Definition & Classification

The term "glaucoma" encompasses a group of eye diseases, not a single entity. Glaucoma is described broadly in terms of aqueous fluid drainage through the trabecular meshwork, the major outflow pathway. There are two types: angle closure glaucoma and open angle glaucoma. Open angle glaucoma is far more common in the United States.

The American Academy of Ophthalmology defines primary angle closure glaucoma as, "An appositional or synechial closure of the anterior chamber angle caused by relative pupillary block in the absence of other causes of angle closure". The American Academy of Ophthalmology defines primary open angle glaucoma as a, "Multifactorial optic neuropathy in which there is a characteristic acquired loss of optic nerve fibers". Classifying glaucoma broadly into angle closure glaucoma or open angle glaucoma is helpful from both a diagnostic and pathophysiological perspective.

Primary angle closure glaucoma

There are several forms of primary angle closure glaucoma. In these, the position of the peripheral iris blocks access of the aqueous fluid to the outflow pathway, i.e. the trabecular meshwork. Primary angle-closure glaucoma is usually episodic. When the angle closes rapidly in an acute episode, the IOP rises rapidly causing symptoms of blurred vision and pain. Some patients describe the appearance of multi-colored halos around lights.

Acute vs. subacute vs. chronic

Acute angle closure describes a situation wherein the entire circumference of the angle has suddenly closed. Pain is the hallmark. The eye is red and the vision is blurred.

Subacute angle closure is associated with milder signs and symptoms because only a portion of the circumference has functionally closed. Closure may be intermittent and repetitive.

Chronic angle closure implies a long-term condition. Misdiagnosing chronic angle closure as open angle glaucoma is a common diagnostic error.

Secondary angle closure glaucoma

The term "secondary angle closure" is assigned when a pre-existing pathological condition of the eye causes the angle to close. Retinal ischemia secondary to diabetic retinopathy or central vein occlusion can result in neovascular glaucoma in which new vessels occlude the angle. If the ciliary body swells as in uveitis, it can push the iris against the trabecular meshwork and occlude the angle.

Primary open-angle glaucoma (POAG)

About 70% of the diagnosed cases of glaucoma are POAG [Fechtner et al., 1997]. The designation of 'Primary' indicates that there is no known association with another disease. Thus OAG, open angle glaucoma, is a broader term that includes both primary and secondary OAG.

Historically, an IOP that was 'diagnostic' of OAG was based on the statistical distribution of IOPs in the general population. According to these historical data, an IOP higher than 21 mm Hg had only a 2.5% probability of occurring in the general population; it was considered diagnostic of OAG. Today, it is understood that because of the overlap of IOP distributions between those with and without OAG, about 1/6 of patients with glaucoma have a "normal IOP", i.e. 21 mm Hg or less. As a consequence, an elevated IOP is now considered one risk factor (albeit very important) for developing glaucomatous optic neuropathy; an elevated IOP by itself is not diagnostic of OAG.

Today, IOP data is integrated with data from other ophthalmological procedures to formulate a diagnosis [Bathija et al., 1998]. These data categories include:

Optic nerve head (ONH) data only,
Visual field data only, or,
ONH and the visual field together.

Of those articles reviewed that do quantify IOP as a diagnostic criterion, about half specify an IOP greater than or equal to 22 mm Hg.

The above figure shows why IOP is a poor criterion to differentiate those with glaucoma from those without glaucoma. In a study conducted in Arizona, 4,774 Hispanics older than 40 years of age were evaluated for glaucoma. The circles (top) represent those people without glaucoma; the boxes (bottom) represent those people with glaucoma [Quigley et al., 2001]. The mean IOPs of the two distributions were similar although the mean was slightly higher among those people with OAG (i.e., right shifted). The shapes of the two distributions were similar. Note that those with an IOP of 31 mm Hg or higher were grouped together; it is only in the group with IOPs of 31 mm Hg or higher that people with OAG (boxes) predominate over those without OAG (circles). In summary, IOP is just one of several risk factors that collectively help to diagnose OAG.

OAG is usually a bilateral disease that affects the eyes asymmetrically. The asymmetrical nature of the damage can be so pronounced that the disease may appear unilateral in the early stages. Over time, however, the contralateral eye will become involved. Only half of the Americans with OAG are aware that they have it.

Ocular hypertension

By comparison to primary open-angle glaucoma, ocular hypertension is characterized by elevated IOP, but the ONHs and visual fields are normal. Ocular hypertension is more common than POAG.

Normal tension glaucoma

Patients in this diagnostic category have visual field loss typical of glaucoma but without elevated IOP. Normal tension glaucoma has been defined as a mean IOP (while not using hypotensive eye drops) that is 24 mm Hg, and:

Open angles as viewed by gonioscopy,
Absence of a secondary cause,
Visual field defect(s) consistent with glaucomatous optic neuropathy,
Progressive damage to the optic nerve due to glaucoma.

References

Bathija R, Gupta N, Zangwill L, et al. Changing definition of glaucoma. J Glaucoma, 1998;7:165-169.

Fechtner RD and Kooner KS. Definitions and classification of glaucoma. Textbook of Ocular Pharmacology. Edited by TJ Zimmerman, et al. Lippincott-Raven Publishers, Phila. 1997, p. 219.

Quigley HA, West S, Rodriguez J, et al. The prevalence of glaucoma in a population-based study in Hispanic subjects. Proyecto VER. Arch. Ophthalmol. 2001;119:1819-1826.

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