Correct Answer: C. Anterior communicating artery
The optic chiasma is located in the suprasellar region, suspended from the diaphragma sellae, and lies directly anterior to the pituitary gland. The anterior communicating artery (AComm) is the most common site of intracranial aneurysms (30–35% of all cerebral aneurysms in Indian populations), and its anatomical course makes it uniquely positioned to compress the optic chiasma. The AComm connects the two anterior cerebral arteries across the midline, running directly above and anterior to the optic chiasma. An aneurysm at this location causes characteristic bitemporal hemianopia due to compression of the inferior nasal crossing fibers of the chiasma first. This is a classic neurosurgical presentation in Indian tertiary centers where cerebral aneurysms are increasingly recognized as a cause of vision loss. The proximity of the AComm to the chiasma (typically 5–10 mm above it) makes it the most likely culprit when an aneurysm is found causing chiasmal damage on imaging. Other arteries in the circle of Willis are either too lateral (MCA, ACA proper) or too posterior (AChoA) to directly compress the chiasma.
Why the other options are wrong
A. Anterior cerebral artery — The ACA proper runs lateral and superior to the optic chiasma, passing over the genu of the corpus callosum. While ACA aneurysms occur, they are less common (5–7% of cerebral aneurysms) and typically cause symptoms related to anterior circulation stroke or frontal lobe compression rather than chiasmal compression. The anatomical course does not place it directly above the chiasma. B. Middle cerebral artery — The MCA is the most common site of aneurysms overall (40–45%), but these are located at the MCA bifurcation in the lateral sylvian fissure, far lateral to the optic chiasma. MCA aneurysms cause hemiparesis, aphasia, or homonymous hemianopia—not bitemporal hemianopia. The anatomical distance and direction make chiasmal compression extremely unlikely. D. Anterior choroidal artery — The anterior choroidal artery is a small branch of the internal carotid artery that supplies the choroid plexus, internal capsule, and parts of the basal ganglia. It arises from the ICA below the posterior communicating artery and runs posteriorly, away from the optic chiasma. Aneurysms here are rare and would not compress the chiasma due to their posterior location.
High-Yield Facts
- Anterior communicating artery aneurysm is the most common cause of subarachnoid hemorrhage and aneurysmal compression of the optic chiasma (30–35% of all cerebral aneurysms).
- Bitemporal hemianopia (loss of bilateral temporal visual fields) is the classic presentation of optic chiasma compression; inferior nasal fibers are compressed first.
- The optic chiasma lies 5–10 mm anterior and superior to the pituitary gland in the suprasellar cistern, directly in the path of the AComm.
- AComm aneurysms typically present with sudden-onset severe headache, neck stiffness, and focal neurological deficits including vision loss.
- In Indian populations, hypertension and smoking are major risk factors for cerebral aneurysm formation, making AComm aneurysms an increasingly recognized cause of vision loss in tertiary centers.
Mnemonics
CHIASMA COMPRESSION ARTERIES AComm (Anterior Communicating) — most common aneurysm causing chiasmal compression. Remember: AComm = Anterior Communicating = Above the chiasma, Common aneurysm site. CEREBRAL ANEURYSM FREQUENCY (Circle of Willis) MCA > AComm > PComm — MCA most common overall (40–45%), but AComm most likely to compress chiasma due to anatomical position directly above it.
NBE Trap
NBE may lure students into selecting MCA (option B) because it is the single most common site of cerebral aneurysms overall; however, the question specifically asks which artery causes chiasmal damage, requiring knowledge of anatomical proximity rather than frequency alone. AComm's direct suprachiasmal location is the discriminating fact.
Clinical Pearl
In Indian neurosurgical practice, a patient presenting with progressive bitemporal hemianopia and imaging showing a suprasellar mass should raise suspicion for AComm aneurysm before assuming pituitary pathology—this distinction is critical because aneurysm rupture risk demands urgent intervention, whereas pituitary adenoma allows more conservative management.
_Reference: Robbins Ch. 28 (Nervous System); Snell's Clinical Neuroanatomy Ch. 7 (Cerebral Circulation); Harrison Ch. 445 (Cerebrovascular Diseases)_