Terminal internal carotid artery berry aneurysm causing acute 3rd cranial nerve palsy
Acute painful left pupillary dilatation. Minimal ophthalmoplegia.
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Small aneurysm arising form the left terminal ICA at the anterior cerebral origin. It projects posteriorly and could well be compressing the 3rd cranial nerve as it passes over the tentorium to enter the lateral wall of the cavernous sinus
Acute 3rd cranial nerve palsy (in the absence of raised intracranial pressure or trauma) is usually painless and due to small vessel ischemia (i.e. underlying vascular disease including diabetes). It is usually pupil-sparing as parasympathetic fibers are more resistant. Traumatic or compressive causes tend to involve the pupil. If pain is a feature then acute expansion of a berry aneurysm must be excluded. It usually arises from the distal internal carotid at the level of the posterior communicator projecting posteriorly. This case is unusual as the terminal ICA at the anterior cerebral origin is the site (it is also unusual to find berry aneurysms here).