Third-nerve palsy can result from pathology at all of the following, EXCEPT:
Brainstem
Subarachnoid space
Cavernous sinus
None of the above
A third-nerve palsy can result from pathology anywhere along its anatomic pathway, beginning with the brainstem, continuing within the subarachnoid space, traversing the cavernous sinus, and terminating within the orbit. Impoant: Contralateral hemiplegia suggests brainstem involvement. In cases of isolated oculomotor palsy, the subarachnoid space is the most likely site of pathology. The differential includes a posterior communicating aery aneurysm, uncal herniation, compressive neoplasms, inflammatory lesions, or trauma. Causes for third-nerve palsy within the cavernous sinus include neoplasms, carotid aery aneurysm, cavernous sinus thrombosis, and carotid-cavernous fistula. Orbital lesions causing third-nerve palsies may be due to inflammation, trauma, neoplasms, and mucoceles. Isolated third-nerve palsy is common with diabetic or hypeensive disease, likely due to microvascular ischemia. In this situation, the palsy rarely affects the pupil, and may be extremely painful. Ref: Levsky M.E., DeFlorio P. (2010). Chapter 2. Ophthalmologic Conditions. In K.J. Knoop, L.B. Stack, A.B. Storrow, R.J. Thurman (Eds), The Atlas of Emergency Medicine, 3e.