

Diagnosis
Rupture of an anterior communicating artery aneurysm with subarachnoid hemorrhage.

Discussion
Most subarachnoid hemorrhages (75%) are caused by rupture of intracranial aneurysms at the base of the brain. Intracranial aneurysms are common lesions with an estimated prevalence in the adult population between 1 and 5%. They are considered to be sporadically acquired lesions, although rare familial forms and association with conditions including autosomal dominant polycystic kidney disease and connective tissue disorders have been described. Intracranial aneurysms occur at arterial branch points in the subarachnoid space at the base of the brain and are most frequently located around the anterior communicating artery (30%), the posterior communicating artery (25%), the middle cerebral artery (20%) and the tip of the basilar artery (7%).

The overall rupture risk is estimated with 1.2% within 5 years. The risk of rupture increases with higher age and female gender. Aneurysms of the posterior circulation have higher rupture risks. However, most intracranial aneurysms are considered not to rupture during life-time. Despite improvements in diagnostic and therapeutic techniques, the mortality rate for subarachnoid hemorrhage is relatively unchanged at approximately 50%. The first 2 weeks after rupture constitute a critical period in which several complications may occur, e.g., rehemorrhage, hydrocephalus, cerebral edema, and vasospasm. Vasospasm may induce secondary cerebral ischemia and is the main reason for mortality of patients who make it to the hospital.

On the basis of the predominant localization of hemorrhagic hyperdensity in particular cisternal areas, unenhanced CT is often able to define the probable origin of the bleed and the location of the aneurysm responsible for hemorrhage. In aneurysm ruptures of the anterior communication artery, hyperdensity prevails in the suprasellar cisterns and the anterior aspect of the interhemispheric fissure, from where it tends to spread to the bifrontal regions. The callosal and cingulate gyri are often outlined by blood, which is also frequently present surrounding the brainstem and within the Sylvian fissures. Aneurysms of the middle cerebral artery branching site invariably produce a unilateral bleed within the Sylvian fissure and often in the adjacent suprasellar cistern.

The Fisher grade classifies the appearance of subarachnoid hemorrhage on CT scans according to the amount of blood and is useful in predicting cerebral vasospasm.

  • group 1: no blood detected
  • group 2: diffuse thin (<1 mm) subarachnoid hemorrhage with no clots.
  • group 3: localised clots and/or layers of blood >1 mm in thickness
  • group 4: intracerebral or intraventricular blood (+/- subarachnoid hemorrhage)

In symptomatic patients CT angiography is the modality of choice for evaluation of intracranial aneurysms. When compared with digital subtraction angiography as the reference standard, CT has average reported specificity rates of 96 to 98% (90 to 94% for aneurysms smaller than 3 mm and up to 100% for aneurysms larger than 4 mm) and sensitivity rates of 96 to 98%. Three-dimensional rendering techniques may prove useful in assisting with treatment planning.

Recommended reading
Brisman et al.
Cerebral Aneurysms
NEJM (2006) 355(9):928

Hacein-Bey and Provenzale
Current Imaging Assessment and Treatment of Intracranial Aneurysms
AJR (2011) 196(1):32

provided by Tim

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