Open-angle glaucoma (OAG) is a chronic bilateral ocular disease characterized by optic nerve damage and a typical pattern of visual field loss. Vision loss can progress to absolute blindness without intervention. Traditionally, elevated intraocular pressure (above 21 mmHg) has been considered a risk factor for progression of glaucoma. However, there is now abundant evidence that up to 30% of patients with progressive glaucoma never have intraocular pressure at that level. The cause of OAG remains unknown, but many risk factors have been identified. These include a large optic nerve cup, thin central cornea, high myopia, older age, positive family history, African or Latino ancestry, diabetes mellitus, systemic hypertension and a history of migraine headaches.
In the United States, 2.2 million people have been diagnosed with OAG and it is estimated that an equal number have undiagnosed OAG. Among all Americans it is the third most common cause of blindness, and in blacks of African ancestry it is the leading cause of blindness. Worldwide, more than 65 million people have glaucoma and 5 million are blind from this condition.
Ocular examination is vital to making a diagnosis of glaucoma and monitoring for progression of the disease. Clinical signs that raise suspicion of glaucoma include intraocular pressure greater than 21 mmHg and asymmetry of IOP. Optic nerve cupping greater than 50% is also suspicious for glaucoma as is asymmetry of optic nerve cupping. The presence of an optic disc hemorrhage is strongly associated with progressing OAG.
Early identification of OAG is the most important element in the treatment of the disease. The American Academy of Ophthalmology Preferred Practice Pattern guidelines for quality eye care recommend periodic comprehensive ocular examinations as the best way to identify patients at risk for developing OAG. The therapeutic goal for OAG patients is preventing initial or progressive optic nerve damage and visual field loss. Setting a target intraocular pressure, generally 20 to 30 percent lower than initial IOP, is helpful in directing care. Treatment is typically initiated with eye drops that lower IOP, balancing response with considerations of side effects, inconvenience of use and cost. Laser therapy may be recommended as an alternative to or in conjunction with eye drops. Lastly, surgery may be recommended for cases that are particularly difficult to manage. Glaucoma surgery can be very effective at lowering IOP, but is associated with the greatest risk of serious complications.
Open angle glaucoma is a common, potentially sight-threatening disease. It is seen in all demographic groups, but is particularly prevalent in people over the age of 65 and in African American adults of all ages. Until the end stage when vision loss is apparent, there are generally no symptoms. OAG can be diagnosed through comprehensive ocular examination and treatment is highly effective at delaying or stopping the progression of the disease. Treatment and monitoring of the disease must be lifelong. Patient compliance with physician-recommended care is essential to successful management.
Ocular hypertension is a condition involving elevated intraocular pressure without evidence of optic nerve injury, as is seen in glaucoma. Elevated intraocular pressure is considered a mean pressure greater than 21 mmHg. This measurement is two standard deviations greater than the mean IOP of 15 to 17 observed in the general population.
In the United States, nearly four million individuals meet the criteria of ocular hypertension. They belong to the much larger group of people considered to be at risk for developing glaucoma. This group also includes those with asymmetric appearing optic discs and patients with strong family histories of glaucoma. Several studies show an exponential increase in the incidence of glaucoma when IOP rises above 25mmHg. For this reason, treatment will often be initiated in ocular hypertension patients reaching this threshold.
Acute Angle Closure Glaucoma
Acute angle-closure glaucoma is caused by the obstruction of fluid outflow in the front segment of the eye due to a blockage of the outflow apparatus by the peripheral iris. The condition usually occurs in patients over the age of 40, and is more common in women. Age is a factor because over time, the lens in the front segment of the eye grows in anterior-posterior diameter and crowds the anterior chamber. The onset of angle-closure glaucoma is generally triggered by a condition called relative papillary block. Relative papillary block involves moderate pupil dilation and may occur when sleep begins, or when pharmacologic agents that dilate the pupil are given. Only those with vulnerable, anatomically narrow angles are thought to be at risk for the development of relative papillary block and angle-closure glaucoma.
Acute angle-closure glaucoma has features that distinguish it from most other types of glaucoma. Intraocular pressure is usually very high, frequently above 50 mmHg. This results in corneal edema with such symptoms as blurring of vision and sometimes rainbow-colored halos around lights. Ocular discomfort and redness may occur, sometimes accompanied by nausea and vomiting. Many patients with acute angle-closure glaucoma initially seek care in an emergency room or urgent care facility where a lack of familiarity with eye disease and limited facilities for examination may result in a delay in diagnosis of glaucoma.
Treatment of acute angle-closure glaucoma involves both eye drops and systemic therapy. Potent eye pressure lowering medication including carbonic anhydrase inhibitors and hyperosmotic agents may need to be administered orally or intravenously to “break” an attack. Eye drops may be effective at controlling eye pressure in less severe cases. Treatment of angle closure must eventually include a surgical intervention to correct the anatomic problem that predisposed the patient to an attack. For most patients, this intervention is a laser iridotomy, which is performed after the eye pressure has been normalized. In some cases, cataract removal may be recommended and performed expeditiously to relieve crowding in the front segment of the eye.
Anatomic Narrow Angles
Narrowing of the space created by the back surface of the cornea and the front surface of the peripheral iris is observed in some patients over 40 years of age, especially as cataract development progresses. This space is the pathway for fluid exiting the eye through a structure called the trabecular meshwork. If the route becomes blocked, eye pressure can elevate dramatically, resulting in all of the symptoms previously described in acute angle-closure glaucoma. Women are more likely than men to have an anatomic narrow angle, and all hyperopic individuals are at risk. If the narrowing is severe, prophylactic peripheral iridectomy is recommended as a means of preventing acute angle-closure glaucoma. An Nd:YAG laser is generally used to perform the procedure, although surgical peripheral iridectomy under anesthesia may be recommended for patients unable to cooperate during laser treatment.