Data extraction and reporting strategies of studies assessing non-central corneal thickness by Pentacam: A review

Highlights

• This review examines commonly used strategies to report Pentacam pachymetry data.

• Some data extraction and reporting strategies can obscure regional corneal thickness asymmetries.

• The chosen approach may influence the interpretation of pachymetry measurements and therefore clinical decision making.

Abstract

Purpose

To review data extraction strategies for the reporting of non-central corneal thickness with the Pentacam system.

Methods

Using predefined search terms, the electronic National Institutes of Health database (PubMed) was searched for studies assessing non-central corneal thickness using the Pentacam instrument. The article titles of the search results were screened for relevance. The abstracts of papers with appropriate titles were retrieved and read. Articles with relevant abstracts were obtained and read in full. The reference list of each article was hand searched to identify further studies. Articles reporting central corneal thickness only were excluded.

Results

Seventeen peer-reviewed studies were identified. Considerable differences in data extraction and reporting of non-central corneal thickness were noted, with non-central pachymetry being assessed at locations between 1.5 and 5.0 mm away from the centre. The terminology used to describe non-central pachymetry was inconsistent. Ring-averaged and single point pachymetry data have been reported. Ring-averaged pachymetry may obscure considerable regional variability in corneal thickness.

Conclusions

The use of different data extraction and reporting strategies can obscure regional corneal thickness asymmetries. This may influence the monitoring of corneal crosslinking outcomes, the interpretation of corneal swelling in contact lens studies and the clinical decision-making in preoperative assessments of refractive surgery patients. For the reliable identification of regional corneal thickness variations point-pachymetry data appears to be preferable over ring-averaged pachymetry.

Introduction

Non-invasive Scheimpflug imaging systems such as the Pentacam (Oculus GmbH, Wetzlar, Germany) provide global pachymetry maps, i.e. corneal thickness (CT) measurements along various corneal meridians. Central (CCT) and non-central corneal thickness (NCCT) are diagnostic parameters used to detect sub-clinical keratoconus in preoperative refractive surgery assessments [1]. NCCT has been considered in corneal crosslinking [2], [3] and may provide useful information for corneal surgery at non-central locations such as intra-stromal ring insertion and limbal relaxing incisions [4].

The Pentacam is a non-contact device that uses a monochromatic blue light (475 nm) and a rotating Scheimpflug camera, which revolves 360 degrees around the optical axis, to scan the anterior segment and its main features have been described previously [5]. In brief, either 25 or 50 images can be obtained per scan and the procedure takes up to 2 s. Prior to image acquisition, the patient is asked to place their chin onto a chinrest and their forehead against a bar and to fixate on a small round target that is located at the centre of the vertical blue light slit. The instrument is centred on the pupil, using real time monitoring on the instrument's screen. A second, camera is used to monitor fixation. The scan is usually initiated automatically, but manual acquisition can be selected. Following image acquisition, the observer can view a wide range of topographic maps. A detailed pachymetric map can be obtained by selecting the appropriate option on the Pentacam system. The pachymetry readings are calculated from the spatial difference between anterior and posterior corneal surface, usually perpendicular to the anterior corneal surface (normal-to-surface tangent measurement, personal communication with manufacturer) [6]. These calculations could be considered to approximate radial pachymetry measurements. The maps contain up to 81 single-point numerical pachymetry readings (Fig. 1) and further data points can be obtained by moving the mouse-based cursor over the map. Visual inspection of the colour-coded maps provides a gross overview of any central and non-central abnormalities. Additional features of the Pentacam software include pachymetry at 3 mm from the centre (Fig. 2) and the illustration of the averaged corneal thickness spatial profile as well as the percentage thickness increase from the centre to the periphery near the limbus, both of which are based on pachymetry readings from ring-averaged thickness along 22 imaginary circles that are centred on the thinnest point (Fig. 3, Oculus Pentacam Interpretation Guideline, 2nd ed) [1]. An advantage of the Pentacam over similar systems is that it can provide non-central pachymetry data up to 5 mm away from the corneal centre, whereas earlier corneal imaging instruments, such as Orbscan, routinely provide data up to 4 and occasionally 4.5 mm away from the centre. Three versions of the Pentacam are currently in clinical use, the Pentacam Basic, Classic, and HR (high resolution). The basic and classic models differ mainly in their software features and the HR model also provides additional hardware updates [5].

While non-contact computerized pachymetry systems are advantageous in reducing inter-observer measurement variability, the complexity of the pachymetry information may present observers with challenges related to the question as to which data should be extracted and compared. For example, observers have the choice between points (Fig. 1, Fig. 2) versus ring-averaged pachymetry (Fig. 3) and will also have to decide from which geographic location to obtain the measurements. The evaluation of peer-reviewed Pentacam studies that report on NCCT confirms that authors have indeed adopted a number of different strategies to extract and report non-central pachymetry measurements. This can be problematic as the use of different data extraction strategies could potentially obscure physiological variations such as changes in pupil size affecting the measurement [7]. Using ring-averaged pachymetry may lead to unexpected results in corneas of generally normal configuration and obscure regional variation in CT. The differences in data extraction and reporting not only make comparisons more difficult, but could potentially affect clinical decision-making, e.g. when assessing the effect of corneal crosslinking outside the corneal centre [2], [3]. To date, there has been little discussion regarding different data extraction strategies. The most commonly used methods in previous studies are single-point and ring-averaged data extraction (see below). Some researchers have provided pachymetry for up to 22 0.4 mm increments from the centre [1], others have reported CT measured at a single distance in close proximity to the central cornea, e.g. 1.5 mm away from the centre [8].

The purpose of this paper is to review data extraction strategies used in different studies to report non-central corneal thickness measurements, with a focus on single-point and ring-averaged data obtained with the Pentacam system.