What are the likely non-aneurysmal causes and management strategies for a patient with non-traumatic diffuse subarachnoid hemorrhage (SAH) where an aneurysm has been ruled out?

Non-Aneurysmal Causes of Subarachnoid Hemorrhage

When aneurysm has been definitively ruled out by adequate vascular imaging, the most common non-aneurysmal cause is perimesencephalic hemorrhage (accounting for approximately 10% of all SAH), followed by rare vascular malformations, vasculitis, reversible cerebral vasoconstriction syndrome, and other uncommon etiologies. 1, 2

Diagnostic Confirmation Requirements

Before accepting a diagnosis of non-aneurysmal SAH, ensure adequate vascular imaging has been performed:

• Digital subtraction angiography (DSA) with 3D rotational angiography is mandatory for diffuse SAH patterns, as CTA misses small aneurysms (<3mm) in up to 14% of cases when initial DSA is negative 1
• For perimesencephalic patterns with negative CTA, some accept this as sufficient, but this remains controversial and DSA is safer 1
• Repeat delayed DSA (at 1-2 weeks) is required if initial DSA is negative in non-perimesencephalic patterns, as small aneurysms may be obscured by vasospasm or technical factors 1, 3
• When blood is located in the sulci, scrutinize imaging specifically for vasculitis and confirm with DSA 1

Classification by Hemorrhage Pattern

Perimesencephalic SAH (52-60% of non-aneurysmal cases)

Blood distribution characteristics:

• Hemorrhage restricted to cisterns surrounding the brainstem and suprasellar cistern 4
• No extension into lateral Sylvian fissures or convexity sulci 1
• No intraventricular or intracerebral hemorrhage 5

Clinical features:

• Younger age at presentation compared to aneurysmal SAH 4
• Less likely to be female or hypertensive 4
• 90% present in good neurological grade (WFNS I-III) 5
• Excellent prognosis with 88-90% achieving favorable outcome (mRS 0-2) 5, 4

Management approach:

• If initial DSA is technically adequate with no vasospasm, repeat DSA may not be necessary 3
• Monitor for hydrocephalus (less common than aneurysmal SAH) 5
• Nimodipine is not indicated as vasospasm risk is minimal 6

Non-Perimesencephalic SAH (40-48% of non-aneurysmal cases)

Blood distribution characteristics:

• Diffuse aneurysmal pattern involving Sylvian fissures, convexity sulci, or interhemispheric fissure 1
• May include intraventricular or intracerebral extension 5

Clinical features:

• More likely to present in poor neurological grade 5
• Higher risk of complications including hydrocephalus and vasospasm 5
• Favorable outcome in only 77% compared to 88% in perimesencephalic pattern 5

Management approach:

• Mandatory repeat DSA at 1-2 weeks even if initial DSA is negative, as occult aneurysms are found in 1.4-14% of cases 1, 3
• Consider MRI/MRA to evaluate for other vascular pathology 1
• Nimodipine should be administered (60mg every 4 hours for 21 days) if patient presents within 96 hours, as vasospasm risk exists 1, 6

Specific Non-Aneurysmal Etiologies

Arteriovenous Malformations (4.2% of non-traumatic SAH)

• Typically produce focal hemorrhage patterns 7
• Require DSA for definitive diagnosis 1
• May be obscured on initial imaging by hematoma or vasospasm 1

Vasculitis

• Suspect when blood is located in sulci with negative initial angiography 1
• DSA should be scrutinized for vessel irregularity, beading, or stenosis 1
• Consider inflammatory markers, CSF analysis, and vessel wall MRI 1

Reversible Cerebral Vasoconstriction Syndrome (RCVS)

• Often presents with convexity SAH 1
• Characterized by thunderclap headaches (may be recurrent) 2
• Requires repeat vascular imaging at 6-12 weeks to document resolution 1

Cerebral Amyloid Angiopathy

• Typically causes convexity SAH in elderly patients 1
• Associated with lobar microbleeds on gradient-echo MRI 1
• Consider in patients >60 years with superficial hemorrhage 1

Other Rare Causes

• Moyamoya disease (0.8% of cases) 7
• Dural arteriovenous fistula 1
• Spinal vascular malformations (may present with intracranial SAH) 1
• Coagulopathy or anticoagulation 2
• Cocaine or sympathomimetic drug use 2

Management Strategy Algorithm

For all non-aneurysmal SAH patients:

1. Transfer to tertiary neurosurgical center for comprehensive evaluation and monitoring 1, 8

2. Blood pressure management:

   • Maintain normotension (systolic BP <160 mmHg) to prevent rebleeding if vascular lesion not definitively excluded 1
   • Avoid hypotension to maintain cerebral perfusion 1

3. Nimodipine administration:

   • Give 60mg every 4 hours for 21 days if presenting within 96 hours for non-perimesencephalic patterns 1, 6
   • Not necessary for classic perimesencephalic SAH with confirmed negative angiography 5

4. Monitor for complications:

   • Hydrocephalus requiring external ventricular drain (less common in non-aneurysmal SAH but still occurs) 1, 5
   • Vasospasm (rare in perimesencephalic, possible in non-perimesencephalic patterns) 1, 5
   • Seizures (occur in up to 20% of SAH patients) 1

5. Repeat vascular imaging protocol:

   • Non-perimesencephalic pattern: Mandatory repeat DSA at 1-2 weeks 1, 3
   • Perimesencephalic pattern with adequate initial DSA: Repeat imaging may be omitted 3
   • If vasculitis suspected: Repeat DSA after treatment initiation 1

6. Long-term follow-up:

   • Screen for physical, cognitive, and psychiatric deficits using validated tools 1
   • Overall prognosis is significantly better than aneurysmal SAH (83% favorable outcome vs 50-70%) 5, 2

Critical Pitfalls to Avoid

• Do not accept negative CTA alone for diffuse SAH patterns - small aneurysms are missed in 14% of cases requiring repeat DSA 1
• Do not assume perimesencephalic pattern guarantees benign etiology - 1.4% still harbor occult aneurysms on follow-up 3
• Do not withhold nimodipine in non-perimesencephalic patterns - vasospasm can still occur and worsen outcomes 1, 6
• Do not discharge patients with non-perimesencephalic SAH without repeat DSA - rebleeding from missed lesions carries 70-90% mortality 9
• Hypertension is less common in non-aneurysmal SAH - its presence should raise suspicion for occult aneurysm 7, 4