Central corneal thickness (CCT) is an important measure for many conditions, including glaucoma. Research shows a thinning cornea increases a patient’s risk for progression from ocular hypertension (OHT) to primary open-angle glaucoma (POAG).1 CCT value also influences intraocular pressure assessment. However, the measurement tool you use can affect the result in some patient populations and not in others, according to a new study.2
While clinicians know optical and ultrasound pachymetry are not interchangeable, the standard ultrasound method poses significant barriers to its use. It require topical anesthesia, relies on examiner experience and necessitates a probe disinfection process. Optical pachymetry overcomes these challenges and is equally reliable, but provides a different measurement that cannot be compared with those of ultrasound pachymetry. Or can it?2
In young and healthy patients, ultrasound pachymetry tends to measure CCT higher than noncontact specular microscopy–based pachymetry, a team of German researchers explain in a recent publication of their findings. They showed the difference in CCT measurements between the two devices disappears in elderly patients (70.6 ± 10.7 years) and even reverses in glaucoma patients, with optical pachymetry recording higher values in that group.
The study included non-contact specular microscopy and ultrasound pachymetry readings 69 eyes of 41 glaucoma patients, 51 eyes of 32 elderly and 50 eyes of 25 young controls (27.2 ± 4.8 years). Young healthy patients had statistically significantly different CCT readings from each tool: 562.1±33.6μm for optical and 565.8±35.8μm for ultrasound pachymetry. Both elderly and glaucoma patients showed no statistical different between the pachymetry readings, although ultrasound provided a slightly thinner CCT measurement than optical pachymetry for glaucoma subjects. Optical pachymetry usually provides thinner readings than ultrasound, suggesting a slight reversal when measuring glaucoma patients, the study suggests. 2
The researchers conclude that the loss of any difference in CCT readings in these two patient populations must be due to the structural and biochemical changes from aging and the glaucoma disease process—meaning the devices could be interchangeable for these specific patients.2 Possibly, they suggest, an increase in collagen crosslinking and a change in the components of the extracellular matrix may develop “in a way that the speed of ultrasound in the cornea becomes higher and as a result CCT values become lower.”
1. Gordon MO, Beiser JA, Brandt JD, et al. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol. 2002;120:714-20.
2. Pillunat KR, Waibel S, Spoerl E, et al. Comparison of central corneal thickness measurements using optical and ultrasound pachymetry in glaucoma patients and elderly and young controls. J Glaucoma. March 6, 2019. [Epub ahead of print].