Pentacam® Corneal Tomography for Screening of Refractive Surgery Candidates: A Review of the Literature, Part I

Mahsaw N Motlagh, Majid Moshirfar, Michael S Murri, David F Skanchy, Hamed Momeni-Moghaddam, Yasmyne C Ronquillo, Phillip C Hoopes, Mahsaw N Motlagh, Majid Moshirfar, Michael S Murri, David F Skanchy, Hamed Momeni-Moghaddam, Yasmyne C Ronquillo, Phillip C Hoopes

Abstract

Corneal tomography and Scheimpflug imaging are frequently used to analyze the corneal surface, especially in the field of cataract and refractive surgery. The Pentacam system is one of the most commonly used commercially available systems for this purpose. Through a rotating Scheimpflug camera, the system is capable of creating a three-dimensional map of the cornea. These advances in tomography have simultaneously enhanced the ability of clinicians to screen surgical candidates and detect subtle corneal changes in diseases such as keratoconus. However, there remains a need to enhance diagnosis in order to recognize mild and early forms of corneal ectasia. As iatrogenic ectasia and keratoconus are dreaded complications of refractive surgery, it is imperative to screen patients appropriately prior to surgery. The Pentacam is one of many systems utilized in the screening process, but the literature has not identified specific protocol nor parameters that are capable of carrying out this process appropriately. Post-operative keratoconus continues to occur despite the advances in technology seen in corneal imaging. Therefore, clear indices for screening are required in order to diagnose early forms of keratoconus and other corneal diseases that may exclude the seemingly asymptomatic patient from undergoing refractive surgery. This article aims to summarize the indices available on the Pentacam system and to identify the most accurate parameters for screening of the refractive surgery candidate.

Keywords: Cornea; Keratoconus; Pentacam; Refractive Surgery; Tomography.

Conflict of interest statement

Ethical issues have been completely observed by the authors. All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship of this manuscript, take responsibility for the integrity of the work as a whole, and have given final approval for the version to be published. No conflict of interest has been presented.

© 2019, Author(s).

Figures

Figure 1
Figure 1
Example of normal (superior) and abnormal (inferior) Placido disk topography. Distortion of the Placido targets is useful in determining topographic changes seen in keratoconus such as inferior steepening
Figure 2
Figure 2
The inferior-superior (I-S) index is calculated as the difference between inferior and superior average dioptric (D) values. The average of five superior points (in blue) above the horizontal meridian are compared to the average of five inferior (in orange) points. The points are spaced in 30-degree intervals and are approximately 3.0-mm from the corneal vertex. Any value >1.4 is suggestive of keratoconus [11, 44].
Figure 3
Figure 3
Calculation of the skewed radial axis index, which corresponds to the angle (α) formed between the two steepest semimeridians above and below the horizontal meridian. The SRAX is equal to 180 minus the smaller of the two angles formed by the radii of the semimeridians. The below algorithm for calculating the composite KISA% index. Abbreviations: K: keratometry value; I-S: inferior-superior asymmetry index; AST: degree of regular astigmatism; SRAX: skewed radial axis index
Figure 4
Figure 4
Proposed ectasia spectrum with corresponding clinical characteristics table
Figure 5
Figure 5
Calculations for sensitivity, specificity, positive predictive value, and negative predictive value based on the presence or absence of keratoconus and refractive index
Figure 6
Figure 6
Area under the curve as determined by true-positive rate against the false-positive rate. Diagnostic accuracy is the area under the red line. The dashed line is equal to an area under the curve of 0.500, which is as accurate as a random guess
Figure 7
Figure 7
The role of refractive index cut-off as demonstrated by the occurrence of false-positives or false-negatives
Figure 8
Figure 8
The Pentacam Road Map. A step-wise algorithm that can assist in screening surgical refractive candidates

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Source: PubMed

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