Oculomotor (III) nerve lesions may be pupil sparing (normal response to light) or pupil-involving (mydriasis, loss of light reflex). The latter situation usually implies a "surgical" cause of oculomotor palsy (e.g., posterior communicating artery aneurysm), especially if extraocular muscle function is relatively preserved. Pupil sparing suggests a "medical" cause (e.g., diabetes mellitus, hypertension) especially if the palsy is otherwise complete (complete ptosis, eye deviated downwards and outwards). This disparity arises because pupillomotor fibers run on the outside of the oculomotor nerve, and are relatively spared by ischemia but are vulnerable to external compression. However, the distinction is not absolute; imaging for an aneurysm (by means of spiral CT, MRA, or catheter angiography) may be necessary if the clinical scenario leaves room for doubt.