Anatomy and Physiology
This article and its sister piece consider the optic nerve. The present article deals with trauma to the optic nerve whereas the complementary article considers disorders affecting it. 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. 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 damage which affects the optic nerve is likely to cause an impairment of sight in one or both eyes.
The optic nerve from each eye meets at the optic chiasm, and half of the nerves from each eye cross over such that a part of 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 damage 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 stroke which 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. A stroke is caused by a blood clot restricting blood flow to part of the brain and can occur as a result of trauma resulting in internal bleeding or after a surgical procedure. Anti-clotting agents (e.g. heparin) are given during surgery to inhibit the body’s natural clotting response, thereby reducing the risk of stroke.
Traumatic Optic Nerve Damage
Optic nerve damage resulting from injury (trauma) is divided into two categories:
(i) Direct traumatic optic nerve neuropathy and
(ii) Indirect traumatic nerve neuropathy
Direct trauma to the optic nerve encompasses penetrative injury to the eye by objects or bone fragments that transect the optic nerve fibers. The point of penetrative injury can be obscured by swelling and redness in the conjunctiva. The optic nerve may also suffer direct trauma as a result of ocular or cranial surgery (to treat the primary trauma, e.g. head injury following a car crash). Penetrative injury to the chiasma by a bone fragment, say, or damage to the blood vessels supplying it will also result in visual impairment (if not total loss of vision). Another form of direct trauma affecting the optic nerve may result from a crush injury (e.g. a displaced cranio orbital fracture).
Indirect trauma to the optic nerve can occur through transmitted forces in head injury; often to the forehead. It may result in traumatic loss of vision without any visible damage to the eye or optic nerve. A frequent cause of indirect optic nerve trauma is blunt force trauma to the orbit of the eye; mid facial trauma and frontal bone damage. Indirect injury to the optic nerve can arise by stretching or tearing forces.
In addition to direct damage to the optic nerve itself, reduction of the blood supply to the nerve is also a major cause of injury. Also physical compression of the nerve by proximal bruising or bleeding will also result in damage.
The incidence of traumatic optic neuropathy is estimated as 2.5% in both mid facial injury and closed head injuries.
Treatment options will depend on the nature and location of the damage to the optic nerve and other complicating medical conditions sustained at the same time. It may involve treatment with steroids and antioxidants or a surgical decompression of the optic nerve.
- The Merck Manuals; Eye disorders: https://www.merck.com/mmhe/sec20/ch235/ch235a.html
- Canadian Institute of Health Research; The Brain From Top To Bottom: https://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: https://www.tedmontgomery.com/the_eye/optcnrve.html
- Dr A. V, Thamburaj, Head Injury and Neuro-opthamology: https://www.thamburaj.com/optic_nerve_injuries.htm