The optic nerve is our most important cranial nerve. As it courses from the eyeball to the brain, it is divided into four segments: intraocular, intraorbital, intracanalicular, and intracranial. Four tests are primarily used to assess its functional integrity and detect optic nerve disorders. These tests are described along with key funduscopic findings. The clinical features of both common as well as notable disorders that occur along the four segments are presented. For example, papilledema and anterior ischemic optic neuropathy involve the intraocular segment, dysthyroid optic neuropathy or optic nerve sheath meningiomas affect the intraorbital segment, traumatic optic neuropathy is mainly within the intracanalicular segment, and pituitary tumors and suprasellar masses compromise the intracranial segment. Ancillary clinical symptoms and signs are highlighted that assist the neurologist in understanding and localizing each disorder along one of these segments. The chief means of confirming the diagnosis, often neuroimaging, are given, and common treatment modalities are provided.

Pupillary evaluation of retinal asymmetry provides a rapid method for detecting and classifying visual field defects. In this patient population, classification agreed with perimetry in 70% of eyes.

Glaucomatous damage to upper and lower retina is often unequal. We have developed a rapid, objective, quantitative measure of asymmetry of retinal sensitivity, using infrared pupillometry and pairs of large stimuli that are symmetric about the horizontal meridian. Results for a group of 11 young subjects free of eye disease indicate that the distribution of asymmetry is close to a normal distribution centered near upper/lower symmetry. Some subjects showed modest amounts of asymmetry, which was relatively uniform within each eye, and between the two eyes, of the subject. This approach to determination of asymmetry within an eye is potentially applicable to testing patients with glaucoma. The narrowness of the distribution should make it possible to detect asymmetries caused by disease.

Iris mechanics limits the amount of pupil contraction and can act to reduce the assessed neuronal integration of the pupil light reflex. Pupil response assessed by using percent contraction amplitude is least affected by mechanical effects and provides a more accurate approximation of afferent input.