Cranial Nerve II, The Optic Nerve
Gross visual acuity can be tested with a Snellen eye chart (or hand held chart if Snellen chart is unavailable). Visual fields can then be tested by having patient cover one eye at a time, facing patient, and determining when patient can detect examiner's hands coming into their field of view. The examiner can gauge the findings based upon when he sees the fingers entering his own field of view. Based upon the position of decreased vision, the defect can be localized. A prechiasmatic lesion results in ipsilateral blind eye (1.). A chiasmatic lesion can result in bitemporal hemianopsia (2.). Post chiasmatic lesions proximal to the geniculate ganglion can result in left (3.) or right homonymous hemianopsia. Lesions distal to the geniculate ganglion can result in upper (4.) or lower (5.) homonymous quadrantopsia.
The pupillary light reflex tests for both CN II and CN III. Light shown in the ipsilateral eye will cause constriction of both eyes provided both CN II and CNIII are functioning properly.
