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I am going for my glaucoma tests. What should I do to prepare for them?
It is a good idea to read a little about glaucoma and carry a small cheat sheet with any questions/ apprehensions that you might have.
There are no specific instructions to be followed for your glaucoma investigations, but you should try and get a good night’s sleep before you go for your visual fields.
It is also advisable to have a light meal before you reach your doctor, since some tests can take time. You should also continue with all of your previously prescribed medications, unless advised otherwise.
In case your doctor has scheduled a water drinking test, you may want to carry a one litre bottle of water, flavoured or otherwise, if you do not want to use the drinking water available at the clinic.
In case a dilated retinal exam or an optic nerve photo has been scheduled, please do remember it will entail about forty five minutes of waiting and you may not be able to drive back alone from the hospital.
Glaucoma investigations are painless, but tiring and often time consuming. You will benefit from carrying something to read with you, as also your sense of humour.
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I have been advised a gonioscopy. It sounds scary.
This is what your doctor sees
Your doctor must have noticed that the anterior compartment or anterior chamber
of your eyes is shallow. Alternatively, you would have been diagnosed with glaucoma, and your doctor needs the information in order to classify the disease, and decide on the future course of action.
In case you have a history of trauma to the eye, the doctor would be looking to see if the trauma has resulted in compromise of the drainage angle of the eye, called angle recession. In case you have diabetic changes on your retina, your doctor may look for new blood vessels in the angle of the eye.
Gonioscopy is a painless procedure, though it is known to be slightly uncomfortable.
Your doctor will put local anesthetic drops in both your eyes which might sting a bit. Thereafter,
you will feel no pain. He/ she will then place a lens in your eye and instruct you to look straight ahead, or towards a particular direction. You will need to keep both your eyes open and look in the direction instructed. The doctor will switch off the lights during the procedure for more accurate results.
Most people tolerate this procedure remarkably well, in some people it may stimulate one the nerves (Vagus) and result in a temporary syncope or giddiness.
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I have to get a visual field. It isn’t an easy test, what are my options?
The visual field test is done to determine your peripheral vision, and is required to diagnose, and monitor glaucoma over time.
A spot that is repeatedly presented in different areas of your peripheral vision, and each time you see it, you are required to click a button. Your doctor will ask you to keep looking ahead at the fixation light, and not look at this moving light, and press the button even if you think you see it.
The visual field test will often frazzle and tire you. You will be tempted to click the button fast in order to finish the test. The result is counter-productive. In case you find yourself getting tired, feel free to tell the operator that you’d like a break.
In case you feel your eyes going dry, feel free to stop and blink, or even use your tear supplements before continuing with the test.
There is a learning curve for the visual field test, so your doctor may ask you to repeat the test to confirm your results. In case you are trigger-happy, or slow to react, the machine tells your doctor about more than acceptable false positives and false negatives, needing you to repeat the test.
In case of advanced glaucoma, your doctor might order a limited field exam for the central ten degree visual field in addition to your regular field test.
For now there isn’t an alternative to the visual field exam, and it remains the most important parameter for monitoring your glaucoma.
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I have been asked to get an OCT. What is that?
The Optical Coherence Tomogram (OCT) takes a very high resolution picture of the optic nerve and the nerve fibres in its immediate vicinity.
You will be asked to keep looking at a bright green star, while the machine takes the high resolution image of your optic nerve. The machine has a database of normal population, specific to your age, gender and ethnicity, to which it compares your optic nerve pictures. The results are presented in a colour coded diagram where the green is good, yellow caution, and the red represents a nerve fibre loss that requires attention.
This is a useful tool for monitoring and diagnosing glaucoma, and is the most important in picking up early disease, as well as monitoring patients with advanced disease.
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What is Diurnal Variation of Intraocular Pressure (IOP)? What are the alternatives available?
The pressure of your eyes undergoes a cyclical variation through the day and night, and this is called its diurnal variation. The peak eye pressures are typically recorded around 4 AM and doctors are worried that the eye pressures recorded during their office hours may miss the peak eye pressures in your eyes. They therefore advise a 24 hour eye pressure monitoring, where your eye pressures are monitored every two hours for a twenty four hour period. This is cumbersome and expensive, and so many doctors ask for a two hourly recording of your eye pressures during the time that their clinic is functioning.
An acceptable surrogate for diurnal variation is the Water Drinking Test. In this, your eye pressures are measured before you drink about 10ml/kg body weight of water over five minutes. Your eye pressures are thereafter recorded every fifteen minutes, for one hour. This gives an approximate idea about the peak eye pressures, fluctuation as well as the time taken for eye to regain the baseline eye pressures.
Continuous eye pressure monitoring:
See more here
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What is my target eye pressure? How does my doctor determine the target?
Your target eye pressure is the acceptable range of eye pressures that will ensure that your glaucoma remains controlled so as to avoid any functional loss of visual field; without compromising your quality of life.
Your doctor keeps into account your baseline eye pressures, race, age, gender, comorbidities and disease severity when arriving at this number.
Typically, the target pressures depend on disease severity, broad guidelines are as follows:
- Severe glaucoma:
- Moderate glaucoma
- Early glaucoma
- Ocular hypertension
Your doctor may also monitor your diurnal variation of IOP and want to decrease the fluctuation of eye pressures, as also peak pressures.
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How is the eye pressure measured?
The technical name for measurement of eye pressure is tonometry. Your doctor may instil local anaesthetic and a dye in the eye to measure your eye pressures with a contact process called Applanation Tonometry.
What your doctor sees during applanation tonometry:
Alternatively, the doctor may use a device called the air-puff or non-contact tonometer which uses a puff of air to measure your eye pressures.
Both processes are painless, and require you to cooperate with your doctor by keeping your eyes open, and not squeezing your eyes shut during the test.
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How can the doctor see my optic nerve?
The doctor can visualise your optic nerve by either an ophthalmoscope or by slit lamp microscopy. The doctor may want to dilate the pupil so as to better examine the shape and color of the optic nerve using a magnifying device with a light source called the ophthalmoscope.
Alternatively, the doctor might use a hand held lens of a high power to visualize your optic nerve on the slit lamp in a process termed slit lamp biomicroscopy.
What your doctor sees when examining your optic nerve:
