The optic nerve.

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.