BLOG: Is Pentacam equal to ultrasound probe for pachymetry?
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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.
The next closest device was the Pentacam, which scored well for observer reproducibility and minimal variability in the data (Lackner et al.). This can likely be explained by the way the Pentacam acquires its data. The device performs a series of sections (called Scheimpflug images) rotating around the optical axis, with each section running through the corneal vertex. As an aside, recall that both the Pentacam and Orbscan are scanning-slit topographers. They differ from the older style of placido disc topography in that they provide information about the posterior cornea as well as the anterior cornea. The Pentacam does a rotational movement, meaning every scan goes through the vertex of the cornea, and the Orbscan performs a translational movement (moving side-to-side), meaning only a few sections contain data from the vertex. For that reason alone you might conclude that the Pentacam would be more accurate for CCT measurements because there are more data points to use.
When comparing Orbscan CCT data to ultrasound, there is also something called the acoustic correction factor (AF) to consider (Radford et al.). Initial studies of the Orbscan found that it consistently overestimated CCT compared to US pachymetry. An AF of 0.92 was developed to apply to the Orbscan data to more accurately represent CCT values. This discrepancy may be due to the fact that Orbscan often incorporates the tear film in the corneal thickness values, which can add 7 to 30 microns to the total CCT measurement (Khaja et al.).
In retrospect, based on the data, I think using the Pentacam was a fine idea to measure our patients’ pachymetry. In fact, one might argue it’s superior or at least equivalent to US pachymetry. Remember, one reason US pachymetry is considered the gold standard is that it was first. And as I said in the first paragraph, just because we’ve done something the same way for a long time doesn’t mean there isn’t a better way. Remember, the US pachymeter isn’t infallible; its accuracy is dependent on the perpendicularity of the probe to the cornea and its reproducibility is dependent on how close it’s placed to the corneal center (Kawana et al.).
In the 1990s the Ocular Hypertension Treatment Study placed ultrasonic pachymeters in all their clinic sites involved in the trial (Brandt et al.). The reason they chose a particular model (DGH-500 Pachette) was because it was one of the only commercially available pachymetry devices at the time, and they wanted to only use devices of the same make and model to assure repeatability. Now that we’re getting deep into the 21st century, I think the data suggests that we can continue to use US pachymetry, but also be comfortable using CCT data from AS-OCT and Pentacam as well.
References:
Brandt JD, et al. Ophthalmology. 2001;doi:10.1016/s0161-6420(01)00760-6.
Kawana K, et al. Br J Ophthalmol. 2004;doi:10.1136/bjo.2003.030361.
Khaja WA, et al. Clin Ophthalmol. 2015;doi:10.2147/OPTH.S81376.
Lackner B, et al. Optom Vis Sci. 2005;doi:10.1097/01.opx.0000180817.46312.0a.
Radford SW, et al. Eye. 2004;doi:10.1038/sj.eye.6700669.