|Year : 2019 | Volume
| Issue : 3 | Page : 78-89
Prevalence of ectatic corneal conditions among keratorefractive candidates
Tarek M Abdullah, Amr I ElAwamry, Ossama T Nada, Walaa M Hamed
Department of Ophthalmology, Faculty of Medicine, Ain Shams University, Cairo, Egypt
|Date of Submission||31-Mar-2019|
|Date of Acceptance||17-May-2019|
|Date of Web Publication||25-Sep-2019|
MBBCh Walaa M Hamed
Group 11, Building 25, Madinaty, Postal code: 19519
Source of Support: None, Conflict of Interest: None
Background Noninflammatory corneal ectasias is a group of diseases including keratoconus (KC), pellucid marginal degeneration (PMD), and keratoglobus. KC is the most common form of corneal ectasia characterized by progressive thinning of the central or inferior cornea, causing corneal steepening and cone formation. The disease typically presents during the second decade of life and is bilateral, but often one eye precedes the other. Detecting the cases of subclinical KC had always been a challenge for refractive surgeons especially when the suggestive clinical signs and symptoms to differentiate subclinical KC from the normal are not observed.
Aim The aim of the study is to determine the prevalence of ectatic corneal conditions among keratorefractive population admitted for refractive surgery at Al-Mashreq Eye Centre, during the period from June 2017 till December 2017, using data from the Scheimpflug tomographer imaging system.
Patients and methods Our study was conducted on patients seeking consultation for refractive surgery at Al-Mashreq Eye Centre during the period from June 2017 to December 2017. We used the Pentacam device to screen 1439 refractive surgery candidates after being subjected to precise ocular examination and full general and ocular history taking.
Results In our study, a total of 42 patients (84 eyes) showed topographic signs of manifest and subclinical KC, making a total prevalence of 3.0%. No cases with PMD or keratoglobus were found, but cases with KC having a topographic pattern mimicking PMD called pellucid-like KC were found in five eyes (three patients). Compound myopic astigmatism was the commonest refractive error in keratoconic eyes (71.4%).
Conclusion Prevalence of KC in refractive population may reflect an approximate idea of its prevalence in general population. Meticulous examination before refractive surgery can forbid bad consequences in patients at high risk of developing post-laser-assisted in situ keratomileusis ectasia and may save patients discovered at mild or moderate KC stages from the ultimate fate of keratoplasty. Moreover, surgeons should be aware that hypermetropia and mixed astigmatism do not eliminate the possibility of an associated ectatic corneal disease.
Keywords: ectatic corneal, keratorefractive candidates, pellucid marginal degeneration
|How to cite this article:|
Abdullah TM, ElAwamry AI, Nada OT, Hamed WM. Prevalence of ectatic corneal conditions among keratorefractive candidates. J Egypt Ophthalmol Soc 2019;112:78-89
|How to cite this URL:|
Abdullah TM, ElAwamry AI, Nada OT, Hamed WM. Prevalence of ectatic corneal conditions among keratorefractive candidates. J Egypt Ophthalmol Soc [serial online] 2019 [cited 2020 Sep 25];112:78-89. Available from: http://www.jeos.eg.net/text.asp?2019/112/3/78/267821
| Introduction|| |
Corneal ectasia is a noninflammatory, bilateral, asymmetrical condition, causing progressive corneal steepening and thinning. Types of corneal ectasia include keratoconus (KC), pellucid marginal degeneration (PMD), keratoglobus (KG), postrefractive ectasia, and wound ectasia after penetrating keratoplasty .
KC is the most prevalent form of corneal ectasia and affects all ethnicities. It is defined as a bilateral, asymmetric degenerative disorder of the eye caused by collagen disorganization in the cornea. The annual incidence of which is two per 100 000, with a prevalence of 54.5 per 100 000 (∼1 per 2000) .
Another form of corneal ectasia is PMD. It is an idiopathic, noninflammatory thinning corneal disorder normally involving the cornea inferiorly, between the 4 and 8 o’clock meridians. The zone of thinning is generally 1–2 mm wide, and separated 1 or 2 mm from the limbus by an area of normal cornea. The area of ectasia is cylindrical, creating an against-the-rule astigmatism .
KG is a rare type of corneal ectasia, characterized by generalized thinning and globular protrusion of the cornea. Both congenital and acquired forms have been shown to occur, and it may be associated with various other ocular and systemic syndromes including the connective tissue disorders .
Refractive surgeries are various elective procedures that modify the refractive status of the eye. Procedures that involve altering the cornea are collectively referred to as ‘keratorefractive surgery.’ Other refractive surgery procedures include the placement of intraocular lens implant either in front of the crystalline lens ‘phakic intraocular lens’ or in place of the crystalline lens ‘refractive lens exchange.’ The first excimer laser tissue ablation keratorefractive surgery performed was photorefractive keratectomy after being approved by the FDA in 1995; subsequently, laser-assisted in situ keratomileusis (LASIK) has become the most commonly performed keratorefractive surgery .
Yet, these surgeries may uncommonly be associated with complications. The most serious long-term complication of keratorefractive surgery is the weakening of the cornea and the development of keratectasia (post-LASIK ectasia) which is a corneal disorder characterized by progressive stromal thinning and central or asymmetric inferior corneal steepening. It has been suggested that central ectasia is associated with low residual stromal bed thickness, whereas inferotemporal ectasia is a sign of preexisting forme fruste keratoconus (FFK) or PMD .
Refractive surgery practice allowed us to see more patients with corneal ectatic dystrophies and other topographic abnormalities than would be expected from the incidence of each of these disorders in the general population; thus, meticulous preoperative screening is one of the key factors that contribute to success in refractive surgery .
Corneal topography is a valuable diagnostic tool for diagnosing subclinical KC and for tracking the progression of the disease. Evolution in KC detection has resulted in continued refinement of indices such as the keratometry, inferior–superior dioptric asymmetry, skew percentage, and astigmatism. Several topographic devices provided additional information through various features, for example, the Orbscan provides data on anterior and posterior elevation and best-fit sphere and a corneal pachymetry map. Later a recently introduced imaging device that provides accurate measurement of corneal power, elevation, and pachymetry is the Pentacam; this device uses a rotating Scheimpflug camera. The Scheimpflug system determines net corneal power, elevation maps, anterior chamber depth, and corneal wave front. It is also an excellent method to detect FFK and subclinical KC .
| Aim of the work|| |
The aim of the study is to determine the prevalence of ectatic corneal conditions among keratorefractive population admitted for keratorefractive surgery at Al-Mashreq Eye Centre, during the period from June 2017 till December 2017, using data from the Scheimpflug tomographer imaging system.
| Patients and methods|| |
This cross-sectional study was conducted on all patients undergoing consultation for laser refractive surgery at Al-Mashreq Eye Centre from the period of June 2017 to December 2017.
During that period, 2878 eyes of 1439 patients were photographed using Pentacam HR system (Oculus, Wetzlar, Germany) as screening device for corneal ectasia ([Figure 1]). Patients who were considered unsuitable for refractive surgery were identified and assessed, as long as there was no reason for exclusion.
The following were the exclusion criteria:
- Patients who were previously diagnosed as having KC or other form of corneal ectasia.
- Patients who previously underwent laser refractive surgery.
- Patients who have not completed 2 weeks without wearing contact lenses.
- Corneal scarring or corneal dystrophies.
- Severe eye dryness.
- Cases with glaucoma and ocular hypertension.
- Patients undergoing other refractive surgeries than laser keratorefractive surgeries.
All patients were subjected to the following:
- General and ocular history taking (history of atopy or other ocular allergies, contact lens wearing, previous ocular surgeries or trauma, and family history of KC).
- Distance visual acuity with and without correction.
- Evaluation of tear film and ocular surface.
- Dilated fundoscopic examination.
- Evaluation of ocular motility and alignment.
- Measurement of intraocular pressure.
- Evaluation of corneal topography (Scheimpflug tomographer imaging system).
Verbal consent was taken from the patients as the study included no extra procedure than the routine medical considerations. All patients’ names were replaced by numbers to maintain privacy of the patients.
According to data collected from Oculus Pentacam HR, cases with abnormal corneal topography were detected and those who met criteria of corneal ectasia were recorded and analyzed.
Topographic patterns characterizing irregularity 
Patterns of the anterior curvature map may take a bow tie shape, a hot spot shape, or an irregular shape:
- Hot spot patterns: oval, round, superior steepening (SS), and inferior steepening (IS).
- Bow tie patterns: symmetric bow tie, asymmetric bow tie/IS, asymmetric bow tie/SS, asymmetric bow tie/skewed steepest radial axis index (SRAX), butterfly pattern, claw pattern, and junctional pattern.
- Irregular pattern: steep areas are mixed with flat areas.
When more than one of the following criteria were found, any of the aforementioned mentioned patterns was considered as frank KC, early stage KC, or at least a case of corneal ectasia suspicion according to the severity and amount of the following criteria .
On the sagittal curvature map
- Steep K more than or equal to 47 D.
- K-max ‘maximal K reading’ more than or equal to 47 D.
- Difference in K-max between the two eyes more than or equal to 2 D.
- Skewed radial axis (SRAX) more than 22°.
- Curvature power at the superior point ‘S’ is greater than that at the inferior point ‘I’ (S–I) more than or equal to 2.5 D.
- Curvature power of inferior point is greater than that at the superior point (I–S) more than or equal to 1.5 D.
On the elevation map
- Skewed or irregular hourglass, isolated island, or tongue-like extension.
- Plus values more than 12 µm on the anterior elevation map (within central 5 mm zone).
- Plus values more than 15 µm on the posterior elevation map (within central 5 mm zone).
Corneal thickness map
- Thickness at the inferior point is less than that at the superior point by more than or equal to 30 µm.
- Thickness at the thinnest location in one eye differs from that of the fellow eye by more than or equal to 30 µm.
- If the thinnest location is displaced from the corneal apex inferiorly or infero-nasally or infero-temporally by a value (Y axis) that exceeds −500 μm, it is a risk factor, and when the value exceeds −1000 μm, it is an important sign of advanced KC.
Pellucid marginal degeneration topography 
- Curvature map: low corneal power along vertical axis, increased power as the inferior cornea is approached, and high corneal power along the inferior oblique meridians, making a crab claw pattern appearance.
- Elevation maps: the ‘kissing bird’ sign appears only if the cone is peripheral in the ‘best-fit sphere’ BFS float mode (only present when the cone is peripheral).
- Corneal thickness map: reveals a thinning of the inferior cornea ‘bell shape,’ and this sign is the hall mark for PMD; the amount of displacement of the corneal thinnest location on Y coordinate is much bigger in PMD than in KC.
Keratoconus curve diagram
In KC and PMD, the red line does not take the same slope of the normal range; instead the curve deviates from the normal range rapidly. In advanced cases, the curve usually takes an inverted passage.
Based to Amsler–Krumeich classification , cases with KC were graded as follows:
The diagnosis of KG is mainly a clinical one (globular corneal protrusion and diffuse thinning from limbus to limbus); however, corneal thickness map will show generalized thinning .
| Results|| |
This study provided an evaluation of prevalence of corneal ectasia among candidates of refractive surgery, with a total of 2878 eyes included.
Data analysis was done by four maps selectable display, as shown in [Figure 2], and refractive display, as shown in [Figure 3].
Among the examined 2878 eyes, only 84 eyes came positive for having KC and subclinical KC (3.0%) ([Figure 4]).
[Table 1] shows the characteristics of studied eyes stratified by KC type. Mean age was 31.62±6.67 years (range, 20–47 years), and the study population included 23 (54.76%) male and 19 (45.24%) female patients ([Figure 5]).
|Figure 5 Comparison between male and female regarding KC grade among the study shows slight male predilection. KC, keratoconus.|
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Patients with subclinical KC represented the majority of cases (51.2%). KC1 was found to be the commonest among other KC grades (22.6%) followed by KC2 (20.2%) and KC3 (6.0%). In the same context, we have not found any eyes with either KG or PMD among the 2878 examined eyes.
Regarding the error of refraction, 71.4% of cases had compound myopic astigmatism (CMA), 14.3% had simple myopic astigmatism (SMA), 10.7% had mixed astigmatism (MA), 2.4% had myopia, and 1.2% showed no error of refraction ([Figure 6]).
|Figure 6 Comparison between different KC grades regarding errors of refraction. KC, keratoconus.|
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[Table 2] shows the mean values of subjective refraction done for patients included in our study and mean values for their Pentacam parameters regarding KC grade. Mean spherical and cylindrical refractive errors were −2.21±2.44 and −2.53±1.93 D, respectively. Mean best-corrected visual acuity was 0.85±0.21 Decimal. Pentacam measurements yielded mean K1 and K2 of 43.81±2.07 and 46.41±2.59 D, respectively. Mean minimal pachymetry value was 494.93±40.78 μm.
|Table 2 Mean values of subjective refraction and Pentacam parameters of studied eyes (stratified by keratoconus grade)|
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[Table 3] shows that five eyes of three (6.0%) patients were diagnosed as pellucid-like KC (PLK). Crab claw pattern in anterior sagittal map was positive in all cases (100%), and kissing bird sign was positive in two (40%) cases ([Figure 7]).
|Table 3 Characteristics of studied eyes with pellucid-like keratoconus (stratified by keratoconus type)|
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|Figure 7 Differences between topographical patterns found among cases with PLK. PLK, pellucid-like keratoconus.|
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Regarding cone location, the cone was paracentral in three (60%) eyes and peripheral in two (40%), and no cases showed central cones in anterior elevation map ([Figure 8]).
|Figure 8 Illustrating cone location in patients with PLK. PLK, pellucid-like keratoconus.|
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A total of three (60%) eyes with PLK had SMA and two (40%) eyes had MA, whereas none of the five eyes had CMA.
| Discussion|| |
KC and PMD are generally progressive conditions in which thinning occurs in the central, paracentral, or peripheral cornea, resulting in asymmetric corneal steepening and reduced spectacle-corrected visual acuity. These two conditions may be separate entities or different clinical expressions of the same ectatic process; in either case, they are currently contraindications for excimer laser surgery, owing to the high risk of developing post-LASIK ectasia .
Published rates for KC prevalence vary from 5 and 23 per 10 000 . Although this would appear to be a reflection of regional differences, it may also represent a disparity in presentation and diagnostic criteria. The use of corneal topography has significantly increased our ability to diagnose mild and subclinical cases of KC .
In the current study, we reviewed the files of 1439 patients undergoing consultation for laser refractive surgery in the period between June 2017 and December 2017 using data from the Scheimpflug tomographer imaging system, and of the total 1439 patients examined, 42 (3.0%) patients were diagnosed to have KC, and no eyes with PMD or KG were found.
This was in contrary to Saro et al.  who reported cases with KC in Sohag University Hospital and Future Centre in Upper Egypt using Scheimpflug camera for screening refractive surgery candidates in the period from April to September 2015, and the prevalence was 17.5%.
Saro and colleagues suggested that the unexpected high prevalence could be owing to the hot dry weather and the increased prevalence of allergic conjunctivitis and eye rubbing in Upper Egypt.
In our study, the mean thinnest location value was 494.93±40.78 μm, whereas in the study by Saro and colleagues, it was 450.71±54.06 µm.
Regarding the stage of KC, the present study found stage 1 KC in 19 (22.6%) eyes of the total 84 eyes of the study sample, making the highest prevalence compared with other stages, followed by stage 2 found in 17 (20.2%) eyes then stage 3 found in five (6.0%) eyes, and the rest of the 43 (51.2%) eyes were diagnosed as subclinical KC; stage 4 KC was not observed among our patients, which may be owing to the far possibility of being accidently discovered at that late stage. These results were similar to the study by Saro and colleagues as KC stage 1 was the commonest (47.1%) followed by stage 2 (35.2%), stage 3 (7.7%), and stage 4 (10%).
In 2002, Hori-Komai et al.  screened 2784 patients requesting refractive surgery and found that 1.62% have KC using corneal topography and the Klyce-Maeda KC index to detect early KC.
In 2003, Ambrósio et al.  used Atlas anterior topography, biomicroscopy, and ultrasonic pachymetry to screen for corneal abnormalities in 1392 candidates for refractive surgery, and 12 (0.8%) were found to have frank KC or suspect KC.
In 2010, Bamashmus et al.  used pachymetry, biomicroscopy, and corneal topography, and of 2091 refractive surgery candidates, 5.8% were classified as poor candidates because of frank KC or FFK.
In 2013, Xu et al.  used ultrasonic pachymetry and corneal Orbscan corneal topography to screen 552 patients seeking refractive surgery and found that 13 (2.3%) patients have Forme Fruste or manifest KC.
In 2014, Hodge et al.  used corneal topography to assess 6101 refractive surgery candidates and 2.5% were diagnosed with KC and suspect KC.
In 2018, Al-Amri  used Pentacam HR system to screen 2931 patients from Aseer province, a southern, high altitude region in Saudi Arabia, and 24.0% were diagnosed as frank KC and 17.5% were diagnosed as KC suspects.
In our study, five eyes of three patients (representing 6.0% of the study sample) were detected showing topographical signs mimicking PMD; however, bell sign was absent in pachymetric map. Thus, they were considered to be PLK. Moreover, two patients had bilateral PLK and 1 patient had PLK in one eye and KC stage 1 in the other. Crab claw pattern in anterior sagittal map was positive in all cases (100%). The cone was paracentral in three (60%) eyes and peripheral in two (40%) eyes, and no cases showed central cones in anterior elevation map. Kissing bird sign was positive in two (40%) cases.
PLK cases’ refractive errors were either SMA or MA. This could be owing to the more peripheral steepening occurring with crab claw pattern causing more flattening in central parts of the cornea, as shown in [Figure 9].
|Figure 9 Four maps selectable display patient number 73, steepening of the peripheral part of the cornea, and flattening of the central part.|
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These findings agree with the study by Mazen and Lara (2012)  as regards crab claw pattern in anterior sagittal map, which was positive in all 11 (100%) cases with PLK. Moreover, according to cone location, the paracentral cone was of the highest prevalence (72.7%). They also found two (18.2%) cases with peripheral cones and kissing bird sign (18.2%) was positive in their anterior elevation map. The study recorded one (9.1%) case with central cone.
No cases of PMD or KG were found in our study which could be owing to the short period of time and the rarity of the disease. Moreover, KG is known to begin at a younger age than that included in our study sample.
Concerning errors of refraction, depending on subjective refraction done for all patients, CMA was the most common refractive error found among patients in our study sample as 62 of 84 eyes were found to have CMA, representing 73.8%. SMA was the second most common refractive error as it was found in 11 eyes, representing 13.09%, followed by MA found in eight (9.52%) eyes. Only two (2.3%) eyes had myopia and one (1.1%) patient showed no error of refraction in one eye.
These results are nearly similar to the study by Qasim and Shahd  who found CMA in majority of keratoconic patients (69.7%), followed by SMA (25.4%) and MA (3.2%) and then simple myopia (1.7%).
The three most common refractive errors (CMA, SMA, and MA) were found among all grades of KC; however, CMA was more common in subclinical KC (36 eyes) and KC1 (15 eyes). SMA and MA were more common in KC2 (six eyes for each). Moreover, one myopic eye was found in KC2 and KCS, whereas one eye with subclinical KC had no error of refraction.
The hyperopic component of MA in KC could be explained in mild forms of KC by the flatter corneal curvature close to emmetropia, whereas in moderate to severe cases could be owing to the inferior displacement of the cone and distortion of corneal surface causing flattening of the central or paracentral parts of the cornea as shown in [Figure 10].
|Figure 10 Four maps selectable display of patient number 1, steepening of corneal curvature in anterior sagittal map, and inferior displacement of the cone with flattening in other areas of corneal surface.|
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| Conclusion|| |
- An estimated prevalence of cases accidently discovered with frank and subclinical KC among keratorefractive surgery candidates was found to be 3.0%, with a slight male predilection (54.8%). Subclinical KC and KC1 represented the highest prevalence compared with other KC stages, being present in 51.2 and 22.6%, respectively. Different types of refractive errors could be present at different grades of KC, yet CMA is the most common refractive error associated with KC, being present in 71.4% of cases.
- A KC pattern that could be confused with PMD pattern called PLK was found in 6.0% of cases.
- No cases of PMD or KG were found.
- Precise preoperative preparations for refractive surgery including general and ocular history taking, uncorrected visual acuity and best-corrected visual acuity, ocular examination with slit lamp biomicroscopy, dilated fundus examination, and evaluation of corneal topography are mandatory before surgery.
- Counseling and educating the candidates planning for surgery about the preoperative examinations and the steps of the procedure.
- Scheimpflug camera-based system (Pentacam) presents plentiful topographic and pachymetric data, which is of massive importance for refractive surgeons to identify corneas with high risk of developing ectasia after surgery and detect cases with subclinical KC that cannot be diagnosed clinically.
- Early diagnosis of KC and other forms of corneal ectasia is of a great value as it may provide these patients a chance to avoid major surgical intervention.
- Although ectatic corneal diseases induce index myopia and irregular astigmatism, hypermetropic patients with suspicious corneal topography should not be eliminated from the possibility of having the disease.
Conflicts of interest
There are no conflicts of interest.
| References|| |
Spadea L, Cantera E, Cortes M, Conocchia NE, Stewart CWM. Corneal ectasia after myopic laser in situ keratomileusis: a long-term study. Clin Ophthalmol 2012; 6:1801–1813.
Weed KH, MacEwen CJ, Giles T, Low J, McGhee CN. The Dundee university Scottish keratoconus study: demographics, corneal signs, associated diseases, and eye rubbing. Eye (Lond) 2008; 22:534–541.
Sridhar MS, Mahesh S, Bansal AK, Nutheti R, Rao GN. Pellucid marginal corneal degeneration. Ophthalmology 2004; 111:1102–1107.
Feder RS, Kshettry P. Noninflammatory ectatic disorders. In Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea: fundamentals, diagnosis and management. 3rd. New York: Elsevier Mosby; 2005.
McLeod SD, Chuck RS, Hamilton DR, Jacobs DS, Katz JA, Keenan JD et al.
Refractiver errors and refractive surgery Preferred Practice Panel. Am Acad Ophthalmol 2013; 19:92.
Binder PS. Analysis of ectasia after laser in situ keratomileusis: risk factors. J Cataract Refract Surg 2007; 33:1530–1538.
Ambrósio R Jr, Wilson SE. Complications of laser in situ keratomileusis: etiology, prevention, and treatment. J Refract Surg 2001; 17:350–379.
Swartz T, Marten L, Wang M. Measuring the cornea: the latest developments in corneal topography. Curr Opin Ophthalmol 2007; 18:325–333.
Mazen MS. Classification and patterns of keratoconus and ectatic corneal disorders: keratoconus (when, why and why not) a step-by-step systematic approach. Jaypee Highlights, 1st ed, 2012. pp. 1–36.
Mazen MS. Topographical criteria and patterns of keratoconus, keratoectasia and ectatic corneal disorders: corneal topography in clinical practice (Pentacam System) basics and clinical interpretation. Jaypee-Highlights, 2nd ed, 2012. pp. 121–143.
Krumeich JH, Daniel J, Knulle A. Live-epikeratophakia for keratoconus. J Cataract Refract Surg 1998; 24:456–463.
Wallang BS, Das S. Keratoglobus. Eye(Lond) 2013; 27:1004–1012.
Lee BW, Jurkunas UV, Harissi-Dagher M, Poothullil AM, Tobaigy FM, Azar DT. Ectatic disorders associated with a claw-shaped pattern on corneal topography. Am J Ophthalmol 2007; 144:154–156.
Espandar L. Keratoconus: overview and update on treatment. Middle East Afr J Opphthalmol 2010; 17:15–20.
Hodge C, Colin C, Gerard S. Investigation of keratoconus in an Australian refractive population. Clin Exp Ophthalmol J 2014; 12304:1–3.
Saro AS, Radwan GA, Mohamed UA, Abozaid MA. Screening for keratoconus in a refractive surgery population of Upper Egypt. Delta J Ophthalmol 2018; 19:19–23. [Full text]
Hori-Komai Y, Toda I, Asano-Kato XX, Tsubota K. Reasons for not performing refractive surgery. J Cataract Refract Surg 2002; 28:795–797.
Ambrósio RJr, Klyce SD, Wilson SE. Corneal topographic and pachymetric screening of keratorefractive patients. J Refract Surg 2003; 19:24–29.
Bamashmus MA, Saleh MF, Awadalla MA. Reasons for not performing keratorefractive surgery in patients seeking refractive surgery in a hospital based cohort in ‘Yemen’. Middle East Afr J Ophthalmol 2010; 17:349–353.
Xu K, McKee HD, Jhanji V. Changing perspectives of reasons for not performing laser-assisted in situ keratomileusis among candidates in a university eye clinic. Clin Exp Optom 2013; 96:20–24.
Al-Amri AM. Prevalence of keratoconus in a refractive surgery population. J Ophthalmol 2018; 2018:1–5.
Mazen MS, Lara NY. Case report, pellucid like keratoconus. F1000Research 2012; 1:48.
Qasim KF, Shahad A. Prevalence of refractive errors in patients with keratoconus among sample of Iraqi population. Open Access J Ophthalmol 2017; 2:1–8.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
[Table 1], [Table 2], [Table 3]