This patient displays signs of oculomotor palsy, with restriction of the eye movements in all directions (except laterally, due to preservation of the sixth cranial nerve, the abducens), and ptosis. Dilatation of the pupil, which fails to react to light, is a sign of intracranial compression of the third, or oculomotor cranial nerve. This should prompt search for an underlying surgical cause of oculomotor palsy. Uncal herniation and aneurysm of the posterior communicating aery are the two most common surgical conditions leading to oculomotor palsy. In the absence of clinical evidence of increased intracranial pressure, it may be assumed that the patient has an aneurysm of the posterior communicating aery until proven otherwise. Cerebral angiography is the investigation of choice to confirm the diagnosis. All of the most common medical causes of oculomotor nerve palsy result in paresis of extraocular movements and ptosis, but the pupillary light reflex is preserved. These conditions include diabetes mellitus , giant cell aeritis , syphilis and systemic hypeension. Carcinoma of the right pulmonary apex may result in Horner syndrome (miosis, ptosis, enophthalmos, and loss of sweating on the affected hemiface) due to infiltration of the cervical autonomic ganglia. Ref: Walker R.A., Adhikari S. (2011). Chapter 236. Eye Emergencies. In J.E. Tintinalli, J.S. Stapczynski, D.M. Cline, O.J. Ma, R.K. Cydulka, G.D. Meckler (Eds), Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 7e.