Optic Nerve Head Avulsion
Optic nerve head avulsion results from injury to the head and/or orbit. Initially, the injury may be difficult to diagnose. The vision is profoundly impacted, and there is no known medical or surgical therapy.
The risk factors for optic nerve head avulsion follow the patterns for other ocular trauma. These factors include: male sex, motor vehicle accidents, altercations, accidental finger pokes (such as with sports), and falls.
Avulsion of the optic nerve head can occur from direct or indirect force resulting in a traumatic optic neuropathy. Direct injury to the optic nerve head, from a penetrating orbital injury, is thought to be less common. Indirect injury can be from rapid torsional force to the globe resulting in shearing at the optic nerve head. Pujari et al. suggested that Bell’s phenomenon, a naturally protective reflex which causes the eye to rotate up and out, may induce changes in the anatomical orientation of the eye, putting torsional strain on the optic nerve during trauma. Rapid increase in intraocular pressure from direct non-penetrating trauma blowing out the nerve is another potential mechanism of injury, though this theory remains controversial. Other potential mechanisms include acute anterior displacement of the globe or retropulsion of the optic nerve, shearing the nerve from the globe.
Axons of the optic nerve are particularly susceptible to injury at the lamina cribrosa because of the loss of myelin and other supportive connective tissues at this site. Because of this, most cases of optic nerve avulsion occur at the junction of the nerve head and the globe, though there are some reports of optic nerve avulsion occurring in more posterior sites.
Prevention is focused on avoidance of potential trauma including use of appropriate eye protection for sports.
In cases where the view to the optic nerve head is clear, the diagnosis can be made by ophthalmoscopy. Initially, in many cases the view of the optic nerve head may be obscured, such as by vitreous hemorrhage. Without a clear view B-scan ultrasonography may be helpful to provide further information. MRI (contraindicated as first line imaging after trauma) and CT imaging may be helpful in some cases, but they have low sensitivity and specificity. As OCT can be impacted by media opacities, this is often not helpful initially.
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