FIGURE 3.

Significance of pupil involvement. Third nerve palsies often have pupillary involvement because the parasympathetic nerves innervating the iris travel with the third nerve. Pupillary involvement is an important diagnostic sign, as usually compressive lesions involve the pupil, whereas microvascular palsies do not. The parasympathetic fibres that control pupillary constriction originate in the Edinger–Westphal nucleus in the midbrain (green/light grey). These fibres then run in the superficial medial portion of the third nerve as it travels in the subarachnoid space and therefore are susceptible to compression from aneurysms arising from the nearby posterior communicating artery aneurysm. A complete third nerve palsy that spares the pupil is not caused by compression from an aneurysm because the parasympathetics fibres are located near the surface of the nerve, and if the nerve compression is severe enough to cause complete paralysis of the extraocular muscles, then the parasympathetic fibres must also be compressed. However, the microvascular pathology affects the blood vessels inside the nerve and therefore could affect all fibres travelling to the muscles and spare the superficial parasymapthetics. If the third nerve palsy is partial and spares the pupil, it is possible it is missing the parasympathetic fibres so could be caused by an aneurysm.