serdarogullari prevalencia queratocono pentacam jovr 2013

8/10/2019 Serdarogullari Prevalencia Queratocono Pentacam JOVR 2013

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Original Article

JOURNAL OFOPHTHALMIC ANDVISIONRESEARCH2013; Vol. 8, No. 3

Prevalence of Keratoconus and SubclinicalKeratoconus in Subjects with Astigmatism

Using Pentacam Derived ParametersHuseyin Serdarogullari1, MD; Mehmet Tetikoglu2, MD; Hatice Karahan1, MD

Feyza Altin1, MD; Mustafa Elcioglu1, MD

1Department of Ophthalmology, Okmeydani Education and Research Hospital, Istanbul, Turkey2Eye Clinic, Mu State Hospital, Mu, Turkey

Purpose: To determine the prevalence of keratoconus (KCN) and subclinical KCNamong subjects with two or more diopters (D) of astigmatism, and to compare Pentacamparameters among these subjects.Methods: One hundred and twenty eight eyes of 64 subjects with astigmatism 2D wereincluded in the study. All subjects underwent a complete ophthalmic examination whichincluded refraction, visual acuity measurement, slit lamp biomicroscopy, retinoscopy,fundus examination, conventional corneal topography and elevation-based topographywith Pentacam. The diagnosis of KCN and subclinical KCN was made by observingclinical findings and topographic features; and confirmed by corneal thickness andelevation maps of Pentacam. Several parameters acquired from Pentacam were analyzedemploying the Mann-Whitney U Test.Results: Mean age of the study population was 29.99.8 (range 15-45) years whichincluded 39 (60.9%) female and 25 (39.1%) male subjects. Maximum corneal power,index of vertical asymmetry, keratoconus index and elevation values were significantly

higher and pachymetry was significantly thinner in eyes with clinical or subclinicalKCN than normal astigmatic eyes (P< 0.05).Conclusion: The current study showed that subjects with 2D or more of astigmatismwho present to outpatient clinics should undergo corneal topography screening forearly diagnosis of KCN even if visual acuity is not affected. Pentacam may providemore accurate information about anterior and posterior corneal anatomy especially insuspect eyes.

Keywords: Keratoconus; Subclinical Keratoconus; Pentacam; Scheimpflug; Astigmatism; CornealTopography

J Ophthalmic Vis Res2013; 8 (3): 213-219.

Correspondence to:Huseyin Serdarogullari, MD. Department of Ophthalmology, Okmeydani Education and ResearchHospital, Talatpasa Mah. Aslangazi Cad. 28 / 18 Okmeydani, Istanbul, Turkey; Tel: +90 5079 477 554, Fax: +90 212 221 7800;email: [email protected]

Received:June 16, 2012 Accepted:January 14, 2013

INTRODUCTION

Keratoconus (KCN) is a chronic, bilateral,non-inflammatory disorder characterized byprogressive steepening, thinning and apical

scarring of the cornea. The annual incidence ofKCN is 2 per 100,000 with a prevalence of 54.5per 100,000 (approximately 1 per 2,000).1-3KCNusually manifests as progressive myopia andirregular astigmatism, with unique slit lamp


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Diagnosing KCN with Pentacam; Serdarogullari et al

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findings. The diagnosis of advanced KCN canbe easily made because of its characteristicbiomicroscopic and topographic findings,but identification of subclinical cases may be

extremely challenging.1,4 Refractive surgerycandidates should be examined meticulouslysince keratorefractive treatment may exacerbatethe clinical progression of this ectasia.5,6

The diagnosis of subclinical KCN can betenuous. The term subclinical KCN is usedto state a premature form of the disease inwhich characteristic keratometric, retinoscopicor slit lamp findings are absent, but mildtopographic changes are consistent with clinicalKCN.7 Identification of subclinical KCN ischallenging as diagnostic criteria should bedetermined.8 It is usually not easy to predictthe natural course of the disease because cornealalterations start generally before a patient isreferred to an ophthalmologist. During puberty,the cornea starts to get thinner and protrudes,causing irregular astigmatism. In the stationaryphase of the disease, the condition may varyfrom mild irregular astigmatism to severethinning.9,10 Several methods with differentcorneal topographers have been introduced toaid the diagnosis of KCN and subclinical KCN.

The Pentacam (Oculus OptikgerteGmbH, Wetzlar, Germany) is a state-of-the-artinstrument that scans the anterior and posteriorcornea with a rotating Scheimpflug camera.Repeatability and reproducibility of cornealthickness and posterior elevation measurementshas been reported to be high.11,12 However,unlike the Orbscan topographer (Bausch &Lomb Inc., Rochester, NY, USA), it is not clearwhat comprises normal or abnormal posteriorelevation of the cornea measured with thistechnique.10,13

The aim of the current study was to determinethe prevalence of KCN and subclinical KCNin subjects with astigmatism of two diopters(2D) or greater using data from the PentacamScheimpflug tomographer.

METHODS

Consecutive subjects (aged 15 to 45 years) with2D or greater of astigmatism, presenting to our

ophthalmology outpatient clinic for routinecheck-ups were included in this cross-sectionalstudy. All subjects underwent a completeophthalmic examination that included visual

acuity measurement with an ETDRS chart,slit lamp biomicroscopy, retinoscopy, fundusexamination and corneal topography (TMS-4;Tomey, Erlangen, Germany). Best correctedvisual acuity (BCVA), retinoscopic, slit lamp

biomicroscopic and funduscopic findings wererecorded. Only subjects with no signs of otherocular pathology were included.

An eye was diagnosed as having KCN ifthere was a scissoring reflex on retinoscopy andcentral or paracentral steepening of the corneaon topography with at least 1 of the followingslit lamp findings: stromal thinning, anterior

bulging of cornea, Vogt striae, Fleischer ring,Descemets breaks, apical scars, and subepithelialfibrosis.14An eye was diagnosed with subclinicalKCN if it was the fellow eye of a patient withKCN or showed the following features: normalcornea by slit lamp biomicroscopy, normalkeratometry, retinoscopy and ophthalmoscopy

but inferior-superior asymmetry or bow-tiepattern with skewed radial axes detected ontangential maps. Subjects not fulfilling these

criteria were classified in the normal astigmaticgroup.A single experienced observer, who was

masked to the patient diagnosis, performed allPentacam measurements. Briefly, the subjectwas asked to place his/her chin on the chinrest and the forehead against the head rest. Thesubject was asked to open both eyes and lookat the fixation target. The examiner aligned the

joystick until the rotating Scheimpflug cameraautomatically captured 25 single images within2 seconds for each eye. The measurements werechecked under the quality specification window;only correct measurements were accepted(comment box reading OK). If the comment

box was marked yellow or red, the examinationwas repeated. Maps with poor centration wererepeated in order to provide a best-fit toric/ellipsoid reference surface. Subjects with cornealsurface irregularity were re-examined withPentacam after using artificial tears for 2 weeks.From the Pentacam examination, flat (K1) and


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steep (K2) keratometric readings, maximumsimulated keratometry (Kmax), corneal thicknessat the thinnest point of the cornea (minimalpachymetry), index of surface variance (ISV),

index of vertical asymmetry (IVA), KCN index(KI), anterior elevation (AE) and posteriorelevation (PE) were recorded into an Excelworksheet.

All statistical analyses were performedusing IBM SPSS Statistics version 20 (SPSS Inc.,Chicago, IL, USA). The bootstrap method wasapplied to eliminate the correlation between eyesof the same subject by treating each subject asa cluster. For group comparisons of continuousvariables, the Mann-Whitney nonparametric testwas used. P values 0.05, for all comparisons).

Table 1 details Pentacam parameters in thethree study groups. There were statisticallysignificant differences between KCN eyes andastigmatic eyes in all Pentacam parametersexcept for cylindrical refractive error (P


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DISCUSSION

The prevalence of KCN among refractive surgerycandidates has been reported to vary from 0.9%to 8.1%.15,16Corneal pachymetry in keratoconiccorneas has been studied with variousinstruments17and many studieshave comparedthe reproducibility and reliability of instrumentswith different operating principles.15,18

The Pentacam, which is based on theScheimpflug principle, measures 138,000elevation points. This technique can be used as ascreening test for detecting KCN since it worksindependent of axis, orientation and position

of the eye. It has been shown that elevation-based topography systems are more accuratethan Placido-based devices in differentiatingnormal eyes from early KCN.16

It is not difficult to detect clinically advancedKCN on the basis of slit lamp biomicroscopicfindings. However, it can be problematicto distinguish between KCN suspects andnormal eyes only with topographic criteria.There are no definite or universally accepted

diagnostic criteria for defining KCN suspects.For instance, anterior corneal topography maypresent increased asymmetry such as localizedsteepening of the inferior cornea without othercharacteristic criteria for both keratoconus andcontact lens-induced corneal warpage. Theremay be asymmetry between the right and theleft eyes, i.e. reduced enantiomorphism. Subjectswith steep keratometric values and progressiveastigmatism may also have subclinical KCN.17

In patients seeking refractive surgery, ahigh incidence of forme fruste KCN and KCNsuspect has been reported in early studies. Someinvestigators stated that this overdiagnosiswas related to the sagittal-based curvaturemeasurements by Placido-based topographysystems.19,20Many patients, who had previously

been labelled as keratoconic, were found to havea displaced corneal apex. It has been pointed outthat these eyes demonstrate elevated I-S ratio,normal pachymetry, orthogonal astigmatismand stable refractions with no other clinical or

Figure 2. Distribution of anterior elevation (AE) ineyes with keratoconus, subclinical keratoconus andastigmatism.

Figure 1. Distribution of minimal pachymetry (Pachy)in eyes with keratoconus, subclinical keratoconus and

astigmatism.

Figure 3. Distribution of posterior elevation (PE) ineyes with keratoconus, subclinical keratoconus andastigmatism.


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topographical feature of KCN.21

Lim et al22found that more than one thirdof subjects with unilateral KCN developedmanifest KCN in the other eye over 8 years.

These authors reported that mean values ofmaximum posterior elevation and irregularitywere significantly higher in KCN and KCN-suspects than control eyes.

Recently, Ucakhan et al15 investigatedseveral Pentacam parameters in subclinicalKCN, KCN and normal eyes. They found thatthe Scheimpflug system could differentiate

between ectatic and normal eyes. In this study,the optimum cut-off point for posterior elevationwas found to be 26.5 m (97.7 % sensitivity and81.0 % specificity). Logistic regression analysisindicated that a model combining corneal power,thickness and elevation data produced the bestpredictive accuracy in KCN and subclinicalKCN.15

In a study by Mihaltz et al23 ROC curveanalysis indicated that posterior elevation wasthe most important criterion in the diagnosisof KCN. A threshold value of 15.5 m hadsensitivity of 95.1% and specificity of 94.3% fordifferentiating normal eyes from KCN. Theseauthors found lower pachymetry readings in

subclinical, early and moderate KCN; however,they did not find significant differences inthese parameters between subclinical KCN andnormal eyes.23

Pinero et al24investigated corneal volume,pachymetry and the correlation between anteriorand posterior corneal shape in normal and KCNeyes. They found lower pachymetry readingsin subclinical and clinical KCN, but found noremarkable differences in these measurements

between subclinical KCN eyes and normaleyes.24

Based on topographic maps from thePentacam, Vejarano25stated that KCN shouldbe highly suspected in eyes with anteriorelevation greater than 15 m and posteriorelevation greater than 20 m using the bestfit toric ellipsoid, corneal thickness less than500 m, and keratometric power greater than47D on the tangential map when all are atthe same corresponding points. Subjects whohad anterior elevation between 12 and 15 m,

posterior elevation between 15 and 20 m witha corresponding location of thinnest pachymetrypoint less than 500 m were diagnosed as KCNsuspects.25

When screening patients for ectasia usingthe Pentacam, higher sensitivity and specificityhave been achieved by combining pachymetricgraphs and indices, and the enhanced elevationmaps provided by the Belin/Ambrosio EnhancedEctasia Display.26In our study, the mean valueswere compatible with the results of other studiesand well correlated with the Belin/AmbrosioEnhanced Ectasia Display.

In our study, 14.1% of patients attendingthe general ophthalmology outpatient clinicwith astigmatism of 2D or greater had somedegree of KCN (6.3% of eyes had KCN and7.8% had subclinical KCN). When we comparethis prevalence with other studies, it is obviousthat higher prevalence rates for KCN will befound as cylindrical power increases. Since thisstudy is limited by sample size, our results maynot reflect the actual prevalence of KCN in thepopulation with 2D of astigmatism. However,our study showed that various parametersacquired from Pentacam measurements canhelp discriminate eyes with varying degrees of

KCN from normal eyes. The analysis of thesemeasurements is important in differentiatingeyes with subclinical KCN from normal eyes.Sometimes analyzing curvature maps separatelymay mislead ophthalmologists in the diagnosisof KCN. It has been emphasized that anteriorelevation, posterior elevation and thinnestpachymetry values appear to be the most crucialcomponents in the diagnosis and follow upof KCN patients.8 While assessing KCN withPentacam, tangential curvature maps shouldalways be combined with corneal thickness andelevation maps.

In conclusion, the diagnosis of KCN is anincreasingly important clinical issue as manymethods are being developed such as cross-linking to halt the progression of this ectaticdisease. The incidence of KCN is higher thanestimated because of the increasing use ofimaging systems such as the Pentacam. Sincekeratoconus induces irregular astigmatism andmyopia, subjects with astigmatism of 2D or


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greater attending to outpatient clinics shouldbe screened with corneal topography for earlydiagnosis even if their visual acuity is notaffected. We think that Pentacam provides more

accurate diagnosis than conventional cornealtopography systems especially in keratoconussuspect eyes.

Conflicts of Interest

None.

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serdarogullari prevalencia queratocono pentacam jovr 2013

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