Friday, June 02, 2006

Section 4-C: Open-Angle Glaucoma

One major type of glaucoma is open-angle glaucoma. The difference between open-angle and angle closure glaucoma is based on examination. The term angle (short for irido-corneal angle; see figure 4-8 below) refers to the drainage angle of the eye, which is between the cornea and the iris. Those with open-angle glaucoma have a widely open drainage angle on examination. The angle is examined with a special lens called a gonioscopy lens (“gonio” means angle). (Figure 4-5)

Figure 4-5. A patient undergoing gonioscopy (same as Figure 1-3).

An open-angle with cupping of the optic nerve and glaucomatous visual field loss leads to the diagnosis of open-angle glaucoma. There are primary and secondary causes of open-angle glaucoma. If there is no identifiable factor causing the glaucoma (i.e., the cause of the glaucoma is unknown), this is referred to as primary open-angle glaucoma (POAG) or chronic open-angle glaucoma (COAG). If there is a known process leading to decreased fluid drainage through the angle, such as blood, inflammatory cells, or pigment, this is called secondary open-angle glaucoma. Glaucoma associated with pigment dispersion syndrome and pseudo-exfoliation syndrome are examples of secondary open-angle glaucoma.

POAG is the most common form of glaucoma in the U.S. POAG can be of two types. It can be associated with either elevated IOP or normal IOP. The former is referred to as POAG with elevated IOP or high pressure glaucoma. The latter is often referred to as normal tension glaucoma (NTG) or low tension glaucoma. NTG has the same characteristics as POAG except the IOP is in the normal range, that is, less than 21 mmHg. One theory regarding the mechanism of injury in NTG is insufficient blood flow leading to optic nerve damage. The POAG and NTG may represent a spectrum of disease all leading to visual loss.
Although NTG patients have IOPs less than 21 mmHg, it has been shown that lowering the IOP to low-normal or even sub-normal range halts or slows the progression of glaucomatous damage. Aside from the IOP, the examination findings are very similar between POAG and NTG. A few differences have been observed. More optic nerve hemorrhages (bleeding spots) are found in NTG than in patients with POAG. The visual field test may also show more central (as opposed to peripheral) loss in NTG. These differences are not absolute and many of the characteristics between the two forms overlap.

The treatment goal of open-angle glaucoma is to lower IOP. This is more challenging in NTG, but lowering IOP has been shown to be effective even in NTG as well as POAG. Treatment modalities include topical medications (glaucoma eye drops), laser to the drainage area or trabecular meshwork, and filtering surgery. Medical and surgical treatment will be covered in more depth in Chapters 7 and 8.

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