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Overview of Glaucoma
Glaucomas are a group of eye disorders characterized by progressive optic nerve damage in which an important part is a relative increase in intraocular pressure (IOP). Glaucoma is the 2nd 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 people of African ethnicity 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.
Glaucomas are categorized as
See table: Open-Angle Glaucoma: Classification Based on Mechanisms of Outflow Obstruction*, see Table: Angle-Closure Glaucoma: Classification Based on Mechanisms of Outflow Obstruction*, and see Table: 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 Figure: Aqueous humor production and flow.). The angle is where > 98% of the aqueous humor exits the eye via either the trabecular meshwork and the Schlemm 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 a known disorder), accounting for > 20 adult types.
Open-Angle Glaucoma: Classification Based on Mechanisms of Outflow Obstruction*
Angle-Closure Glaucoma: Classification Based on Mechanisms of Outflow Obstruction*
Developmental Abnormalities of the Anterior Chamber Angle Causing Glaucoma: Classification Based on Mechanisms of Outflow Obstruction*
Axons of retinal ganglion cells travel through the optic nerve carrying visual information 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 externally measured pressure and nerve damage is complicated. Of people with IOP > 21 mm Hg (ie, ocular hypertension), only about 1 to 2%/yr (about 10% over 5 yr) develop glaucoma. Additionally, about one third of patients with glaucoma do not have IOP > 21 mm Hg (known as low-tension glaucoma or normal-tension glaucoma).
One factor may be that externally measured IOP does not always reflect true IOP; the cornea may be thinner than average, which leads to a higher IOP measurement, or thicker than average, which leads to a lower IOP measurement, inside the eye than externally measured IOP. Another factor may be that a vascular disorder compromises blood flow to the optic nerve. Also, it is likely that there are factors within the optic nerve that affect susceptibility to damage.
IOP is determined by the balance of aqueous secretion and drainage. Elevated IOP is caused by inhibited or obstructed outflow, not oversecretion; a combination of factors in the trabecular meshwork appear to be involved. 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 vary with the type of glaucoma, but the defining characteristic is optic nerve damage as evidenced by an abnormal optic disk (see Optic nerve appearance) and certain types of visual field deficits (see Visual field defects).
IOP may be elevated or within the average range. (For techniques of measurement, see Testing.)
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, measurement of central corneal thickness, 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 elevated IOP can occur in the absence of glaucoma and is not essential for making the diagnosis.
Screening for glaucoma 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.
Patients with characteristic optic nerve and corresponding visual field changes are treated regardless of IOP measurement. Lowering the IOP is the only clinically proven treatment. For chronic adult and juvenile glaucomas, the initial target IOP measurement is at least 20 to 40% below pretreatment readings.
Three methods are available: drugs, laser surgery, and incisional surgery. The type of glaucoma determines the appropriate method or methods.
Drugs and most laser surgeries (trabeculoplasty) modify the existing aqueous secretion and drainage system.
Traditional incisional surgeries (eg, guarded filtration procedures [trabeculectomy], glaucoma drainage implant devices [tube shunts]) create a new drainage pathway between the anterior chamber and subconjunctival space. Newer incisional surgeries attempt to enhance trabecular or uveoscleral outflow without creating a full-thickness fistula.
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 based on 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.
Glaucoma is common, often asymptomatic, and contributes heavily to blindness worldwide.
Suspect glaucoma if patients have elevated IOP, optic nerve abnormalities on ophthalmoscopy, or a family history of glaucoma.
Do not rule out glaucoma because IOP is not high.
Screen patients > 40 yr or with risk factors every 1 to 2 yr, based mainly on results of ophthalmoscopy and FDT (to assess visual fields).
Treat by decreasing IOP.
Prophylactically decrease IOP if > 30 mm Hg, even if glaucoma is absent.
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