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Over many years, glaucoma has been defined by an intraocular pressure of more than 20 mm Hg. Incompatible with this (now obsolete) definition of glaucoma was the ever larger number of cases that have been reported in medical literature in the 1980s and 1990s who had the typical signs of glaucomatous damage, like optic nerve head excavation and thinning of the retinal nerve fiber layer, while ...
Conversely, optic nerve damage may occur with normal pressure, known as normal-tension glaucoma. [27] In case of above-normal intraocular pressure, the mechanism of open-angle glaucoma is believed to be the impeded exit of aqueous humor through the trabecular meshwork, while in closed-angle glaucoma, the iris blocks the trabecular meshwork. [ 2 ]
The optic nerve can be damaged when exposed to direct or indirect injury. Direct optic nerve injuries are caused by trauma to the head or orbit that crosses normal tissue planes and disrupts the anatomy and function of the optic nerve; e.g., a bullet or forceps that physically injures the optic nerve.
A good analogy to better understand the cup-to-disc ratio is the ratio of a donut hole to a donut. The hole represents the cup and the surrounding area the disc. If the cup fills 1/10 of the disc, the ratio will be 0.1. If it fills 7/10 of the disc, the ratio is 0.7. The normal cup-to-disc ratio is less than 0.5.
Ocular hypertension (OHT) is defined by intraocular pressure being higher than normal, in the absence of optic nerve damage or visual field loss. [13] [14] Ocular hypotension, hypotony, or ocular hypotony, is typically defined as intraocular pressure equal to or less than 5 mmHg.
Best known is the higher risk of normal tension glaucoma, a disease with an impaired regulation of blood flow in a large number of patients. [9] If glaucomatous damage occurs despite normal eye pressure or if glaucomatous damage is progressive despite normalized intraocular pressure, frequently Flammer syndrome is the cause.
Since OCT is not influenced by disc size, it can be used in differentiating normal and glaucomatous eyes with large discs. [6] Since the clinical appearance of megalopapilla resembles that of glaucomatous optic disc, it should be differentiated from Normal tension glaucoma and other pseudo-glaucomatous diseases. [6]
Secondary glaucoma is a collection of progressive optic nerve disorders associated with a rise in intraocular pressure (IOP) which results in the loss of vision. In clinical settings, it is defined as the occurrence of IOP above 21 mmHg requiring the prescription of IOP-managing drugs. [ 1 ]