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A 40 year old woman yesterday had the sudden onset of left periocular pain. Her eye care provider found left ptosis, mydriasis, and adduction, supraduction, and infraduction deficits. Her medical history is unremarkable
Given these clinical signs, what is the most likely location of the aneurysm?
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 often of the intradural “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 intradural, so if they rupture, they bleed into the subarachnoid space 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. That’s when
you want to detect them, because at that stage, they can be occluded and prevented from rupturing.
Where does that expanded aneurysm occur? At the junction of the internal carotid and posterior communicating arteries
Less common sites of aneurysms causing third nerve palsies 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 berry 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!
You are right if you were thinking that third nerve palsies can also be caused by aneurysms of the cavernous sinus. A cavernous third nerve palsy may be isolated or accompanied by sixth nerve palsy, fourth nerve palsy, trigeminal hyperesthesia or hypesthesia, or Horner syndrome. But such cavernous aneurysms are almost always entirely extradural, which means that if they rupture, they cause carotid-cavernous fistulas rather than subarachnoid hemorrhages. Such fistulas are not life-threatening and are amenable to treatment at a later date.
Where does that expanded aneurysm occur? At the junction of the internal carotid and posterior communicating arteries
Modern computed tomographic (CT) angiography and magnetic resonance (MR) angiography are about 96% likely to detect berry 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!
You are right if you were thinking that third nerve palsies can also be caused by aneurysms of the cavernous sinus. A cavernous third nerve palsy may be isolated or accompanied by sixth nerve palsy, fourth nerve palsy, trigeminal hyperesthesia or hypesthesia, or Horner syndrome. But such cavernous aneurysms are almost always entirely extradural, which means that if they rupture, they cause carotid-cavernous fistulas rather than subarachnoid hemorrhages. Such fistulas are not life-threatening and are amenable to treatment at a later date.