What is a Tonometer and the Types of Methods?
A tonometer is an instrument that measures the internal pressure of the eye, which is also known as intraocular eye pressure (IOP). An eye doctor (ophthalmologist) can use tonometry to diagnosis glaucoma - an eye condition where fluid accumulates in the patient’s eye, causing high eye pressure. The high pressure can damage the optic nerve at the back of the eye, and may even result in blindness if left untreated.
The IOP of 12 - 22 mmHg indicates that the patient is normal, and most people with the value above 20 mm Hg have a risk of glaucoma. However, eye pressure is unique to each person - so some people can develop glaucoma with the normal range of IOP readings.
People with glaucoma usually don’t have symptoms, but they can have problems with eyesight as they age. Therefore, it is essential to carry out a routine eye test, as it is the only way to determine if we have this eye condition. Early screening can help protect a patient’s eyesight, and prevent vision loss.
For most patients with glaucoma, they will only need eye drops to help decrease the pressure in their eye. In rare cases, laser treatment or surgery to help control their eye pressure and prevent blindness.
There are different methods and tonometers to measure the pressure inside a patient’s eye. Here are several methods that are available:
Applanation Tonometry
Goldmann tonometry is the most common type of applanation tonometry and is considered as the gold standard IOP test for many practitioners. There are several Goldmann tonometers: Type R which is attached to a slit lamp and Type T which can be fitted to the slit lamp when desired.
Anaesthetic eye drops are given to the patient to numb their eyes. The eye doctor will use a thin strip of paper containing an orange dye (fluorescein dye) to stain the surface of the patient’s eye – this helps to increase the accuracy of the test. Cobalt blue light is shone into the eye to illuminate the fluorescein dye. The slit lamp moves towards the eye until the tip of the tonometer probe gently touches and indents the cornea. The Goldmann tonometer can measure the pressure by flattening your cornea slightly - the eye doctor will need to adjust the tension until they get a proper reading.
Impression Tonometry
Schiotiz tonometry is the most common type of impression tonometry, also referred to as indentation tonometry. The Schiotz tonometer detects the eye pressure by measuring the depth of corneal indentation that’s caused by a small metal plunger that rests on the cornea. The higher the pressure, the harder it is to push and indent the cornea – extra weights may be added to make the plunger push harder. A calibrated scale measures the movement of the plunger. Modern eye care practitioners rarely use indentation tonometry, whereas general practitioners and the military occasionally use this method.
Non-Contact Tonometry
Non-contact tonometry is also referred to as the "air puff" test, is also a mass screening device. A gentle puff of air is blown into the patient’s eye, and the cornea is flattened without any discomfort. The non-contact tonometer records the IOP by detecting the change in light reflected off the cornea. Non- contact tonometers are ideal for busy clinical environments and are a quick and simple way to examine high-pressure levels in the patient’s eye. Children or sensitive adults prefer this method because it doesn’t touch their eye – but results aren’t as accurate as using Goldmann tonometry.
Electronic Tonometry
An electronic tonometer is a portable instrument that resembles a writing pen. The eye doctor gently uses the electronic tonometer to flatten the patient’s cornea. This method is not as reliable or accurate as Goldmann tonometry, but it is ideal for a busy practitioner because of the ease of use and portability. Also, an electronic tonometer is used to measure the IOP in children and non-cooperative adults as it is a non-invasive method.
What Tonometer Should I Buy?
When deciding on what tonometer you should buy, it all boils down to the number of patients you’ll be treating and the level of diagnosis. If patients need a simple routine check-up, the non-contact tonometer and the electronic tonometer is ideal for rapid IOP analysis. For older adults or those who have signs and symptoms of any eye problems, the applanation tonometry would be a more reliable and accurate method to identify glaucoma. Ensure you have thought carefully what the best tonometer to purchase, or if you’re teaching ophthalmology at a college/university, make sure you buy the correct tonometer for your students to learn.