Journal of Refractive Surgery

Review Supplemental Data

A Systematic Review of Subclinical Keratoconus and Forme Fruste Keratoconus

Maria A. Henriquez, MD, PhD; Marta Hadid, MD; Luis Izquierdo Jr, MD, PhD

Abstract

PURPOSE:

To identify the definitions used for the terms sub-clinical keratoconus and forme fruste keratoconus in published articles.

METHODS:

This was a prospective, systematic literature review of the electronic database in PubMed, the Cochrane Library, and LILACS Database of all studies using the keywords “subclinical keratoconus” and/or “forme fruste keratoconus” until August 18, 2017. Two independent reviewers analyzed the data. The inclusion criteria for articles were having analyzed subclinical keratoconus or forme fruste keratoconus eyes with a sample size greater than 10 eyes; containing the definition of subclinical keratoconus or forme fruste keratoconus; and the quality of published reports was assessed using standards quality index methods. The following aspects of the selected articles were then analyzed: inclusion criteria for definition and technology used.

RESULTS:

A total of 198 and 95 studies, respectively, including the definition of subclinical keratoconus and forme fruste keratoconus were collected in an initial search, of which 165 and 73 studies, respectively, were excluded. Definitions for subclinical keratoconus and forme fruste keratoconus included the criteria of having keratoconus in the fellow eye in 72.72% (24 of 33) and 77.27% (17 of 22) of the articles, respectively. A total of 96.97% (32 of 33) and 90.90% (20 of 22) of the studies used more than one parameter to define subclinical keratoconus and forme fruste keratoconus, respectively. The most common extra parameters included normal slit-lamp examination and cornea on slit-lamp biomicroscopy and inferior-superior asymmetry and/or bowtie pattern with skewed radial axes.

CONCLUSIONS:

This review demonstrates the lack of unified criteria to define subclinical keratoconus and forme fruste keratoconus. According to the literature review, the most common subclinical keratoconus definition used refers to an eye with topographic signs of keratoconus and/or suspicious topographic findings under normal slit-lamp examination and keratoconus in the fellow eye and the most common forme fruste keratoconus definition refers to an eye with normal topography, normal slit-lamp examination, and keratoconus in the fellow eye.

[J Refract Surg. 2020;36(4):270–279.]

Abstract

PURPOSE:

To identify the definitions used for the terms sub-clinical keratoconus and forme fruste keratoconus in published articles.

METHODS:

This was a prospective, systematic literature review of the electronic database in PubMed, the Cochrane Library, and LILACS Database of all studies using the keywords “subclinical keratoconus” and/or “forme fruste keratoconus” until August 18, 2017. Two independent reviewers analyzed the data. The inclusion criteria for articles were having analyzed subclinical keratoconus or forme fruste keratoconus eyes with a sample size greater than 10 eyes; containing the definition of subclinical keratoconus or forme fruste keratoconus; and the quality of published reports was assessed using standards quality index methods. The following aspects of the selected articles were then analyzed: inclusion criteria for definition and technology used.

RESULTS:

A total of 198 and 95 studies, respectively, including the definition of subclinical keratoconus and forme fruste keratoconus were collected in an initial search, of which 165 and 73 studies, respectively, were excluded. Definitions for subclinical keratoconus and forme fruste keratoconus included the criteria of having keratoconus in the fellow eye in 72.72% (24 of 33) and 77.27% (17 of 22) of the articles, respectively. A total of 96.97% (32 of 33) and 90.90% (20 of 22) of the studies used more than one parameter to define subclinical keratoconus and forme fruste keratoconus, respectively. The most common extra parameters included normal slit-lamp examination and cornea on slit-lamp biomicroscopy and inferior-superior asymmetry and/or bowtie pattern with skewed radial axes.

CONCLUSIONS:

This review demonstrates the lack of unified criteria to define subclinical keratoconus and forme fruste keratoconus. According to the literature review, the most common subclinical keratoconus definition used refers to an eye with topographic signs of keratoconus and/or suspicious topographic findings under normal slit-lamp examination and keratoconus in the fellow eye and the most common forme fruste keratoconus definition refers to an eye with normal topography, normal slit-lamp examination, and keratoconus in the fellow eye.

[J Refract Surg. 2020;36(4):270–279.]

Keratoconus is a progressive and asymmetric disease characterized by steepening, distortion, and apical thinning of the cornea.1 Inclusion criteria for keratoconus diagnosis are well defined1–3; however, terms such as subclinical keratoconus and forme fruste keratoconus are still unclear and imprecise. At times the definitions overlap.

Initially, diagnosis of keratoconus was based on slit-lamp examination and clinical signs, and then topographic signs were included as diagnostic criteria.1 More recently, with the advent of Scheimpflug imaging analysis and anterior segment optical coherence tomography (OCT), early diagnosis of the disease was achieved due to information such as pachymetric, epithelial, elevation, and aberrometry data, among others.3 At this point, many indicators have been suggested to diagnose forme fruste keratoconus and sub-clinical keratoconus. However, there is currently no clear definition of these terms, and considering that the prevalence of keratoconus is higher today than previously reported and that the diagnostic technology allows higher accuracy than was previously used, it is imperative to press deeper into the imprecise field of criteria used in these definitions.

The purpose of this study was to review the existing literature on subclinical keratoconus and forme fruste keratoconus definitions used for diagnosis and to describe which are the most-used criteria for these two entities to unify criteria.

Methods

Search Strategy and Inclusion Criteria

A systematic review was performed of primary research articles published in scientific databases, developed under an internationally recommended methodology, to create a reliable and replicable summary of the best evidence available. The protocol was approved by the Scientific Ethics Committee of the Instituto de Ojos Oftalmosalud, Lima, Peru. We searched in the following electronic databases: Cochrane Central Register of Controlled Trials (CENTRAL) (which contains the Cochrane Register of the Eyes and Vision Group); PubMed, using Medical Subject Headings (MeSH) terms and clinical queries; and Latin American and Caribbean Health Sciences Literature Database (LILACS). We reviewed all studies in these databases using the key words “subclinical keratoconus” (all fields) and “forme fruste keratoconus” (all fields) using dates through August 18, 2017. The reference lists of the articles were also examined.

Types of Publication

We primarily searched for articles published in scientific databases. The types of studies included were: prospective comparative studies; prospective cross-sectional, observational studies; prospective case series studies; prospective case–control studies; retrospective comparative studies; retrospective consecutive non-randomized studies; and retrospective cross-sectional studies. The inclusion criteria for articles was having analyzed subclinical keratoconus or forme fruste keratoconus eyes with a sample size greater than 10 eyes; containing the definition of subclinical keratoconus or forme fruste keratoconus; articles without a year-of-publication filter; and English language. Study participants were adult patients with subclinical keratoconus and/or forme fruste keratoconus.

Exclusion Criteria

Duplicated data, studies with a sample size of less than 10 eyes or patients, literature reviews, case reports, letters to the editor, comments to the editor, conferences, protocols, and other unrelated studies were deleted for each study analyzed.

Data Extraction and Assessment of Study Quality

Two independent reviewers analyzed the articles (MAH, MH), and any disagreements were resolved by a third party or by consensus between the reviewers (LI). The two independent authors reviewed the selected articles to find the keratoconus definitions used and the methodological quality and characteristic of each article. The information was entered into an Excel (Microsoft Corporation, Redmond, WA) spreadsheet for analysis.

The following information was extracted: authors, year of publication, title of the study, journal of publication, parameters used to define subclinical keratoconus and forme fruste keratoconus, technology used, and sample size. The quality of published reports was assessed using standard quality index methods adapted from Downs and Black4 and Deeks et al.5

Results

Subclinical Keratoconus

A total of 198 studies concerning the definition of subclinical keratoconus were collected in an initial search, of which 105 were excluded. A total of 93 were selected for a full evaluation and literary review, of which 60 studies were eliminated. In total, 33 articles6–38 were evaluated for subclinical keratoconus terminology (Table 1, Figure A, available in the online version of this article).

Inclusion Criteria to Define Subclinical Keratoconus Used in Each of the Articles Analyzed

Table 1:

Inclusion Criteria to Define Subclinical Keratoconus Used in Each of the Articles Analyzed

Flow chart describing the selection of studies about subclinical keratoconus definition.

Figure A.

Flow chart describing the selection of studies about subclinical keratoconus definition.

Studies were evaluated using 14 questions we formulated to assess their quality (Figure B, available in the online version of this article). A “yes” answer to each quality-assessment question was considered a positive measure. According to the quality index formulary, 100% (33 of 33) of the sample selected had adequate hypotheses or objectives described and a satisfactory sample size and 93.94% (31 of 33) had main outcomes described in the introduction or methods.

Bar graph showing the proportion of studies about subclinical keratoconus terminology: per quality item.

Figure B.

Bar graph showing the proportion of studies about subclinical keratoconus terminology: per quality item.

The most frequently cited criterion used to define subclinical keratoconus was to have keratoconus in the fellow eye (72.72%; 24 of 33), followed by “Normal-appearing cornea on slit-lamp biomicroscopy, keratometry, retinoscopy, and ophthalmoscopy” in 45.45% (15 of 33) of the studies. Table 2 shows the most frequently criteria used in the studies.

Frequency of Parameters Used in Subclinical Keratoconus Definition in the Analyzed Studies

Table 2:

Frequency of Parameters Used in Subclinical Keratoconus Definition in the Analyzed Studies

Thirty-two (96.97%; 32 of 33) of the studies used more than one parameter to define subclinical keratoconus. Only one study used one criterion to define subclinical keratoconus. Hashemi et al.12 used the keratoconus severity score as a unique criterion, but it is a grading scheme that included several parameters.

The most frequently cited criteria combinations used to define subclinical keratoconus were: (1) normal-appearing cornea on slit-lamp biomicroscopy, keratometry, retinoscopy, and ophthalmoscopy; (2) inferior–superior asymmetry and/or bow-tie pattern with skewed radial axes; and (3) diagnosis of keratoconus in the fellow eye in 21.71% (7 of 33) of the studies. Table 3 shows the most commonly used combinations.

Most Frequently Cited Criteria Combinations Used to Define Subclinical Keratoconus and Forme Fruste Keratoconusa

Table 3:

Most Frequently Cited Criteria Combinations Used to Define Subclinical Keratoconus and Forme Fruste Keratoconus

Some criteria have been used in a contradictory, imprecise, or ambiguous way. For example, “Normal-appearing cornea on slit-lamp biomicroscopy, keratometry, retinoscopy, and ophthalmoscopy” was used in 45.45% (15 of 33) of the studies, whereas “Lack of any keratoconus-related findings/signs in the slit-lamp biomicroscopy” was used in 33.33% (11 of 33) of them. “Inferior–superior asymmetry and/or bow-tie pattern with skewed radial axes” was used in 36.36% (12 of 33) of the studies, whereas “inferior steepening or asymmetric bow tie pattern (unspecific value)” was used in 21.71% (7 of 33). “Paracentral inferior–superior dioptric asymmetry difference in 1.40 to 1.90 D” was used in 6.06% (2 of 33) of the studies, whereas “inferior–superior asymmetry lower than 1.40 D” was used in 12.12% (4 of 33) of them. “KISA index between 60% and 100%” was used in 9.09% (3 of 33) of the studies, whereas “KISA% index lower than 60%” was used in 9.09% (3 of 33) of the studies. “Keratoconus Severity Score (without explanation of which value was included)” was used in 6.06% (2 of 33), whereas “KSS of 0, 1, or 2” was used in 6.06% (2 of 33). “Maximum keratometry ≥ 47.00 D” was used in 3.03% (1 of 33), and “Maximum keratometry ≥ 47.00 D” in 12.12% (4 of 33) of the studies.

These were the sample sizes of the articles analyzed for definition of subclinical keratoconus: 30.30% (10 of 33) had fewer than 20 patients, 42.42% (14 of 33) had 21 to 50 patients, 21.21% (7 of 33) had 50 to 100 patients, and 6.06% (2 of 33) had more than 100 patients.

Forme Fruste Keratoconus

A total of 95 studies on the definition of forme fruste keratoconus were collected in an initial search, of which 48 were excluded. A total of 47 were selected for a full evaluation and literature review, of which 25 studies were eliminated. In total, 22 articles39–59 were evaluated for forme fruste keratoconus analysis (Table 4, Figure C, available in the online version of this article).

Inclusion Criteria to Define Forme Fruste Keratoconus Used in Each of the Articles Analyzed

Table 4:

Inclusion Criteria to Define Forme Fruste Keratoconus Used in Each of the Articles Analyzed

Flow chart describing the selection of studies about forme fruste keratoconus definition.

Figure C.

Flow chart describing the selection of studies about forme fruste keratoconus definition.

We evaluated the quality of the studies using criteria similar to those described above. All 22 articles (100%) had an adequate sample size, a comparative group, an adequate hypothesis or objective described, a description of patients' characteristics, findings that were clearly described, an appropriate statistical test, and an accurate results measure of value and reliability (Figure D, available in the online version of this article).

Bar graph showing the proportion of studies about terminology and definition of forme fruste keratoconus: per quality item.

Figure D.

Bar graph showing the proportion of studies about terminology and definition of forme fruste keratoconus: per quality item.

The three variables most used to define forme fruste keratoconus in the articles evaluated were: keratoconus in the fellow eye in 77.27% (17 of 22), normal topography in 59.09% (13 of 22), and normal slit-lamp examination in 40.90% (9 of 22).

Of these studies, 90.90% (20 of 22) used more than one parameter to define forme fruste keratoconus. Only two studies used one parameter. Zhang et al.47 used the Keratoconus Severity Score (KSS) as unique criteria, but it is a grading scheme that included several parameters, and Kirwan et al.58 used a superior–inferior power difference in a 4-mm central zone of more than 1.50 diopters (D).

The most frequently cited criteria combinations used to define forme fruste keratoconus were normal topography, normal slit-lamp examination, and keratoconus in the fellow eye in 31.81% (7 of 22) studies, followed by lack of any keratoconus-related findings of signs in the slit-lamp biomicroscopy, KISA% values between 60% and 100%, and keratoconus in the fellow eye in 9.09% (2 of 22) of the studies (Table 5).

Frequency of Parameters Used on Forme Fruste Keratoconus Definition in the Analyzed Studies

Table 5:

Frequency of Parameters Used on Forme Fruste Keratoconus Definition in the Analyzed Studies

Some criteria were used in an unspecific or contradictory way in the articles. For example, “normal-appearing cornea on slit-lamp biomicroscopy, keratometry, retinoscopy, and ophthalmolscopy” was used in 40.90% (9 of 22), and “Lack of any keratoconus-related findings/signs in the slit-lamp biomicroscopy” in 13.63% (3 of 22) of the studies. “Inferior–superior power difference in 4-mm central zone more than 1.50 D” was used in 9.09% (2 of 22) of the studies, whereas “Paracentral inferior–superior dioptric asymmetry ≤ 1.4” was used in 4.54% (1 of 22), and “area of inferior or superior steepening (unspecific value)” was used in 4.54% (1 of 22). “KISA% index between 60% and 100%” was used in 9.09% (2 of 22) of the studies, whereas “KISA% index less than 60%” was used in 4.54% (1 of 22) of them. “KSS of 1 or 2, regardless of the status of the fellow eye” was used in 9.09% (2 of 22), whereas “KSS of 0, 1, or 2 as long as the other eye with keratoconus has a KSS ≥ 3” was used in 4.54% (1 of 22) of the studies.

The sample size of the articles analyzed for the definition of forme fruste keratoconus were as follows: 22.22% (6 of 22) of the total articles had a sample of 21 to 30 patients; 11.11% (3 of 22) had 31 to 40 patients; 22.77% (5 of 22) had 41 to 50 patients, 7.40% (2 of 22) had 51 to 60 patients; and 13.63% (3 of 22) had 61 to 70 patients.

Discussion

A clinical diagnosis means that the identification of the disease underlying a patient's complaints is based merely on signs, symptoms, and medical history of the patient, rather than on laboratory examination or medical imaging. In this way, clinical keratoconus is defined by the evidence of one or more slit-lamp biomicroscopic findings, including conical protrusion of the cornea at the apex, Fleischer rings, Vogt striae, and corneal stromal thinning.1–3 However, keratoconus diagnosis based only on clinical signs will lead to its diagnosis at the latest stage of the disease, so a commonly used definition is the presence of at least one clinical sign plus topographic criteria. This then ceases to be only a clinical diagnosis; it also includes medical imaging.

The term “subclinical” by dictionary denotation60 is “not detectable or producing effects that are not detectable by the usual clinical tests”; usual tests for keratoconus diagnosis currently include corneal topography and tomography. Therefore, although many studies may be using the term “subclinical” to mean disease evident on imaging but not on “clinical” examination, this definition appears to be incorrect and outdated. In our review, the three most common variables used in the articles to define subclinical keratoconus were to have: “keratoconus in the fellow eye” in 72.72% (24 of 33); “normal-appearing cornea on slit-lamp biomicroscopy, keratometry, retinoscopy, and ophthalmoscopy” in 45.45% (15 of 33) of the studies; and “inferior-superior asymmetry and/or bowtie pattern with skewed radial axes” in 36.36% (12 of 33) of the studies.

Forme fruste (from the French, “crude, or unfinished, form”) is an atypical or attenuated manifestation of a disease or syndrome, with the implications of incompleteness, partial presence, or aborted state. In 1938, Amsler used photographic Placido disk technology to describe early corneal topographic changes and coined the term “form fruste keratoconus.”61–63 The results of our study show that the three variables most used to define forme fruste keratoconus in the articles evaluated were: keratoconus in the fellow eye in 77.27% (17 of 22) of the studies, normal topography in 59.09% (13 of 22) of the studies, and normal slit-lamp examination in 40.90% (9 of 22) of the studies.

The review also shows that most of the articles (subclinical keratoconus: 72.72% [24 of 33] and forme fruste keratoconus: 77.27% [17 of 22]) included having keratoconus in the fellow eye as a diagnostic criterion. This means that a bilateral early diagnosis of the disease without clinical expression is difficult. Also, according to the literature examined, there is no consensus about how many and which signs of a suspicious topography are necessary to distinguish subclinical keratoconus or forme fruste keratoconus from “keratoconus suspect or abnormal topography” in a patient when neither eye has keratoconus. Thus, to diagnose the early form of keratoconus from an abnormal topography is still a challenge today, and will require consensus on what features are relevant.

A suspicious topography is defined as a topography that includes asymmetric bow-tie, which is asymmetric steepening in any direction greater than 0.50 D but less than 1.00 D as compared with the region 180 degrees opposite the steepest region with no skewed radial axis, and inferior steepening of skewed radial axis, which includes significant skewed radial axis (20 degrees or greater) with or without inferior steepening or 1.00 D or more as compared with the region 180 degrees opposite the steepest region, but an inferior-superior value less than 1.40 D.44 According to our results, some of the topographic parameters used for both definitions (sub-clinical keratoconus and forme fruste keratoconus) are not clearly diagnostic criteria for “keratoconus” and can overlap the “suspicious topographic definition.”

After analyzing the currently used terminology, the lack of unified criteria is evident. The purpose of the current study is not to generate definitions; however, the most commonly used criteria suggest diagnosing “subclinical keratoconus” only in those eyes with normal slit-lamp examination, topographic/tomographic signs of keratoconus or suspicious topography, and keratoconus in the fellow eye. The diagnosis of forme fruste keratoconus definition was reserved for those eyes with normal slit-lamp examination, normal topography, and keratoconus in the fellow eye. These definitions seem not able to offer initial diagnoses of these conditions in a useful way independent of the condition of the other eye.

This review also reflects some gaps in information. Some criteria related to epithelial imaging,64 wavefront aberrations,55 corneal biomechanics,65 and posterior elevation66 that have been associated with early diagnosis of keratoconus have not been included as inclusion criteria in the majority of the studies. Finally, it remains doubtful if the term “subclinical keratoconus” is adequate, considering that keratoconus is a bilateral disease and the clinical signs of keratoconus in the fellow eye would mean that the disease was no longer subclinical in its presentation.

This review reflects the lack of unified criteria to define subclinical keratoconus and form fruste keratoconus, and shows that, in the majority of the cases, both definitions require the presence of keratoconus in the other eye, which make it extremely difficult to define the disease in its early form independent of the status of the other eye.

References

  1. Rabinowitz YS. Videokeratographic indices to aid in screening for keratoconus. J Refract Surg. 1995;11(5):371–379.
  2. Gomes JAP, Tan D, Rapuano CJ, et al. Group of Panelists for the Global Delphi Panel of Keratoconus and Ectatic Diseases. Global consensus on keratoconus and ectatic diseases. Cornea. 2015;34(4):359–369. doi:10.1097/ICO.0000000000000408 [CrossRef]
  3. Martínez-Abad A, Piñero DP. New perspectives on the detection and progression of keratoconus. J Cataract Refract Surg. 2017;43(9):1213–1227. doi:10.1016/j.jcrs.2017.07.021 [CrossRef]
  4. Downs SH, Black N. The feasibility of creating a checklist for the assessment of the methodological quality both of randomised and non-randomised studies of health care interventions. J Epidemiol Community Health. 1998;52(6):377–384. doi:10.1136/jech.52.6.377 [CrossRef]
  5. Deeks JJ, Dinnes J, D'Amico R, et al. International Stroke Trial Collaborative GroupEuropean Carotid Surgery Trial Collaborative Group. Evaluating non-randomised intervention studies. Health Technol Assess. 2003;7(27):iii–x, 1–173. doi:10.3310/hta7270 [CrossRef]
  6. Sideroudi H, Labiris G, Georgantzoglou K, Ntonti P, Siganos C, Kozobolis V. Fourier analysis algorithm for the posterior corneal keratometric data: clinical usefulness in keratoconus. Ophthalmic Physiol Opt. 2017;37(4):460–466. doi:10.1111/opo.12386 [CrossRef]
  7. Peña-García P, Peris-Martínez C, Abbouda A, Ruiz-Moreno JM. Detection of subclinical keratoconus through non-contact tonometry and the use of discriminant biomechanical functions. J Biomech. 2016;49(3):353–363. doi:10.1016/j.jbiomech.2015.12.031 [CrossRef]
  8. Shetty R, Rao H, Khamar P, et al. Keratoconus screening indices and their diagnostic ability to distinguish normal from ectatic corneas. Am J Ophthalmol. 2017;181:140–148. doi:10.1016/j.ajo.2017.06.031 [CrossRef]
  9. Vinciguerra R, Ambrósio R Jr, Roberts CJ, Azzolini C, Vinciguerra P. Biomechanical characterization of subclinical keratoconus without topographic or tomographic abnormalities. J Refract Surg. 2017;33(6):399–407. doi:10.3928/1081597X-20170213-01 [CrossRef]
  10. Martínez-Abad A, Piñero DP, Ruiz-Fortes P, Artola A. Evaluation of the diagnostic ability of vector parameters characterizing the corneal astigmatism and regularity in clinical and subclinical keratoconus. Cont Lens Anterior Eye. 2017;40(2):88–96. doi:10.1016/j.clae.2016.11.008 [CrossRef]
  11. Sideroudi H, Labiris G, Georgatzoglou K, Ditzel F, Siganos C, Kozobolis V. Fourier analysis of videokeratography data: clinical usefulness in grade I and subclinical keratoconus. J Cataract Refract Surg. 2016;42(5):731–737. doi:10.1016/j.jcrs.2016.01.049 [CrossRef]
  12. Hashemi H, Beiranvand A, Yekta A, Maleki A, Yazdani N, Khabazkhoob M. Pentacam top indices for diagnosing subclinical and definite keratoconus. J Curr Ophthalmol. 2016;28(1):21–26. doi:10.1016/j.joco.2016.01.009 [CrossRef]
  13. Feizi S, Yaseri M, Kheiri B. Predictive ability of Galilei to distinguish subclinical keratoconus and keratoconus from normal corneas. J Ophthalmic Vis Res. 2016;11(1):8–16. doi:10.4103/2008-322X.180707 [CrossRef]
  14. Li Y, Chamberlain W, Tan O, Brass R, Weiss JL, Huang D. Subclinical keratoconus detection by pattern analysis of corneal and epithelial thickness maps with optical coherence tomography. J Cataract Refract Surg. 2016;42(2):284–295. doi:10.1016/j.jcrs.2015.09.021 [CrossRef]
  15. Tummanapalli SS, Potluri H, Vaddavalli PK, Sangwan VS. Efficacy of axial and tangential corneal topography maps in detecting subclinical keratoconus. J Cataract Refract Surg. 2015;41(10):2205–2214. doi:10.1016/j.jcrs.2015.10.041 [CrossRef]
  16. Cui J, Zhang X, Hu Q, Zhou W-Y, Yang F. Evaluation of corneal thickness and volume parameters of subclinical keratoconus using a Pentacam Scheimflug system. Curr Eye Res. 2016;41(7):923–926. doi:10.3109/02713683.2015.1082188 [CrossRef]
  17. Steinberg J, Casagrande MK, Frings A, et al. Screening for subclinical keratoconus using swept-source Fourier domain anterior segment optical coherence tomography. Cornea. 2015;34(11):1413–1419. doi:10.1097/ICO.0000000000000568 [CrossRef]
  18. Piñero DP, Pérez-Cambrodí RJ, Soto-Negro R, Ruiz-Fortes P, Artola A. Clinical utility of ocular residual astigmatism and topographic disparity vector indexes in subclinical and clinical keratoconus. Graefes Arch Clin Exp Ophthalmol. 2015;253(12):2229–2237. doi:10.1007/s00417-015-3169-x [CrossRef]
  19. Steinberg J, Katz T, Lücke K, Frings A, Druchkiv V, Linke SJ. Screening for keratoconus with new dynamic biomechanical in vivo Scheimpflug analyses. Cornea. 2015;34(11):1404–1412. doi:10.1097/ICO.0000000000000598 [CrossRef]
  20. Galletti JD, Ruiseñor Vázquez PR, Fuentes Bonthoux F, Pförtner T, Galletti JG. Multivariate analysis of the Ocular Response Analyzer's corneal deformation response curve for early keratoconus detection. J Ophthalmol. 2015;2015:496382. doi:10.1155/2015/496382 [CrossRef]
  21. Jafarinasab MR, Shirzadeh E, Feizi S, Karimian F, Akaberi A, Hasanpour H. Sensitivity and specificity of posterior and anterior corneal elevation measured by Orbscan in diagnosis of clinical and subclinical keratoconus. J Ophthalmic Vis Res. 2015;10(1):10–15. doi:10.4103/2008-322X.156085 [CrossRef]
  22. Steinberg J, Aubke-Schultz S, Frings A, et al. Correlation of the KISA% index and Scheimpflug tomography in ‘normal’, ‘sub-clinical’, ‘keratoconus-suspect’ and ‘clinically manifest’ keratoconus eyes. Acta Ophthalmol. 2015;93(3):e199–e207. doi:10.1111/aos.12590 [CrossRef]
  23. Muftuoglu O, Ayar O, Hurmeric V, Orucoglu F, Kilic I. Comparison of multimetric D index with keratometric, pachymetric, and posterior elevation parameters in diagnosing subclinical keratoconus in fellow eyes of asymmetric keratoconus patients. J Cataract Refract Surg. 2015;41(3):557–565. doi:10.1016/j.jcrs.2014.05.052 [CrossRef]
  24. Sahebjada S, Xie J, Chan E, Snibson G, Daniel M, Baird PN. Assessment of anterior segment parameters of keratoconus eyes in an Australian population. Optom Vis Sci. 2014;91(7):803–809. doi:10.1097/OPX.0000000000000295 [CrossRef]
  25. Ruiseñor Vázquez PR, Galletti JD, Minguez N, et al. Pentacam Scheimpflug tomography findings in topographically normal patients and subclinical keratoconus cases. Am J Ophthalmol. 2014;158(1):32–40.e2. doi:10.1016/j.ajo.2014.03.018 [CrossRef]
  26. Serdarogullari H, Tetikoglu M, Karahan H, Altin F, Elcioglu M. Prevalence of keratoconus and subclinical keratoconus in subjects with astigmatism using Pentacam derived parameters. J Ophthalmic Vis Res. 2013;8(3):213–219.
  27. Ozgurhan EB, Kara N, Yildirim A, Bozkurt E, Uslu H, Demirok A. Evaluation of corneal microstructure in keratoconus: a confocal microscopy study. Am J Ophthalmol. 2013;156(5):885–893.e2. doi:10.1016/j.ajo.2013.05.043 [CrossRef]
  28. de Sanctis U, Aragno V, Dalmasso P, Brusasco L, Grignolo F. Diagnosis of subclinical keratoconus using posterior elevation measured with 2 different methods. Cornea. 2013;32(7):911–915. doi:10.1097/ICO.0b013e3182854774 [CrossRef]
  29. Ramos-López D, Martínez-Finkelshtein A, Castro-Luna GM, et al. Screening subclinical keratoconus with Placido-based corneal indices. Optom Vis Sci. 2013;90(4):335–343. doi:10.1097/OPX.0b013e3182843f2a [CrossRef]
  30. Ahmadi Hosseini SM, Mohidin N, Abolbashari F, Mohd-Ali B, Santhirathelagan CT. Corneal thickness and volume in subclinical and clinical keratoconus. Int Ophthalmol. 2013;33(2):139–145. doi:10.1007/s10792-012-9654-x [CrossRef]
  31. Arbelaez MC, Versaci F, Vestri G, Barboni P, Savini G. Use of a support vector machine for keratoconus and subclinical keratoconus detection by topographic and tomographic data. Ophthalmology. 2012;119(11):2231–2238. doi:10.1016/j.ophtha.2012.06.005 [CrossRef]
  32. Uçakhan ÖÖ, Cetinkor V, Özkan M, Kanpolat A. Evaluation of Scheimpflug imaging parameters in subclinical keratoconus, keratoconus, and normal eyes. J Cataract Refract Surg. 2011;37(6):1116–1124. doi:10.1016/j.jcrs.2010.12.049 [CrossRef]
  33. Miháltz K, Kovács I, Kránitz K, Erdei G, Németh J, Nagy ZZ. Mechanism of aberration balance and the effect on retinal image quality in keratoconus: optical and visual characteristics of keratoconus. J Cataract Refract Surg. 2011;37(5):914–922. doi:10.1016/j.jcrs.2010.12.040 [CrossRef]
  34. Piñero DP, Alió JL, Alesón A, Escaf Vergara M, Miranda M. Corneal volume, pachymetry, and correlation of anterior and posterior corneal shape in subclinical and different stages of clinical keratoconus. J Cataract Refract Surg. 2010;36(5):814–825. doi:10.1016/j.jcrs.2009.11.012 [CrossRef]
  35. Bühren J, Kook D, Yoon G, Kohnen T. Detection of subclinical keratoconus by using corneal anterior and posterior surface aberrations and thickness spatial profiles. Invest Ophthalmol Vis Sci. 2010;51(7):3424–3432. doi:10.1167/iovs.09-4960 [CrossRef]
  36. Lema I, Sobrino T, Durán JA, Brea D, Díez-Feijoo E. Subclinical keratoconus and inflammatory molecules from tears. Br J Ophthalmol. 2009;93(6):820–824. doi:10.1136/bjo.2008.144253 [CrossRef]
  37. de Sanctis U, Loiacono C, Richiardi L, Turco D, Mutani B, Grignolo FM. Sensitivity and specificity of posterior corneal elevation measured by Pentacam in discriminating keratoconus/subclinical keratoconus. Ophthalmology. 2008;115(9):1534–1539. doi:10.1016/j.ophtha.2008.02.020 [CrossRef]
  38. Bühren J, Kühne C, Kohnen T. Defining subclinical keratoconus using corneal first-surface higher-order aberrations. Am J Ophthalmol. 2007;143(3):381–389. doi:10.1016/j.ajo.2006.11.062 [CrossRef]
  39. Awad EA, Abou Samra WA, Torky MA, El-Kannishy AM. Objective and subjective diagnostic parameters in the fellow eye of unilateral keratoconus. BMC Ophthalmol. 2017;17(1):186. doi:10.1186/s12886-017-0584-2 [CrossRef]
  40. Naderan M, Jahanrad A, Farjadnia M. Ocular, corneal, and internal aberrations in eyes with keratoconus, forme fruste keratoconus, and healthy eyes. Int Ophthalmol. 2018;38(4):1565–1573. doi:10.1007/s10792-017-0620-5 [CrossRef]
  41. Pahuja N, Shroff R, Pahanpate P, et al. Application of high resolution OCT to evaluate irregularity of Bowman's layer in asymmetric keratoconus. J Biophotonics. 2017;10(5):701–707. doi:10.1002/jbio.201600106 [CrossRef]
  42. Fujimoto H, Maeda N, Shintani A, et al. Quantitative evaluation of the natural progression of keratoconus using three-dimensional optical coherence tomography. Invest Ophthalmol Vis Sci. 2016;57(9):OCT169–OCT175. doi:10.1167/iovs.15-18650 [CrossRef]
  43. Hashemi H, Beiranvand A, Khabazkhoob M, et al. Corneal elevation and keratoconus indices in a 40- to 64-year-old population, Shahroud Eye Study. J Curr Ophthalmol. 2016;27(3–4):92–98. doi:10.1016/j.joco.2015.10.007 [CrossRef]
  44. Freitas G de O, Ambrósio R Jr, Ramos I, et al. Astigmatic vector analysis of posterior corneal surface: a comparison among healthy, forme fruste, and overt keratoconic corneas. Am J Ophthalmol. 2016;167:65–71. doi:10.1016/j.ajo.2016.04.008 [CrossRef]
  45. Ruiz Hidalgo I, Rodriguez P, Rozema JJ, et al. Evaluation of a machine-learning classifier for keratoconus detection based on Scheimpflug tomography. Cornea. 2016;35(6):827–832. doi:10.1097/ICO.0000000000000834 [CrossRef]
  46. Luz A, Lopes B, Hallahan KM, et al. Discriminant value of custom Ocular Response Analyzer waveform derivatives in forme fruste keratoconus. Am J Ophthalmol. 2016;164:14–21. doi:10.1016/j.ajo.2015.12.020 [CrossRef]
  47. Zhang L, Danesh J, Tannan A, Phan V, Yu F, Hamilton DR. Second-generation corneal deformation signal waveform analysis in normal, forme fruste keratoconic, and manifest keratoconic corneas after statistical correction for potentially confounding factors. J Cataract Refract Surg. 2015;41(10):2196–2204. doi:10.1016/j.jcrs.2015.11.011 [CrossRef]
  48. Ayar O, Ozmen MC, Muftuoglu O, Akdemir MO, Koc M, Ozulken K. In-vivo corneal biomechanical analysis of unilateral keratoconus. Int J Ophthalmol. 2015;8(6):1141–1145.
  49. Mohammadpour M, Etesami I, Yavari Z, Naderan M, Abdollahinia F, Jabbarvand M. Ocular Response Analyzer parameters in healthy, keratoconus suspect and manifest keratoconus eyes. Oman J Ophthalmol. 2015;8(2):102–106. doi:10.4103/0974-620X.159255 [CrossRef]
  50. Sideroudi H, Labiris G, Giarmoukakis A, Bougatsou N, Kozobolis V. Contribution of reference bodies in diagnosis of keratoconus. Optom Vis Sci. 2014;91(6):676–681. doi:10.1097/OPX.0000000000000258 [CrossRef]
  51. Ye C, Ng PK-F, Jhanji V. Optical quality assessment in normal and forme fruste keratoconus eyes with a double-pass system: a comparison and variability study. Br J Ophthalmol. 2014;98(11):1478–1483. doi:10.1136/bjophthalmol-2013-304494 [CrossRef]
  52. Fukuda S, Beheregaray S, Hoshi S, et al. Comparison of three-dimensional optical coherence tomography and combining a rotating Scheimpflug camera with a Placido topography system for forme fruste keratoconus diagnosis. Br J Ophthalmol. 2013;97(12):1554–1559. doi:10.1136/bjophthalmol-2013-303477 [CrossRef]
  53. Smadja D, Touboul D, Cohen A, et al. Detection of subclinical keratoconus using an automated decision tree classification. Am J Ophthalmol. 2013;156(2):237–246.e1. doi:10.1016/j.ajo.2013.03.034 [CrossRef]
  54. Kozobolis V, Sideroudi H, Giarmoukakis A, Gkika M, Labiris G. Corneal biomechanical properties and anterior segment parameters in forme fruste keratoconus. Eur J Ophthalmol. 2012;22(6):920–930. doi:10.5301/ejo.5000184 [CrossRef]
  55. Saad A, Gatinel D. Evaluation of total and corneal wavefront high order aberrations for the detection of forme fruste keratoconus. Invest Ophthalmol Vis Sci. 2012;53(6):2978–2992. doi:10.1167/iovs.11-8803 [CrossRef]
  56. Johnson RD, Nguyen MT, Lee N, Hamilton DR. Corneal biomechanical properties in normal, forme fruste keratoconus, and manifest keratoconus after statistical correction for potentially confounding factors. Cornea. 2011;30(5):516–523. doi:10.1097/ICO.0b013e3181f0579e [CrossRef]
  57. Saad A, Gatinel D. Topographic and tomographic properties of forme fruste keratoconus corneas. Invest Ophthalmol Vis Sci. 2010;51(11):5546–5555. doi:10.1167/iovs.10-5369 [CrossRef]
  58. Kirwan C, O'Malley D, O'Keefe M. Corneal hysteresis and corneal resistance factor in keratoectasia: findings using the Reichert Ocular Response Analyzer. Ophthalmologica. 2008;222(5):334–337. doi:10.1159/000145333 [CrossRef]
  59. Chan C, Ang M, Saad A, et al. Validation of an objective scoring system for forme fruste keratoconus detection and post-LASIK ectasia risk assessment in Asian eyes. Cornea. 2015;34(9):996–1004. doi:10.1097/ICO.0000000000000529 [CrossRef]
  60. Merriam-Webster's Collegiate Dictionary, 3rd ed. Springfield, MA: Merriam-Webster Incorporated; 2008.
  61. Randleman JB, Trattler WB, Stulting RD. Validation of the Ectasia Risk Score System for preoperative laser in situ keratomileusis screening. Am J Ophthalmol. 2008;145(5):813–818. doi:10.1016/j.ajo.2007.12.033 [CrossRef]
  62. Amsler M. Le kératocône fruste au Javal. Ophthalmologica. 1938;96(2):77–83. doi:10.1159/000299577 [CrossRef]
  63. Amsler M. Kératocõne classique et kératocône fruste; arguments unitaires. Ophthalmologica. 1946;111(2–3):96–101. doi:10.1159/000300309 [CrossRef]
  64. Silverman RH, Urs R, RoyChoudhury A, Archer TJ, Gobbe M, Reinstein DZ. Combined tomography and epithelial thickness mapping for diagnosis of keratoconus. Eur J Ophthalmol. 2017;27(2):129–134. doi:10.5301/ejo.5000850 [CrossRef]
  65. Luz A, Lopes B, Hallahan KM, et al. Enhanced combined tomography and biomechanics data for distinguishing forme fruste keratoconus. J Refract Surg. 2016;32(7):479–494. doi:10.3928/1081597X-20160502-02 [CrossRef]
  66. Villavicencio OF, Gilani F, Henriquez MA, Izquierdo L Jr, Ambrosio RA Jr, Belin MW. Independent population validation of the Belin/Ambrósio Enhanced Ectasia Display: implications for keratoconus studies and screening. Int J Keratoconus Ectatic Corneal Diseases. 2014;3(1):1–8. doi:10.5005/jp-journals-10025-1069 [CrossRef]

Inclusion Criteria to Define Subclinical Keratoconus Used in Each of the Articles Analyzed

Author, Journal, and Year of PublicationInclusion Criteria for Diagnosis for Subclinical KeratoconusTechnology Used for Diagnosis
Sideroudi et al.6Ophthalmic Physiol Opt, 2017D, E, FPentacam
Peña-Garcia et al.7J Biomech, 2016F, D, RORA, Corvis ST
Shetty et al.8Am J Ophthalmol, 2017A, B, C, DPentacam, Galilei, or Sirius
Vinciguerra et al.9J Refract Surg, 2017D, C, G, H, RCorvis ST, Pentacam, OPD III, or CSO
Martínez-Abad et al.10Cont Lens Anterior Eye, 2017A, B, D, I, JSirius
Sideroudi et al.11J Cataract Refract Surg, 2016D, E, FPentacam
Hashemi et al.12J Curr Ophthalmol, 2016MPentacam HR
Feizi et al.13J Ophthalmic Vis Res, 2016A, B, O, KGalilei
Li et al.14J Cataract Refract Surg, 2016F, B, Q, IOCT-Fourier
Tummanapalli et al.15J Cataract Refract Surg, 2015D, R, F, POrbscan IIz
Cui, et al.16Curr Eye Res, 2016A, B, C, DPentacam
Steinberg et al.17Cornea, 2015D, UCorvis ST
Piñero et al.18Graefes Arch Clin Exp Ophthalmol, 2015A, I, D, JPentacam
Steinberg et al.19Cornea, 2015D, P, UPentacam, SS-OCT
Galletti et al.20J Ophthalmol, 2015F, T, DOCT, Placido disk topography, and aberrometry, ORA
Jafarinasab et al.21J Ophthalmic Vis Res, 2015A, B, O, KOrbscan IIz
Steinberg et al.22Acta Ophthalmol, 2015D, UPentacam
Muftuoglu et al.23J Cataract Refract Surg, 2015F, D, LPentacam HR
Sahebjada et al.24Optom Vis Sci, 2014A, IIOLMaster, Pentacam
Ruiseñor Vázquez et al.25Am J Ophthalmol, 2014D, TPentacam HR
Serdarogullari et al.26J Ophthalmic Vis Res, 2013A, D, IPentacam
Ozgurhan et al.27Am J Ophthalmol, 2013A, B, D, J, WSirius or ConfoScan 4
de Sanctis et al.28Cornea, 2013A, B, C, DPentacam
Ramos-López et al.29Optom Vis Sci, 2013A, BCSO
Ahmadi Hosseini et al.30Int Ophthalmol, 2013A, BPentacam
Arbelaez et al.31Ophthalmology, 2012F, D, Q, L, ISirius
Uçakhan et al.32J Cataract Refract Surg, 2011A, B, DPentacam
Miháltz et al.33J Cataract Refract Surg, 2011F, E, S, VTop Model System, Hartmann-Shack wavefront sensor
Piñero et al.34J Cataract Refract Surg, 2010A, D, I, JPentacam
Bühren et al.35Invest Ophthalmol Vis Sci, 2010C, D, F, POrbscan IIz, Axial-keratometric data, MATLAB
Lema et al.36Br J Ophthalmol, 2009V, NBiomicroscope, EyeSys Corneal System, Orbscan II
de Sanctis et al.37Ophthalmology, 2008A, B, C, DPentacam
Bühren et al.38Am J Ophthalmol, 2007C, D, M, P, FOrbscan IIz

Frequency of Parameters Used in Subclinical Keratoconus Definition in the Analyzed Studies

ParameterFrequency
Diagnosis of keratoconus in the fellow eye72.72% (24/33)
Normal-appearing cornea on slit-lamp biomicroscopy, keratometry, retinoscopy, and ophthalmoscopy45.45% (15/33)
Inferior–superior asymmetry and/or bow-tie pattern with skewed radial axes36.36% (12/33)
Lack of any keratoconus-related findings/signs in the slit-lamp biomicroscopy33.33% (11/33)
No history of contact lens use, ocular surgery, or trauma21.21% (7/33)
Corneal topography showing an abnormal, localized steepening, or central/inferior steepening or asymmetric bow-tie pattern or claw-shaped pattern on topography21.71% (7/33)
One of the following signs: steep keratometric curvature greater than 47.00 D, oblique cylinder greater than 1.50 D, or central corneal thickness less than 500 µm12.12% (4/33)
Inferior–superior asymmetry lower than 1.40 D and/or maximum keratometry of 47.00 D or less12.12% (4/33)
KISA% index between 60% and 100% in the eye with subclinical keratoconus9.09% (3/33)
KISA% index lower than 60%9.09% (3/33)
Normal topography (with no asymmetric bowtie and no focal or inferior steepening pattern)9.09% (3/33)
No topography finding significant enough to be diagnosed as clinical keratoconus/corneas with subtle signs of keratoconus but without evidence of clinical keratoconus6.06% (2/33)
Keratoconus Severity Score (no specified number was used for subclinical keratoconus definition)6.06% (2/33)
Abnormal biomicroscopic findings including Vogt's striae and Fleischer ring > 2 mm or skewed radial axis > 21° or > 20°, or keratoconus predicting index > 30% or > 0.3 or keratoconus severity index > 30%, and abnormal keratoconus index6.06% (2/33)
Corrected distance visual acuity of 20/20 or better (Snellen)6.06% (2/33)
Keratoconus Severity Score 0, 1, or 26.06% (2/33)
Paracentral inferior–superior dioptric asymmetry difference in 1.40 to 1.90 D gradient6.06% (2/33)
Belin/Ambrósio Enhanced Ectasia total deviation index (BAD-D) from the Pentacam < 1.60 standard deviations3.03% (1/33)
Corvis Biomechanical Index (CBI) score > 0.5 in both eyes3.03% (1/33)
Simulated central corneal power > 47.20 D but less than 48.70 D3.03% (1/33)
Maximum keratometry ⩾ 47.00/47.20 D3.03% (1/33)
Elevation of the posterior corneal surface (unspecified quantitative value)3.03% (1/33)

Most Frequently Cited Criteria Combinations Used to Define Subclinical Keratoconus and Forme Fruste Keratoconusa

Frequently Cited Criteria Combinations Including 3 ParametersStudies That Used This Combination
To define subclinical keratoconus
  Normal-appearing cornea on slit-lamp biomicroscopy, keratometry, retinoscopy, and ophthalmoscopy21.71% (7/33)
  Inferior–superior asymmetry and/or bow-tie pattern with skewed radial axes
  Diagnosis of keratoconus in the fellow eye

  Inferior–superior asymmetry and/or bow-tie pattern with skewed radial axes12.12% (4/33)
  No history of contact lens use, ocular surgery, or trauma
  Diagnosis of keratoconus in the fellow eye

  Normal-appearing cornea on slit-lamp biomicroscopy, keratometry, retinoscopy, and ophthalmoscopy12.12% (4/33)
  Corneal topography showing an abnormal localized steepening or central/inferior steepening or asymmetric bow-tie pattern or claw-shape pattern on topography
  Diagnosis of keratoconus in the fellow eye

To define forme fruste keratoconus
  Normal topography31.81% (7/22)
  Normal slit-lamp examination
  Keratoconus in the fellow eye

  Lack of any keratoconus-related findings/signs in the slit-lamp biomicroscopy9.09% (2/22)
  KISA% values between 60% and 100%
  Keratoconus in the fellow eye

Inclusion Criteria to Define Forme Fruste Keratoconus Used in Each of the Articles Analyzed

Author, Journal, and Year of PublicationInclusion Criteria for TerminologyTechnology Used for Diagnosis
Awad et al.39BMC Ophthalmol, 2017A, B, C, D, EPentacam
Naderan et al.40Int Ophthalmol, 2018A, D, EOPD Scan II
Pahuja et al.41J Biophotonics, 2017A, D, EPentacam, OCT
Fujimoto et al.42Invest Ophthalmol Vis Sci, 2016A, D, EOCT
Hashemi et al.43J Curr Ophthalmol, 2016F, G, HPentacam version 1.17r72
Freitas G de O et al.44Am J Ophthalmol, 2016A, EPentacam
Ruiz Hidalgo et al.45Cornea, 2016E, SPentacam
Luz et al.46Am J Ophthalmol, 2015K, A, EORA
Zhang et al.47J Cataract Refract Surg, 2015MORA, Orbscan IIz, Galilei
Ayar et al.48Int J Ophthalmol, 2015A, D, EPentacam, ORA
Mohammadpour et al.49Oman J Ophthalmol, 2015I, J, N, OORA
Sideroudi et al.50Optom Vis Sci, 2014E, L, OPentacam
Ye et al.51Br J Ophthalmol, 2014A, EAS OCT, OCT
Fukuda et al.52Br J Ophthalmol, 2015A, D, E3D CAS-OCT, Scheimpflug camera with topography
Smadja et al.53Am J Ophthalmol, 2013A, E, OGalilei
Kozobolis et al.54Eur J Ophthalmol, 2012E, L, OPentacam, ORA, Topolyzer wavelight
Saad & Gatinel55Invest Ophthalmol Vis Sci, 2012A, D, E, P, QOPD, Orbscan IIz
Johnson et al.56Cornea, 2011E, R, MORA, Orbscan IIz
Saad & Gatinel57Invest Ophthalmol Vis Sci, 2010P, Q, EOrbscan IIz, OPD Scan
Kirwan et al.58Ophthalmologica, 2008HORA
Chan et al.59Cornea, 2015A, DOrbscan IIz, Tomey, SCORE analyzer

Frequency of Parameters Used on Forme Fruste Keratoconus Definition in the Analyzed Studies

ParameterFrequency
Keratoconus in the fellow eye77.27% (17/22)
Normal topography59.09% (13/22)
Normal slit-lamp examination40.90% (9/22)
Lack of any keratoconus-related findings/signs in the slit-lamp biomicroscopy13.63% (3/22)
Inferior–superior power difference in 4-mm central zone more than 1.50 D9.09% (2/22)
KISA% values between 60% and 100%9.09% (2/22)
NCN score: non-null score similarity to keratoconus for the contralateral eyes9.09% (2/22)
Mean keratometry < 47.00 D4.54% (1/22)
Paracentral inferior–superior dioptric asymmetry ⩽ 1.40 D4.54% (1/22)
Apex of the cone not centered at the 6-o'clock semi-meridian4.54% (1/22)
Corneal thickness at the apex of the cone is approximately 30 mm thinner than the corresponding distance above the pupil center4.54% (1/22)
Area of inferior or superior steepening (unspecific value) or minor topographic asymmetry4.54% (1/22)
Corneal steepness > 47.00 D4.54% (1/22)
KISA% index less than 60%4.54% (1/22)
KSS of 1 or 2, regardless of the status of the fellow eye9.09% (2/22)
Oblique cylinder > 1.50 D4.54% (1/22)
NCN score: null score similarity to suspect keratoconus and keratoconus4.54% (1/22)
KSS of 0, 1, or 2 as long as the other eye with keratoconus has a KSS of ⩾ 34.54% (1/22)
Asymptomatic Pentacam tomography and elevation4.54% (1/22)
Authors

From the Research Department, Instituto de Ojos Oftalmosalud, Lima, Peru.

The authors have no financial or proprietary interest in the materials presented herein.

The authors thank Jose Chauca, MSc, for helping with the statistical analysis and Carmen Maldonado, MSc, for technical support and data collection.

AUTHOR CONTRIBUTIONS

Study concept and design (MAH, LI); data collection (MH); analysis and interpretation of data (MAH, MH); writing the manuscript (MAH, MH); critical revision of the manuscript (MAH, LI); supervision (MAH, LI)

Correspondence: Maria A. Henriquez, MD, PhD, Instituto de Ojos Oftalmosalud, Av. Javier prado este 1142, San Isidro, Lima 27-Peru. E-mail: mariahenriquez1610@gmail.com

Received: August 06, 2019
Accepted: February 10, 2020

10.3928/1081597X-20200212-03

Sign up to receive

Journal E-contents