Sandbox:Bagheri2

CRANIAL NERVE II

Editor-In-Chief: C. Michael Gibson, M.S., M.D. Associate Editor(s)-in-Chief:

Synonyms and Keywords:

Overview

The optic nerve, also known as cranial nerve II, or simply as CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived from optic stalks during the seventh week of development and is composed of retinal ganglion cell axons and glial cells; it extends from the optic disc to the optic chiasma and continues as the optic tract to the lateral geniculate nucleus, pretectal nuclei, and superior colliculus.

The optic nerve has been classified as the second of twelve paired cranial nerves but it is technically part of the central nervous system, rather than the peripheral nervous system because it is derived from an out-pouching of the diencephalon (optic stalks) during embryonic development. As a consequence, the fibers of the optic nerve are covered with myelin produced by oligodendrocytes, rather than Schwann cells of the peripheral nervous system, and are encased within the meninges. Peripheral neuropathies like Guillain–Barré syndrome do not affect the optic nerve. However, most typically the optic nerve is grouped with the other eleven cranial nerves and is considered to be part of the peripheral nervous system.

The optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in peripheral nerves. Fiber tracts of the mammalian central nervous system have only limited regenerative capabilities compared to the peripheral nervous system. Therefore, in most mammals, optic nerve damage results in irreversible blindness. The fibers from the retina run along the optic nerve to nine primary visual nuclei in the brain, from which a major relay inputs into the primary visual cortex.

The optic nerve component lengths are 1 mm in the globe, 24 mm in the orbit, 9 mm in the optic canal, and 16 mm in the cranial space before joining the optic chiasm. There, partial decussation occurs, and about 53% of the fibers cross to form the optic tracts. Most of these fibers terminate in the lateral geniculate body.

Based on this anatomy, the optic nerve may be divided in the four parts as indicated in the image at the top of this section (this view is from above as if you were looking into the orbit after the top of the skull had been removed): 1. the optic head (which is where it begins in the eyeball (globe) with fibers from the retina; 2. orbital part (which is the part within the orbit). 3. intracanicular part (which is the part within a bony canal known as the optic canal); and, 4. cranial part (the part within the cranial cavity, which ends at the optic chiasm).

From the lateral geniculate body, fibers of the optic radiation pass to the visual cortex in the occipital lobe of the brain. In more specific terms, fibers carrying information from the contralateral superior visual field traverse Meyer's loop to terminate in the lingual gyrus below the calcarine fissure in the occipital lobe, and fibers carrying information from the contralateral inferior visual field terminate more superiorly, to the cuneus.

Historical Perspective

• Discovery of cranial nerves started in the 2nd century by Galen, but these 12 cranial nerves that we recognize today was enumerated by Samuel Sömmerring in 1778.

physiology

The optic nerve contains 1.2 million nerve fibers. This number is low compared to the roughly 100 million photoreceptors in the retina, and implies that substantial pre-processing takes place in the retina before the signals are sent to the brain through the optic nerve.

The eye's blind spot is a result of the absence of retina where the optic nerve leaves the eye. This is because there are no photoreceptors in this area.

Role in disease

Main article: List of eye diseases and disorders

Damage to the optic nerve typically causes permanent and potentially severe loss of vision, as well as an abnormal pupillary reflex, which is diagnostically important. The type of visual field loss will depend on which portions of the optic nerve were damaged. Generally speaking:

• Damage before the optic chiasm causes loss of vision in the visual field of the same side only.
• Damage in the chiasm causes loss of vision laterally in both visual fields (bitemporal hemianopia). It may occur in large pituitary adenomata.
• Damage after the chiasm causes loss of vision on one side but affecting both visual fields: the visual field affected is located on the opposite side of the lesion.

Injury to the optic nerve can be the result of congenital or inheritable problems like Leber's Hereditary Optic Neuropathy, glaucoma, trauma, toxicity, inflammation, ischemia, infection (very rarely), or compression from tumors or aneurysms. By far, the three most common injuries to the optic nerve are from glaucoma, optic neuritis (especially in those younger than 50 years of age) and anterior ischemic optic neuropathy (usually in those older than 50).

Glaucoma is a group of diseases involving loss of retinal ganglion cells causing optic neuropathy in a pattern of peripheral vision loss, initially sparing central vision.

Optic neuritis is inflammation of the optic nerve. It is associated with a number of diseases, most notably multiple sclerosis.

Anterior Ischemic Optic Neuropathy is a particular type of infarct that affects patients with an anatomical predisposition and cardiovascular risk factors.

Ophthalmologists, particularly those sub specialists who are neuro-ophthalmologists, are often best suited to diagnose and treat diseases of the optic nerve.

The International Foundation for Optic Nerve Diseases IFOND sponsors research and information on a variety of optic nerve disorders and may provide general direction.

Screening

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Physical Examination

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X-ray

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OR

An x-ray may be helpful in the diagnosis of [disease name]. Findings on an x-ray suggestive of/diagnostic of [disease name] include [finding 1], [finding 2], and [finding 3].

OR

CT scan

There are no CT scan findings associated with [disease name].

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[Location] CT scan may be helpful in the diagnosis of [disease name]. Findings on CT scan suggestive of/diagnostic of [disease name] include [finding 1], [finding 2], and [finding 3].

OR

There are no CT scan findings associated with [disease name]. However, a CT scan may be helpful in the diagnosis of complications of [disease name], which include [complication 1], [complication 2], and [complication 3].

MRI

There are no MRI findings associated with [disease name].

OR

[Location] MRI may be helpful in the diagnosis of [disease name]. Findings on MRI suggestive of/diagnostic of [disease name] include [finding 1], [finding 2], and [finding 3].

OR

There are no MRI findings associated with [disease name]. However, a MRI may be helpful in the diagnosis of complications of [disease name], which include [complication 1], [complication 2], and [complication 3].

Other Imaging Findings

There are no other imaging findings associated with [disease name].

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[Imaging modality] may be helpful in the diagnosis of [disease name]. Findings on an [imaging modality] suggestive of/diagnostic of [disease name] include [finding 1], [finding 2], and [finding 3].

Other Diagnostic Studies

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[Diagnostic study] may be helpful in the diagnosis of [disease name]. Findings suggestive of/diagnostic of [disease name] include [finding 1], [finding 2], and [finding 3].

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Other diagnostic studies for [disease name] include [diagnostic study 1], which demonstrates [finding 1], [finding 2], and [finding 3], and [diagnostic study 2], which demonstrates [finding 1], [finding 2], and [finding 3].