Overview of Tonometry
Tonometry is a normal process that eye doctors use to test the pressure in your eyes. This test measures the pressure inside your eyeball, called intraocular pressure (IOP), using a special machine. This measurement is important because it’s always the same all over the eye, whether in the front area or in the back.
The regular range for eye pressure is 10 to 21 millimeters of mercury (mmHg). This number comes from measuring the eye pressure in healthy people. One of the main conditions that can affect the pressure in your eyes is glaucoma. This is a serious eye disease that gradually damages the optic nerves, leading to problems in your eyesight, or even blindness in serious cases. There are many factors that can cause glaucoma. The most significant one is ocular hypertension (OHT), a condition where the eye pressure is more than 21 mmHg.
Doctors have several ways to treat high eye pressure and prevent further damage from glaucoma. These can range from eye-drops that reduce the pressure, to laser treatment, or even surgery. All these treatments aim to reduce the eye pressure to a safer level to avoid or slow down the progress of glaucoma.
Doctors have been using many ways to measure the pressure inside the eye since the 19th century. The most accurate way, however, is through a method called manometry, which involves inserting a probe into the eye. However, this method is too risky and impractical to use in daily check-ups.
Ever since the first tonometer, a machine used for measuring eye pressure, was built by von Graefe in 1863, many new methods have been proposed. These different ways are all based on principles of physics to provide an accurate, specific, and real estimate of the eye pressure. This is very important for managing the patient’s condition in routine examinations. The tonometers can be classified into different types based on how they measure the eye pressure. These include applanation tonometry, indentation tonometry, rebound tonometry, Dynamic Contour tonometry, and continuous IOP monitoring.
Anatomy and Physiology of Tonometry
The eye’s pressure, or IOP, is balanced by the production and drainage of a fluid called the aqueous humor. High eye pressure can come from too much fluid being produced, or not enough being drained. Most of the time, glaucoma, which is a condition that leads to blindness, happens when not enough aqueous humor is drained. There are many types of glaucoma, and most of them lead to high eye pressure. Some of the common types include primary open-angle glaucoma, closed-angle glaucoma, and secondary glaucoma. Also, there’s congenital glaucoma, which is something babies can have from birth.
Many types of open-angle glaucoma can silently threaten your sight. This is because they don’t affect your straight-ahead vision, don’t cause pain or irritation, and slowly limit the range of what you can see over many years in eyes with high eye pressure. Going without symptoms for a long time with high eye pressure can lead to irreversible damage to the optic nerve, thinning of the retinal nerve layer, and damage to your field of vision. That’s why tonometry, a method to measure the eye’s pressure, is very important. This is especially true for people who have a higher risk of developing glaucoma. Effective tonometers should be well-tuned, easy to use, and provide accurate results consistently. Ideally, they should not be affected by external and eye-specific factors.
Why do People Need Tonometry
Tonometry is a test that measures the pressure inside your eye, which is also known as intraocular pressure (IOP). It’s a key test to diagnose and manage glaucoma, a condition that can cause damage to your eye’s optic nerve and may lead to blindness if not treated. The test is usually done during a routine eye exam to check for glaucoma, especially if you’re at risk.
Besides glaucoma, it’s also important to measure IOP if you’ve had an eye injury that didn’t cause the eyeball to rupture, or if you have an eye infection or inflammation. If you’re going to have eye surgery, your doctor will measure your IOP before and after the procedure. Basically, keeping an eye on your IOP is crucial for managing any conditions that may lead to glaucoma and for slowing down the progression of any glaucoma-related damage to your eyes.
Here are some specific situations when you would need tonometry:
- As part of an eye exam to check for ocular hypertension (OHT) and glaucoma. This is especially important if you have known risk factors for these conditions.
- For managing patients who either have OHT, are already undergoing treatment for glaucoma, or are at risk of developing glaucoma in the future.
- To help diagnose and manage various eye conditions that can cause high IOP. These include pigment dispersion syndrome, eye infections, pseudoexfoliation syndrome (flakes of material coming off the outer layer of the lens in your eye), Posner-Schlossman syndrome (a rare condition with attacks of high pressure in one eye), inflammation of the eye, and uveitis (swelling and irritation of the middle layer of the eye).
- For people who respond to steroids with an increase in eye pressure, or those who’ve had eye surgery or trauma.
When a Person Should Avoid Tonometry
When you go for an eye examination, your eye doctor (ophthalmologist) may measure the pressure inside your eye. This pressure is known as IOP, or intraocular pressure. This information is very useful for a number of eye conditions. But there are some circumstances when it could be difficult to use the traditional methods to measure IOP, or even harmful. Here they are:
If your eye is ruptured or your doctor thinks it might be: Adding pressure to the eye could make the situation worse by causing further damage to the eye, and force out the fluid inside the eye. You’ll often know if your eye is ruptured if you’ve recently injured it and you’re in a lot of pain.
If there’s an eye infection present like keratoconjunctivitis in one eye, the doctor should make sure to clean the instrument or use a fresh cover before they measure IOP in the other eye. This is so that the infection isn’t passed from one eye to the other. And it’s important to do the same when using the same device for different patients, to avoid spreading infections.
For people with corneal abrasions (scratches on the cornea – the clear layer at the front of the eye), corneal ulcers, corneal scars, and for those with astigmatism (a condition in which the eye isn’t perfectly round but more oval): Tonometry can sometimes worsen these conditions and may also not give accurate readings when there is scarring or high astigmatism present, so it’s crucial for the doctor to choose the right type of tonometry in these situations.
If you can’t tolerate eye drops (used to numb the eye before tonometry) because of an allergy or any other reason, there are alternative methods to measure IOP that your doctor can consider which do not require these drops, like the iCare and “air puff” methods.
Some tonometry devices like the commonly used Goldmann device, require your co-operation and require you to be in an upright position. These devices may not be suitable for everyone, particularly if you’re bedridden or a young child. In such cases, devices that don’t require your active participation and can be used while lying down (like the iCare device) may be more suitable.
Equipment used for Tonometry
Applanation Tonometry is a method doctors use to measure the pressure inside your eyes, also known as intraocular pressure (IOP). This can be important for diagnosing and managing conditions like glaucoma. One of the main tools they use is the Goldmann Applanation Tonometer (GAT).
The GAT, invented by Hans Goldmann in 1948, is considered the gold standard for eye pressure measurements. It works by pressing a small, flat-tipped instrument against your eye to see how much force it takes to slightly flatten a part of your cornea (the clear, front surface of your eye). The more pressure inside your eye, the more force it will take to flatten it.
During a GAT measurement, your doctor will first put some numbing drops in your eyes, and then apply a fluorescein dye. They use a special blue light to make the dye glow, and a small instrument to flatten a part of your cornea. The light helps your doctor see a ring on your cornea created by the instrument. They adjust the amount of force used until the ring is a certain size, then read your eye pressure from the instrument’s scale.
There are many factors that can affect the accuracy of these measurements, such as the thickness of your cornea, the condition of your eye’s surface, and past eye surgeries. While the GAT is considered the best way to measure eye pressure, it is not perfect. For example, it requires skill to use correctly, and it can’t be used on patients who can’t sit upright at a slit-lamp microscope or on eyes with severely scarred corneas.
There are other similar instruments, like the Perkins and Draeger hand-held tonometers, which work on the same principle. These are useful in situations where a GAT isn’t suitable, such as in the emergency department, or in surgery settings. But, like the GAT, these also require skill to use and may provide less stable measurements.
There are also “air puff” eye pressure measurements, called non-contact tonometry (NCT). These blow a small puff of air onto your eye and measure the force of the air bouncing back. These are useful as they don’t need numbing drops and can be better tolerated by some patients. However, they are often not as accurate as the GAT test, particularly in patients with glaucoma or other risk factors.
Newer versions of air puff tonometers, like the Ocular Response Analyzer (ORA), also measure how the cornea moves in response to the puff of air. This additional information can help doctors better understand the risk of glaucoma progression. However, these are not useful on patients who cannot sit at a desktop instrument.
How is Tonometry performed
When measuring the pressure inside your eye, also known as the intraocular pressure (IOP), it’s important to take a few precautions. One of these is to make sure that the person measuring your eye pressure isn’t putting pressure on your eyelid, which might increase your eye pressure and give a false result. It’s also essential that you’re comfortable and not squinting during the measurement because that can also increase eye pressure.
Your position during this measurement is important too. Different devices used for this test might need you sitting up or laying down. For instance, a certain type of device called a Goldmann applanation tonometer can only be used if you’re sitting upright, while rebound tonometry (using a device called I-Care) can only be used if you’re horizontal.
Before and after your test, the tonometer will be cleaned and disinfected to prevent the spread of diseases and avoid any eye injuries from leftover debris. Some tonometers have disposable covers that keep them clean.
There are some other factors that might influence the accuracy of the eye pressure reading. The thickness and flexibility of the centre of your cornea (the clear front part of the eye) can impact the results. When the cornea is thin and flexible, the reading might be lower, and when it’s thick and rigid, it might be higher. Other things that can affect the accuracy of the test include astigmatism (an irregularly shaped cornea), holding your breath, and any natural changes in eye pressure throughout a heartbeat cycle.
The most trusted tool for measuring eye pressure right now is the Goldmann applanation tonometer (GAT). This device is especially useful for managing patients at risk of high eye pressure and glaucoma treatment. If any other tonometers give a high eye pressure reading, the GAT is used to double-check it.
Possible Complications of Tonometry
Various issues may arise from tonometry, a test often used to measure the pressure in your eyes. These complications include a scratch on the surface of the eye (corneal abrasion), worsening of a severe eye injury (globe rupture), getting an infection, or having a reaction to eye medicines. However, it’s important to note that the likelihood of these problems occurring is extremely low, usually less than 1%. If necessary, doctors can use contact-free methods or a technique called rebound tonometry. Even so, the traditional method (called Goldmann Applanation Tonometry or GAT) remains the best option in most cases.
What Else Should I Know About Tonometry?
Checking the pressure inside your eye (known as intraocular pressure) is important for keeping track of glaucoma. Glaucoma is a condition that gradually damages your optic nerve, which connects your eye to your brain, and can lead to vision loss. However, you can slow down the progression of glaucoma by reducing the pressure inside your eye. As of now, this is the only factor that we can change to control the progression of glaucoma.
Doctors can prescribe medications to lower the pressure in your eye, and they will monitor this pressure over time using a tool called a tonometer. Using this tool has proven to help slow the loss of peripheral vision (what you can see on the side when looking straight ahead) and central vision (what you see when you look straight ahead) due to glaucoma.
Tonometry can also help detect dangerously high eye pressure quickly, such as in conditions like acute angle-closure glaucoma and after eye injuries. Acute angle-closure glaucoma occurs when the liquid inside the eye is suddenly blocked, causing a rapid increase in pressure and is a serious eye emergency. If this happens, it’s important to quickly lower the pressure inside your eye to prevent loss of vision.
