Most Common Cause of Non-Aneurysmal Subarachnoid Hemorrhage
Perimesencephalic hemorrhage is the most common cause of non-aneurysmal subarachnoid hemorrhage, accounting for approximately 10% of all spontaneous SAH cases and representing the majority of the 15% of SAH cases that are non-aneurysmal. 1
Understanding the Spectrum of Non-Aneurysmal SAH
When aneurysmal SAH is excluded (which accounts for 70-85% of all spontaneous SAH), the remaining 15% of cases fall into distinct categories with different etiologies and prognoses 1:
Perimesencephalic SAH (Most Common Non-Aneurysmal Type)
- Represents 10% of all spontaneous SAH cases and is the predominant form of non-aneurysmal hemorrhage 1
- Blood distribution is characteristically restricted to the perimesencephalic cisterns on CT imaging 2, 3
- The presumed etiology is rupture of small veins or capillaries that are not detectable on conventional angiography 1
- Venous pathology, including transverse sinus thrombosis and intracranial venous hypertension, has been documented as a causative mechanism 4
- This pattern has an excellent prognosis with virtually no risk of rebleeding or long-term complications 3, 5
Other Non-Aneurysmal Causes (Less Common)
- Cerebral amyloid angiopathy and reversible cerebral vasoconstriction syndrome are rare causes of diffuse non-aneurysmal SAH 1
- Occult aneurysms that rupture and then thrombose with parent vessel occlusion may present as angiogram-negative non-perimesencephalic SAH, particularly in elderly hypertensive patients 6
Critical Diagnostic Distinction
The pattern of blood distribution on initial CT is crucial for risk stratification:
- Perimesencephalic pattern (blood confined to perimesencephalic cisterns): Benign course with excellent outcomes in nearly 100% of cases 3, 5
- Diffuse/aneurysmal pattern (blood in basal cisterns and Sylvian fissures): Carries significantly higher risk of complications including vasospasm, hydrocephalus, and poor outcomes similar to aneurysmal SAH 7, 5
Imaging Workup for Angiogram-Negative SAH
- When initial CTA or DSA is negative in patients with an aneurysmal bleeding pattern, repeat DSA should be performed days to weeks later, as this detects an additional 10% of previously missed aneurysms 2, 1
- For true perimesencephalic pattern with negative CTA, some authorities consider this sufficient, though this remains controversial 2
Common Pitfall to Avoid
Do not assume all angiogram-negative SAH has a benign prognosis. While perimesencephalic SAH has excellent outcomes, patients with diffuse non-perimesencephalic patterns on CT have a 6-fold lower odds of good outcome compared to perimesencephalic patterns and require intensive monitoring for vasospasm and other complications similar to aneurysmal SAH 7, 5.