Subarachnoid Hemorrhage and Vascular Malformations
CVA · EP 05 · STROKE
Before You Listen
- Prerequisites: Circle of Willis anatomy and the territories of the anterior, middle, and posterior cerebral arteries (CVA-01); the cranial nerve (CN) III oculomotor pathway including parasympathetic outflow; intracerebral hemorrhage management and the closed-skull pressure dynamics from CVA-04; non-contrast computed tomography (CT) interpretation for blood density.
- Runtime: 1 hour 4 minutes.
- Topic in one line: the saccular (“berry”) aneurysm at Circle-of-Willis bifurcations as the cause of 80-85% of spontaneous subarachnoid hemorrhages (SAH); the three highest-frequency sites (anterior communicating artery 30%, posterior communicating artery 25%, middle cerebral artery bifurcation 20%); the painful pupil-involving cranial nerve III palsy of a posterior communicating artery aneurysm; the thunderclap headache in 97% of cases and the 30% sentinel-leak rate driving the CT-then-lumbar-puncture (LP) algorithm; the Hunt-Hess clinical grade and the Fisher radiographic grade (Fisher 3 carries the highest vasospasm risk, not Fisher 4); the vasospasm window from day 3-5 through day 14, resolving by day 21; transcranial Doppler (TCD) middle cerebral artery (MCA) velocity >120 cm/s with Lindegaard ratio >3 to confirm focal spasm; nimodipine 60 mg PO every 4 hours for 21 days as the only proven pharmacologic agent (never IV); the International Subarachnoid Aneurysm Trial (ISAT) ~7% absolute risk reduction favoring coiling over clipping; the syndrome of inappropriate antidiuretic hormone (SIADH) versus cerebral salt wasting (CSW) trap; the Spetzler-Martin grading system for arteriovenous malformations (AVMs); the cavernous malformation as the angiographically occult “popcorn lesion” most associated with seizures; and the dural arteriovenous fistula (dAVF), in which cortical venous drainage transforms a benign lesion into a dangerous one.
Vignette. A 47-year-old woman with autosomal dominant polycystic kidney disease arrives in the emergency department after sudden onset of “the worst headache of my life” while gardening 4 hours ago. She had a brief loss of consciousness at onset, vomited twice, and now has neck stiffness and photophobia. Examination shows a drowsy but arousable patient (Glasgow Coma Scale 13), no focal motor deficit, but a fixed, dilated right pupil with ptosis and the right eye deviated down and out. Non-contrast head CT shows hyperdense blood layered in the basal cisterns and the interhemispheric fissure with mild ventricular enlargement.
State the Hunt-Hess grade, name the most likely aneurysm location and explain the cranial nerve finding, list the next two diagnostic-and-management steps, predict the highest-risk delayed complication and the day it is most likely to begin, and explain why phenytoin should be avoided if seizure prophylaxis is selected.
(Answer at the end of this chapter)
Section 1: Berry Aneurysms, Aneurysm Locations, and the PCOM-CN III Trap
Bottom line: 80-85% of spontaneous SAH comes from a saccular aneurysm at a Circle-of-Willis bifurcation; ACOM 30%, PCOM 25%, MCA 20%; a painful pupil-involving CN III palsy is a PCOM aneurysm pressing the parasympathetic surface fibers until proven otherwise.
Subarachnoid hemorrhage accounts for roughly 3% of all strokes but commands an outsized portion of board questions because it strikes a younger population and carries devastating mortality. Roughly 80-85% of spontaneous cases come from rupture of a saccular aneurysm, historically called a berry aneurysm. The label is misleadingly benign; functionally the lesion is a blister on the sidewall of a high-pressure arterial branch point.
Normal cerebral arteries have a robust tunica media (smooth muscle) and an internal elastic lamina that absorb the pulsatile shock of every heartbeat. At the bifurcation points within the Circle of Willis, both layers are congenitally deficient. Decades of pulsatile hemodynamic stress against an unreinforced T-junction balloon the wall outward. The aneurysm enlarges incrementally; at a critical wall tension it ruptures and ejects arterial blood under systolic pressure into the cerebrospinal fluid (CSF) cisterns surrounding the brain.
Three locations dominate. The anterior communicating artery (ACOM) sits at the midline bridge between the two anterior cerebral arteries and accounts for approximately 30% of ruptured aneurysms, the single most common site. Blood concentrated in the interhemispheric fissure on CT suggests an ACOM source. The posterior communicating artery (PCOM) accounts for approximately 25% and connects the internal carotid to the posterior cerebral artery. The middle cerebral artery bifurcation accounts for approximately 20%; blood pooled in the Sylvian fissure on CT suggests an MCA aneurysm. Basilar tip and other posterior circulation sites round out the remainder.
The PCOM aneurysm deserves its own slot because of an anatomic relationship that is one of the highest-yield pearls in neurology. As the PCOM courses through the interpeduncular cistern it runs immediately adjacent to CN III (oculomotor nerve). When a PCOM aneurysm expands or ruptures, it compresses CN III externally. The internal topography of the nerve dictates the order of symptoms.
The parasympathetic fibers that drive pupillary constriction travel on the superficial outer surface of CN III. Because they are on the outside, they are crushed first by an expanding aneurysm pressing in from the surface. Without parasympathetic tone the iris sphincter is paralyzed and the sympathetic dilator operates unopposed. The pupil blows to a fixed mydriatic state, and a blown pupil is the earliest sign. Only as compression deepens into the core of the nerve do the somatic motor fibers fail: ptosis from the levator palpebrae superioris, then paralysis of the superior rectus, inferior rectus, medial rectus, and inferior oblique. The remaining functional muscles, lateral rectus (CN VI) and superior oblique (CN IV), pull the eye into the classic down-and-out position.
The complete triad in order is therefore mydriasis first, then ptosis, then down-and-out deviation. A new painful pupil-involving CN III palsy is a neurosurgical emergency: assume a PCOM aneurysm is compressing the nerve until imaging proves otherwise, because the next event in the natural history is rupture. The board contrast is diabetic (microvascular) CN III palsy: ischemia of the vasa nervorum strikes the deep core of the nerve where the somatic motor fibers run, sparing the surface parasympathetic fibers because they have a separate pial blood supply. Diabetic CN III palsy presents with ptosis and down-and-out deviation but a normal reactive pupil. The single discriminator is the pupil. Pupil involved means external compression and emergent imaging; pupil spared means microvascular disease and outpatient workup.
Source: Custom schematic diagram based on classic neuroanatomy internal topography.
High Yield — Berry aneurysms and CN III
- Saccular (“berry”) aneurysm at a Circle-of-Willis bifurcation = 80-85% of spontaneous SAH; bifurcations are weak because the tunica media and internal elastic lamina are congenitally deficient.
- Top three sites: ACOM 30% > PCOM 25% > MCA bifurcation 20%.
- Pupil-involving CN III palsy = PCOM aneurysm until proven otherwise (parasympathetic fibers ride the superficial surface and are compressed first).
- Diabetic CN III palsy = ptosis + down-and-out + pupil spared (vasa nervorum ischemia hits the deep core, surface parasympathetics have separate pial blood supply).
- Order of CN III findings in compression: mydriasis → ptosis → down-and-out.
Board Trap — Pupil involvement is the discriminator
The novice instinct is to call any CN III palsy ischemic if the patient has diabetes. Wrong if the pupil is blown. A new painful pupil-involving CN III palsy is an aneurysm pressing on the nerve from the outside, and the next step is emergent CT angiography or catheter angiography, not a hemoglobin A1c. Reverse the trap and remember: pupil spared = inside-out ischemia; pupil blown = outside-in compression.
At these bifurcations, both the tunica media and the internal elastic lamina are completely lacking or severely thinned out. So you have maximum hemodynamic pounding hitting the exact spot where the pipe wall is the thinnest.
— CVA-05 podcast, ~04:40