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Pachymetry

Eye doctors will perform a wide range of exams during the glaucoma diagnosis process, however there is one exam that is critical to ensuring an accurate diagnosis

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.

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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.

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What happens during a pachymetry exam?

There are a number of different ways to perform a pachymetry test, ranging from smaller handheld ultrasound probes to larger scanning machines. 
With the handheld devices, the clinician will often numb your eye before delicately placing the small probe on your cornea, the front of your eye. On larger machines, patients will be set on a headrest in front of the device and have their corneal thickness measured through a laser beam.

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.

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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.