One of the commonest causes of a third nerve palsy is pressure on the nerve from a posterior communicating artery aneurysm, which lies parallel to the nerve on the anterior aspect of the brainstem. As the aneurysm abuts the outside of the oculomotor nerve [III], it involves the parasympathetic fibers, which lead to a predominance of the loss of pupillary function over general function.
The aneurysm was imaged with an angiogram.