Primary open-angle glaucoma is a syndrome of optic nerve damage associated with an open anterior chamber angle and an elevated or sometimes average intraocular pressure (IOP). Symptoms are a result of visual field loss. Diagnosis is by ophthalmoscopy, gonioscopy, visual field examination, and measurement of central corneal thickness and IOP. Treatment includes laser surgery and/or topical medications (eg, prostaglandin analogs, beta-blockers) and often requires incisional surgery to increase aqueous drainage.
(See also Overview of Glaucoma.)
Etiology of Primary Open-Angle Glaucoma
Although open-angle glaucomas can have numerous causes (see table Open-Angle Glaucoma: Classification Based on Mechanisms of Outflow Obstruction), 60 to 70% of cases in the United States have no identifiable cause and are termed primary open-angle glaucoma. Both eyes usually are affected, but typically not equally.
Risk factors for primary open-angle glaucoma include
Older age
Positive family history
Black race
Thinner central corneal thickness
Systemic hypertension
In Black people, glaucoma is more severe and develops at an earlier age, and blindness is 6 to 8 times more likely.
Pathophysiology of Primary Open-Angle Glaucoma
Intraocular pressure (IOP) can be elevated or within the average range.
In primary open-angle glaucoma, there is an abnormality of extracellular matrix in the part of the trabecular meshwork (TM) that is immediately adjacent to the Schlemm canal (called the juxtacanalicular TM) similar to accelerated aging; trabecular meshwork cellularity may also decrease.
Elevated-pressure glaucoma
Two thirds of patients with glaucoma have elevated (> 21 mm Hg) IOP. Aqueous humor drainage is inadequate, whereas production by the ciliary body is normal. Identifiable mechanisms (ie, secondary open-angle glaucomas) are not present. Secondary mechanisms include developmental anomalies, scarring caused by trauma or infection, and plugging of channels by detached iris pigment (ie, pigment dispersion syndrome) or abnormal protein deposits (eg, pseudoexfoliation syndrome).
Normal-pressure glaucoma or low-tension (low-pressure) glaucoma
In at least one third of patients with glaucoma, IOP is within the average range, but optic nerve damage and visual field loss typical of glaucoma are present. These patients have a higher incidence of vasospastic diseases (eg, migraines, Raynaud syndrome) than the general population, suggesting that a vascular disorder compromising blood flow to the optic nerve may play a role. Glaucoma occurring with average-range IOP is more common among Asian patients.
Symptoms and Signs of Primary Open-Angle Glaucoma
Early primary open-angle glaucoma symptoms are uncommon. Usually, the patient becomes aware of visual field loss only when optic nerve atrophy is marked; the typically asymmetric deficits contribute to delay in recognition. However, some patients have complaints, such as missing stairs if their inferior visual field has been lost, noticing portions of words missing when reading, or having difficulty with driving earlier in the course of the disease.
Examination findings include an unobstructed open angle on gonioscopy and characteristic optic nerve appearance and visual field defects. Intraocular pressure (IOP) may be normal or high but is almost always higher in the eye with more optic nerve damage.
Optic nerve appearance
The optic nerve head (ie, disk) is normally a slightly vertically elongated circle with a centrally located depression called the cup. The neurosensory rim is the tissue between the margin of the cup and the edge of the disk, and is composed of the ganglion cell axons from the retina.
Photo courtesy of Douglas Rhee, MD
Characteristic optic nerve changes include
Increased cup:disk ratio (particularly an increasing ratio over time)
Thinning of the neurosensory rim
Pitting or notching of the rim
Nerve fiber layer hemorrhage that crosses the disk margin (ie, Drance hemorrhage or splinter hemorrhages)
Vertical elongation of the cup
Quick (ie, sharp change in the course of angulations in the exiting blood vessels [called bayoneting])
Thinning of the neurosensory rim (optic nerve or retinal nerve fiber layer) over time alone can be diagnostic of glaucoma regardless of the IOP or visual field and is the initial sign of damage in 40 to 60% of cases. In other cases, the initial sign of damage is some visual field change.
Wedge-shaped dark areas may develop, reflecting damage to the retinal nerve fiber layer.
Photo courtesy of Douglas Rhee, MD.
WESTERN OPHTHALMIC HOSPITAL/SCIENCE PHOTO LIBRARY
Photo courtesy of Douglas Rhee, MD.
Visual field defects
Visual field changes caused by lesions of the optic nerve include
Nasal step defects (which do not cross the horizontal meridian—an imaginary horizontal line between the upper and lower parts of the visual field)
Arcuate (arc-shaped) scotomata extending nasally from the blind spot
Temporal wedge defects
Paracentral scotomata
In contrast, deficits of the more proximal visual pathways (ie, from the lateral geniculate nucleus to the occipital lobe) involve quadrants or hemispheres of the visual field; thus, deficits do not cross the vertical meridian.
Diagnosis of Primary Open-Angle Glaucoma
Visual field testing
Ophthalmoscopy
Measurement of central corneal thickness and intraocular pressure (IOP)
Exclusion of other optic neuropathies
Diagnosis of primary open-angle glaucoma is suggested by the examination, but similar findings can result from other optic neuropathies (eg, caused by ischemia, cytomegalovirus infection, or vitamin B12 deficiency).
Before a diagnosis of normal-pressure glaucoma can be established, the following factors may need to be ruled out:
Inaccurate IOP readings
Large diurnal fluctuations (causing intermittent normal readings)
Optic nerve damage caused by previously resolved glaucoma (eg, a previously elevated IOP due to corticosteroid use or uveitis)
Intermittent angle-closure glaucoma
Other ocular or neurologic disorders that cause similar visual field defects
Central corneal thickness is measured to help interpret the result of IOP measurement.
Optic disk photography or a detailed optic disk drawing is helpful for future comparison. The frequency of follow-up examinations varies from weeks to months, depending on the patient’s reliability, severity of the glaucoma, and response to treatment.
Treatment of Primary Open-Angle Glaucoma
Decreasing intraocular pressure (IOP) 20 to 40%
Sometimes surgery, such as laser trabeculoplasty or guarded filtration procedure
Vision lost by glaucoma cannot be recovered. The goal is to prevent further optic nerve and visual field damage by lowering IOP. The target level is 20 to 40% below pretreatment readings or the IOP at which damage is known to have occurred. In general, the greater the damage caused by glaucoma, the lower the IOP must be to prevent further damage. If damage progresses, the IOP goal is lowered further and additional therapy is initiated.
Initial treatment used to be medical therapy, proceeding to laser therapy and then incisional surgery if the target IOP is not met; however, subsequent evidence suggests that initial treatment with laser trabeculoplasty preserves visual fields and results in less frequent subsequent incisional glaucoma surgeries than treatment with medications (1). Surgery may be the initial treatment if IOP is extremely high, the patient does not wish to use or has trouble adhering to medical therapy, or if there is significant visual field damage at presentation.
Medical therapy
Multiple medications are available (see table Medications Used to Treat Glaucoma
Patients taking topical glaucoma medications should be taught passive eyelid closure with punctal occlusion to help reduce systemic absorption and associated adverse effects, although the effectiveness of these maneuvers is controversial. Patients who have difficulty instilling drops directly onto the conjunctiva may place the drop on the nose just medial to the medial canthus, then roll the head slightly toward the eye so that the liquid flows into the eye.
Typically, to gauge effectiveness, clinicians start medications in one (one-eye trial) or both eyes.
Medications Used to Treat Glaucoma
Medication | Dose/Frequency | Mechanism of Action on Eye | Comments |
|---|---|---|---|
Prostaglandin analogs (topical) | |||
1 drop at bedtime | Increase uveoscleral outflow rather than altering conventional (trabeculocanalicular) aqueous outflow | Increased pigmentation of the iris and skin Possible worsening of uveitis Elongated and thickened eyelashes Muscle, joint, and back pain Rash | |
1 drop at bedtime | |||
1 drop at bedtime | |||
1 drop at bedtime | |||
Beta-blockers (topical) | |||
1 drop once or twice a day | Decrease aqueous production; do not affect pupil size | Systemic adverse effects (eg, bronchospasm, depression, fatigue, confusion, erectile dysfunction, hair loss, bradycardia) May develop insidiously and be attributed by patients to aging or other processes | |
1 drop once or twice a day* | |||
1 drop once or twice a day | |||
Levobetaxolol | 1 drop twice a day* | ||
1 drop once or twice a day | |||
Metipranolol | 1 drop once or twice a day | ||
Carbonic anhydrase inhibitors (oral or IV) | |||
125–250 mg orally four times a day (or 500 mg orally twice a day using extended-release capsules) or 500 mg IV single dose for acute lowering | Decrease aqueous production | Used as adjunctive therapy Cause fatigue, altered taste, anorexia, depression, paresthesias, electrolyte abnormalities, kidney calculi, and blood dyscrasias Possibly nausea, diarrhea, weight loss | |
25–50 mg orally two or three times a day | |||
Carbonic anhydrase inhibitors (topical) | |||
1 drop two or three times a day | Decrease aqueous production | Low risk of systemic effects, but may cause bad taste in mouth and/or rash | |
1 drop two or three times a day | |||
Rho kinase inhibitor | |||
1 drop at bedtime | Increases conventional aqueous outflow | May develop conjunctival hyperemia (redness), corneal verticillata, subconjunctival petechial hemorrhages | |
Miotics, direct-acting (cholinergic agonists; topical)† | |||
1 drop two or three times a day | Cause miosis, increase aqueous outflow | Less effective as monotherapy than beta-blockers Possible need for higher strengths in patients with darker-pigmented pupils Hinder dark adaptation | |
1 drop two to four times a day | |||
Miotic, indirect-acting (cholinesterase inhibitors; topical)† | |||
1 drop once or twice a day | Causes miosis, increases aqueous outflow | Very long acting: Irreversible inhibition; can cause cataracts and retinal detachment; should be avoided in angle-closure glaucoma because of the extreme miosis; hinders dark adaptation Systemic effects (eg, sweating, headache, tremor, excess saliva production, diarrhea, abdominal cramps, nausea) more likely than with direct-acting miotics May still be an option in pseudophakic patients | |
Osmotic diuretics (oral, IV)‡ | |||
1–1.5 g/kg body weight orally (may repeat 8–12 hours later) | Cause increased serum osmolarity, which draws fluid from eye | Used for acute angle closure Has adverse systemic effects Can rarely cause cerebral hemorrhage and acute, decompensated heart failure Ineffective in patients with moderate to severe renal failure | |
0.5–2.0 g/kg body weight IV over 30–45 minutes (may repeat 8–12 hours later) | |||
Alpha-2-selective adrenergic agonists (topical) | |||
1 drop two to three times a day | Decrease aqueous production; may increase uveoscleral aqueous outflow; may cause mydriasis | < 2 years Systemic effects (eg, hypertension, tachycardia) less common than with nonselective agonists | |
1 drop two to three times a day§‡ | |||
* Beta-1-selective. | |||
† Miotics are rarely used. | |||
‡ For acute use only. | |||
Surgery
Surgery for primary open-angle glaucoma and normal-pressure glaucoma includes laser trabeculoplasty, a guarded filtration procedure, and procedures that enhance only a portion of the drainage pathway.
Selective laser trabeculoplasty (SLT) uses a pulsed double-frequency neodymium:yttrium-aluminum-garnet laser. SLT and argon laser trabeculoplasty (ALT) are equally effective initially, but SLT may have greater effectiveness in subsequent treatments. SLT is emerging as the standard of care for the initial treatment for newly diagnosed patients.
Argon laser trabeculoplasty (ALT) may also be considered but is rarely used in lieu of SLT. Laser energy is applied to either 180º or 360º of the trabecular meshwork to improve the drainage of aqueous humor. Within 2 to 5 years, about 50% of patients require additional drug therapy or surgery because of insufficient IOP control.
A guarded filtration procedure is the most commonly used filtration procedure. A hole is made in the limbal sclera (trabeculectomy), which is covered by a partial-thickness scleral flap that controls egress of aqueous from the eye to the subconjunctival space, forming a filtration bleb. Adverse effects of glaucoma filtration surgery include acceleration of cataract growth, pressures that are too low, and transient accumulation of fluid in the choroidal space (ie, choroidal effusion) during the perioperative period.
Patients with trabeculectomies are at increased risk of bacterial endophthalmitis and should be instructed to report any symptoms or signs of bleb infection (blebitis) or endophthalmitis (eg, worsening vision, conjunctival hyperemia, pain) immediately.
Partial-thickness procedures bypass portions of the outflow pathways, unlike full-thickness procedures, even if guarded, that create a direct conduit between the anterior chamber and subconjunctival space. In general, trabecular meshwork bypass procedures appear to be safer but less effective than trabeculectomy:
The ab externo approach (an approach from outside the eye), including viscocanalostomy, deep sclerectomy, and canaloplasty, involves a deep dissection of greater than > 98% thickness of the scleral passage, leaving a window of Descemet membrane and/or the inner wall of the Schlemm canal and trabecular meshwork. The canal is dilated by using a viscoelastic solution (in viscocanalostomy) or a microcatheter (in canaloplasty). Deep sclerectomy generally relies on the formation of a conjunctival bleb.
In the ab interno approach (an approach from inside the eye), a device is used to remove (eg, with ab interno goniotomy or trabeculectomy) or bypass (eg, with some stent procedures) the trabecular meshwork, creating direct communication between the anterior chamber and collecting channels. No bleb is formed.
Treatment reference
1. Gazzard G, Konstantakopoulou E, Garway-Heath D, et al: Laser in glaucoma and ocular hypertension (LiGHT) trial: Six-year results of primary selective laser trabeculoplasty versus eye drops for the treatment of glaucoma and ocular hypertension. Ophthalmology 130(2):139-151. doi: 10.1016/j.ophtha.2022.09.009
Key Points
Primary open-angle glaucoma is usually related to elevated intraocular pressure (IOP) but may occur with normal IOP.
Vision loss due to glaucoma cannot be recovered.
Begin diagnostic evaluation with ophthalmoscopy, measurement of IOP, and visual field testing.
Aim to decrease IOP by 20 to 40%.
Consider incisional surgery if laser surgery and/or topical medications are not effective or if visual loss is severe.
