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The glaucomas are a group of eye disorders characterized by progressive optic nerve damage at least partly due to increased intraocular pressure (IOP). Glaucoma is the 3rd most common cause of blindness worldwide and the 2nd most common cause of blindness in the US, where it is the leading cause of blindness for blacks and Hispanics. About 3 million Americans and 14 million people worldwide have glaucoma, but only half are aware of it. Glaucoma can occur at any age but is 6 times more common among people > 60 yr.
The glaucomas are categorized as open-angle or closed-angle (angle-closure—see Table 1: Glaucoma: Open-Angle Glaucoma: Classification Based on Mechanisms of Outflow Obstruction* , Table 2: Glaucoma: Angle-Closure Glaucoma: Classification Based on Mechanisms of Outflow Obstruction*, and Table 3: Glaucoma: Developmental Abnormalities of the Anterior Chamber Angle Causing Glaucoma: Classification Based on Mechanisms of Outflow Obstruction*). The “angle” refers to the angle formed by the junction of the iris and cornea at the periphery of the anterior chamber (see Fig. 1: Glaucoma: Aqueous humor production and flow. ). The angle is where > 98% of the aqueous humor exits the eye via either the trabecular meshwork and Schlemm's canal (the major pathway, particularly in the elderly) or the ciliary body face and choroidal vasculature. These outflow pathways are not simply a mechanical filter and drain but instead involve active physiologic processes.
Glaucomas are further subdivided into primary (cause of outflow resistance or angle closure is unknown) and secondary (outflow resistance results from another disorder), accounting for > 20 adult types.
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Fig. 1
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Aqueous humor production and flow.
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Most of the aqueous humor, produced by the ciliary body, exits the eye at the angle formed by the junction of the iris and cornea. It exits primarily via the trabecular meshwork and Schlemm's canal.
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Table 3
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Developmental Abnormalities
of the Anterior Chamber Angle Causing Glaucoma: Classification Based
on Mechanisms of Outflow Obstruction*
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Means
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Disorders
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High insertion of peripheral iris
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Axenfeld-Rieger syndrome
Peters' anomaly
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Incomplete development of trabecular meshwork or Schlemm's canal
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Congenital (infantile) glaucoma
Glaucomas associated with other developmental abnormalities
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Fine strands that contract to close angle
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Aniridia
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*Clinical examples cited; not an inclusive list of the glaucomas.
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Adapted from Ritch R, Shields MB, Krupin T: The Glaucomas, ed. 2. St. Louis, Mosby, 1996, p. 720; with permission.
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Pathophysiology
Axons of retinal ganglion cells travel through the optic nerve carrying images from the eye to the brain. Damage to these axons causes ganglion cell death with resultant optic nerve atrophy and patchy vision loss. Elevated IOP (in unaffected eyes, the average range is 11 to 21 mm Hg) plays a role in axonal damage, either by direct nerve compression or diminution of blood flow. However, the relationship between pressure and nerve damage is variable. Of people with IOP > 21 mm Hg (ie, ocular hypertension), only about 1 to 2%/yr (roughly 10% over 5 yr) develop glaucoma. Additionally, about 1/3 of patients with glaucoma do not manifest IOPs > 21 mm Hg (known as low-tension glaucoma or normal-tension glaucoma).
IOP is determined by the balance of aqueous secretion and drainage. Elevated IOP is caused by inhibited or obstructed outflow, not oversecretion. In open-angle glaucoma, IOP is elevated because outflow is inadequate despite an angle that appears unobstructed. In angle-closure glaucoma, IOP is elevated when a physical distortion of the peripheral iris mechanically blocks outflow.
Symptoms and Signs
Symptoms and signs vary with the type of glaucoma, but the defining characteristic is optic nerve damage as evidenced by an abnormal optic disk (see Glaucoma: Optic nerve appearance) and certain types of visual field deficits (see Glaucoma: Visual field defects).
IOP may be elevated or within the average range. (For techniques of measurement, see Approach to the Ophthalmologic Patient: Testing)
Diagnosis
Glaucoma should be suspected in a patient with any of the following:
Such patients (and those with any risk factors) should be referred to an ophthalmologist for a comprehensive examination that includes a thorough history, family history, examination of the optic disks (preferably using a binocular examination technique), formal visual field examination, IOP measurement, and gonioscopy (visualization of the anterior chamber angle with a special mirrored contact lens prism). Glaucoma is diagnosed when characteristic findings of optic nerve damage are present and other causes (eg, multiple sclerosis) have been excluded. Elevated IOP makes the diagnosis more likely but is not essential.
Screening:
Screening can be done by primary physicians by checking visual fields with frequency-doubling technology (FDT) perimetry and ophthalmoscopic evaluation of the optic nerve. FDT perimetry involves use of a desktop device that can screen for visual field abnormalities suggestive of glaucoma in 2 to 3 min per eye. Although IOP should be measured, screening based only on IOP has low sensitivity, low specificity, and low positive predictive value. Patients > 40 yr and those who have risk factors for open-angle or angle-closure glaucoma should receive a comprehensive eye examination every 1 to 2 yr.
Treatment
Patients with characteristic optic nerve and corresponding visual field changes are treated regardless of IOP. Lowering the IOP is the only clinically proven treatment. For chronic adult and juvenile glaucomas, the initial target IOP is at least 20% below pretreatment readings.
Three methods are available: drugs, laser surgery, and incisional surgery. The type of glaucoma determines the appropriate method. Drugs and most laser surgeries (trabeculoplasty) modify the existing aqueous secretion and drainage system. Most incisional surgeries (eg, guarded filtration procedures [trabeculectomy] or glaucoma drainage implant devices [tube shunts]) create a new drainage system.
Prophylactic IOP lowering in patients with ocular hypertension delays the onset of glaucoma. However, because the rate of conversion from ocular hypertension to glaucoma in untreated people is low, the decision to treat prophylactically should be individualized by the presence of risk factors, magnitude of IOP elevation, and patient factors (ie, preference for drugs vs surgery, drug adverse effects). Generally, treatment is recommended for patients with IOP > 30 mm Hg even if the visual field is full and the optic nerve disk appears healthy, because the likelihood of damage is significant at that IOP level.
Last full review/revision August 2008 by Douglas J. Rhee, MD
Content last modified August 2008
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