Primary Open-Angle Glaucoma
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 topical drugs (eg, prostaglandin analogs, beta-blockers) and often requires laser or incisional surgery to increase aqueous drainage.
(See also Overview of 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 US have no identifiable cause and are termed primary open-angle glaucoma. Both eyes usually are affected, but typically not equally.
Open-Angle Glaucoma: Classification Based on Mechanisms of Outflow Obstruction*
Risk factors for primary open-angle glaucoma include
Positive family history
Thinner central corneal thickness
In people of African ethnicity, glaucoma is more severe and develops at an earlier age, and blindness is 6 to 8 times more likely.
IOP can be elevated or within the average range.
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).
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 Asians.
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. IOP may be normal or high but is almost always higher in the eye with more optic nerve damage.
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.
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 angulations in the course of 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.
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
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 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.
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 is usually drug therapy, proceeding to laser therapy and then incisional surgery if the target IOP is not met. Surgery may be the initial treatment if IOP is extremely high, the patient does not wish to use or has trouble adhering to drug therapy, or if there is significant visual field damage at presentation.
Multiple drugs are available (see Table: Drugs Used to Treat Glaucoma). Topical agents are preferred. The most popular are prostaglandin analogs, followed by beta-blockers (particularly timolol). Other drugs include alpha-2-selective adrenergic agonists, carbonic anhydrase inhibitors, and cholinergic agonists. Oral carbonic anhydrase inhibitors are effective, but adverse effects limit their use.
Patients taking topical glaucoma drugs 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 drugs in one (one-eye trial) or both eyes.
Drugs Used to Treat Glaucoma
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.
Argon laser trabeculoplasty (ALT) may be the initial treatment for patients who do not respond to or who cannot tolerate drug therapy. Laser energy is applied to either 180º or 360º of the trabecular meshwork to improve the drainage of aqueous humor. Within 2 to 5 yr, about 50% of patients require additional drug therapy or surgery because of insufficient IOP control.
Selective laser trabeculoplasty (SLT) uses a pulsed double-frequency neodymium:yttrium-aluminum-garnet laser. SLT and ALT are equally effective initially, but SLT may have greater effectiveness in subsequent treatments. SLT may also be considered for initial treatment.
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.
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 trabeculectomy) or bypass (eg, with some stent procedures) the trabecular meshwork, creating direct communication between the anterior chamber and collecting channels or between the anterior chamber and suprachoroidal space. No bleb is formed.
In general, these procedures appear to be safer but less effective than trabeculectomy.
Primary open-angle glaucoma is usually related to elevated 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%.
Begin treatment with topical drugs (eg, prostaglandin analogs such latanoprost or tafluprost, beta-blockers such as timolol).
Consider surgical treatment if drugs are not effective or if visual loss is severe.