A 55-year-old, right-handed man comes to the emergency department due to recent onset of severe, throbbing, right-sided headache and double vision. His medical history includes poorly controlled hypertension and chronic tobacco use. Neurologic examination shows that he is awake, alert, and oriented and can follow commands. Visual fields and optic fundi are normal. The position of the right eye is down and out with ipsilateral ptosis, and the right pupil is dilated and nonreactive to both light and accommodation. Left eye examination is normal. Based on this patient's neurologic deficits, CT angiography of the head is most likely to reveal an aneurysm arising from which of the following locations in the image below?
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This patient has right oculomotor nerve (CN III) palsy secondary to a compressive aneurysm arising from the junction of the right posterior communicating artery with the internal carotid artery. Saccular aneurysms typically arise from branch points on the circle of Willis, and most (85%) affect the anterior circulation (eg, anterior communicating, posterior communicating, and middle cerebral arteries). Chronic smoking and poorly controlled hypertension are risk factors. Unruptured aneurysms are usually asymptomatic, but patients may experience headache and cranial neuropathies due to mass effect.
CN III is a pure motor nerve that exits the midbrain between the posterior cerebral and superior cerebellar arteries and courses along the posterior communicating artery, making it particularly susceptible to injury from posterior communicating artery aneurysms. It carries general visceral efferent fibers on its surface (for the pupillary light and near-reflex pathways) and general somatic efferent fibers within its interior (innervating superior rectus, inferior rectus, medial rectus, inferior oblique, and levator palpebrae superioris muscles). Consequently, aneurysmal compression of CN III produces mydriasis (due to superficial parasympathetic fiber damage) with diplopia, ptosis, and down and out deviation of the ipsilateral eye (due to somatic efferent fiber injury).
(Choice A) The junction of the anterior communicating artery and anterior cerebral artery is the most common location for saccular aneurysms; however, aneurysms in this region typically compress the central optic chiasm, causing bitemporal hemianopia.
(Choices B and C) Saccular aneurysms of the middle cerebral artery typically occur at its distal bifurcation and do not usually affect the oculomotor nerve.
(Choice E) Posterior communicating artery aneurysms cause symptoms on the same side as the lesion due to damage to the ipsilateral oculomotor nerve. A left posterior communicating artery aneurysm would cause left-sided mydriasis, ptosis, and eye deviation.
(Choices F, G, and H) Approximately 15% of saccular aneurysms arise from the posterior circulation (eg, branches of the basilar and vertebral arteries). Although these aneurysms could potentially cause oculomotor palsy by compressing the oculomotor nerve as it exits the midbrain, this presentation is rare and usually associated only with very large aneurysms that cause multiple neurologic deficits.
Educational objective:
The oculomotor nerve (CN III) is most susceptible to injury from ipsilateral posterior communicating artery aneurysms. Aneurysmal compression of CN III produces mydriasis (due to superficial parasympathetic fiber damage) with diplopia, ptosis, and down and out deviation of the ipsilateral eye (due to somatic efferent fiber injury).