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Correct! The combination of neuro-ophthalmic deficits described here suggests a left third nerve palsy. Intracranial aneurysm is an uncommon--but the most feared--cause of third nerve palsy. When an aneurysm causes a third nerve palsy, it is of the extradural “berry” type, in which focal congenital defects in the intima and media at arterial branch points cause bubble-like expansion that worsens with aging, systemic hypertension, and other conditions that weaken arterial walls. Berry aneurysms are prone to rupture with a mortality rate of 50%. But mere expansion without rupture is sufficient to cause third nerve palsy if the expanded aneurysm measures at least 3 millimeters in diameter. The third nerve is most likely to be compressed by an expanding (or ruptured) aneurysm at the junction of the internal carotid and posterior communicating aneurysm

Less common sites of aneurysms are the junction of basilar and superior cerebellar arteries and the junction of the basilar and posterior cerebral arteries

Modern computed tomographic (CT) angiography and magnetic resonance (MR) angiography are about 96% likely to detect aneurysms that cause third nerve palsies, provided the interpreting radiologist is experienced enough to detect the often subtle abnormalities. If you suspect a recent-onset third nerve palsy, you must order such a non-invasive vascular imaging study immediately, particularly in a patient in whom the most common ischemic palsy is less favored, because rupture of an expanding aneurysm could be imminent—and fatal!

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