Refractive Surgery Pearls in Topography

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Introduction

Topography is derived from the Greek: “topos” (place) and “graphein” (to draw).  Corneal topography relates to a representation of the geometrical properties of the corneal surface it is a technique for measuring and evaluating its shape and curvature. [1][2] The advances in refractive surgery techniques have created the necessity for precise analysis of the corneal surface and shape. The conventional keratometer only measures close estimations of the anterior corneal curvature but is insufficient for the assessment of patients for refractive surgery. In recent years, rapid advances in corneal topography and tomography have paralleled those of excimer laser refractive surgery, and computerized corneal topography has become the standard of care in clinical practice.[1]

History

In the seventeenth century, Christopher Scheiner compared pictures formed by the impression of marbles on the cornea. [3] In 1880, Placido created a disk, involving the reflection of a series of concentric highly contrasting rings.[4] In the late nineteenth century, Javal and Schoitz, created the keratometer. This device focuses on the 3 - 4mm central area of the cornea, and assumes that the rest of the cornea has a sphero-cylindrical surface. More recently, the photokeratoscopes have been developed to assess the rest of the corneal surface. [1]

Preoperative Assessment

Diagnostic criteria for subclinical keratoconus.jpg

A fundamental component of the refractive surgery evaluation is the preoperative assessment of the cornea with topography. Its main goal is to detect epithelial irregularities and stromal abnormalities, evaluate corneal astigmatism, refractive stability or undetected corneal diseases.[5] It is essential that the patient maintains an adequate position and fixation in order to ensure a high-quality image.[1]

Laser refractive surgery is now widely used. In order to avoid ectasia secondary to the procedure, it is mandatory the detect subclinical keratoconus before surgery. There are several tools we can use to assess this disorder: corneal topography, pachymetry, and corneal aberrometry. Recent studies have reported tomographic measurements using optical coherence tomography and corneal biomechanical indices. Several factors can increase the risk of corneal ectasia after the procedure, such as age, keratometric indices, corneal elevation data, and corneal thickness.[6]

Incipient or subclinical keratoconus is usually asymptomatic and can be undetected in routine clinical practice. It is the most significant risk factor for the development of ectasia after laser refractive surgery.[7] To assess incipient keratoconus, it is necessary to use multiple parameters, algorithms and predictive models.[6]

Topographic maps

Dioptric values in the topographic map are represented with colors: warmer colors illustrate steeper curvatures (higher D), and cooler colors exhibit flatter curvatures (lower D). Near the periphery, normal corneas tend to flatten, and it is represented with cooler colors. Both eyes tend to have mirror topographies (enantiomorphism). [8]

Topographic patterns

In 1996, Rabinowitz et al., created a database of videokeratography patterns and quantitative indices based on the normal corneas of 390 eyes. They proposed 10 different topographical patterns.[9] They are:

Rabinowitz et al. Topographic Patterns

Studies have shown a good accuracy of the mean keratometry (K) in detecting keratoconus. The cutoff points between 45.2 D and 45.7 D have a sensitivity >80% and specificity >70%. This parameter is useful for differentiating subclinical keratoconus and normal eyes, [10][11][12] Astigmatism has been shown to be higher in keratoconus. The cutoff point is 2.5 D, it has an acceptable ability for the screening of keratoconus (sensitivity and specificity >75%). The specificity decreases in incipient keratoconus (<65%).[11]

Elevation map

Anterior Elevation Map “Regular Ridge” Posterior Elevation Map “Incomplete Ridge”
Elevation Maps

Elevation maps illustrate the variation in height within the measured corneal surface and some reference surface. The most common reference is a sphere, referred to as “best-fit sphere” (BFS). Other shapes include the "best-fit asphere (BFA) or best-fit toric asphere (BFTA)". The values lying above the reference surface are represented with warmer colors, while cooler colors indicate areas below it.[8] There are several patterns described for topography maps: “island, regular ridge, irregular ridge, incomplete ridge, and unclassified”.[13] Jafarinasab et al. studied the sensitivity and specificity of the anterior and posterior corneal elevation by Orbscan II topography (Bausch and Lomb, Rochester, NY, USA). They determined an optimal cutoff point for the posterior elevation of 51 µm for keratoconus and 35 µm for subclinical keratoconus. While for the anterior elevation the optimal cutoff points were 19 µm for keratoconus and 16 µm for subclinical keratoconus.[14] In Pentacam Scheimpflug corneal tomography, the posterior elevation: normal ≤+17 µm, suspicious +18 µm - + 20 µm and risky >+20 µm. In the anterior map: normal ≤+12 µm, suspicious +13 µm + 15 µm and risky >+15 µm.[15] Recently, the posterior elevation has been determined as an excellent predictor of keratoconus. Most of the studies have reported a sensitivity and specificity of >90%.[10][12] When detecting incipient keratoconus, there is significant variability. De Sanctis et al. detected a sensitivity of 73.3% and specificity of 86.5% for posterior elevation in subclinical keratoconus.[16] Jafarinasab et al. obtained a lower sensitivity of 50.00% and specificity 88.65%.[14]

Pachymetric map

Pachymetric Map
The distribution of corneal thicknesses is represented in pachymetric maps. It provides a global map of corneal thickness which eliminates errors due to probe decentration with the manual ultrasound. An accurate measurement is essential before refractive surgery, as it is an important tool in the diagnosis and progression of corneal ectasias.[8] It can be determined by the rotating Scheimpflug camera (Pentacam) and the Scheimpflug camera combined with Placido corneal topography (Sirius). These systems measure the corneal thickness at different points of the cornea.[6]

The pachymetric map can useful in detecting incipient keratoconus. The diagnostic criteria are a central corneal thickness (CCT) less than 500 mm, in addition to topographic asymmetry. Also, a study has shown a high probability (97.5%) of corneal ectasia when there is a difference between the central and minimum thickness of more than 27 mm.[17]

Although the pachymetric parameters can accurately detect keratoconus, there is variability in the cutoff points between different studies according to the sample or the device used. Therefore, we should not use this parameter alone to detect keratoconus. When detecting subclinical keratoconus, sensitivities, and specificities of less than 70% have been reported in all studies.[6]

Pachymetric measurements assessed with OCT are similar to those obtained with Scheimpflug systems.[18]The Optical coherence tomography can determine the difference between the thickness of the upper and lower area of the cornea. Also, it shows the vertical location of the thinnest point of the cornea. Studies have shown a high precision in detecting keratoconus.[6]

Reinstein et al. determined the total corneal thickness, as well as the epithelial and stromal thickness in patients with keratoconus. They found that there was a localized central thinning surrounded by an annulus of thick epithelium (epithelial doughnut pattern). The position of the thinnest point in the epithelium was displaced 0.48 ± 66 mm temporally and 0.32 ± 0.67 mm, and by 0.31 ± 0.45 mm and 0.45 ± 0.37 mm inferiorly in the stroma.[19]


Schallhorn et al., distinguished between corneal ectasia and contact lens-related warpage by OCT epithelial thickness and corneal topography maps. The keratoconus and forme fruste keratoconus eyes had coincident topographic steepening with epithelial thinning. Also, the locations of maximum axial power agreed with the thinnest epithelial value in 90% of the cases. On the other hand, the warpage eyes had topographic steepening with epithelial thickening and normal pachymetry. The maximum mean power agreed with the maximum epithelial thickness (93%). Topographic maps and epithelial thickness assess with optical coherence tomography are synergistic tools to differentiate both entities.[20]

Indices

The next indices are used to distinguish normal corneas from those with a pathology (keratoconus or forme fruste keratoconus) and are based on the pachymetry, anterior corneal curvature, and elevation patterns.[8]

Keratoconus percentage index (KISA%) value

The keratoconus percentage index (KISA%) value is obtained from 3 videokeratographic parameters:

  • Central keratometric power
  • I–S index
  • Corneal simulated astigmatism
  • SRAX index


The SRAX index has an accuracy of 99.6% in detecting keratoconus. The KISA % index varies from 60% to 100% in subclinical keratoconus.[21]

Randleman Ectasia Risk Score System

Randleman.jpg

In 2008, Randleman et al. studied the epidemiologic features of ectasia after refractive surgery. There were included 171 ectasia cases. They evaluated the preoperative characteristics, including: “age, gender, spherical equivalent refraction, pachymetry, and topographic patterns, type of surgery performed, flap thickness, ablation depth, and residual stromal bed (RSB) thickness”; and postoperative features: time elapsed since the appearance of ectasia. They created a “risk factor stratification scale”, considering each characteristic. This is an important method to assess risk of ectasia after LASIK.[22]

Randleman2.jpg

PTA Index

PTA Index

Santhiago et al., investigated the association between the percent tissue altered (PTA) and the development of ectasia in patients with normal topography, who underwent LASIK surgery. They included 30 eyes with normal topography that developed ectasia and another group of 174 eyes without this complication.

The equation for PTA is: PTA = (FT + AD) / CCT

(PTA) Percent Of Tissue Altered

(FT) Flap Thickness

(AD) Ablation Depth

(CCT) Preoperative Central Corneal Thickness

They concluded that PTA values below 40.0% correlate with the low incidence of ectasia. This formula is a precise method for assessing ectasia risk, and it is more sensitive than the individual components that comprise it.[23]

Index Rabinowitz-McDonnell

Index Rabinowitz-McDonnell

They described a new index for the assessment of keratoconus. Three criteria were considered:

  • “Central K (central steepening)”: is used to distinguish central cones.
  • “I-S values (reflecting inferior-superior dioptric asymmetry)”: Is the refractive power difference between the five inferior points and the superior points. It is used to detect inferior corneal steepening.
  • “SRAX (relative skewing of the steepest radial axes above and below the horizontal meridian)”.


They found, that a central K greater than 47.20 D, I-S higher than 1.2 and SRAX index above 21°, identified 98% of the patients with keratoconus.[24]

Conclusions

Corneal topography is an essential tool to perform the preoperative assessment in refractive surgery.[5] It is important to diagnose a variety of corneal abnormalities that can lead to poor outcomes with LASIK, such as keratoconus, forme fruste keratoconus, and pellucid marginal degeneration.[25] Also, we need to determine the presence of astigmatism and refractive stability. It is especially relevant to screen patients carefully and to diagnose those who have an elevated risk for ectasia. Some artifacts can be confused with keratoconus: "Displaced apex syndrome, CL-induced warpage, a prominent tear meniscus, misalignment when obtaining the topography, dry eye disease with poor precorneal tear film and accidental external pressure on the globe".[5]

In LASIK, PRK, SMILE and LASEK, it is relevant to assess the refractive error and corneal topography and determine whether it will lead to an abnormally steep or flat corneal curvature. The risk for buttonholes is bigger for steep corneas, whereas the risk for free caps is higher for flatter corneas. These complications are reported mainly with the use of mechanical microkeratomes; on the other hand, femtosecond LASIK flaps do not seem to have these complications.[25] Postoperative corneal topography is helpful in determining the ablation bed, the quality of the surgery and the uniformity of laser. The study should be performed at least one week after surgery. It is also useful for detecting ectasia and evaluating progression. "Corneal topography-guided custom ablation patterns" is a helpful technique to prevent undesirable postoperative results.[5]
Refractive Surgery Pearls in Topography

References

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