Central corneal thickness: Which way to measure?
Accurate measurement of central corneal thickness is crucial in current ophthalmologic examination. Preoperative evaluation in refractive surgery and accurate intraocular pressure (IOP) determination in glaucoma needs a correctly measured value of central corneal thickness (CCT).
Measuring CCT has been traditionally performed by ultrasonic pachymetry (UP) devices and these devices are accepted as gold standard in this application. Today, noncontact devices, such as optical low coherence reflectometry (OLCR), partial coherence interferometry (PCI) and anterior segment optical tomographies are also available for CCT measurement. These non contact devices indeed have a low risk of infection and trauma that might occur during ultrasonic pachymetry.
One might ask if these devices could replace the ultrasonic pachymetry for CCT measurement. Regarding optical low coherence reflectometry; in a recent study of ours, which has been published in Cornea,1 no statistically significant difference between OLCR and UP was observed.
In that study, we measured the bilateral CCT of 50 emmetropes with optical lowcoherence reflectometry (Lenstar LS 900, Haag Streit, Koniz, Switzerland) and ultrasound pachymetry (US 4000, Nidek, Japan). OLCR is an optical biometer. It uses an 820 Ám superluminescent diode with a Gaussian shaped spectrum to provide high axial resolution and the effect of time domain interferometric or coherent superposition of light waves to measure ocular distances. With this device, many optical components including CCT, anterior chamber depth, lens thickness, axial length, keratometry and pupil size can be measured.
We also wanted to measure the level of interexaminer reproducibility with the aforementioned devices and as such used two separate examiners who consecutively measured CCT in 16 eyes. We have found the intraclass correlation coefficient as 99.3% for Lenstar and 99.2% for ultrasound pachymetry.
We have observed both a high correlation (95.8%) and a high level of aggreement (r = 0.966) between two methods. Additionally, the reproducibility between different examiners was also high for both. Even if CCT measurements that are obtained with OLCR and UP demonstrate very strong correlation and agreement, and the amount of difference in recordings between these two instruments may be clinically acceptable, Lenstar and ultrasound pachmetry should not be used interchangeably for CCT measurements in the evaluation of patients with glaucoma or in suspect of glaucoma and in patients who are candidates for refractive procedures.