You’ve probably had your blood pressure taken at some point in your life – it’s a common procedure that helps doctors get an idea of how healthy your heart is.
But have you ever had your eye pressure checked?
Just like high blood pressure can be a sign of cardiovascular problems, high eye pressure (also called intraocular pressure or IOP) can be a sign of eye problems, such as glaucoma.
Glaucoma is a condition that results in damage to the optic nerve due to an increase in pressure within the eye. Over time, glaucoma can cause blindness if it goes untreated.
Fortunately, there are types of tonometers that can measure your eye pressure.
A tonometry test – which is an integral part of a comprehensive eye examination – can help your doctor determine if you may be at risk for glaucoma.
The type of tonometer the optometrists or ophthalmologists choose to use will depend on several factors, including the patient’s age and eye condition.
What Is A Tonometer?
A tonometer is a medical device used to measure the intraocular pressure (IOP) of the eye.
Intraocular pressure is a key factor in diagnosing glaucoma, and regular monitoring with tonometry tests can help catch the condition early.
There are a few different types of tonometers, but they all work in basically the same way.
They use a small amount of air pressure to measure the thickness of the cornea, and this reading is used to calculate the IOP.
They often measure in millimeters of mercury (mmHg).
This measurement helps doctors determine if someone has high eye pressure, which could indicate glaucoma.
The most well-known types of tonometers are the Goldmann applanation tonometer, which is considered to be the most accurate type, and the Perkins handheld tonometer.
Who Needs A Tonometry Test?
A tonometry test is a comprehensive eye examination to measure the intraocular pressure in the eye.
It is often recommended if a person is at risk for glaucoma.
Your healthcare provider may still recommend a tonometry test if you are experiencing any symptoms of glaucoma, such as eye pain, blurred vision, or difficulty seeing at night.
Even if you don’t have any symptoms, other factors can increase your risk of developing glaucoma.
These factors include:
- Family history of glaucoma
- Are African, Asian, or Hispanic descent
- Age over 40 years old
- Have high blood pressure
- Suffer from diabetes
- Use corticosteroid medications
- Are severely nearsighted
- Have had an eye injury or surgery
It is also recommended for people with other eye conditions, such as cataracts or uveitis.
Having any of these risk factors doesn’t mean you’re guaranteed to get glaucoma, but it does mean that your doctor will monitor your eye pressure closely with tonometry tests.
What Happens During A Tonometry Test?
During a tonometry test, the doctor or optometrist will apply a small amount of anesthetic eye drops to dilate your pupils.
This will help numb your eye and reduce the discomfort of the test.
Your doctor will then use a paper strip stained with orange dye to touch the surface of your eye. This is done in order to help increase the accuracy of the test.
Once your eye is numbed, they will place a “slit-lamp” in front of you.
Because of this, the doctor may have you rest your chin and forehead on supports while they move the tonometer toward your eye until it touches your cornea.
They’ll then flatten your cornea just slightly to get an accurate reading. You might feel pressure during this process, but it shouldn’t be too painful.
After the test is complete, your doctor may ask you to wait a few minutes for the anesthetic to wear off before you leave.
However, even if tonometry is a quick and easy process with minimal risk of danger, there are always a few risks associated with any medical procedure.
The most common risk is that the tonometer could scratch your cornea. This is why your doctor will apply the anesthetic eye drops – to minimize the risk of discomfort and injury.
Other risks include:
- Infection
- Bleeding of the eye
- Inflammation
Although these risks are rare, it’s essential to be aware of them before undergoing a tonometry test.
What Do the Tonometry Test Results Mean?
The tonometry test results will give your doctor a general reading of your eye pressure, but they will also have to do other types of tests to determine your risk for glaucoma accurately.
According to the Glaucoma Research Foundation, a normal IOP reading is between 12 and 22 mmHg. If your IOP is higher than that, you may be at risk of developing glaucoma.
However, it’s important to note that not everyone with high eye pressure will develop glaucoma.
Your doctor may also order a visual field test, optic nerve examination, and measurement of the thickness of your cornea to make a definitive diagnosis.
Sometimes your eye pressure may be high while your optic nerve looks normal, suggesting “ocular hypertension.”
In this case, your doctor would order further vision tests to determine the cause of the high eye pressure.
The doctor may still want to monitor how your eye pressure changes over time so they can determine whether or not it could lead to glaucoma. Or, they may recommend treating it right away with eye drops or surgery to prevent vision loss.
Types Of Tonometry
Your doctor may use a few types of tonometry to test your eye pressure.
Each type has its own benefits and drawbacks, so it’s important to understand the differences before going in for your test.
The most common types of tonometry are:
Applanation Tonometry
This is the most common type of tonometry. It’s based on the Imbert-Fick principle, which in short states that “the pressure inside an ideal, dry, thin-walled sphere equals the force necessary to flatten its surface divided by the area of flattening (P = F/A, where P = pressure and A = area).”
During this test, your doctor will place a small, flat tonometer tip against your cornea and gently flatten it with pressure to get an accurate reading.
The pressure is then recorded by the machine that also calculates your eye pressure based on the force needed to flatten your cornea.
However, it can be somewhat uncomfortable, and some people may feel a lot of pressure during the test.
Goldmann And Perkins Applanation Tonometry
Goldmann Applanation. This type of tonometry is considered the gold standard and is the most accurate way to measure your eye pressure.
It is mostly used to measure the force required to flatten the cornea.
The device is usually attached to a slit lamp microscope so your doctor can get a closer look at your eye while they’re taking the measurement.
During the test, your doctor will place the probe on your cornea and apply a small amount of anesthetic eye drops followed by fluorescein dye.
A small cobalt blue light is used to help illuminate the dye and the tonometer.
The doctor then gently presses the probe onto your cornea until it flattens, taking several measurements to ensure accuracy.
This test usually takes about 20 seconds and won’t hurt or require additional anesthetic eye drops.
Perkins Applanation Tonometry. This type of tonometry uses a similar measurement method as Goldmann, but it is portable handheld and utilizes the same applanation prisms.
It measures the pressure required to flatten your cornea by applying light pressure with a probe to the center of the cornea, but it’s important to note that Perkins tonometry does not attach to a slit lamp microscope as Goldmann types do.
This means it’s more difficult to gauge the quality of your cornea or apply anesthetic eye drops before applying pressure.
That said, Perkins types are mostly used for patients who:
- Are in the supine position
- Are unable to position in a slit lamp due to physical difficulties
- Aren’t being examined with a slit lamp in the doctor’s office
Non-Contact Tonometry (NCT) or “air puff” Tonometry
Widely known as the “air-puff” test, NCT is considered the most comfortable type of tonometry.
This type of tonometry is considered to be an alternative to Goldmann types because it can be used for patients who cannot hold still, are too young, or physically unable to remain stationary during the exam.
It’s also helpful when cataract surgery is planned in the future due to the danger of corneal abrasions that can occur with traditional types of tonometry.
During this test, the device will emit a short puff of air onto the surface of your cornea to flatten it, and an automated machine measures the time it takes for the air to bounce back.
This means it is a non-invasive technique that doesn’t touch your eye at all.
That said, NCT is not as accurate as Goldmann types and should only be used as a back up measure if other types of tonometry are unavailable.
The most common type of non-contact tonometry is the ocular response analyzer.
Ocular Response Analyzer
The ocular response analyzer is a relatively new, non-invasive type of tonometry that uses a column of air to measure the force required to applanate the cornea.
It works by emitting an increasing air pressure onto the cornea until it’s indented.
The machine then records the difference in pressure at the two applanation points.
This difference in pressures is used to calculate the corneal elasticity (i.e., hysteresis).
Additionally, the ocular response analyzer is quick and easy to use. It can also adjust for corneal thickness and help monitor glaucoma progression over time; that’s more accurate by considering the biomechanical properties of your cornea to calculate an intraocular pressure (IOP) measurement.
The drawback with this type of tonometry is that it is not a good option for patients who:
- Have high eye pressure
- Are taking medication that can affect their vision for up to 24 hours after treatment
- Have a cataract
- Do not have good eye coordination or are unable to follow instructions while the test is being conducted accurately
Indentation Tonometry
The Indentation Tonometer provides an effective means of assessing intraocular pressure (IOP).
The test is performed by placing a plunger containing a known weight on the central cornea and measuring the depth of corneal indentation made by the plunger.
The greater the force required to flatten the cornea, the higher the intraocular pressure. This means that a force will indent much further into a soft eye as opposed to a hard eye.
Also, note that extra weight is added to the plunger for very high IOP levels to increase the measuring force.
By increasing pressure with an increment, a graph is produced that can be used to estimate intraocular pressure (IOP) accurately.
The types of indentation tonometry include Schiotz types, pneumotonometer types, and the “Tono-Pen.”
Schiotz Tonometer
This type of indentation tonometer measures the depth of indentation of the cornea by placing a plunger with a known weight over the central cornea and then making measurements at different indentation levels to calculate IOP.
It consists of an open footplate, which is depressed against the patient’s eye, compressing the surrounding soft tissue.
The amount of force applied to the plunger by the footplate determines that amount of degree of indentation in the cornea created when the plunger is removed from the cornea.
This translates to IOP readings, which are calculated through a conversion table.
Pneumotonometer
A pneumotonometer is a handheld device that uses a controlled airstream to measure the IOP.
It consists of a silicone tip used to gently indent the corneal tissue with pressure from a controlled airflow.
The tip measures 5mm in diameter and is attached to a floating piston.
When the cornea and the tip are pushed forward, the pressure forces both surfaces flat, allowing intraocular pressure (IOP) to be recorded when an equilibrium point is reached.
The pressure found within the system is displayed in millimeters of mercury (mm Hg).
While the silicone tip is difficult to disinfect and can be a breeding ground for bacteria, this type of tonometry is still considered safe for screening purposes because it does not contact the eye.
Tono-Pen
The Tono-Pen is a handheld, battery-operated tonometer that uses applanation and indentation to measure the intraocular pressure (IOP) of a patient.
It combines a small plunger connected to a strain gauge that extends from an applanating surface with an adjustable footplate.
As the tonometer touches the cornea, the plunger is met with increased resistance from the eye.
As a result of this applanation, the force generated is shared between the plunger and the footplate, causing a momentary slight decrease in the rising pressure.
This decrease in the rising force is used to determine IOP based on empirical studies that assess the area of applanation.
Typically, the Tono-Pen is considered easy to use, safe, and reliable, requiring minimal training. It provides an estimate of IOP derived from the average of multiple readings with a standard deviation of the measures.
Additionally, it can be used on patients with irregular corneas. The only drawback of this device is that it may not be accurate for elevated or lowered IOP levels.
Rebound Tonometry
The rebound tonometry is yet another type of tonometry, which uses a magnetized probe to measure the deceleration of the probe after it gets in contact with the cornea to estimate the IOP.
This method of tonometry measures the force exerted on the probe by the cornea when it rebounds back to its original state after being compressed.
It is distinct from other forms of tonometry in that it presumes that individuals with a greater eye pressure or eyes with a harder cornea will experience a quicker deceleration of the probe than people with a low IOP or softer eye.
The greater the force that acts upon the probe, the more likely it is that a person has high eye pressure or hard corneas.
So, if the probe experiences a faster deceleration, it means that the person likely has high pressure, which is how IOP is measured.
The device then calculates the velocity in mm HG based on the probe’s deceleration.
Unfortunately, this device can be affected by other biomechanical properties of the cornea, such as corneal hysteresis and corneal resistance factor.
Pascal Dynamic Contour Tonometer
The Pascal Dynamic Contour Tonometer (DCT) is based on the Pascal principle, which states that the change in pressure is transmitted entirely throughout the volume of a fluid contained in an enclosed space.
Unlike other types of tonometers, the DCT measurements are less influenced by the corneal thickness and it utilizes a tiny piezoelectric sensor positioned in its tip to record the pressure changes that occur when the surrounding tissue expands and contracts with IOP.
The tip of the DCT also has a contour. When the contour is applied to the cornea, it causes the shape of the cornea to match or conform to the desired contour.
DCT is also found to be more effective than other types of tonometers because it can measure IOP trans corneally, thus eliminating the effects of ocular structures on the measures obtained.
In addition, the DCT can measure the ocular pulse amplitude.
Tips for Efficient Tonometry Test
No matter which type of tonometer is used, there are a few key things to keep in mind when taking tonometry measurements:
- Make sure that the patient is not wearing contact lenses. If they are, ask them to remove their lenses for about an hour before the tonometry test
- If patients have undergone cataract surgery, make sure to avoid testing near the incision site. The incision could be sealed with a solution or gel that would interfere with the tonometry reading
- Instruct patients to sit comfortably while they’re being scanned. This prevents them from paying attention to the tonometer tip, lowering their chance of flinching as it gets close to their eye
- Have patients maintain the opposite eye open wide and unblinking. If they clench, it will be more difficult for them to maintain that position
- The patient’s pupil should be fully dilated. If their pupil is constricted due to the light in the room, it could cause a falsely low IOP reading
- Encourage your patients to relax their faces and eyelids as much as possible because tension might interfere with pressure readings
- Make sure to clean the tonometer probe before and after each use with a sterile wipe. This way, you’ll prevent bacteria from getting into the eye and causing an infection
- Make sure the patient is looking ahead to avoid any potential changes in IOP measurements that could be caused by different gaze directions
- After obtaining a reading, check the cornea for any damage that may have been caused by the sensitivity to the tonometer probe. In this case, discontinue the use of the probe and try another method
- Lastly, always make sure to calibrate the tonometer before taking any measurements. This ensures that the readings are accurate
To Wrap Up
Tonometry is an important part of eye care because it can help to diagnose conditions like glaucoma. There are a few types of tonometers available, and each has its own benefits and drawbacks. And now that you know about these different types of tonometers, you can choose the one that is best suited for your needs. Remember to keep the patient’s comfort in mind and to follow the tips listed above for accurate readings.
Happy testing!
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WAGGONER A. (January 1, 2019). Tonometry types, tips, and tricks. Retrieved from, https://www.ophthalmicprofessional.com/issues/2019/january-2019/tonometry-types,-tips,-and-tricks
Troy L. Bedinghaus. (2020) How Tonometry Eye Pressure Test Works. Retrieved from, https://www.verywellhealth.com/tonometry-eye-pressure-test-3421842
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What Is Tonometry? Retrieved from, https://www.webmd.com/eye-health/what-is-tonometry
Frequently Asked Questions
Q: What Types Of Tonometers Can Be Found In An Ophthalmologist’s Office?
A: There are different types of tonometers that can be found in an ophthalmologist’s office – those types include the DCT, the Goldmann applanation tonometer, and the Perkins applanation tonometer. Each type has its own benefits and drawbacks. Speak with your ophthalmologist to determine which types will be the best fit for you.
Q: Why Is It Important To Use A Tonometer?
A: Tonometry is an important part of eye care because it can help to diagnose conditions like glaucoma. By taking tonometry measurements, you can get an accurate reading of the intraocular pressure (IOP) in a patient’s eye. This information can be used to help diagnose and treat glaucoma.
Q: How Often Should I Have A Tonometry Test?
A: It is recommended that people have a tonometry test every year. However, if you are at risk for glaucoma or if you have been diagnosed with the condition, you may need to have more frequent tests. Speak to your ophthalmologist to find out what’s best for you.
Q: What Should I Do If I Experience Pain During A Tonometry Test?
A: If you experience pain during a tonometry test, discontinue the use of the probe and try another method. It is possible that the probe was inserted too deep into the eye or that it is causing damage to the cornea. Speak to your ophthalmologist for further instructions.
