This patient's presentation is consistent with a third nerve palsy. The presence of a third nerve palsy, in addition to a unilateral blown pupil, suggests third nerve compression and requires urgent neurosurgical evaluation.
Third nerve palsies are characterized by an eye that is "down and out." This position results from unopposed action of the superior oblique and the lateral rectus, which are controlled by the fourth and sixth cranial nerves, respectively. The most important exam findings to look for in patients with a third nerve palsy is an ipsilateral, dilated pupil that does not respond to light ("blown pupil"). A patient with a third nerve palsy with pupillary involvement should be considered to have a posterior communicating artery (PCOM) aneurysm until proven otherwise. Neurosurgical evaluation is indicated, including cerebrovascular imaging with possible coiling of the aneurysm.
Vega et. al discuss the management of intracranial aneurysms. They note that, although PCOM aneurysms are the most well known, the anterior communicating and middle cerebral are also very common locations for aneurysms. While enlarging aneurysms often present as cranial nerve palsies, ruptured aneurysms that produce a subarachnoid hemorrhage are not associated with cranial nerve palsies, as nerve compression resolves with aneurysm rupture.
Anan et. al perform a retrospective case study to determine whether the size of a PCOM aneurysm correlates with the risk of having a third nerve palsy on presentation. They find that 27% of patients with a PCOM aneurysm presented with a third nerve palsy, but the size of the aneurysm was not associated with the likelihood of having this presentation. Rather, the distance between the internal carotid artery and the skull base was a better predictor of third nerve palsy, such that patients with a short distance and a small PCOM aneurysm were still likely to present with a third nerve palsy.
Illustration A shows a non-compressive third nerve palsy in a patient with diabetes. Illustration B shows the course of the third cranial nerve, and its proximity to the posterior communicating artery. Illustration C shows the structure of the circle of Willis, including common locations of aneurysms.
Answer 2: Bilateral dilated pupils are associated with medication effects (e.g. cocaine and marijuana).
Answers 3-5: Ptosis is associated with 3rd-nerve palsies due to impaired innervation of the levator palpebrae. Since ptosis can be present in both compressive and vasculopathic 3rd-nerve palsies, this feature alone is not an indication for urgent neurosurgical intervention. Rather, evaluation of the pupil should be considered as a more useful indication of pathophysiology.
Vega C, Kwoon JV, Lavine SD. Intracranial aneurysms: current evidence and clinical practice. Am Fam Physician. 2002 Aug 15;66(4):601-8.
PMID:12201551 (Link to Abstract)
Anan M, Nagai Y, Fudaba H, Kubo T, Ishii K, Murata K, Hisamitsu Y, Kawano Y, Hori Y, Nagatomi H, Abe T, Fujiki M. Third nerve palsy caused by compression of the posterior communicating artery aneurysm does not depend on the size of the aneurysm, but on the distance between the ICA and the anterior-posterior clinoid process. Clin Neurol Neurosurg. 2014 Jun 4;123C:169-173.
PMID:24968189 (Link to Abstract)