The effect of corneal thickness and corneal curvature on pneumatonometer measurements

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  • Abstract

    Purpose. The purpose of this study was to investigate theinfluence of corneal topography and thickness on intraocularpressure (IOP) and pulse amplitude (PA) as measured usingthe Ocular Blood Flow Analyzer (OBFA) pneumatonometer(Paradigm Medical Industries, Utah, USA).

    Methods. 47 university students volunteered for this cross-sectional study: mean age 20.4yrs, range 18 to 28yrs; 23male, 24 female. Only the measurements from the right eyeof each participant were used. Central corneal thickness andmean corneal radius were measured using Scheimpflug biometry and corneal topographic imaging respectively. IOPand PA measurements were made with the OBFA pneu-matonometer. Axial length was measured using A-scan ultrasound, due to its known correlation with these cornealparameters. Stepwise multiple regression analysis was usedto identify those components that contributed significantvariance to the independent variables of IOP and PA.

    Results. The mean IOP and PA measurements were 13.1 (SD3.3) mmHg and 3.0 (SD 1.2) mmHg respectively. IOP mea-surements made with the OBFA pneumatonometer correlatedsignificantly with central corneal thickness (r = +0.374, p =0.010), such that a 10 mm change in CCT was equivalent toa 0.30mmHg change in measured IOP. PA measurementscorrelated significantly with axial length (part correlate =-0.651, p < 0.001) and mean corneal radius (part correlate =+0.459, p < 0.001) but not corneal thickness.

    Conclusions. IOP measurements taken with the OBFA pneu-matonometer are correlated with corneal thickness, but notaxial length or corneal curvature. Conversely, PA measure-ments are unaffected by corneal thickness, but correlatedwith axial length and corneal radius. These parameters

    should be taken into consideration when interpreting IOP andPA measurements made with the OBFA pneumatonometer.

    Keywords: pneumatonometer; intraocular pressure mea-surement; pressure pulse amplitude; corneal thickness;corneal curvature

    Introduction

    The measurement of intraocular pressure (IOP) is of clinicalimportance in the detection and monitoring of primary open-angle glaucoma (POAG).1,2 The influence of corneal topog-raphy on the measurement of IOP by Goldmann applanationtonometry is well documented3 and IOP measurements areknown to be influenced by corneal thickness4 and cornealcurvature.5 These influences are of clinical concern as erro-neous IOP measurements may lead to mislabelling of glaucoma patients and healthy subjects.6 Tomlinson andLeighton7 found the mean corneal radius to be flatter inpatients with normal-tension glaucoma than those withPOAG or normal patients. Copt et al.,8 on correcting IOP forcorneal thickness, calculated that 31% of their patients withnormal-tension glaucoma would be reclassified as havingPOAG and that 56% of their ocular hypertensive patientswould be classified as normal.

    The principle of pneumatonometry uses elastic platetheory.9 A probe tip containing a central tube and surroundedby side exhausts is capped with a thin silastic membrane (Fig. 1). A constant flow of gas passes down the central tube,forcing open a small gap between membrane and tube edge,and is expelled through the side exhausts. When placedagainst a cornea, the cornea is flattened to the outer edges of

    Received: April 11, 2002Accepted: September 4, 2002

    Correspondence: Sarah L. Hosking, Neurosciences Research Institute, School of Life & Health Sciences, Aston University, Aston Triangle,Birmingham, B4 7ET, UK. Tel: +44 (0)121 359 3611 ext. 5172, Fax: +44 (0)121 333 4220, E-mail: s.l.hosking@aston.ac.uk

    The effect of corneal thickness and corneal curvature on

    pneumatonometer measurements

    Andrew J. Morgan, Justine Harper, Sarah L. Hosking and Bernard Gilmartin

    Neurosciences Research Institute, School of Life & Health Sciences, Aston University, Birmingham, UK

    Current Eye Research 0271-3683/02/2502-107$16.002002, Vol. 25, No. 2, pp. 107112 Swets & Zeitlinger

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  • 108 A.J. Morgan et al.

    the probe tip and the membrane gap is forced closed. The gaspressure within the tube rises until it balances the forces onthe other side of the membrane (namely the IOP and themembrane mechanics of the cornea) and the gas can onceagain escape. At this balance point the gas pressure withinthe tube, in comparison to atmospheric pressure, is propor-tional to the IOP and it is from this that a measure of IOP ismade. In addition, when placed against the cornea for anumber of seconds, the high frequency response of the pneu-matonometer provides a method of measuring the small fluctuation in IOP that occurs with each heart beat.10 Theamplitude of this IOP pulsation (PA) originates from therhythmic dilation and contraction of the intraocular, primar-ily choroidal, vasculature.11 The PA measurement has shownclinical utility because it is diminished in low-tension glau-coma patients compared to healthy controls and amplified inocular hypertensive patients compared to POAG patients.12,13

    It has been postulated that these variations are a manifesta-tion of anomalous ocular blood flow: low-tension glaucomapatients exhibiting an impoverished supply and ocular hyper-tensive patients an enhanced and possibly protective bloodflow.14

    IOP measurements taken with pneumatonometers areused clinically15 and PA measurements are principally usedin research.16 However there have been few reports on the

    influence of corneal dimensions on these measurements.Only the association between corneal thickness and IOPmeasurements by pneumatonometry has been investigatedand results have been mixed: some investigators concludethat IOP measurements are significantly influenced bycorneal thickness17 and others that they are not.18,19 Thepurpose of this study was to investigate the influence ofcorneal topography and thickness on the pneumatonometricmeasurements of IOP and PA.

    Materials and methods

    Subjects

    47 university students volunteered for this cross-sectionalstudy: mean age 20.4yrs, range 18 to 28yrs; 23 male, 24female. In order to confirm that all eyes included in the studywere healthy, subjects underwent a full eye examination. Asone of the purposes of this study was to investigate the rela-tionship between pneumatonometer measurements and meancorneal curvature, it was felt that high degrees of cornealastigmatism or causes of corneal distortion could be a sourceof error. Subjects were therefore excluded from the study ifthey had greater than 2.00 dioptres of astigmatism, had wornhard or rigid contact lenses, or had any ocular pathology. Following the tenets of the Helsinki declaration, the studyreceived approval from Aston Universitys ethical committeeand each subject gave written informed consent before takingpart. All measurements were taken with the subject in aseated position, between the hours of 10.00 am and 4.00 pm,by the same investigator (JH).

    Instruments

    The pneumatonometer used in this study was the OcularBlood Flow Analyzer (OBFA; Paradigm Medical Industries,Utah, USA) which is based on the original design of Durhamet al.20 and its later modifications.21,22 After anaesthetising the cornea with 0.4% benoxinate hydrochloride (Minims,Chauvin, UK), average IOP and PA measurements were auto-matically calculated from one continuous IOP recording(approximate recording time 510 seconds) that was suffi-cient to encapsulate five similar IOP pulses. Mean cornealradius was measured with a computerised corneal topogra-pher (EyeSys 2000 Corneal Analysis System, Spectrum Ophthalmic, UK) by taking the average of three readings. Ameasure of central corneal thickness (CCT) was calculatedby taking the average of three corneal width images producedfrom a Scheimpflug camera system (CASE-S, Marcher Enter-prises, Hereford, England). In addition, because of the knowncorrelations between corneal radius with axial length23 andaxial length with pulse amplitude,24 axial length measure-ments were taken on all subjects. Axial length was calculatedautomatically as the average of 10 A-scan ultrasound read-ings (Storz Omega Compu-Scan Biometric Ruler, StorzInternational, St Louis, USA). In order to assess repeatabil-

    Figure 1. Schematic of a pneumatonometer probe in contact witha cornea: P1, air pressure within probe; P2, intraocular pressure; AT,total air flow entering probe; AB, proportion of air flow escapingfrom the air bearing; AP, proportion of air flow escaping from theprobe head.

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  • Corneal thickness and curvature in pneumatonometry 109

    ity of measurements, coefficients of reliability for each parameter were calculated and are shown in Table 1. Thecoefficient of reliability, sometimes called the intraclass correlation coefficient, estimates the average correlationbetween all possible pairs of observations.25 For repeatedmeasurements, as in this study, the coefficient (CR) is calcu-lated via a one-way ANOVA as

    where SSb and SSt are the sums of squares between subjectsand in total, respectively, and k is the number of measure-ments taken.

    Analysis

    As physiological data from the two eyes of a single subjectcorrelate highly, combining right and left eye measurementscan lead to erroneous statistical significa