Pachymetry
What is it for?
Pachymetry is for measuring corneal thickness – specifically, a pachymeter measures the depth of the thinnest point of your cornea.
Hand-held pachymetry is traditionally used primarily to measure the central area of the cornea where the cornea is thinnest. It takes single point measurements of the corneal thickness.
Corneal pachymetry can also be mapped using optical devices such as the tomography scanners described above (MS-39, Pentacam, etc). The gold standard for pachymetry measurement by optical devices is an optical coherence tomography (OCT) scanner.
Very high-frequency (VHF) digital ultrasound is the most precise way to measure the thickness of the cornea. In addition, it provides 3D thickness profiles of the individual layers of the cornea, allowing improved diagnostic capabilities for both pre-operative and post-operative analysis.
Both tomography and VHF digital ultrasound provide important information about the shape of the cornea that is used to screen for abnormalities such as keratoconus.
What actually happens?
There are three main types of pachymetry:
Hand-held ultrasound probes: A topical anaesthetic drop is placed in each eye to numb the surface of the eye for approximately 15 minutes. The optometrist then sits or stands in front of you and gently holds open your eyelid. They will then gently touch a hand-held probe onto the surface of the eye to take readings of the thickness of the cornea.
Very high-frequency ultrasound 3D scanning: In VHF digital ultrasound, you have your eyes numbed with anaesthetic drops, and have your eye positioned over a watertight rubber eyepiece. The eyepiece is then filled with warm saline solution (like artificial tears). Several arc-scans of the entire cornea are made to generate the thickness profile of each individual layer within the cornea, most importantly the front layer of skin on the cornea (the epithelium). This scan can also be used to measure the thickness of the corneal flap following LASIK.
Optical pachymetry devices: You rest your chin on a padded support and stare straight into the examining instrument. The clinician sits in front of you and aligns the instrument by having you focus on a visual target. They will ask you to open your eyes widely to take a number of multi-coloured pictures that are printed and added to your medical record. The pictures are like maps, where the different colours show the thickness profile of your cornea and some optical devices can also measure the epithelium.
How does it feel?
Hand-held ultrasonic pachymetry is not painful – anaesthetic eye drops are used, so you will not feel the probe touching the eye.
During VHF digital ultrasound scanning, the eye is comfortable in the warm eye bath. The measurements are taken through sound waves that travel through the saline solution – so no instruments will touch the eye.
In optical pachymetry scans, the patient feels nothing (as in a topography/ tomography scan).
How does it benefit you?
Along with front and back surface tomography, measuring the thickness of your cornea and epithelium is one of the most important safety factors in laser eye surgery. Using a pachymeter together with a topography device provides very accurate data and ensures that the thickness of your cornea is within safety limits. Your surgeon uses these measurements to determine whether you are suitable for surgery and which type of treatment is the safest option for you.
VHF digital ultrasound scanning is also one of the most sensitive ways of diagnosing keratoconus – an eye condition that prevents patients from having laser eye surgery.
The most advanced and accurate pachymetry device is the Artemis Insight 100 VHF digital ultrasound scanner. This measures corneal thickness more accurately than OCT, tomography or handheld ultrasound machines. It produces a 3D image that displays the thinnest point of the cornea with great accuracy, and shows a profile of the depth of your cornea. This technology is currently only available in a handful of clinics around the world.
