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The Optic Nerve
The optic nerve is also referred to as cranial nerve II (there are 12 paired cranial nerves). The function of the optic nerve is to transmit the images formed by light striking photo-sensitive cells at the retina, in the back of the eye, to the brain where they are interpreted. While none of us may actually have eyes in the backs of our heads, the center of the brain responsible for interpreting this visual information is actually located there. Signals travel from the eyes through the optic nerve to the visual cortex in the brain, which is located at the back of the head. Each eye is connected to the visual cortex by the optic nerve, which consists of a bundle of more than a million individual nerve fibers sheathed in myelin. Any condition that affects the optic nerve is likely to cause impairment of sight in one, or both, eyes.
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Tracing The Origin Of Optic Nerve Problems
The optic nerve from each eye meets at the optic chiasm and half of the nerves from each eye cross over so that some of the images formed in the right eye are processed on the left part of the visual cortex and vice versa. This means that the patterns of sight loss that a patient experiences can be used to diagnose where the damage is within the eye and optic nerve. Damage to an eye or optic nerve occurring before the optic chiasm will result in vision loss in the affected eye. Damage to the optic chiasm itself will cause the outer part of the patient’s field of vision to be lost in both eyes. A condition called homonymous hemianopia can be the result of a brain tumor or stroke that affects the optic nerve after it leaves the optic chiasm. Such a condition will cause one side of the visual field to be lost in both eyes.
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The Four Major Types of Optic Nerve Disorders
Optic nerve disorders can be caused by a number of conditions, other than the brain tumor or stroke referred to above. There are four main classes of problems that can lead to optic nerve disorders:
- Ischemic optic neuropathy – a condition where the blood supply to the optic nerve is compromised by a blockage
- Optic neuritis – a condition where the optic nerve is inflamed at some point
- Papilledema – this condition involves the swelling of the optic nerve at the eye(s) due to increased pressure in, or surrounding, the brain
- Toxic amblyopia (or nutritional amblyopia) – a condition caused by exposure of the optic nerve to poisonous substances (methanol, lead, antifreeze etc) or due to malnutrition.
Ischemic optic neuropathy can be diagnosed by examining the back of the eye with an ophthalmoscope. The condition can have a rapid onset (minute to hours) or take place over a longer time. Vision is lost in the centre of the field of vision of the affected eye(s) and then spreads, potentially resulting in blindness in the affected eye. The treatment involves controlling the condition that led to the reduced blood supply (diabetes; blood pressure problems and other causes).
Optic neuritis is also diagnosed using an ophthalmoscope. It is usually due to multiple sclerosis (MS), which attacks the myelin sheath of nerves, but can be due to a viral infection or a tumor. Often MRI will be needed to confirm the diagnosis of MS. The condition can cause mild to severe vision loss and may affect color vision. The onset of vision loss occurs over a period of days.
Papilledema is also diagnosed using an ophthalmoscope but MRI or computerised tomography may be needed to confirm diagnosis and monitor treatment. The cause is often a brain tumor or abscess, but it can also result from a head injury, particularly if it involves intracranial bleeding or an infection in the brain. Symptoms are transient, usually only lasting for a few seconds at a time and may involve blurred, double or flickering vision or total loss of vision.
Toxic amblyopia is diagnosed following a careful history taking and can be confirmed with toxicology or vitamin measurements. A blind spot may develop in the field of vision and enlarge over a period of days to weeks causing deterioration of vision. Usually both eyes are affected. If the cause is ingestion of antifreeze (ethylene glycol) or methanol, vision loss may be rapid.
Glaucoma may also result in permanent damage to the optic nerve resulting from increased intraocular pressure (for more information, see http://www.brighthub.com/science/medical/articles/63631.aspx).
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- The Merck Manuals; Eye disorders: http://www.merck.com/mmhe/sec20/ch235/ch235a.html
- Canadian Institute of Health Research; The Brain From Top To Bottom: http://thebrain.mcgill.ca/flash/i/i_02/i_02_cr/i_02_cr_vis/i_02_cr_vis.html
- Ted Montgomery, Anatomy, Physiology and Pathology of the Human Eye: http://www.tedmontgomery.com/the_eye/optcnrve.html