In his blog, Doug Rett, OD, FAAO, digs deeper into questions every eye doctor has had at one point in his or her career. He sees patients and works with students and residents at the optometry clinic for the VA Boston Healthcare System and will use his blog to bring the teaching points he sees in clinic to a larger audience. Contact him at

Disclosure: Rett has no relevant financial disclosures.

BLOG: Is Pentacam equal to ultrasound probe for pachymetry?

Several years ago, the hospital where I work decided that simply cleaning the ultrasound probe with an alcohol wipe after pachymetry wasn’t properly following the manufacturer’s instructions of our particular ultrasonic pachymeter (which technically calls for immersing the probe tip in alcohol for a full 10 minutes). The hospital asked us to send the probe down to the sterilization clinic for proper cleaning after each pachymetry use.

This prompted a lot of eye-rolling in the eye clinic (at least the proper place for such facial tics), and we pointed to the lack of any patient harm in the years and years that we’ve been doing it our way.

But honestly, over the years I’ve come to appreciate standard-operating-procedure changes like this, when they have the patient’s best interest at heart. At least there is a governing body here who is willing to face the wrath of annoyed doctor-grumblings and make a change in the clinic despite the inertia of “that’s-the-way-we-do-it-because-that’s-way-we’ve-always-done-it.”

So, our clinic started sending down the pachymetry probe to be properly cleaned after each use. But this led to an obvious problem: We can’t do pachymetry in the meantime until the probe is returned. So we bought a second pachymeter (only because the company didn’t sell probes individually, which bothered me to no end, but I digress...) but still had times when we were unable to perform pachymetry. Necessity being the mother of invention, I started using our Pentacam device (Oculus) to give central corneal thickness (CCT) measurements for our glaucoma suspect patients. This worked great and checked three boxes: gave accurate, repeatable and documentable CCT values; yielded a bonus corneal topography; and was noncontact, so no need to worry about sterilization. But a thought nagged at me: Are these results equivalent to the results from ultrasonic pachymetry? I thought this month we could review the data and find out for ourselves.

There are several studies that compare the Pentacam, the Orbscan (Bausch + Lomb) and/or anterior segment (AS) OCT to ultrasonic (US) pachymetry, and I’ve listed several in the references (not every study compared all devices). In many studies, the AS-OCT device was shown to have the best agreement with US pachymetry (Khaja et al.). This should not be too much of a surprise because the two devices have a similar mechanism of action: They both use reflection of waves to determine the depth of interfaces. Ultrasounds use sound waves, and OCTs use light waves, but they both work by wavefront technology. Realize that to measure CCT properly with OCT, you would need an OCT with AS software capabilities.