Overview of Applanation Tonometry

Tonometry is a test that measures the pressure inside your eyes, known as intraocular pressure (IOP). This test is important because it can help detect glaucoma. Glaucoma is a disease that gradually causes loss of vision and can ultimately lead to blindness if not identified and treated promptly.

Regular eye exams should include tonometry to screen not only for glaucoma but also other eye diseases. If you have glaucoma, high eye pressure, or are at risk for these conditions, your doctor would want to keep a close eye on your intraocular pressure.

Generally, normal eye pressure ranges from 10 to 21 mmHg, a unit of measurement for pressure. It’s based on the average pressure in the eyes of healthy individuals, with less than 2% having IOP greater than 21 mmHg.

Lower than normal eye pressure or hypotonus can be due to a variety of causes, including inflammation inside the eye (uveitis), eye injuries, detached retina, or complications after eye surgery. High eye pressure is typically a sign of glaucoma.

Previously, a diagnosis of glaucoma was simply based on an IOP reading greater than 21 mmHg. Nowadays, however, the definition has expanded to include observations of any functional abnormalities or physical defects in the eye.

Your doctor will work to identify a personalised target IOP range for you. They aim to establish a range that poses the least likely risk of damage to the optic nerve or loss of peripheral vision or deterioration of an existing problem due to elevated eye pressure.

If you are diagnosed with high eye pressure or glaucoma, your doctor may prescribe treatments to lower your eye pressure. Treatment plans can include the usage of eye drops, laser treatments, or surgery. As such, it is crucial to ensure the instruments used for measuring eye pressure are accurate and reliable, as these readings directly impact patient management plans.

The most direct way to measure true IOP involves inserting a probe into the front chamber of your eye. But as you can imagine, this technique is quite invasive, so it’s usually only performed in animal studies and not in routine clinical practice.

Different types of instruments called tonometers have been developed over time. All of these instruments aim to provide precise readings of eye pressure by establishing the amount of force needed to flatten or indent the surface of the eye. However, bear in mind that each tonometer type comes with its own set of considerations and potential influences on accuracy and precision.

Among the different types of tonometers, the Goldmann applanation tonometry (GAT) method is the most widely accepted and the gold standard in clinics. GAT measures eye pressure by determining the amount of force required to flatten a certain area of the cornea. The harder your eyeball, the more force it will take to flatten the corneal surface, hence, indicating higher eye pressure. Other forms of tonometers built around similar principles include the Perkins applanation tonometer, non-contact tonometers, and the Ocular Response tonometer (ORA).

Why do People Need Applanation Tonometry

Tonometry is a simple test that the eye doctor uses regularly to measure the pressure inside your eyes. This process is essential for:

* Regular check-ups to screen for glaucoma, a common eye condition that can cause vision loss if not treated.[7]
* Detecting and confirming a condition called Ocular Hypertension (OHT) and all types of glaucoma. OHT is when your eye pressure is consistently above the normal range, which can lead to glaucoma over time.[8]
* Keeping an eye on your eye pressure if you have OHT or glaucoma, to make sure your treatment is working.[9]
* Recognizing, treating, and diagnosing a variety of eye health issues that may cause a rise in eye pressure. These conditions could include Posner-Schlossman syndrome (a rare inflammation of the eye), uveitis (inflammation of the middle layer of the eye), long-term steroid use, or pigment dispersion syndrome (a condition where pigment cells clog the drainage area of the eye).[10][11]
* Checking your eye pressure before and after you have eye surgery.[12]

This test is also used to diagnose, manage and monitor patients who have an infection or inflammation in their eyes, or who have had an eye injury that did not result in a break or rupture of the outer layer of the eye (known as the globe).[13]

A tonometry test is a painless and quick method to check your eye health, providing vital information to your doctor to prevent, diagnose, and treat various eye conditions.

When a Person Should Avoid Applanation Tonometry

There are certain situations where a certain type of eye test, called applanation, should be avoided or used with caution because of the way it is performed. Some issues include:

The different methods used in this test involve touching the surface of the eye with the instrument tip or with a puff of air. So, it’s not safe for people with open wounds or injuries in the eye.

Two versions of this test, called the Goldmann and Perkins, involve using local eye drops for numbing and a dye called fluorescein. If a patient has problems with these substances, it’s better to measure their eye pressure with other tools that don’t involve touching the eye or using these substances.

There’s also a risk of spreading eye infection to the other eye or to other patients when doing this test. Therefore, tests that involve touching the eye or puffing air should be carefully cleaned, or better yet, avoided.

The Goldmann and Perkins tests involve looking at rings formed from a prism in the device after it touches the eye. So people with scars in the center of their eyes, irregular eye shape, unhealed scratches on their eyes, or ulcerations in the cornea should avoid having their eye pressure measured with these devices.

Most of these eye pressure measurement devices require the patient to be cooperative and sit upright, except for the Perkins device, which can be used when a patient is lying down and is portable. This could be challenging for children who may have difficulty cooperating, as well as for patients who are bedridden or unable to cooperate.

Equipment used for Applanation Tonometry

Applanation tonometry is a technique used by doctors to measure the pressure inside your eyes. This technique was established by two scientists named Imbert and Fick in the late 1800s. The method works on the principle that the force required to flatten the cornea, which is the clear front surface of your eye, can be used to calculate the internal eye pressure.

The first applanation tonometer, an instrument to measure the eye pressure, was built by Adolph Weber in 1867, but it wasn’t widely used because of its limitations such as being prone to errors and hard to use in a clinical setting. A much more effective version of this instrument was developed by Hans Goldmann in 1948. This instrument, known as the Goldmann Applanation Tonometer (GAT), is what your doctor likely uses during your eye exam and it is considered the gold standard for eye pressure measurement. The GAT predicts that corneal thickness and elasticity doesn’t vary significantly between individuals.

However, further studies suggest that corneal thickness, elasticity, and the liquid layer over the cornea can influence the measurements taken by GAT, leading to inaccuracies in some cases. For instance, eyes with thicker corneas have a tendency to display overestimated GAT readings, while eyes with thinner corneas can show understated measurements.

The GAT machine is small and lightweight. It needs to be attached to another machine, a slit-lamp, to take measurements. The machine needs to be checked periodically with a control weight bar to ensure it’s measuring accurately.

Some other types of applanation tonometry devices include the Perkins tonometer, the Air-puff tonometer and the Ocular Response Analyzer (ORA). The Perkins tonometer, invented by ES Perkins in 1965, is a portable version of the GAT and can be used in patients that cannot be placed in front of a slit-lamp. While this can be an advantage, it’s a more difficult instrument to use and requires more training.

The Air-puff tonometer, first proposed in the 1970s, blows a puff of air into the eye and measures the pressure based on how much the eyeball resists the air puff. It doesn’t require direct contact with the eye nor any anesthesia drops. However, its measurements tend to be less accurate compared to GAT, especially at high and low pressure levels.

The ORA is a newer version of non-contact tonometry developed by Luce in 2005, which not only measures eye pressure, but also examines the eye’s physical properties. This countertop instrument assesses eye pressure by using air impulses, and it also needs to be calibrated regularly to ensure its accuracy.

How is Applanation Tonometry performed

Contact tonometry is a procedure used to measure the pressure inside your eye, also known as intraocular pressure (IOP). This pressure is often measured using devices known as Goldmann Applanation Tonometer (GAT) and Perkins tonometers. This procedure involves these devices making direct contact with the clear front surface of your eye, known as the cornea.

The way the GAT and Perkins measure the pressure is identical, with the only difference being that the GAT requires the use of an equipment called a slit-lamp, which gives a magnified view of the eye. Perkins, on the other hand, is portable and can be used while the patient is lying down.

In this procedure, eye-numbing drops and a specialized dye are applied to your eyes. Following this, the tip of the tonometer, which has a special kind of prism, lightly touches the central portion of your cornea. The tonometer then calculates the force required to flatten a specific part of the cornea, which helps measure your eye’s pressure.

This procedure uses a blue light to identify the boundaries of the semi-circles made by the prism on the tip of the tonometer. The eye doctor then adjusts the force on the eye by turning a dial on the tonometer until two images line up on the device. The pressure inside your eye is then noted down.

While these methods are typically accurate, they do have some disadvantages. They can be more complex and time-consuming, and may be less accurate with scarred or irregular shaped corneas. Additionally, achieving accurate readings can be influenced by factors such as central corneal thickness, the flexibility of the cornea, thickness of tears, usage of the dye, a high degree of astigmatism, surgery history, and edema (swelling) of the corneas.

Non-contact tonometry (NCT) is another way of measuring eye pressure, which involves using a puff of air. The patient has to simply sit in front of the instrument, which then releases a small puff of air onto the eye, and the pressure is calculated internally through the device’s software.

NCT is quicker and simpler than the other methods as it does not require numbing drops or a specialized dye to be used. It’s a handy tool for certain patients and for screening procedures, but it’s not usually recommended for routine use, especially for patients with high eye pressure or glaucoma.

Possible Complications of Applanation Tonometry

The process of measuring eye pressure, known as applanation tonometry, should be approached with caution or avoided in certain situations. This is particularly true for a type called contact tonometers such as GAT and Perkins – these procedures involve direct contact with the cornea, the clear, dome-shaped surface that covers the front of the eye.

Special consideration should be given if the patient’s eyes have scratches (abrasions), serious injuries or, in extreme cases, burst (globe rupture). The procedure should also be avoided if there are signs of an eye infection or if the patient can’t tolerate fluorescein (a yellow dye used to examine the eye) or local anesthesia drops (medicine used to numb the eyes before the procedure).

What Else Should I Know About Applanation Tonometry?

Measuring the pressure inside your eyes (IOP or intraocular pressure) is crucial for finding, monitoring, and treating a condition called glaucoma. Glaucoma affects your vision by damaging the optic nerve at the back of your eye. It’s unfortunately irreversible, but with early detection and proper management, we can slow its progression.

Treatments like eye drops, laser therapy or even surgery might be applied to prevent the start or worsening of glaucoma. All of these treatments aim to reduce the pressure in your eyes, which is the only glaucoma risk factor we can actively control so far.

There are several ways to measure eye pressure, but the gold standard is the use of a tool called GAT (Goldmann applanation tonometry). In some special situations, we may use other methods, but in most cases, we stick with the tried-and-true GAT.

Frequently asked questions

1. How does Applanation Tonometry help in detecting and monitoring glaucoma? 2. What is the normal range for intraocular pressure (IOP) and what does it indicate if my IOP is outside of this range? 3. Are there any risks or limitations associated with Applanation Tonometry that I should be aware of? 4. Are there any alternative methods for measuring eye pressure that may be more suitable for my specific situation? 5. How often should I undergo Applanation Tonometry to monitor my eye health and the effectiveness of my glaucoma treatment?

Applanation tonometry is a procedure used to measure the pressure inside the eye. It involves applying a small amount of pressure to the cornea using a device called a tonometer. This procedure is commonly used to diagnose and monitor conditions such as glaucoma, and it can help determine the effectiveness of treatment options.

You may need Applanation Tonometry to measure your eye pressure, which is important in diagnosing and managing conditions such as glaucoma. However, there are certain situations where this test should be avoided or used with caution, such as if you have open wounds or injuries in the eye, problems with the substances used in the test, or if you have certain eye conditions or difficulties cooperating with the test. It is important to discuss with your healthcare provider if Applanation Tonometry is appropriate for you.

You should not get Applanation Tonometry if you have open wounds or injuries in your eye, if you have problems with the substances used in the test, if you are at risk of spreading eye infection, if you have scars, irregular eye shape, unhealed scratches, or ulcerations in the cornea, or if you are unable to cooperate or sit upright during the test.

There is no mention of a recovery time for Applanation Tonometry in the provided text.

To prepare for Applanation Tonometry, the patient should be aware that the procedure involves direct contact with the cornea, so it should be avoided if there are scratches, serious injuries, or signs of an eye infection. The patient should also inform the doctor if they cannot tolerate fluorescein or local anesthesia drops. Additionally, the patient should be aware that there are other methods to measure eye pressure, but the Goldmann applanation tonometry (GAT) is the most widely accepted and commonly used method.

The complications of Applanation Tonometry include: - Caution or avoidance in certain situations, particularly for contact tonometers such as GAT and Perkins. - Special consideration should be given if the patient's eyes have scratches, serious injuries, or burst. - The procedure should be avoided if there are signs of an eye infection. - The procedure should be avoided if the patient can't tolerate fluorescein or local anesthesia drops.

There are no specific symptoms mentioned in the text that would require Applanation Tonometry. However, the test is used for regular check-ups, detecting and confirming Ocular Hypertension and glaucoma, monitoring eye pressure, diagnosing various eye health issues, and checking eye pressure before and after eye surgery.

There is no specific information provided in the given text about the safety of Applanation Tonometry in pregnancy. It is recommended to consult with a healthcare professional or an eye doctor for personalized advice regarding the safety of this procedure during pregnancy.

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