1. Lillequist membrane separates:
- the interpeduncular from prepontine cisterns (mesencephlalic part here shown by m)
- supracellar from interpeduncular cisterns (diencephalic shown here by d).
- Diencephalic is thicker and more often competent, which isolates the chiasmatic/supracellar cistern, while the mesecephalic is less likely competent
- Blood in interpeduncular cistern = low pressure pre-truncal source of bleed.
- Blood in supracellar/chiasmatic cistern = raises concern about aneurysmal bleed

2. Ruptured A-Comm aneurysm pattern of SAH

{image source}
Data on 81 patients from {a paper in AJNR} from the 1980s correlating pattern of hemohrrage with location of aneurysm

3. A-comm aneurysms (from greenberg)
-  Interhemispheric blood in almost all cases, and intraparenchyma hemorrhage in ~2/3 of cases, IVH in ~80%, acute hydrocephalus in about 25%.
- Frontal lobe infarcts happen in ~20%, usually within several days
- Bilateral ACA territory infarcts (causing prefrontal lobotomy like symptoms of abulia, apathy) are rare, but one cause is vasospasm after A-comm rupture.
4. Angiograph of a-comm aneurysms: 
- Check via angio if the aneurysm fills from both sides of the carotid vs one; if you think it's only one side, make sure by holding pressure on that side and injecting the other side again just to see if there's any collateral filling. Essentially, are you going to be able to get proximal control by clamping one ICA?
- Also check via angio if each ACA fills from its own side - this tells you whether or not you can trap the aneurysm. If both ACAs are supplied by one ICA and you trap the aneurysm, you'll give the person a unilateral ACA stroke.
5. Pterional approach to A-comm aneurysms:
- The classic approach. dissect down sylvian fissure as you retract frontal lobe, find ICA, follow anterior to find A1 and acomm. {Great article from WFNS} on surgical approaches to a-comm aneurysms. Same article is source of below image.

- If aneurysm is too high and would require excessive retraction on frontal lobe, you can either extend craniotomy to OZ, or do a cortisectomy into gyrus rectus (may cause neuropsych deficits)

"Illustration showing the difference in working angle obtained when the pterional approach (light blue) is converted to an orbitozygomatic approach (dark blue). The net difference is 10°, which provides a more shallow and wider exposure when the lateral wall and orbital roof are removed" {source for image and text}
6. When to do a L-sided pterional craniotomy: 
- For a-comm aneurysms you generally want to do a R crani, except:
- When dome of the aneurysm is pointing R, such that you would encounter dome before neck in a R crani
- When aneurysm is only supplied by L side; better proximal control with L crani
- There are multiple aneurysms, some on the L.
7. Other approaches to a-comm aneurysms
- Subfrontal: good for aneurysms that are pointing superiorly accompanied by significant frontal clot that you can evacuate at same time.
- Anterior interhemispheric: pros: less brain retraction; cons: suboptimal proximal control, low-forehead incision. Contraindicated in anteriorly pointing aneurysms.
8. Recurrent artery of Heubner 
- Often encountered in Acomm aneurysm surgery. Don't cut this.
- Most often comes off A2

From a {Rhoton} paper in JNS in the 70s: 

 More variant anatomy:

 photo from same rhoton paper:

More photo from internet: 

What happens when you cut Heubner: stroke of ipsilateral caudate, anterior internal capsule, and putamen. May cause hemiparesis of face and UE. 

9. A1 perforators
- As you can see from the above rhoton images, there are many perforators off A1 and the A-comm itself that go to the optic tract/chiasm and anterior perforated substance.
10. Anterior perforated substance:
- Posterior to trigone of olfactory nerve, anaterior to optic tract
- Important because the Anterior choroidal and many of the lenticulostriate perforators off M1 and A1 that supply the internal capsule and caduate, putamen, globus pallidus go through it

The following image is from {this paper}