Glaucoma is the leading cause of blindness in people over 40. People who have glaucoma do not know it because it usually destroys eyesight without any symptoms of pain. Early diagnosis and treatment is imperative.
Glaucoma by definition is a progressive disease that damages nerves carrying visual information to the brain. During an eye examination, if the bundle of nerves leaving the back of the eye (called the optic nerve) appears thin, more specialized glaucoma testing is required depending on your age, medical history and risks of developing eye disease. When evaluating patients at risk for glaucoma, the doctors at the Toronto Eye Clinic recommend the following tests:
This test measures the thickness of your cornea. Numbing eye drops are instilled and then your eye doctor uses an instrument that emits ultrasound waves to measure your corneal thickness. This is important because an unusually thin or thick cornea can affect the accuracy of your eye pressure readings. If you have thick corneas, your eye pressure reading may read artificially high even though you may not have glaucoma. Similarly, people with thin corneas can have normal pressure readings and still be at risk for glaucoma. The OHTS study concluded that people with thin corneas have a greater risk of developing glaucoma over a lifetime. This test is not covered by OHIP.
VISUAL FIELD TEST (PERIMETRY 24-2)
Your visual field is the area in front of you that you see including central and peripheral (side) vision. The visual field test measures your peripheral vision. Often early peripheral vision changes related to glaucoma go undetected by a patient. Your eye doctor uses a computer program that flashes small lights as you look into a special instrument. You press a button every time you see a light while staring at a central fixation light. This test may take 15-20 minutes to complete.
Figure 1. Normal Visual Field. This is a grid print out of a visual field where the patient can see small targets.
Figure 2. Abnormal Visual Field. In this grid printout, the dark black shaded areas and squares show where glaucoma has caused loss in peripheral vision.
HEIDELBERG RETINAL TOMOGRAPH (HRT3)
This instrument is a laser ophthalmoscope, which precisely measures and analyses the shape of your optic nerve. It is a scanning laser, not a treatment laser and is not harmful to the eye. Repeated measurements of the optic nerve with the HRT3 unit can pick up glaucoma damage earlier than visual field testing (structural change occurs before functional change). The HRT3 has been shown to be superior to all other optic nerve imaging techniques available. The first scan is a baseline and future scans look for progression in nerve loss. This test takes only moments to complete and does not usually require pupil dilation. This test is not covered by OHIP.
Figure 3. HRT of a normal optic nerve. The health of this optic nerve is within normal limits. The red represents the cupping, which is small, and the blue and green represent healthy peripheral nerve fibers.
Figure 4. HRT of an abnormal optic nerve. The red represents a large cup and is due to damage and thinning of healthy nerve fibers. As you can see, the blue and green takes up much less of the optic nerve.
What will the tests tell us?
Conducting these tests does not mean you have glaucoma; it means that you MAY BE at risk of developing glaucoma in the future and are considered a glaucoma suspect. Since glaucoma is a progressive disease, it is important to establish a baseline of measurements so that we can monitor for any glaucomatous changes overtime.
When will I find out the results of the test?
Since these preliminary tests are only to establish baseline information, you will NOT receive the results after they are taken. You will be automatically booked for your next annual exam with the eye doctor. At that one-year visit, you will receive your yearly vision and health check, previous test results will be discussed, and the HRT, OCT, and 24-2 Visual Field tests will be scheduled to be repeated. Keep in mind that a diagnosis of glaucoma requires 3 HRT, OCT and visual field tests that indicate progressive optic nerve damage. The combination of HRT and OCT are able to detect structural loss of nerve tissue leading to optic nerve disease. The visual field test will detect functional loss of vision over time.
OPTICAL COHERENCE TOMOGRAPHY (OCT)
The OCT is a diagnostic test that provides high-resolution, cross-sectional imaging of ocular tissues. It is predominantly used for imaging of the back of the eye to measure retinal nerve thickness entering the optic nerve and macular health. OCT is used to study and monitor diseases such as glaucoma and age-related macular degeneration. This test is not covered by OHIP.
WHAT WILL THESE TESTS TELL US?
Conducting these tests does not mean you have glaucoma; it means that you MAY BE at risk of developing glaucoma in the future and therefore are considered a glaucoma suspect. Since glaucoma is a progressive disease, it is important to establish a baseline of measurements so that we can monitor for any glaucomatous changes overtime. Therefore if an initial test is outside of the normal limits, this it not necessarily a cause for concern. It is more important to determine if test results are changing over time.
Figure 5. OCT Analysis of left and right optic nerves. This is an example of what the eye doctor will interpret from the OCT scan. The software plots out a graph, called the TSNIT curve, which shows where there is thinning in the optic nerve. Red indicates damaged areas of the optic nerve and green indicates healthy areas of the optic nerve.
Figure 6. OCT Asymmetry analysis. This is also part of the OCT software and it compares the relative macular thickness between the left and right optic nerves. Both optic nerves are split into 8x8 grid cells and the average thickness is calculated for each cell. Purple represents areas of thinning in the optic nerve.
HRT FEE $110 • OCT FEE $77 • PRESSURE $15 • PACHYMETRY $25 • VISUAL FIELD $50
Primary Open-angle Glaucoma
Intraocular Pressure (IOP). Although IOP is no longer considered a diagnostic criterion, POAG is more likely to occur at a higher IOP. Multiple randomized, controlled trials have shown that a reduction in IOP slows the progression of visual-field defects and prevents the onset of POAG. IOP is now thought to be one of many factors that cause optic neuropathy leading to glaucoma. Many people with glaucoma have normal IOP and, conversely, some with elevated IOP show no signs of optic neuropathy. IOP undergoes diurnal variations, and elevation in IOP is suspected to be worse after falling asleep. Since pharmacologic treatment of POAG focuses on lowering IOP, an understanding of the process would be beneficial to pharmacists.
Aqueous humor (AH) is produced by the epithelium of the ciliary body and is used to supply nourishment to the cornea and lens. AH is secreted into the posterior chamber, flows into the anterior chamber, and then drains from there. A decrease in the outflow of AH from the anterior chamber increases IOP. There are two mechanisms by which AH is drained from the anterior chamber, the conventional and unconventional pathways. The conventional pathway involves the outflow of AH through Schlemm's canal. The trabecular meshwork controls the flow of AH into Schlemm's canal and ultimately the bloodstream. The unconventional pathway is a collection of pathways and involves the seepage of AH through optic tissues. The most common of these pathways is the uveoscleral route (Figure 1).
Conventional and Unconventional Aqueous Humor Pathways
Optic Nerve Damage. Retinal ganglion cell axons converge in the optic nerve head and exit through the lamina cribrosa. The degeneration of retinal ganglion cells in the optic nerve head is the end result of multiple processes and ultimately leads to vision loss. Increased IOP can lead to stress on the retinal ganglion cell axons by reducing the flow of important neurotrophic factors for the axon's function. This stress also leads to the release of degenerative substances such as tumor necrosis factor alpha (TNF-α). This causes damage to retinal ganglion cell axons. As retinal ganglion cells begin to die, the nerve fiber layer begins to thin and the cup at the top of the nerve head begins to increase in size. The lamina cribrosa begins to bow as well, and this increases the amount of cupping. The cup-to-disc ratio is used to assess optic neuropathy.
ACG is characterized by blockage of AH outflow due to the closure of the angle between the iris and cornea. This closure results in the pinching-off of access to the trabecular meshwork. The decrease in outflow can cause a gradual increase in IOP or, more commonly, a rapid increase in IOP, leading to pain and permanent vision loss.
Intraocular Pressure (IOP). Although IOP is no longer considered a diagnostic criterion, POAG is more likely to occur at a higher IOP. Multiple randomized, controlled trials have shown that a reduction in IOP slows the progression of visual-field defects and prevents the onset of POAG. IOP is now thought to be one of many factors that cause optic neuropathy leading to glaucoma. Many people with glaucoma have normal IOP and, conversely, some with elevated IOP show no signs of optic neuropathy. IOP undergoes diurnal variations, and elevation in IOP is suspected to be worse after falling asleep
Glaucoma is described as optic nerve damage that leads to visual dysfunction. Generally, POAG is bilateral, asymmetric, and asymptomatic until significant peripheral vision loss occurs. ACG could include prodromal symptoms. It could also be an acute situation with distinct symptoms.
A thorough patient history should be conducted at the start of every comprehensive adult eye evaluation. Visual acuity is measured at both near and far distances. Both pupils are assessed for restricted constriction of the affected pupil(s). An anterior segment examination is conducted to assess the integrity of the cornea and anterior and posterior chambers. Tonometry measures IOP to determine the level of pressure elevation, if present. The time of day and instrument used are recorded due to diurnal variations. A goniolens (gonioscope) measures the angle between the iris and cornea in order to differentiate between POAG and ACG. The optic nerve head and retinal nerve fiber layer are examined for characteristic changes with glaucoma. The cup-to-disc ratio is used to assess the amount of glaucomatous atrophy, and anything >0.5 is suggestive of atrophy. Perimetry measures the visual field and is used to judge the extent of peripheral vision loss. Although glaucoma is a bilateral process, the level of severity is not necessarily congruent between both eyes
Pharmacologic Treatment( 1st Option)
Medications for the treatment of glaucoma are aimed at lowering IOP through two mechanisms, decreasing AH production and increasing AH outflow. It is recommended that IOP be lowered to a target level. That level is generally 20% below the baseline as measured several times. Prostaglandin analogues and beta-blockers are currently the most frequently used agents. Due to their once-daily dosing and effectiveness, prostaglandin analogues are generally selected as first-line options in treatment.
Beta-blockers. Topical beta-blockers are one of the most commonly used classes of medications in the treatment of POAG. They produce an IOP-lowering effect by reducing the production of AH by the ciliary body. Local side effects that occur with the beta-blockers consist of stinging, burning, irritation, inflammation, and blurred vision. The local side effects are normally minor. Side effects are generally short-lived and will go away in time.
Systemic side effects with beta-blockers are rare, but it is important to be aware of them. These include bradycardia, hypotension, bronchospasm, serum lipid changes, and masking of hypoglycemia. The nonselective beta-blockers are contraindicated in patients with asthma or chronic obstructive pulmonary disease (COPD) due to their ability to induce bronchospasm. All topical beta-blockers are contraindicated in patients with sinus bradycardia, secondor third-degree heart block, congestive heart failure, atherosclerosis, and diabetes. Topical beta-blockers should also be avoided in patients taking oral beta-blockers.
Prostaglandin Analogues. Topical prostaglandin analogues are usually the first choice by prescribers for POAG. In a large meta-analysis, it was concluded that prostaglandin analogues showed a greater 24-hour IOP reduction than timolol and other POAG medications. In a separate systematic review that included tafluprost, all of the prostaglandin analogues except tafluprost were shown to have significantly greater IOP reduction than timolol. It was also shown that bimatoprost was most effective at achieving a 30% reduction in IOP, which was the goal of the study. Prostaglandin analogues lower IOP by increasing the uveoscleral outflow of AH. It is suspected that bimatoprost also increases AH outflow through the trabecular meshwork. The prostaglandins are dosed once daily at night, where they have shown to be most effective.
Systemic side effects are rare with these medications. Local adverse reactions are more common and include conjunctival hyperemia, lengthening and darkening of eyelashes, irreversible altered iris pigmentation, cornea inflammation, and macular edema.
Adrenergic Agonists. Dipivefrin is a nonspecific adrenergic agonist. It is a prodrug of epinephrine and decreases IOP by increasing AH outflow through the uveoscleral route and trabecular meshwork. Dipivefrin is less effective at lowering IOP than other treatment options and is poorly tolerated. Therefore, it is rarely used. There are currently no commercial dipivefrin products available in the U.S.
Brimonidine and apraclonidine are alpha2 adrenergic agonists that lower IOP by decreasing AH production. Brimonidine also increases uveoscleral outflow. Brimonidine lowers IOP at a similar level to timolol and can be considered as an initial therapy. Brimonidine can be used as a monotherapy or as adjunctive to beta-blockers or prostaglandin analogues, while apraclonidine is considered to be a second-line agent. Both are indicated for the treatment of glaucoma.
An ocular allergic-like reaction is the most common side effect with these agents. It occurs at a lower rate with brimonidine, but it is still significant and generally causes these medications to be discontinued. Other local events include burning, stinging, and irritation upon administration. Systemic side effects are rare, but dizziness, drowsiness, and dry mouth may occur. Caution should be used with coadministration of central nervous system (CNS) depressants, as these glaucoma agents could exacerbate CNS depression.
Cholinergics. Cholinergic agents lower IOP by increasing AH outflow through the trabecular meshwork. These agents are rarely used because of multiple daily dosing and adverse effects. The local adverse events include retinal detachment, myopia, miosis, and pupillary block. Systemic side effects would be typical cholinergic effects such as sweating, salivation, nausea, vomiting, diarrhea, and bradycardia.
Carbonic Anhydrase Inhibitors. These agents lower IOP by blocking the secretion of sodium and bicarbonate ions into the AH, thereby inhibiting the production of AH. Topical formulations (brinzolamide and dorzolamide) are considered for monotherapy or adjunctive therapy in those who do not achieve effective control with other medications. These are well tolerated, with adverse events including transient burning and stinging, blurred vision, tearing, and corneal edema. Dorzolamide is reported to produce more stinging than brinzolamide, while brinzolamide can cause more blurred vision than dorzolamide. Systemic effects are rare and are generally caused by oral formulations, although topical formulations can cause altered taste.
Oral formulations (acetazolamide and methazolamide) are used for those who do not effectively respond to maximum topical therapy. Systemic side effects are common and include malaise, depression, metallic taste, anorexia, diarrhea, kidney stones, metabolic acidosis, and decreased libido.
Cholinesterase Inhibitor. The cholinesterase inhibitor echothiophate widens the trabecular meshwork by inhibiting the destruction of acetylcholine and lowers IOP by increasing AH outflow. Side effects are a major reason for limited use. These include fibrinous iritis, iris cysts, conjunctival thickening, and nasolacrimal occlusion. In addition, this drug has limited commercial availability and should only be used in nonresponsive patients.
Combination Products. Products that combine timolol with brimonidine or dorzolamide are available. A product that combines brimonidine and brinzolamide is also available. Combination therapy should be considered when initial therapy produces only a partial response. Advantages of using one product instead of two include less total preservative exposure, no washout effect, a single copay, and a possible improvement in adherence to treatment.
Preservative-free Products. Recent research has shown that preservatives like benzalkonium chloride (BAK) in eyedrops could be causing a significant increase in local adverse events, such as dry eyes. These studies also showed that preservative-free eyedrops lead to a significant decrease in local adverse events. Studies have also found equal therapeutic efficacy between preservative-free and preserved products. Products with multiple daily dosing are more likely to cause side effects because of an increase in the amount of BAK These studies looked at products available primarily in Canada and Europe. The only currently available preservative-free product in the U.S. is Cosopt PF, a combination of timolol and dorzolamide.
Absorption of certain medications, especially beta-blockers and drugs that can produce anticholinergic effects, may produce serious systemic side effects. Therefore, pharmacists should counsel patients on the proper instillation of eyedrops, including the use of punctal occlusion (pressing on the bridge of the nose to prevent the drops from entering the nasolacrimal duct). Simply closing the eye (but not blinking) is an equally effective alternative. Either of these procedures should be done for about 2 minutes.
Adherence is a problem with glaucoma patients. In one study, 45% of patients were shown to take <75% of their doses.
Nonpharmacologic Treatment(2nd Option)
Laser Trabeculoplasty. This procedure is considered in patients who fail to be adherent to medication regimens, are unable to administer eyedrops, or cannot tolerate topical medications. Trabeculoplasty uses a very focused beam of light to cause increased drainage through the trabecular meshwork. The effects of this may wear off over time and, therefore, long-term benefit is uncertain.
Trabeculectomy. This is the most common surgical procedure used to lower IOP. It involves creating a new pathway for the drainage of AH via bypassing the trabecular meshwork. Trabeculectomy is generally considered after topical agents and trabeculoplasty have been deemed insufficient at controlling IOP. It may also be considered as an initial therapy when IOP is extremely elevated.
Cyclodestructive Procedures. These are used when medicaland surgical treatments have failed and glaucoma is highly advanced. During cyclodestructive procedures, the ciliary body is intentionally damaged so that AH production is permanently reduced.