Central corneal thickness and corneal diameter in premature infants

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Diagnosis/Therapy in OphthalmologyCentral corneal thickness andcorneal diameter in prematureinfantsCaitriona Kirwan,1 Michael OKeefe1,2,3 and Susan Fitzsimon11Childrens University Hospital, Dublin, Ireland2Rotunda Hospital, Dublin, Ireland3University College Dublin, Dublin, IrelandABSTRACT.Background: Central corneal thickness is significantly greater in full-term infantsthan in adults. Very little is known about corneal thickness in premature infants.Methods: Measurements of central corneal thickness and horizontal cornealdiameter were carried out in 35 premature babies (70 eyes) undergoing screeningfor retinopathy of prematurity. Initial measurements were taken at 31 weeksgestational age and at intervals until term was reached.Results: Babies born at 31 weeks have very thick corneas which show aprogressive and statistically significant decrease to term. Conversely, horizontalcorneal diameter shows a progressive significant increase to term. A very stronginverse correlation was found between these two parameters.Conclusion: We demonstrate that premature infants have thick corneas andsmall corneal diameters. Central corneal thickness decreases dramatically from 31 weeks to term and is mirrored by a significant increase in corneal diameter.Key words: central corneal thickness corneal diameter premature infantsActa Ophthalmol. Scand. 2005: 83: 751753Copyright# Acta Ophthalmol Scand 2005.doi: 10.1111/j.1600-0420.2005.00559.xIntroductionA number of studies have demon-strated that central corneal thicknessin full-term infants is significantlygreater than in adults (Ehlers et al.1976; Autzen & Bjornstrom 1991).Furthermore, a previous study of cor-neal thickness in premature infantsdemonstrated a decrease in cornealthickness from 33 weeks gestationalage to term (Portellinha & Belfort1991; al-Umran & Pandolfi 1992). Theaim of this study was to record centralcorneal thickness and horizontal cor-neal diameter in infants at 31 weeksgestational age, to document thechange from this time to term and todetermine if a correlation existsbetween central corneal thickness andcorneal diameter.MethodsA prospective study of changes in cen-tral corneal thickness and horizontalcorneal diameter in premature infantsundergoing screening for retinopathyof prematurity was carried out. Eyeswith other abnormalities such as catar-acts were excluded from the study.Screening commenced in all infantsbetween 30 and 32 weeks gestationalage at which time initial measurementswere taken. Subsequent measurementswere taken at 23-week intervalsas appropriate until full-term wasreached.Central corneal thickness measure-ments were recorded using a handheldultrasound pachymeter (DGH 55; DGHTechnology Inc., Exton, Pennsylvania,USA). This pachymeter utilizes a soundvelocity of 1640 m/second. Threerecordings were taken in each eye andthe pachymeter was programmed so thateach recording represented an average of20 measurements. The recording withthe lowest standard deviation waschosen to represent the central cornealthickness. Horizontal diameter wasdetermined using callipers and takingwhite-to-white measurements. Allinfants had lid speculae inserted andmeasurements were recorded at thesame time as funduscopy was performedwhen screening for retinopathy ofprematurity (ROP).Measurements were recorded atvarying gestational ages: 3032 weeks,3435 weeks, 3738 weeks and then atterm (3941 weeks). Using a singledata point from each baby, the meancentral corneal thickness values in eachgestational age group were determinedand compared using the two-tailedACTA OPHTHALMOLOGICA SCANDINAVICA 2005751t-test. Similar analysis was conductedon the mean corneal diameter in eachgroup, again utilizing one data pointfrom each baby in each age group.The correlation between central cor-neal thickness and corneal diameter ineach group was determined using thePearson product moment correlationcoefficient.ResultsThere were 70 eyes of 35 babies (20boys, 15 girls) in the study. The meangestational age at birth was 28.3 weeks(range 2332 weeks) and the meanbirth weight was 1006 g kilograms(range 4901380 g). Initial measure-ments were recorded at a mean gesta-tional age of 31 weeks and at intervalsuntil term. Mean central corneal thick-ness at the initial measurement wasfound to be 691 mm (Table 1). Thiswas associated with a mean horizontalcorneal diameter of 8.0 mm. Centralcorneal thickness measurementsshowed a progressive decline from theinitial to the final measurement atterm, at which time a mean of 564 mmwas obtained (Fig. 1). Conversely, hor-izontal corneal diameter showed a con-tinuous increase over the same periodto a mean value of 9.6 mm at term(Fig. 2). Determination of the Pearsonproduct moment correlation coefficientrevealed a very strong inversecorrelation between central cornealthickness and corneal horizontal dia-meter from initial measurement toterm ( 0.97); decreasing central cor-neal thickness was very strongly asso-ciated with increasing corneal diameter(Fig. 3).DiscussionThe issue of corneal thickness hasattracted increasing attention since theOcular Hypertension Treatment Studyidentified central corneal thickness asimportant in the diagnosis of glaucoma(Brandt et al. 2001). It concluded thatmeasurement of central corneal thick-ness should be an essential part of thework-up of patients with ocularhypertension. Thinner corneas havelower intraocular pressure readingsand thicker corneas have higher read-ings. Central corneal thickness variesamongst racial groups and in thosewith glaucoma syndromes (Shimmyoet al. 2003). Increased central and per-ipheral corneal thickness in newbornsis thought to be due to prolonged eyeclosure (al-Umran & Pandolfi 1992).Decreasing thickness after birth isthought to be due to improved hydra-tion control. Thickness decreases invalue from that found in prematureand term babies to an adult value inchildren aged 24 years. A previousstudy in 1991 examined central cornealthickness in 13 eyes of prematureinfants born at less than 33 weeksgestational age (Portellinha & BelfortTable 1. Gestational age, central cornealthickness and horizontal corneal diameter at3032, 3435, 3738 and 3941 weeks gesta-tional age.Gestational Central Horizontaage corneal corneal(weeks) thickness diameter(mm) (mm)3032Mean 691 8.0SD 87 0.23435Mean 648* 8.5*SD 72 0.33738Mean 605* 8.9*SD 59 0.33941Mean 564* 9.6*SD 34 0.5* Statistically significantly at p < 0.05.500550600650700750800Gestational age (weeks)Mean central corneal thickness(microns)3032 3435 3738 3941Fig. 1. Graph showing the change in mean central corneal thickness from 30 weeks gestationalage to term.7.58.59.510.5Gestational age (weeks)Mean horizontal cornealdiameter (mm)3032 3435 3738 3941Fig. 2. Graph showing the change in mean horizontal corneal diameter from 30 weeks gestationalage to termACTA OPHTHALMOLOGICA SCANDINAVICA 20057521991). It demonstrated a progressivedecrease in thickness from 33 weeks toterm. It found no correlation betweengestational age, birth length and weightand central corneal thickness.The exact mechanism that deter-mines central corneal thickness has inour view not been fully elucidated.Premature infants of less than30 weeks gestational age have hazycorneas and in most infants this pre-cludes fundal examination. However,in the vast majority of infants there issufficient corneal clarity after 31 weeksto permit funduscopy. This findingindicates that central corneal hydrationplays an important role in determiningcorneal thickness in babies up to31 weeks gestational age. After this webelieve that other factors influencethickness. This study shows that adecrease in corneal thickness was clo-sely correlated with an increase in cor-neal diameter. This finding suggeststhat growth of the eye, with possibleremodelling and stretching of collagenfibres, plays a significant role in thereduction of central corneal thickness.The mandatory screening for ROP ofpremature infants born under 31 weeksgestational age and under 1500 g pro-vided us with a population of prema-ture infants for this study (Cryotherapyfor Retinopathy of PrematurityCooperative Group 1996). We wereable to measure central corneal thick-ness and corneal diameter at varyingintervals from 31 weeks to term. Therecent introduction of a predictablehandheld pachymetry instrumentenabled easy measurement of centralcorneal thickness.In conclusion, this study demon-strates that premature infants at31 weeks gestational age have thickcorneas and small corneal diameters.They undergo a dramatic shift betweenthis time and term, with a reduction incorneal thickness that is mirrored by anincrease in corneal diameter.Referencesal-Umran KU & Pandolfi MF (1992): Cornealdiameter in premature infants. Br JOphthalmol 76: 292293.Autzen T & Bjornstrom R (1991): Central cor-neal thickness in premature babies. ActaOphthalmol (Copenh) 69: 251252.Brandt JO, Beiser JA, Kass MA, Gordon MO& Ocular Hypertension Treatment Study(OHTS) Group (2001): Central cornealthickness in the Ocular HypertensionTreatment Study (OHTS). Ophthalmology108: 17781788.Cryotherapy for Retinopathy of PrematurityCooperative Group (1996): Multicentretrial of cryotherapy for retinopathy ofprematurity: Snellen visual acuity andstructural outcome at 5.5 years afterrandomization. Arch Ophthalmol 114:417424.Ehlers N, Sorensen T, Bramsen T & PoulsenEH (1976): Central corneal thickness innewborns and children. Acta Ophthalmol(Copenh) 54: 285290.Portellinha W & Belfort R Jr (1991): Centraland peripheral corneal thickness in new-borns. Acta Ophthalmol (Copenh) 69:247250.Shimmyo M, Ross AJ, Moy A & Mostafavi R(2003): Intraocular pressure, Goldmannapplanation tension, corneal thickness andcorneal curvature in Caucasians, Asians,Hispanics and African Americans. Am JOphthalmol 16: 603613.Received on February 22nd, 2005.Accepted on July 12th, 2005.Correspondence:Professor Michael OKeefeSuite 5Mater Private HospitalEccles StreetDublin 1IrelandTel: 353 1 885 8626Fax: 353 1 885 8490Email: mokeefe@materprivate.ie5506006507007.5 8 8.5 9 9.5 10Horizontal corneal diameter (mm)Central corneal thickness (microns)TimeFig. 3. Graph showing a strong inverse correlation between mean central corneal thickness andmean horizontal corneal diameter from 31 weeks gestational age to term.ACTA OPHTHALMOLOGICA SCANDINAVICA 2005753

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