Central corneal thickness in high myopia

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  • Central corneal thickness in highmyopiaLene Pedersen, Jesper Hjortdal and Niels Ehlers

    Department of Ophthalmology, Aarhus University Hospital, Arhus Sygehus, Arhus,

    Denmark

    ABSTRACT.Purpose: To examine if central corneal thickness (CCT) is different in emme-tropia and high myopia.Methods: 57 emmetropic subjects (0 to + 1.5 D) and 48 high myopes (all morethan 6 D in spherical equivalent refraction) were studied. CCT was measuredby a HaagStreit Optical Low-Coherence Reflectometry (OLCR) pachymeter,a recently developed high precision pachymeter with a standard deviation (SD)for repeated measurements of 1 mm.Results: Mean CCT for the emmetropic group was 538.6 mm (SD = 32.1), and forthe myopic group 527.7 mm (SD = 35.0). Neither the mean CCT nor the variancefrom the two groups showed a statistically significant different (p > 0.05).Conclusion: CCT is not systematically altered in myopia. The process by whichthe myopia progresses does not to a measurable degree influence the centralcornea.

    Key words: central corneal thickness high myopia optical low - coherence reflectometry

    pachymeter

    Acta Ophthalmol. Scand. 2005: 83: 539542Copyright# Acta Ophthalmol Scand 2005.

    doi: 10.1111/j.1600-0420.2005.00498.x

    IntroductionAlthough the true aetiology of myopiais still unknown (Goldschmidt 2003),the cornea is responsible for approxi-mately two-thirds of optical refractionand its role in myopia has consequentlybeen studied intensely over the years.Known possible changes in the highlymyopic eye are all located in the pos-terior segment: staphyloma, myopicconus, choroidal atrophy, thinning ofthe retina and sclera. Changes in theanterior segment associated with myo-pia are still under debate. Carney et al.(1997), among others, found that themyopic cornea has a steeper central

    corneal curvature, while Chang et al.(2001) found no correlation betweencorneal curvature and central cornealthickness (CCT). The myopic eye isknown to be longer than the normalemmetropic eye (see, e.g. Touzeauet al. 2003). If this is the result of gen-eral growth, one might expect the cor-nea to have grown to be thicker than isnormal, in which case a correlationwith body mass index (BMI) mightexist. If instead the myopic eye is largerdue to a mechanism similar to that of aballoon being inflated, one wouldexpect the cornea to be thinner thannormal, according to a simple

    stretching theory. An emmetropiceye could then be compared to asphere, and a myopic eye to a prolatespheroid.von Bahr (1956), Price et al. (1999)

    and Touzeau et al. (2003), amongothers, looked for a connection betweenCCT and myopia. Measurements weretaken with different types of pachy-meters and with different set-ups, andwith inconclusive results (Table 1).Today, there are many methods of

    measuring CCT (Ehlers & Hjortdal2004), primarily based upon opticaland ultrasound principles.The precision (standard deviation,

    SD) of manual optical pachymetry is8 mm, with intraobserver errors of56 mm and interobserver errors of20 mm (Olsen et al. 1980). Ultrasoundpachymetry has better precision (Tam& Rootman 2003), but results mayvary due to applanation force and dif-ferences between types of instruments(Salz et al. 1983). The Orbscan system() is an automatic and non-contactoptical pachymeter and topographwith a precision similar to that of man-ual optical pachymetry (Yaylali et al.1997).The apparatus used in this study was

    an optical low-coherence reflectometry(OLCR) pachymeter. (Bohnke et al.(1999) reported a precision of about1 mm (SD), an intrasession reproducibil-ity around 0.91.2 mm and a high inter-session reproducibility. Other studieshave compared the OLCR pachymeterwith the ultrasound pachymeter. Walti(1998) found the OLCR pachymeter tobe faster and more precise (lower SD)than an ultrasound pachymeter. TheCCT values were found to be

    ACTA OPHTHALMOLOGICA SCANDINAVICA 2005

    539

  • significantly higher when measuredwith an ultrasound pachymeter thanwhen measured with the OLCR pachy-meter (Ventura et al. 2001; Genth et al.2002). Today, the OLCR seems to bethe most precise instrument to measureCCT.The purpose of this study was to

    determine whether CCT differs inemmetropia and high myopia usingthe OLCR pachymeter. Documenteddifferences between these groupsmight contribute to the ongoing discus-sion on the aetiology and pathogenesisof myopia.

    Material and MethodsSubjects

    All subjects were white and agedbetween 18 and 55 years. Subjectswith previous eye surgery, glaucoma,diabetes mellitus or other acute orchronic diseases possibly affecting thecorneal thickness were excluded.The emmetropic group included 57

    subjects, all with normal visual acuityand refraction (self-reported to be from0 to 1.5 D spheq).Thegroupconsistedof volunteers sourced from hospital staff,students and patients relatives.The myopic group was recruited from

    subjects referred for refractive surgeryfor high myopia. Only eyes with a mini-mum of 6 D in spherical equivalent

    refraction and a maximum of 2 D cor-neal astigmatism were accepted.The refractive criteria were set to

    ensure that only highly myopic eyeswere included in the group and thatcases with subclinical keratoconuswere excluded. On this basis, the studyincluded 48 highly myopic subjects withan average subjective refraction of 8.93 D (ranging from 6 to 15.75D, SD 2.02 D) in spherical equiva-lent and an average astigmatism of 0.81 D cylinder.Those who normally used contact

    lenses (24 subjects) did not use them forat least 48 hours before examination.The subjects in both groups answered

    questions on height and weight, fromwhich BMI was calculated.Central corneal thickness and axial

    length were measured in both eyes,without touching or medicating theeyes, but data from only one eye fromeach subject was included. The datapertained to the right eye in 99 subjects,but to the left eye in six subjects. Thiswas due to previous retinal detachmentin the right eye in one case, a previoussevere trauma in the right eye in onecase, the fact that the right eye did notfulfil the refractive criteria in threecases, and because the right eye wasunder topical anaesthetic in one case.Table 2 summarizes data character-

    izing the two groups, the only statistic-ally significant difference being the

    axial length, which was 3 mm longerin the myopic group.

    Equipment

    The OLCR pachymeter was attachedto a BC 900 slit-lamp (both fromHaag-Streit,) (Ballif et al. 1997; Genthet al. 2002). The OLCR pachymeterwas calibrated and used as describedin the instruction manual. A cornealrefractive index of n 1.376 wasassumed. The OLCR pachymeter isan automatic non-contact pachymeterbased on interferometry. The pachymeteris easy to use and an average CCTmeasurement (based on 20 scans) takes28 seconds including adjustment of theapparatus (Walti 1998).All measurements were performed

    by the same examiner (LP). EachOLCR measurement is the result of 20consecutive scans, where the five upperand five lower scan values are deleted.The OLCR pachymeter calculates amean CCT and a standard deviationfrom the remaining 10 scans.To confirm the reported precision of

    the OLCR pachymeter, the right eyesof five normal subjects were measured10 times. Each subject was repositionedbetween each measurement. The mea-surements of one subject were com-pleted within 30 mins. The averageintersession precision (SD) for five sub-jects was 1.0 mm corresponding with

    Table 1. Overview of previously published papers with information on myopia and central corneal thickness.

    Authors and year Country Equipment No of subjects Refractive range Results

    CCT and myopia

    Total Myopic

    Kunert et al. 2003 India Ultrasound/

    Orbscan

    615 615 Up to 20 D sph(< 3 D cyl)

    Thicker CCT in high myopic

    Touzeau et al. 2003 France Orbscan 95 95 eyes 9.16 to19.23 D spheq

    Thinner CCT when myopic

    Srivannaboon 2002 Thailand Orbscan 280 280 0.5 to 18 D sph Thinner CCT in high myopicChang et al. 2001 Taiwan Ultrasound 216 ? Roughly 7

    to 22 D spheqThinner in high myopic

    Liu & Pflugfelder 2000 China Orbscan 30 30 0.75 to 10.25 D sph No correlationCho & Lam 1999 China Ultrasound 151 ? 1.63 to 13.50 D spheq No correlation (including high myopic)Price et al. 1999 USA Ultrasound 450 ? 0 to 30 D sph No correlationTanaka et al. 1996 Brazil Ultrasound 70 25 3.2 to 25.5 D spheq No correlationAlsbirk 1978 Greenland Optical 325 ? ? Thinner CCT when myopic

    Ehlers & Hansen 1976 Denmark Optical 101 ? ? No correlation

    Hansen 1971 Denmark Optical 113 ? 3.0 up to 5 D No correlationMartola & Baum 1968 USA Optical 121 ? Up to more than 6 D No correlation

    (Boston) (tendency to be thicker)

    von Bahr 1956 Sweden Optical 125 12 eyes More than 3 to morethan 4 D

    Thinner CCT when myopic (> 4 D)

    Blix 1880 Sweden Optical 8 2 Hypermetropic to myopic No difference

    ACTA OPHTHALMOLOGICA SCANDINAVICA 2005

    540

  • previous findings (Bohnke et al. 1999).High and low CCT values were deter-mined with the same SD.

    Ethical approval

    The study was conducted in accord-ance with a protocol approved by thelocal ethics committee of Arhus inaccordance with the Helsinki DeclarationII. Informed consent was obtainedfrom all persons included in the study.

    Statistics

    The material was tested by kurtosis andskewness tests and was found not todeviate from a normal distribution.Paired t-test was used to establishwhether there was any differencebetween right and left eyes. Studentst-tests and F-tests were used to com-pare mean values and variancesbetween the two groups.

    ResultsThe mean CCT for the emmetropicgroup was 538.6 mm (SD 32.1 mm,range 459.9606.0 mm), and for the myo-pic group 527.7 mm (SD 35.0 mm,

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