Etiology of SAH

1. Trauma
- Most common
- Head injury, MVA
2. Aneurysm 
- If you exclude traumatic causes, 75-80% of SAH is aneurysmal
- Congenital (rare) - birth trauma, congenital aneurysm
- Trauma - s/p knife/GSW to head, or s/p surgery or radiation
- Mycotic - bacteria/fungi get into vaso vasorum of vessels and weaken vessel walls
- Flow related (most common)
- Dissection
3. AVM : 
- AVM bleed is typically intraparenchymal
- SAH with AVM typically is from an aneurysm associated with AVM - high flow = flow related aneurysm proximal to AVM
4. Vasculitis/vasculopathy
- Autoimmune
- Infection
5. Perimesencephalic 
- Typically benign, unknown cause
- Rarely recur
- Small vessel injuries in basilar cisterns
6. Anticoagulation
- Small vessel leak/compromise is common, and anticoagulation prevents the spontaneous resolution
7. Spinal AVM
- blood can track to brain
8. Sickle cell 
- Sickling in vaso vasorum - ischemia of and weakening of blood vessels
9. Drug related 
- Can cause infections or weakening of blood vessels
- Cocaine (additives in cocaine rather than cocaine itself)
- amphetamines

10. Pituitary apoplexy 

- Rapid growth (usu from tumor), outstrips blood supply, necrosis/hemorrhage, blood leaks into SAH 

11. Unknown 

- 14-22% no etiology is ever found

- typically good outcomes, no sequelae other than HA

- <1% chance recurrence