Pachymetry comes from the Greek word Pachys, which means ‘thick’. Corneal pachymetry is the measurement of corneal thickness. There are several primary reasons why you would need to do this.
Why is the corneal thickness important in diagnosing glaucoma?
Glaucoma is a progressive optic neuropathy that can lead to blindness and one of its primary risk factors is high intraocular pressure (IOP). There are numerous studies that show a link between thinner corneas and a higher risk of developing glaucoma.
Also, tonometry (the measurement of the IOP) relies on an assumption of the patient’s corneal centre thickness (CCT). If the patient’s CCT is a lot different to the assumed thickness, the tonometer might significantly over or underestimate the IOP. This could lead to false positives and false negatives and may mean some people ‘slip through the net’ of glaucoma screening until they’ve lost some level of vision unnecessarily. Measuring CCT and then correcting the IOP result has been shown to provide a better diagnostic result and improves patient outcomes.
What other conditions can a pachymeter help diagnose?
Another condition that can cause sight-threatening damage is Keratoconus. This is a form of Ectasia (chronic thinning of the cornea). It typically develops in childhood and the teens and gradually there is central thinning of the cornea leading to misshaping and reduced vision (the cornea becomes ‘cone shaped’ – hence keratoconus). Screening for this is important and measuring CCT is a key tool.
There are conditions where the cornea is too thick, such as after excessive soft contact lens wear or from Fuch’s endothelial dystrophy. Again, screening for these problems requires pachymetry.
How do modern pachymeters work?
Modern pachymeters rely either on ultrasound devices which (handheld sometimes) contact the cornea and therefore require anaesthetic and risk cross-infection or corneal abrasion – including potentially Covid, or they are large desktop-based devices using Scheimpflug cameras, OCT or other optical methods. These tend to be expensive and non-portable products.
What happens during a pachymetry test?
There are two types of pachymetry tests: optical and ultrasonic (ultrasound). Here’s how each type works:
Optical pachymetry uses imaging systems, such as optical coherence tomography. You will sit in front of the machine, resting your head and chin for support. The equipment then takes measurements from images, without touching your eye.
For ultrasonic pachymetry, numbing drops are first applied to your eye. Your provider then uses a sterilized handheld pachymeter to briefly touch your cornea and take measurements. This method employs ultrasound waves to determine corneal thickness.
A pachymetry test is a valuable diagnostic tool for assessing corneal health and preparing for surgical procedures. Understanding how it works and what to expect can help alleviate any concerns and ensure you take informed steps towards optimal eye health.
What is the average corneal thickness?
In general the average corneal thickness measures between 540 and 560 micrometres. Cornea's over 560 micrometres are considered thick, and very thick corneas can be found in ranges above 600 micrometres.
In 2004 a study by G.F. Hyman & J.R. Mehta found that corneal thickness can vary by race, with results indicating that thinner corneas (below 540 micrometres) are common with the African-Caribbean population. With thin corneas, there is a higher chance of misdiagnosis from the underestimation of intraocular pressure - the fluid pressure in the eye. Conversely, there may be an overestimation of intraocular pressure when the corneas are very thick.
If your pachymetry test yields abnormal results, your provider will determine the next steps for treating conditions like glaucoma or preparing you for eye surgery. Ensuring the cornea is adequately thick can help minimize the risk of corneal ectasia, a condition where the cornea bulges forward.
Is there a non-contact pachymeter?
There is one device that is handheld and uses optical technology, thus removing the need for anaesthesia and avoids cross-infection risk. It is small, portable, and considerably more affordable than the larger desktop-based products. There is no risk of cross-infection or corneal abrasion because it is fully non-contact. Click the button below if you'd like to learn more.