Cranial Nerve III, The Oculomotor Nerve

Extraocular movements are tested by having patient follow examiner's finger throughout 360° range. Pupils are first assessed for size and symmetry. Anisocoria (unequal pupils) of up to 0.5 mm is fairly common, and benign provided pupillary reaction to light is normal. Pupillary reflex is tested in a darkened room. It is performed by shining light in one pupil and observing constriction of ipsilateral pupil (direct reflex) and contralateral pupil (contralateral reflex). If abnormality is found with light reaction, pupillary accomodation can be tested by having patient focus on object at distance, then focus on object 10 cm from nose. Pupils should converge and constrict at close focus.

Pupillary abnormalities may be a result of either CN II or CN III lesions. A CN II lesion (blind eye) will not react to direct light (and will not elicit a consensual pupillary constriction) but will constrict if light is shown in the opposite eye. A Horner's syndrome (sympathetic chain lesion) can also present as pupillary abnormality. In Horner's, the affected pupil is smaller but constricts to both light and near vision and may be associated with ptosis and anhydrosis. In a CN III lesion, the affected pupil is fixed and dilated and may be associated with ptosis and lateral deviation (due to unopposed action of CN VI). Small pupils that do not react to light but do constrict with near vision (accommodate but do not react) can be seen by central nervous system syphilis (Argyll-Robertson pupil).