Primary Open-Angle Glaucoma (POAG)

Primary Open-Angle Glaucoma (POAG)

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Primary open-angle glaucoma (POAG) is described distinctly as a multifactorial optic neuropathy that is chronic, progressive, and irreversible, with a characteristic acquired loss of optic nerve fibers. Such loss develops in the presence of open anterior chamber angles, characteristic visual field abnormalities, and intraocular pressure that is too high for the continued health of the eye. It manifests by cupping of the optic disc (see the image below) in the absence of other known causes of the disease. [1, 2]

Because of the silent nature of glaucoma, patients usually don’t present with any symptoms or visual complaints until late in the disease course, particularly with primary open-angle glaucoma. However, narrow/closed-angle glaucoma and secondary glaucomas can cause a rapid rise in intraocular pressure, which is usually symptomatic, particularly when intraocular pressure is 35 mm Hg or more.

Significant attention should be given to the following in the patient’s clinical history:

See Clinical Presentation for more detail.

Screening the general population for primary open-angle glaucoma is most effective if targeted toward those at high risk, such as African Americans and elderly individuals, especially if the screening consists of intraocular pressure measurements combined with assessment of optic nerve status.

Examination of patients with suspected primary open-angle glaucoma includes the following:

Lab Tests

Laboratory tests that may be used to rule other causes for optic neuropathy in patients suspected of having normal-tension glaucoma include the following:

Imaging studies

The following imaging studies may be used to evaluate patients with suspected primary open-angle glaucoma:

Investigational imaging modalities in the evaluation and management of patients with primary open-angle glaucoma include the following:

See Workup for more detail.

Current medical therapy for primary open-angle glaucoma is limited toward lowering intraocular pressure. A rational approach to choosing antiglaucoma medications should minimize the number of medications and the probability of significant adverse effects.

If one medication is not adequate in reaching the target pressure, a second medication should be chosen that has a different mechanism of action, so that the 2 drug therapies will have an additive effect.

Pharmacotherapy

Medications used in the management of primary open-angle glaucoma include the following:

Laser therapy

Laser can be used as primary or adjunctive treatment. It is indicated in cases of noncompliance with medications or if the patient is on maximum tolerated medical therapy and needs further intraocular pressure reduction.

The following are laser options that may be used for primary open-angle glaucoma:

Surgery

Surgery is indicated in primary open-angle glaucoma when glaucomatous optic neuropathy worsens (or is expected to worsen) at any given level of intraocular pressure and the patient is on maximum tolerated medical therapy.

The following are surgical options that may be used for primary open-angle glaucoma:

Minimally invasive glaucoma surgery (MIGS) consists of newer techniques that hold potential as surgical options in primary open-angle glaucoma, including the following:

See Treatment and Medication for more detail.

The definition of glaucoma has changed drastically since its introduction around the time of Hippocrates (approximately 400 BC). The word glaucoma came from the ancient Greek word glaucosis, meaning clouded or blue-green hue, most likely describing a patient having corneal edema or rapid evolution of a cataract precipitated by chronic elevated pressure. Over the years, extensive refinement of the concept of glaucoma has continued to the present date.

Glaucoma is currently defined as a characteristic progressive degeneration of the optic nerve, which may also lead to specific visual field defects over time. This process can be slowed by adequate lowering of intraocular pressure (IOP). Nevertheless, some controversy still exists as to whether IOP should be included in the definition, as some subsets of patients can exhibit the characteristic optic nerve damage and visual field defects while having an IOP within the normal range. The generic term glaucoma should only be used in reference to the entire group of glaucomatous disorders as a whole, because multiple subsets of glaucomatous disease exist. A more precise term should be used to describe the glaucoma, if the specific diagnosis is known.

Primary open-angle glaucoma (POAG) is glaucoma in the presence of open anterior chamber angles. It manifests by cupping of the optic disc (shown in the image below), in the absence of other known causes of glaucomatous disease. [1, 2]

POAG may develop in the absence of documented elevated IOP. This condition has been termed normal-tension or low-tension glaucoma.

People who maintain elevated pressures in the absence of nerve damage or visual field loss exist. They are considered at risk for glaucoma and have been termed ocular hypertensives (see Ocular Hypertension). POAG is a major worldwide health concern, because of its usually silent, progressive nature, and because it is one of the leading preventable causes of blindness in the world. With appropriate screening and treatment, glaucoma usually can be identified and its progress arrested before significant effects on vision occur.

The exact cause of glaucomatous optic neuropathy is not known, although many risk factors have been identified, to include the following: elevated IOP, family history, race, age older than 40 years, and myopia.

Elevated IOP is the most studied of these risk factors because it is the main clinically treatable risk factor for glaucoma. Multiple theories exist concerning how IOP can be one of the factors that initiates glaucomatous damage in a patient. Two of the major theories include the following: (1) onset of vascular dysfunction causing ischemia to the optic nerve, and (2) mechanical dysfunction via cribriform plate compression of the axons.

In addition to vascular compromise and mechanically impaired axoplasmic flow, contemporary hypotheses of possible pathogenic mechanisms that underlie glaucomatous optic neuropathy include excitotoxic damage from excessive retinal glutamate, deprivation of neuronal growth factors, peroxynitrite toxicity from increased nitric oxide synthase activity, immune-mediated nerve damage, and oxidative stress. The exact role that IOP plays in combination with these other factors and their significance to the initiation and progression of subsequent glaucomatous neuronal damage and cell death over time is still under debate; the precise mechanism is still a hot topic of discussion.

However, IOP is the only clinical risk factor that has been able to be successfully manipulated to date. Categorizing and managing patients based on their IOP and determining when IOP should be treated to prevent optic nerve damage became the forefront issue of glaucoma management for most of the last half of the 20th century.

Several studies over the years have shown that as IOP rises above 21 mm Hg, the percentage of patients developing visual field loss increases rapidly, most notably at pressures higher than 26-30 mm Hg. A patient with an IOP of 28 mm Hg is about 15 times more likely to develop field loss than a patient with a pressure of 22 mm Hg. Therefore, a patient population of those with elevated IOP should not be thought of as homogeneous. Furthermore, before initiating treatment of a patient based on a specific IOP measurement, the following factors should be considered regarding that IOP level obtained:

Variability of tonometry measurements per examiner (usually found to be about 10%, or 1-2 mm Hg)

Effect corneal thickness has on accuracy of IOP measurements (see Other Tests)

Diurnal variation of IOP (often highest in the early morning hours, but maximum IOP can be at any time of day in some patients)

In addition, remember that while normal eyes have a diurnal variation of approximately 3-4 mm Hg, glaucomatous eyes have even higher variation (>10 mm Hg). Note: Multiple readings should be taken over time and should be considered with correlative evidence of visual field and optic nerve examination before any diagnosis or therapy is rendered.

A study by Costa et al supports the need to more accurately assess the relationship of 24-hour IOP to 24-hour diastolic perfusion pressure in patients with glaucoma. Future methodology that performs noninvasive, real-time IOP measurements throughout the 24 hours of the day may enable a more complete understanding of the roles that IOP and blood pressure have to the etiology of glaucomatous damage and progression of the disease. [3]

Other points of importance when considering a diagnosis of POAG are described below.

Disc cupping and nerve fiber layer loss of up to 40% have been shown to occur before actual visual field loss has been detected. Therefore, visual field examination cannot be the sole tool used to determine when a patient has begun to sustain undeniable glaucomatous damage, and it should not be used in isolation as the benchmark for treatment.

In cases where POAG is associated with increased IOP, the cause for the elevated IOP generally is accepted to be decreased facility of aqueous outflow through the trabecular meshwork. Occurrence of this increase in resistance to flow has been suggested by multiple theories, to include the following:

An obstruction of the trabecular meshwork by accumulated material

A loss of trabecular endothelial cells

A reduction in trabecular pore density and size in the inner wall endothelium of the Schlemm canal

A loss of giant vacuoles in the inner wall endothelium of the Schlemm canal

A loss of normal phagocytic activity

Disturbance of neurologic feedback mechanisms

Other processes thought to play a role in resistance to outflow include altered corticosteroid metabolism, dysfunctional adrenergic control, abnormal immunologic processes, and oxidative damage to the meshwork.

Numerous other undetermined factors are considered to be at work in the pathogenesis of glaucoma. Basic and clinical science research continues to play a role in the search for such factors that contribute to the development and prognosis of the patient with POAG.

United States

Multiple population studies (eg, Framingham, Beaver Dam, Baltimore, Rotterdam, Barbados, Egna-Neumarkt) have been performed to estimate the prevalence of eye disease, including that of POAG and those individuals with ocular hypertension (OHT) who are at risk for POAG.

Estimates of the prevalence of glaucoma in studies involving only the United States suggest the following: glaucoma is a leading cause of irreversible blindness, second only to macular degeneration; only one half of the people who have glaucoma may be aware that they have the disease; and more than 2.25 million Americans aged 40 years and older have POAG.

More than 1.6 million have significant visual impairment, with 84,000-116,000 bilaterally blind in the United States alone. These statistics emphasize the need to identify and closely monitor those at risk of glaucomatous damage.

In a white population at risk for glaucoma, visual field loss can be expected to develop in about 3% of subjects over 10 years of follow up without treatment. Risk increases with age and IOP. [4]

In the United States, 3-6 million people, including 4-10% of the population older than 40 years, are currently without detectable signs of glaucomatous damage using present-day clinical testing, but they are at risk due to IOP of 21 mm Hg or higher. Roughly 0.5-1% per year of those individuals with elevated IOP will develop glaucoma over a period of 5-10 years. The risk may be declining to less than 1% per year, now that ophthalmoscopic and perimetric techniques for detecting glaucomatous damage have improved significantly.

International

Glaucoma is the second leading cause of blindness in the world (surpassed only by cataracts, a reversible condition). More than 3 million people are bilaterally blind from POAG worldwide, and more than 2 million people will develop POAG each year.

Over a 5-year period, several studies have shown the incidence of new onset of glaucomatous damage in previously unaffected patients to be about 2.6-3% for IOPs 21-25 mm Hg, 12-26% incidence for IOPs 26-30 mm Hg, and approximately 42% for those higher than 30 mm Hg.

The Ocular Hypertension Treatment Study (OHTS) found that the overall risk for patients with IOPs ranging from 24-31 mm Hg but with no clinical signs of glaucoma have an average risk of 10% of developing glaucoma over 5 years, with that risk being cut in half if patients are preemptively started on IOP-lowering therapy. Significant subsets of higher and lower risk exist when pachymetry (central corneal thickness [CCT]) is taken into account (see the image below).

Some patients’ first sign of morbidity from elevated IOP can be presentation with sudden loss of vision due to a central retinal vein occlusion (CRVO), the second most common risk factor for CRVO behind systemic hypertension.

See References for additional resources.

Prevalence of POAG is 3-4 times higher in blacks than in Caucasians; in addition, blacks are up to 6 times more susceptible to optic disc nerve damage than Caucasians. A higher prevalence of larger cup-to-disc ratios exists in the normal black population as compared with white controls.

Glaucoma is the most common cause of blindness among people of African descent. They are more likely to develop glaucoma early in life, and they tend to have a more aggressive form of the disease.

The Barbados Eye Study over 4 years showed a 5 times higher incidence of developing glaucoma in a group of black ocular hypertensives as compared with a predominantly white population.

Some population studies have found the mean IOP in blacks to be higher than Caucasian controls. Other studies (eg, Baltimore) found no difference. Consequently, further study needs to be conducted to clarify this issue.

Furthermore, the OHTS has suggested that black patients overall may have a thinner average central corneal thickness, thereby leading to underdiagnosis of elevated pressure, and consequently, exposure to higher risk of developing glaucoma. Therefore, pachymetry measurement is particularly important in establishing a baseline for African-American patients who are glaucoma suspects.

Reports on sex predilection also differ. Although some age-controlled studies have reported significantly higher mean IOP values in women than in men, others have failed to find such a difference, while others have even shown males to have a higher prevalence of glaucoma.

Age older than 40 years is a risk factor for the development of POAG, with up to 15% of people affected by the seventh decade of life.

Consequently, glaucoma is found to be more prevalent in the aging population, even after compensating for the fact that mean IOP slowly rises with increasing age.

However, the disease itself is not limited to only middle-aged and elderly individuals.

Prognosis is generally good for patients with POAG. With careful follow-up care and compliance with therapy, the vast majority of patients with POAG retain useful vision throughout their lifetime.

Patient education is essential for successful treatment of glaucoma. The patient who understands the chronic, potentially progressive nature of glaucoma is more likely to comply with therapy. Numerous handouts are available to patients, including the following:

“Understanding and Living with Glaucoma: A Reference Guide for People with Glaucoma and Their Families,” Glaucoma Research Foundation, 1-800-826-6693.

“Glaucoma Patient Resource: Living More Comfortably with Glaucoma,” Prevent Blindness America, 1-800-331-2020.

For patient education resources, see the Eye and Vision Center. Also, see the patient education articles Primary Open-Angle Glaucoma, Glaucoma FAQs, Normal-Tension Glaucoma, Glaucoma Medications, and Ocular Hypertension.

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Kristin Schmid Biggerstaff, MD Assistant Professor, Department of Ophthalmology, Baylor College of Medicine; Staff Physician, Michael E DeBakey Veterans Affairs Medical Center

Kristin Schmid Biggerstaff, MD is a member of the following medical societies: American Academy of Ophthalmology, American Glaucoma Society, American Society of Cataract and Refractive Surgery

Disclosure: Nothing to disclose.

Albert P Lin, MD Assistant Professor, Department of Ophthalmology, Baylor College of Medicine; Staff Physician, Michael E DeBakey Veterans Affairs Medical Center; Ophthalmologist, Ophthalmology Consultants of Houston

Albert P Lin, MD is a member of the following medical societies: American Academy of Ophthalmology, American Glaucoma Society, Harris County Medical Society, Texas Medical Association

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Martin B Wax, MD Professor, Department of Ophthalmology, University of Texas Southwestern Medical School; Vice President, Research and Development, Head, Ophthalmology Discovery Research and Preclinical Sciences, Alcon Laboratories, Inc

Martin B Wax, MD is a member of the following medical societies: American Academy of Ophthalmology, American Glaucoma Society, Society for Neuroscience

Disclosure: Nothing to disclose.

Inci Irak Dersu, MD, MPH Associate Professor of Clinical Ophthalmology, State University of New York Downstate College of Medicine; Attending Physician, SUNY Downstate Medical Center, Kings County Hospital, and VA Harbor Health Care System

Inci Irak Dersu, MD, MPH is a member of the following medical societies: American Academy of Ophthalmology, American Glaucoma Society

Disclosure: Nothing to disclose.

Neil T Choplin, MD Adjunct Clinical Professor, Department of Surgery, Section of Ophthalmology, Uniformed Services University of Health Sciences

Neil T Choplin, MD is a member of the following medical societies: American Academy of Ophthalmology, American Glaucoma Society, Association for Research in Vision and Ophthalmology, California Association of Ophthalmology, San Diego County Ophthalmological Society, Society of Military Ophthalmologists

Disclosure: Nothing to disclose.

Jerald A Bell, MD Staff Physician, Department of Ophthalmology, Billings Clinic

Jerald A Bell, MD is a member of the following medical societies: American Academy of Ophthalmology

Disclosure: Nothing to disclose.

The authors and editors of Medscape Reference gratefully acknowledge the contributions of previous coauthors, Robert J Noecker, MD, and Emily Patterson, MD, to the development and writing of this article.

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