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Optic Nerve - Structure, Function, Testing |
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Written by Dr. Hannah de Guzman
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Optic Nerve Structure and Function
The optic nerve is what connects the eye to the brain. The optic disc is the end of the optic nerve that can be seen within your eye when you eye doctor uses certain lenses and instruments. The optic disc can progressively change in appearance and progressively lose function when it is affected by glaucoma.
All of the approximately one million nerve fibers that receive light signals from the outside world pass through the optic nerve to reach the brain. In glaucoma the number of living, functioning nerve fibers decreases at a rate much faster than would occur through the normal aging process. Their death may be due to increased intraocular pressure (IOP), lack of blood flow, some other mechanism not yet discovered, or a combination of mechanisms. When the nerve fibers die they leave an empty space where they used to be. If enough nerve fibers die the empty space on the optic disc becomes visible to the ophthalmologist examining the patient.
Optic Nerve Testing
Because the optic disc tends to change in appearance over time in patients with glaucoma, your eye doctor may have optic disc photographs or optic disc imaging tests done. These tests enable your doctor to compare your disc appearance at every check-up with the baseline photos and/or imaging test results taken during an earlier visit. Having a basis for comparison makes it easier to detect change.
Optic disc photographs and imaging tests allow your eye doctor to see structural changes due to glaucoma. Visual field testing or perimetry allows your eye doctor to see functional changes. Various machines can be used for this purpose. The most commonly used method involves asking the patient to look at a central target while flashes of light or other kinds of visual stimuli are projected all around the central target. The patient has to press a button whenever a stimulus is seen. The machine then records and analyzes what stimuli were seen and what were not and the result is sent to the eye doctor for interpretation. The test is not painful and not too uncomfortable but it can be tiring or stressful due to the intense concentration required. It is helpful to get a good night’s sleep the night before your test. The test takes from 2 to 20 minutes per eye depending on the machine, the test program used, and the speed of the patient’s responses.
References:
- Ritch R, Shields MB, Krupin T (Eds). The Glaucomas, 2nd Edition. St. Louis, Missouri, USA, 1996, Mosby-Year Book, Inc.
- Epstein DL, Allingham RR, Schuman JS (Eds). Chandler and Grant’s Glaucoma, 4th Edition. Baltimore, Maryland, USA, 1997, Williams & Wilkins.
- South East Asian Glaucoma Interest Group. Asia-Pacific Glaucoma Guidelines. Sydney, Australia, 2003-2004, SEAGIG.
- European Glaucoma Society. Terminology and Guidelines for Glaucoma 2nd Ed. Savona, Italy, 2003, EGS.
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Last Updated on Sunday, 26 July 2009 19:49 |
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Primary Open Angle Glaucoma |
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Written by Dr. Hannah de Guzman
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Primary Open Angle Glaucoma aka POAG
Primary open angle glaucoma (POAG) develops due to the interaction of multiple factors some of which are known and others that are as yet undiscovered. Some of the known risk factors for glaucoma include intraocular pressure (IOP), age, genes, and race. POAG is the most prevalent type of glaucoma worldwide and is a leading cause of preventable, irreversible blindness worldwide (as opposed to cataracts which cause reversible blindness).
In the anterior chamber angle there is a sieve-like structure called the trabecular meshwork (TM) that is responsible for draining the fluid from within the eye. When the trabecular meshwork malfunctions fluid drainage slows down. This causes fluid, and therefore pressure, to build up within the eye. The increased IOP is believed to be the cause of the optic nerve damage in POAG.
Primary open angle glaucoma can occur over a wide spectrum of IOP. In some cases the IOP is within the normal range. This variant of POAG is called normal tension glaucoma (NTG), normal pressure glaucoma (NPG), low tension glaucoma (LTG), or low pressure glaucoma (LPG). In these cases the optic nerve damage is thought to be caused by an increased disc susceptibility to slight increases in pressure, disease of the blood vessels causing decreased blood flow to the optic nerve, or a combination of these and other unknown factors. Normal tension glaucoma patients often have a history of diseases involving the blood vessels such as diabetes, hypertension, migraine, atherosclerosis, ischemic heart disease, and stroke.
POAG Symptoms
The symptoms of POAG in the early and moderate stages of optic nerve damage are usually so subtle that the vast majority of patients don’t notice them. These can include poor dark adaptation or night vision, frequent bumping into objects at the sides, and difficulty tracking fast-moving objects (e.g. golf ball or tennis ball). It is only when the POAG is at a more advanced stage that the patients are likely to notice poor peripheral vision and sometimes even poor central vision.
POAG Diagnosis
The diagnosis of POAG is made by assessing the patient’s history pertaining to risk factors, the ophthalmic examination, AND any diagnostic tests that were done (e.g. visual field test). Unlike in other diseases there is no single test or exam finding alone that can diagnose POAG. It not like diabetes mellitus, for example, where one or two blood sugar level determinations are often enough to make the diagnosis.
POAG Treatment
Medications, laser and surgery can all be used to treat POAG by lowering IOP. Which method to use first and how to combine the various methods is a decision made based on factors affecting the individual patient such as severity of the glaucoma, age, lifestyle, preference, economic status and the likelihood of progression to visual impairment. Most doctors prefer to use eye drops as their first choice when treating POAG but circumstances vary from patient to patient and the doctor may sometimes decide to use laser or surgery as the initial treatment. Aside from eye treatments, POAG patients who are smokers should stop smoking and those who are hypertensive or diabetic should strive for better control of their medical conditions.
Primary open angle glaucoma patients should not be worried about losing their sight overnight. Because of the very slow progression of optic nerve damage it takes years or even decades of no treatment or inadequate treatment for the glaucoma to reach the point where sight is completely and irreversibly lost. Only a small percentage of POAG patients become bilaterally, completely blind although many become visually impaired. Early detection, appropriate treatment, and patient compliance with treatment can slow down the rate of progression of damage so that it equals the normal rate of optic nerve deterioration due to aging.
POAG patients need to undergo periodic monitoring of their condition at the doctor’s clinic. The eye doctor looks for changes in the optic nerve and in other parts of the eye and checks the intraocular pressure. The eye doctor may occasionally request for a new visual field test to be able to compare the new test results with the older results.
People with first degree relations with POAG, the elderly, people who are myopic (near-sighted), those with diabetes or hypertension, and those with any other risk factors for glaucoma should have their eyes checked. Even if an initial screening shows no signs of glaucoma the screening should be repeated periodically as the person ages because of the increased risk of POAG with increased age.
References:
- Goldberg I. How common is glaucoma worldwide? In: Glaucoma in the 21st Century. London, UK. 2000, Mosby International Ltd.
- Ritch R, Shields MB, Krupin T (Eds). The Glaucomas, 2nd Edition. St. Louis, Missouri, USA, 1996, Mosby-Year Book, Inc.
- Epstein DL, Allingham RR, Schuman JS (Eds). Chandler and Grant’s Glaucoma, 4th Edition. Baltimore, Maryland, USA, 1997, Williams & Wilkins.
- South East Asian Glaucoma Interest Group. Asia-Pacific Glaucoma Guidelines. Sydney, Australia, 2003-2004, SEAGIG.
- European Glaucoma Society. Terminology and Guidelines for Glaucoma 2nd Ed. Savona, Italy, 2003, EGS.
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Last Updated on Sunday, 26 July 2009 19:50 |
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Intraocular Pressure, Intraocular Pressure Measurement |
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Written by Dr. Hannah de Guzman
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“Normal” Intraocular Pressure
One of the main risk factors for the development of glaucoma is a relatively increased intraocular pressure (IOP). The normal level of IOP varies from person to person. The statistically “normal” IOP is 16.5 mmHg (millimeters of mercury) with pressures above 20 mmHg considered statistically “elevated”. These values were obtained by averaging the IOP results of thousands of normal individuals with a wide range of pressures (from 10 to 35). For example, if a person with a usual IOP of 9 mmHg were to later have an IOP of 19 mmHg that would be a significant elevation even though it’s still a statistically normal IOP. Thus, a person may have an IOP that is increased relative to their normal IOP but is still not increased compared to the rest of the population (absolute increase). Ophthalmologists always keep this in mind when making the diagnosis of glaucoma and deciding what IOP to aim for when treating glaucoma patients.
Intraocular Pressure Fluctuation
The actual level of IOP is not the only aspect of IOP that matters in glaucoma. Our IOP normally fluctuates throughout the day. Glaucoma patients tend to have a wider daily fluctuation in their IOP (difference between peak IOP and trough IOP). This is called the diurnal variation. The IOP reading taken at the doctor’s office is the measurement for that specific moment only and does not indicate what the IOP levels have been the rest of the day. This is why glaucoma patients or glaucoma suspects are sometimes asked to have several IOP measurements taken in a single day. This is procedure is called phasing or diurnals.
Intraocular Pressure Lowering
Intraocular pressure is not the only risk factor for glaucoma but it is the one that ophthalmologists (and some glaucoma patients, too) are most preoccupied with. This is because IOP is the only risk factor that we are able to modify. We have many drugs and treatments that can lower IOP but there are no treatments (yet!) that can decrease the effects of aging or that can change a person’s genetic make-up. (Indeed, gene therapy may already be available for certain diseases but, unfortunately, research has not yet even uncovered all of the genes that predispose to the various types of glaucoma.) Evidence has shown that lowering intraocular pressure and decreasing intraocular pressure fluctuation are effective for preventing glaucoma progression.
Intraocular Pressure Measurement
Intraocular pressure can be measured using various instruments. The current gold standard instrument for measuring IOP is the Goldmann applanation tonometer (GAT or AT). It is called the gold standard because it gives the most accurate, reliable, and reproducible measurements. It measures IOP by flattening an area of the cornea and measuring the amount of pressure needed to flatten that area of the cornea. Because the instrument needs to come into contact with the eye your doctor will first put anesthetic drops to numb your eye and fluorescein dye drops to make your tear film more visible during the measurement process. The other instruments for measuring IOP work in different ways and have varying degrees of accuracy. There are promising new IOP-measuring instruments being studied and being introduced into the market and one of those instruments may one day replace the AT as the gold standard for measuring IOP.
Central Corneal Thickness and Pachymetry
Most methods of IOP measurement including AT can be affected by corneal thickness. An unusually thick or thin cornea can cause the IOP measurements to be erroneously high or erroneously low, respectively. A central corneal thickness outside if the normal range has also been associated with a difference in the risk of glaucoma or glaucoma progression. Those with thinner corneas seem to have a slightly higher risk of having glaucoma. If your doctor suspects that your corneal thickness will have a role in the management of your glaucoma you may be asked have your corneal thickness measured. This simple test is called pachymetry and there are different kinds of instruments that can be used. The most commonly available method involves lightly touching an ultrasonic probe to the anesthetized cornea for a few seconds.
References:
- Kahn HA et al. The Framingham eye study. 1. Outline and major prevalence findings. American Journal of Epidemiology 1977; 106:17.
- Heijl et al. Reduction of intraocular pressure and glaucoma progression: Results from the Early Manifest Glaucoma Trial (EMGT). Archives of Ophthalmology 2003; 121:48-56.
- Asrani et al. Large diurnal fluctuations in intraocular pressure are an independent risk factor in patients with glaucoma. Journal of Glaucoma 2000; 9:134-142.
- The AGIS Investigators. The Advanced Glaucoma Intervention Study (AGIS): 7. The relationship between control of intraocular pressure and visual field deterioration. American Journal of Ophthalmology 2000; 130:429-440.
- Collaborative Normal Tension Glaucoma Study Group. Comparison of glaucomatous progression between untreated patients with normal tension glaucoma and patients with therapeutically reduced intraocular pressures. American Journal of Ophthalmology 1998; 126:487-97.
- Kass et al. The Ocular Hypertension Treatment Study: A randomized trial determines that topical ocular hypotensive medication delays or prevents the onset of primary open-angle glaucoma. Archives of Ophthalmology 2002; 120:701-713.
- Ritch R, Shields MB, Krupin T (Eds). The Glaucomas, 2nd Edition. St. Louis, Missouri, USA, 1996, Mosby-Year Book, Inc.
- Epstein DL, Allingham RR, Schuman JS (Eds). Chandler and Grant’s Glaucoma, 4th Edition. Baltimore, Maryland, USA, 1997, Williams & Wilkins.
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Last Updated on Sunday, 26 July 2009 19:49 |
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Primary Angle Closure Glaucoma |
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Written by Dr. Hannah de Guzman
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Primary Angle Closure Glaucoma aka PACG
Primary angle closure glaucoma (PACG) is second to primary open angle glaucoma (POAG) as the most prevalent type of glaucoma worldwide but it is the cause of more blindness than POAG. Those who are at increased risk of developing PACG are people with smaller eyeballs, people who are hyperopic (far-sighted), females, the elderly, and some ethnic groups particularly those of Sino-Mongolian (Chinese) or Eskimo ancestry.
Primary angle closure glaucoma occurs when the iris bows forward and obstructs the anterior chamber angle. When the anterior chamber angle is obstructed by the iris, fluid can no longer drain out of the eye. Since fluid production continues as usual despite the impaired drainage, the fluid and the intraocular pressure build up within the eye. The increased IOP is believed to be the cause of the optic nerve damage in PACG.
The obstruction of the anterior chamber angle by the iris can occur suddenly or gradually. When the obstruction occurs suddenly this causes the IOP to increase suddenly and the patient may experience a combination of sudden eye pain, blurring of vision, redness, iridescent vision (seeing rainbows around lights), headache on the side of the affected eye, and nausea or vomiting. These attacks are usually precipitated by conditions which cause the pupil to dilate naturally such as dim illumination and episodes of extreme emotion. When the obstruction attack is prolonged and doesn’t resolve on its own this is called acute angle closure (AACG) and is considered an ocular emergency. The sooner treatment is started, the easier it is to treat and the better the outcome of treatment. Delaying treatment can lead to adverse consequences such as extremely elevated eye pressure that can only be relieved by surgery.
In some cases the obstruction attack is sudden but is also transient, resolves spontaneously, and recurs every so often. This is called intermittent angle closure (IACG) or sub-acute angle closure. While it is not an ocular emergency, the eye doctor must be consulted promptly because early treatment is more successful and repeated attacks of IACG can lead to permanent damage to the anterior chamber angle, permanently increased IOP, and the risk of IOP-related damage to the optic nerve if left untreated.
In many cases, the obstruction occurs gradually and the IOP increases so slowly that the patient does not experience any symptoms at all. This is called chronic angle closure glaucoma (CACG). The term CACG is also sometimes used for cases of AACG or IACG that were left untreated or were unsuccessfully treated.
PACG Diagnosis
The diagnosis of PACG is made clinically. After taking the patient’s history, the eye doctor examines the eye while paying special attention to the IOP and the anterior chamber angle. Closure or obstruction of the angle is visible through the cornea when a special lens (called a gonioscope) is used. Visual field tests are not always done initially but may be requested later in the course of treatment especially when optic nerve damage is suspected.
PACG Treatment
Laser treatment is usually the first step in treating PACG. The laser is used to relieve the obstruction by creating a hole in the iris (this creates an alternative fluid pathway) or by pulling the iris away from the angle. The laser treatment is also meant to prevent further attacks of angle obstruction. For cases that are diagnosed early laser treatment is often enough. In cases where the PACG is diagnosed at a later stage and some parts of the angle are already permanently obstructed, laser treatment may not be enough and medications or surgery may also be needed.
Even if laser treatment is initially successful, PACG patients still need to undergo periodic monitoring of their condition. Sometimes, laser treatments lose their effect as time passes. During check-ups, the eye doctor looks for changes in the optic nerve and in other parts of the eye, checks the intraocular pressure, and makes sure that the old laser procedure (if present) continues to serve its purpose. The eye doctor may occasionally request for a visual field test as an initial test and to be able to compare the new test results with older results.
Sometimes laser treatment is initially successful but another attack of angle closure occurs after a short time. This can occur if the original laser hole closes up slightly or in cases of plateau iris. Plateau iris is a special type of PACG in which the iris curves in a particular way that it is still able to obstruct the angle even after an initial laser iridotomy has already created an alternative fluid pathway. If another attack occurs after the initial laser treatment, it may need to be repeated or a different type of laser procedure may be needed to supplement the initial procedure.
PACG patients rarely lose sight overnight. Because of the slow progression of optic nerve damage it takes years or sometimes even decades of no treatment or inadequate treatment for the glaucoma to reach the point where sight is completely and irreversibly lost. However, once in a while patients can have attacks of severely increased IOP that disrupts the blood flow to the retina. This can cause sudden, severe visual loss that doesn’t improve even when IOP is lowered. This usually happens to those who are not being treated. More patients go blind from PACG than from POAG so early detection, appropriate treatment, and patient compliance with treatment and follow-up visits are much more critical in PACG. If diagnosed promptly and treated adequately it is possible to minimize IOP control problems and even prevent optic nerve damage from occurring in the first place.
PACG Risk Factors
The elderly, hyperopes, those with PACG symptoms, and those with other risk factors for PACG should have their eyes checked. Even if an initial screening shows no signs of glaucoma the screening may need to be repeated periodically as the person ages because of the increased risk of PACG with increased age.
Some prescription and over-the-counter medications can cause attacks of angle closure glaucoma. This is indicated on the package insert or the label of the medication. This is more likely to happen in those who are at risk of angle closure who have not yet had laser treatment than in those who are already under treatment. Those who are at risk of PACG should inform their eye doctor if they need to take a medication that lists glaucoma or angle closure as a side effect or a contraindication.
References:
- Goldberg I. How common is glaucoma worldwide? In: Glaucoma in the 21st Century. London, UK. 2000, Mosby International Ltd.
- Ritch R, Shields MB, Krupin T (Eds). The Glaucomas, 2nd Edition. St. Louis, Missouri, USA, 1996, Mosby-Year Book, Inc.
- Epstein DL, Allingham RR, Schuman JS (Eds). Chandler and Grant’s Glaucoma, 4th Edition. Baltimore, Maryland, USA, 1997, Williams & Wilkins.
- South East Asian Glaucoma Interest Group. Asia-Pacific Glaucoma Guidelines. Sydney, Australia, 2003-2004, SEAGIG.
- European Glaucoma Society. Terminology and Guidelines for Glaucoma 2nd Ed. Savona, Italy, 2003, EGS.
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Last Updated on Sunday, 26 July 2009 19:51 |
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