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ORIGINAL ARTICLE
Year : 2020  |  Volume : 33  |  Issue : 2  |  Page : 563-568

Comparison between axial and tangential corneal topography maps in localization of cone in keratoconus


1 Department of Ophthalmology, Faculty of Medicine, Menoufia University, Menoufia, Egypt
2 Department of Ophthalmology, Menoufia University Student's Hospital, Menoufia, Egypt

Date of Submission22-Jan-2019
Date of Decision10-Feb-2020
Date of Acceptance18-Feb-2020
Date of Web Publication27-Jun-2020

Correspondence Address:
Rofaida M Serag EL-Deen
Shebin El-kom, Menoufia
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/mmj.mmj_36_19

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  Abstract 


Objectives
The objectives of this study were to compare axial maps versus tangential maps in the localization of a keratoconus cone using pentacam and to describe the cone characteristics for the different stages of keratoconus.
Background
Cone location is an essential factor in the selection of intrastromal corneal ring segments' specification used in the treatment of keratoconus.
Patients and methods
This was a retrospective case series' study conducted between October 2017 and October 2018 on a sample of 100 eyes with keratoconus. Corneal topography with a rotating Scheimpflug device was carried out. Cone location, cone decentration, and morphological patterns were analyzed. Cone location was determined for each eye with both axial and tangential maps and compared for matching. A correlation between kmaxlocation and cone location according to axial and tangential maps was also carried out.
Results
The matching rate between axial and tangential maps in the total sample was 100, 5, and 9.6% for central, paracentral, and peripheral groups, respectively, which was statistically significant (P < 0.001). For stages I, II, and III, the results were statistically significant (P < 0.001). Correlations between kmaxand tangential map were greater than correlations between kmaxlocation and axial map (P < 0.001). For cone apex decentration, the most frequent value of decentration was between 0.5 and 1 mm in the different stages. For morphological pattern, the most frequent cone morphology in different stages was a nipple-shaped cone.
Conclusion
Analysis of axial and tangential corneal topography maps showed a significant discrepancy in cone location in the form of 'centralization tendency' on the tangential map. No correlations between stage, morphology, or decentration were revealed.

Keywords: axial map, cone location, keratoconus, tangential map


How to cite this article:
EL-Sebaey AR, Ibrahim AM, Serag EL-Deen RM. Comparison between axial and tangential corneal topography maps in localization of cone in keratoconus. Menoufia Med J 2020;33:563-8

How to cite this URL:
EL-Sebaey AR, Ibrahim AM, Serag EL-Deen RM. Comparison between axial and tangential corneal topography maps in localization of cone in keratoconus. Menoufia Med J [serial online] 2020 [cited 2020 Oct 22];33:563-8. Available from: http://www.mmj.eg.net/text.asp?2020/33/2/563/287786




  Introduction Top


Keratoconus is a noninflammatory, ectatic corneal disorder, characterized by corneal protrusion and thinning, leading to corneal irregularity and decreased visual acuity[1]. The estimated prevalence of keratoconus in the general population varies from 50 to 230 per 100 000, and it is higher among candidates screened for refractive surgery[2]. In most cases, the disease is bilateral and mostly diagnosed in the second or third decade of life[3].

Several risk factors, such as constant eye rubbing, the presence of systemic diseases (e.g. sleep apnea), floppy eyelid syndrome, allergies, and eczema, as well as family history, have been defined for the development of this corneal disease[4].

Diagnosis of keratoconus is based on clinical findings (corneal protrusion, corneal thinning, Vogt's striae, Fleischer ring, anterior stromal scars, or corneal hydrops) along with keratometry readings, keratoscopy, and corneal topography[5].

Corneal topography is currently one of the most important tools in the ophthalmology field for the imaging diagnosis of corneal diseases related to the alteration of corneal morphology, and more concretely to the diagnosis of keratoconus. This noninvasive technique provides a series of indices that permit a new diagnostic approach from corneal topographies, which are based on the univariate and multivariate quantitative detection systems[6].

The pentacam is a Scheimpflug imaging system that provides direct anterior and posterior elevation and pachymetry measurements from the cornea. As such, data from this system are independent of a reference axis or orientation and more accurately represent the true corneal shape. Furthermore, because the system can measure the center of the cornea, the far corneal periphery, and the posterior corneal surface, it can generate pachymetry maps, and localize and characterize any ectatic change easily[7].

Cone location is an essential factor in the selection of intrastromal corneal ring segments' (ICRS) specification used for keratoconus treatment. Cone location by pentacam examination depends on multiple maps of the pentacam, including sagittal (axial) curvature map, anterior elevation map, posterior elevation map, and thickness map[8].

The aim of the study was to compare axial maps versus tangential maps in the localization of the cone in keratoconus using pentacam and to describe the cone characteristics for the different stages of keratoconus.


  Patients and Methods Top


This retrospective case series' study was conducted at Menoufia University Hospital and TIBA Eye Center in Shebin El-kom during the period spanning between October 2017 and October 2018 on patients with keratoconus. The study was conducted after approval of the institutional review board and a written consent was taken from every patient before participation in the study. The study included a sample of 100 eyes with keratoconus. Corneal topography with a rotating Scheimpflug device (WaveLight Allegretto Oculyzer, WaveLight GmbH, Am Wolfsmantel, Erlangen, Germany) was carried out after obtaining Menoufia ethical committee approval. Inclusion criteria were (a) patients with clinical keratoconus, diagnosed with pentacam, (b) aged between 15 and 40 years old, (c) with no visual dysfunctions other than keratoconus.

History of keratorefractive surgery on the operative eye, history of recurrent uveitis, history of herpes simplex virus keratitis, and patients with severe ocular and systemic pathologies (e.g., diagnosed autoimmune disease, systemic connective tissue disease, glaucoma, cataract, diabetic retinopathy, and age-related macular degeneration) were the exclusion criteria of this study.

For each patient, the age, sex, eye side, stage of keratoconus (according to topographic keratoconus classification), the thinnest location, Q value, topographic corneal astigmatism, kmax(front) value, kmax(front) location, cone location according to axial map, cone location according to tangential map, morphological pattern of the cone, and decentration of the thinnest location from the corneal apex were recorded.

Topographic keratoconus classification allows classification of keratoconus into five grades, 0 (normal) to 4 (severe keratoconus)[9], and depends on seven anterior surface pentacam-derived topometric indices: index of surface variance, index of vertical asymmetry, keratoconus index, central keratoconus index, index of height asymmetry, index of height decentration, and index of minimum radius of curvature[10].

Cone location was labeled as central, paracentral, or peripheral according to the extent of the cone area. A circle that encompasses the two warmest colors was delineated, and its diameter was measured. The cone was considered central or paracentral if more than half of the diameter fell within the central 3 or 5 mm, respectively. Otherwise, it was labeled as a peripheral one. Cone location was determined for each case with both axial and tangential maps and compared for matching. Correlation between kmax location and cone location according to axial and tangential maps was also performed.

Morphological classification included morphological patterns (classified into nipple, oval, and globus cone); a tangential map of the topographer was selected to best distinguish the three cones, and the cones were identified and then measured in millimeters using the two predominant colors indicating corneal steepening, which were then marked as the two reference points when analyzing the size of the cone. Morphological patterns included nipple cone less than 5 mm, oval cone 5–6 mm, and globus cone more than 6 mm.

Decentration (D) (mm) of the thinnest pachymetry point from the apex was calculated using trigonometry. For the right triangle, the hypotenuse can be obtained as the square root of the sum of the square value of each leg. Decentration is calculated as the square root of the sum of x coordinate squared plus y coordinate squared: D=√(x2+ y2) D1: less than 0.5 mm, D2: more than 0.5–1.0 mm, D3: more than 1–1.5 mm, D4: more than 1.5 mm.

Statistical analysis

Data were fed to the computer and analyzed using IBM SPSS software package, version 20.0. (IBM Corp., Armonk, New York, USA). Qualitative data were described using number and percent. The Kolmogorov–Smirnov test was used to verify the normality of distribution. Quantitative data were described using range (minimum and maximum), mean, SD, and median. Significance of the obtained results was judged at the 5% level. χ2 test was used for categorical variables to compare between different groups; Fisher's exact or Monte Carlo correction and correction for χ2 were used when more than 20% of the cells had an expected count less than 5.


  Results Top


Hundred eyes of 61 patients were included; there were 50 male and 50 female patients and 51 right eyes and 49 left eyes. There were 67 patients more than or equal to 30 years of age and 33 patients more than 30 years of age for whom corneal topography with a rotating Scheimpflug device was carried out. [Table 1] shows the demographic and clinical data of the patients.
Table 1: The demographic and clinical data of the patients (n=100)

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The results revealed a statistically significant (P < 0.001) matching rate between axial and tangential maps in the total sample of 100, 5, and 9.6% for central, paracentral, and peripheral groups, respectively. Moreover, the paracentral group showed the greatest discrepancy, wherein 38 of 40 (95%) were labeled as central on the tangential map, although they were displayed as paracentral on the axial map [Table 2].
Table 2: Relation between axial map and tangential map in total sample (n=100)

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In stage I, the matching rate was 100, 8.3, and 26.3% for central, paracentral, and peripheral groups, respectively, which was statistically significant (P < 0.001), with the greatest discrepancy in the paracentral group, wherein 11 of 12 (91.6%) were labeled as central on the tangential map, although they were displayed as paracentral on the axial map. In stage II, the matching rate was 100, 5.3, and 0.0% for central, paracentral, and peripheral groups, respectively, which was statistically significant (P < 0.001), with the greatest discrepancy in the paracentral group, wherein 18 of 19 (94.7%) were labeled as central on the tangential map, although they were displayed as paracentral on the axial map. In stage III, the matching rate was 100, 0.0, and 0.0% for central, paracentral, and peripheral groups, respectively, which was statistically significant (P = 0.011), with the greatest discrepancy in the paracentral group, wherein nine of nine (100%) were labeled as central on the tangential map, although they were displayed as paracentral on the axial map. [Table 3] shows the matching rate in the different stages.
Table 3: Relation between axial map and tangential map in different stages of keratoconus

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There was a significant correlation between kmax location and cone location; the correlation between kmax and tangential maps was greater than the correlation between kmax location and axial maps (P < 0.001), as the matching rate between kmax location and cone location by the axial map was 100, 2.5, and 3.8% for central, paracentral, and peripheral groups, respectively, but the matching rate between kmax location and cone location by the tangential map was 98.4, 57.6, and 40.0% for central, paracentral, and peripheral groups, respectively [Figure 1].
Figure 1: Graphical representation shows the correlation between cone location according to axial map and tangential map in relation to kmax(front) location.

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For cone apex decentration, the most frequent value of decentration was between 0.5 and 1 mm in different stages: stage I was 62.9%, stage II was 64.9%, and stage III was 46.4%, which was statistically significant (P = 0.445) [Figure 2].
Figure 2: Graphical representation shows the relation between the stage of keratoconus and decentration.

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For morphological pattern, the most frequent cone morphology in different stages was nipple shaped: in stage I, it was 45.7%; in stage II, it was 73%, and in stage III, it was 85.7%, which was statistically significant (P < 0.001) [Figure 3].
Figure 3: Graphical representation shows the relation between the stage of keratoconus and morphological patterns.

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  Discussion Top


Keratoconus, the leading cause of corneal transplantation in developed countries, is a noninflammatory ectatic disorder that usually manifests at puberty when the cornea assumes a conical shape due to a gradually progressive thinning of the corneal stroma. It is almost always a bilateral and asymmetrical condition that leads to irregular astigmatism and high myopia and, consequently, significant visual impairment[11].

ICRS have been proposed and investigated as an additive surgical procedure for keratoconus correction[12], aiming at visual improvement and delaying the need for a corneal graft in patients with keratoconus[13]. Cone location is an essential factor in the selection of ICRS specification used for keratoconus treatment[8].

Cone location can be determined by different ways; the first challenge in locating the cone is the circle that it lies within. The cone can be labeled in different ways, with no agreement for a standard terminology. One of these ways is to be labeled as central or eccentric, pointing to be in the central 3 mm or outside this circle[14].

Another way is to be in the central 2 mm circle or outside, thus labeled as central or eccentric, respectively[15]. Others define it as central, paracentral, or peripheral cone that usually point to be in the central 3 mm, 5 mm, or outside those circles[16]. Others, whom we follow, consider the cone area as a defining criterion, which is when more than 50% of the cone lies within the central 3 mm circle or the central 5 mm circle, and it is labeled as central and paracentral, respectively. Otherwise, it is a peripheral cone[17].

Haddad et al.[18], Gordillo et al.[19], and Wilde et al.[20] used the axial map as a guide for ring selection and implantation without providing an evidence-based rationale, except for being 'easy and less noisy' for the interpretation. However, the tangential map, which provides more details, is supposed to be the right guide. Tangential maps were more effective in detecting the subclinical keratoconus and rigid gas-permeable contact lens fitting in keratoconus[21],[22].

Tummanapalli and colleagues conducted their study including 71 keratoconus suspect patients and 76 normal participants. For each patient and, in both maps, four parameters were determined: the location of corneal apex and its decentration, magnitude of the apex (maximum curvature), corneal irregularity in the 3 and 5 mm zones, and the anterior–posterior ratio of maximum curvature, and stunning results were obtained[21].

First, the corneal apex was not the same in both maps. In the tangential map, the anterior corneal apex was consistently closer to the map center. Second, posterior apex curvature was consistently significantly higher on the tangential than the axial map, and this was surprisingly evident in the control group as well as the keratoconus suspect group. Third, a strong correlation was detected between anterior and posterior curvature on the tangential map in keratoconus suspect (r=−0.81) and controls as well (r=−0.82). Finally, sensitivity and specificity of anterior–posterior curvatures' ratio were calculated using the receiver operating characteristics curves. The tangential map had significantly higher sensitivity (98.5%) and specificity (98.7%(at the cut-off value of more than or equal to − 6.97. Similarly, posterior irregularity indices at 3 and 5 mm zones showed higher sensitivity and specificity in the differentiation of a normal eye from a keratoconus suspect eye[21].

In the American Academy of Optometry Conference, the tangential map was first recommended for the follow-up of cone progression, as it is more accurate and more sensitive to changes in the apical power. Moreover, it was recommended for the apical clearance method of contact lens fitting in keratoconus[22].

Szczotka and Thomas compared three measures of apical power in keratoconus. Thirty patients with various grades of keratoconus (mild, moderate, and severe) were assessed using EyeSys System 2000 (version 3.04) to map the front corneal surface. Central steep K, axial apical power, and tangential apical power were retrieved and compared in the three grades of keratoconus. The greater apical power was consistently greater with the tangential map) average of 1.28, 2.37, and 6.3 D in mild, moderate, and advanced keratoconus, respectively)[23],[24].

The tangential map assumes invaluable capability to detect the corneal periphery, and the wide practice and the better outcomes obtained reflect and potentiate the reliability of the tangential map in contact lens fitting compared with the axial one[25].

Our study found that the most prevalent cone morphology was nipple cone in all three stages of keratoconus. Hence, if the cone begins with a nipple morphology, continuous rubbing will cause degradation of the epithelium, resulting in a larger area of thinning, leading to a change in the corneal morphology[26].

Our finding is similar to other studies, which found that the most prevalent cone locations were central and paracentral, with nipple morphology being associated with these locations[5],[27].

Our results revealed that the most frequent value of decentration was between 0.5 and 1 mm in the different stages. This agrees with Munsamy et al.[27]. Others revealed an association between the stage of keratoconus and the cone apical decentration for moderate keratoconus with a value of 3–4 mm off the corneal center[28].


  Conclusion Top


Analysis of axial and tangential corneal topography maps showed a significant discrepancy in cone location in the form of 'centralization tendency' on the tangential map. No correlations between stage, morphology, or decentration were revealed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

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