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Ocular components values and their intercorrelations in Saudi Arabians
Ophthal. Physiol. Opt. Vol. 19, No. 6, pp. 489±497, 1999 # 1999 The College of Optometrists. Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain 0275-5408/99 $20.00 + 0.00
PII: S0275-5408(99)00008-3
Ocular components values and their intercorrelations in Saudi Arabians Ebi Peter Osuobeni1 Department of Optometry, College of Applied Medical Sciences, King Saud University, P.O. Box 10219, Riyadh 11433, Saudi Arabia Summary There are no previous studies on the dimensions of ocular components and their intercorrelations in adult Saudi Arabians. In this study ocular components were measured by ultrasonography and keratometry in 152 adult Saudis. Their ages ranged from 16 to 50 years. The males had significantly longer anterior chamber depth (ACD), vitreous chamber depth (VCD), axial length (AL) and axial length/corneal radius ratio (AL/CR) than females. However, females had non-significantly steeper corneas than males. The AL/CR ratio had the highest correlation with refractive error. Corneal radius of curvature was positively correlated with VCD and AL. Lens thickness was negatively correlated with the VCD, AL and AL/CR ratio. Myopes had significantly deeper ACD, VCD, and higher AL/CR ratio than nonmyopes. Myopes had significantly thinner lenses than hyperopes. The average values of the optical components and their intercorrelations are similar to reported values obtained from subjects of other races but with comparable refractive errors. # 1999 The College of Optometrists. Published by Elsevier Science Ltd. All rights reserved
Introduction
that the refractive state of the eye tends towards emmetropia. A breakdown in this regulated growth pattern, or emmetropization, leads to the development of refractive error. There are extensive studies on the dimensions of the optical components in Caucasian populations (Sorsby et al., 1957; Larsen, 1971a±d) and Asian subjects (Garner et al., 1990; Lam and Goh, 1991; Goh and Lam, 1994; Lam et al., 1994). Taw®k et al. (1993) observed that there were no prior studies to determine the average values of the optical components in fullterm newborns in the Arabian region. Consequently, they went on to study 200 Saudi Arabian infants. Ocular dimensions measured include axial length, anterior chamber depth and lens thickness. They concluded that the average values that were obtained for the measured variables were not signi®cantly dierent from those of surveys performed on other populations in the world. The present study was carried out to determine the average values of the dimensions of the optical components and their intercorrelations in adult Saudi Arabians. Currently, such values are not available. The results of this study will be useful in understand-
The prevalence of refractive error is dependent on various factors including age and race. Asians as a group tend to have a higher prevalence of myopia than nonAsians (Lam and Goh, 1991; Goh and Lam, 1994; Lam et al., 1994). The type and magnitude of the refractive error of an eye is determined by the relationships between the dimensions of its optical components. The relevant optical components include the corneal power and radius of curvature, anterior chamber depth, lens thickness and power, vitreous chamber depth and the axial length. The average values of the dimensions of the optical components are also dependent on race, age and gender. The high prevalence of emmetropia in the general population implies that the growth of the optical components must be coordinated such that an excess in the dimension of one component is oset by a reduction adjustment in the dimension of another so 1
Tel./fax: 00966-1-4191077; e-mail:[email protected].
Received: 21 July 1998 Revised form: 1 February 1999
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ing ocular growth patterns and the bases of the development of refractive errors among Saudi Arabians of Arab descent.
Method A total of 152 subjects (75 males and 77 females) took part in this study. All subjects were Saudi Arabian citizens of Arab origin. Their ages ranged from 16 to 50 years with an average of 22.68 years. The subjects were recruited from the student and sta populations of the College of Applied Medical Sciences, King Saud University, Riyadh, Saudi Arabia. Informed consent was obtained from all participating subjects. Each subject was objectively refracted using a Nidek autorefractor (ARK-900) followed by a subjective refraction. Both objective and subjective refractions were done without cycloplegia. The objective refractions were performed by two student assistants while the subjective re®nements were done by the author. Subjects were categorized into refractive groups based on the following subjective refraction ®ndings: emmetropes were those with spherical equivalent of the refractive error E20.50 D, hyperopes had refractive error > + 0.50 D, while myopes were those subjects with spherical equivalent of the refractive error >ÿ0.50 D. The corneal radius of curvature of each eye was measured by using a Bausch & Lomb type manual keratometer. The keratometer calibration was routinely veri®ed by using a steel ball of known radius (6.75 mm, 50.0 D). The keratometer readings on the female subjects were all made by a female student assistant while a male student assistant measured the central corneal radius of all the male subjects. However, both assistants followed exactly the same
procedure. The corneal radius of curvature along the two principal meridians was determined. The average radius of curvature was calculated and used for all subsequent analyses. All ocular components dimensions were measured by the author using a Nidek Echoscan (Model US3300, Japan) equipped with a 10 MHz probe. The subjects corneas were anesthetized, each with one drop of 0.4% Novesin (Benoxinate hydrochloride 4 mg, CIBA Vision, Switzerland). Measurements were made with the subject lying in a supine position while viewing a distant target. The lids were held apart with the ®ngers, care being taken not to apply excessive pressure on the globe. The probe was then lightly placed on the cornea. The waveform was viewed to ensure that it had four clean peaks each from the cornea, anterior and posterior lens surfaces and the retina. A minimum of six acceptable measurements were made. The instrument automatically determined and displayed the average value and standard deviation of each ocular component.
Results and analyses Table 1 shows a summary of some descriptive statistics for each of the measured variable in this study. Initial analyses (one-sample t-tests) showed that for all subjects, there were no signi®cant dierences between the right and left eye values of all measured variables. Only the right eye values are therefore shown in Table 1. Age distribution Most of the male subjects (N = 20, 26.7%) had ages between 18.74 and 20.32 years with a midpoint at 19.53 years. The overwhelming majority of the male
Table 1. Descriptive statistics of the measured variables Gender Age (years) SPH (D) KV (D) KH (D) KAV (D) ACD (mm) LT (mm) VCD (mm) AL (mm) AL/CR Average Standard deviation Minimum Maximum
M F B M F B M F B M F B
22.71 22.66 22.68 4.80 5.39 5.09 16.00 19.00 16.00 42.00 50.00 50.00
ÿ1.11 ÿ0.79 ÿ0.95 1.73 1.41 1.58 ÿ7.63 ÿ5.00 ÿ7.63 1.38 1.75 1.75
43.30 43.81 43.56 1.48 1.71 1.62 39.68 37.11 37.11 46.85 46.75 46.85
42.78 43.08 42.93 1.41 1.91 1.69 39.00 34.27 34.27 45.75 46.62 46.62
43.05 43.44 43.25 1.40 1.67 1.55 39.56 39.15 39.15 46.25 46.21 46.25
3.32 3.08 3.20 0.33 0.40 0.38 2.45 2.07 2.07 3.88 4.08 4.08
3.72 3.73 3.72 0.25 0.28 0.27 3.04 3.30 3.04 4.33 4.70 4.70
16.80 16.36 16.58 1.01 0.94 0.99 15.19 14.54 14.54 19.59 18.92 19.59
23.81 23.16 23.48 0.99 1.03 1.06 21.94 20.91 20.91 26.68 25.89 26.68
3.03 2.97 3.00 0.12 0.14 0.13 2.84 2.42 2.42 3.49 3.30 3.49
SPH = spherical equivalent of the refractive error, KV and KH corneal radius of curvature along the vertical and horizontal meridians; KAV = average corneal radius of curvature; ACD = anterior chamber depth; LT = lens thickness; VCD = vitreous chamber depth; AL = axial length; AL/CR = axial length/corneal radius ratio; M = Males; F = females; B = males and females
Ocular components values in Saudis: E. P. Osuobeni subjects (N = 74, 98.67%) were aged less than 40 years. Only one male subject (1.33%) was aged over 40 years. Thirty-®ve female subjects (45.45%) had ages between 19.11 and 21.21 years with a midpoint at 20.16 years. Seventy-six of the female subjects (98.7%) were aged less than 40 years. Only one female subject (1.33%) was over 40 years. A two-sample t-test showed that there was no signi®cant dierence between the average male and female ages (t = ÿ0.053, df = 150, p = 0.957).
Spherical equivalent of the refractive error The majority of the male subjects (N = 41, 54.67%) were myopic, with an average spherical equivalent of refractive error of ÿ2.13 D. Twenty-nine (38.67%) were emmetropic, with an average spherical equivalent of 0.00 D. Only ®ve of the male subjects (6.67%) were hyperopic with an average spherical equivalent error of +0.83 D. A majority of the female subjects (N = 43, 55.84%) were emmetropic, with an average spherical equivalent of the refractive error of 0.01 D. Thirty were myopic (38.96%) with an average spherical equivalent of ÿ2.20 D, while, only four (5.20%) were hyperopic, with an average spherical equivalent error of +1.09 D. A twosample t-test demonstrated that the average spherical equivalent errors of the male and female subjects were
not signi®cantly p = 0.215).
dierent
(t = 1.245,
491
df = 150,
Corneal curvature The female cornea was slightly more curved (smaller radius of curvature) along the major meridians (Table 1). Although the mean corneal radius of curvature was also slightly smaller than that of the males, the dierence was not signi®cant (t = ÿ1.465, df = 150, p = 0.145). For all subjects, the average corneal radius of curvature varied non-signi®cantly with the spherical equivalent of the refractive error (F = 0.878, df = 44, p = 0.6809) and the type of refractive error (F = 0.991, df = 2, p = 0.374). However, as shown in Table 2, myopes tended to have the steepest corneas, followed by the emmetropes and, lastly, the hyperopes. Anterior chamber depth Males had deeper average anterior chamber depth than females (Table 1). The dierence in the average anterior chamber depths was signi®cant (t = ÿ4.137, df = 150, p = 0.000). For the male (F = 1.02, df = 55, p = 0.50), females (F = 0.68, df = 60, p = 0.86) and all subjects (F = 1.213, df = 90, p = 0.2118), the anterior chamber depth did not vary signi®cantly with the magnitude of the spherical equiv-
Table 2. Average values of measured variables of myopic, emmetropic and hyperopic subjects Gender
Variable
Myopes (<ÿ0.50 DS)
Emmetropes (ÿ0.50 to +0.50 DS)
Hyperopes (> + 0.50 DS)
M F B M F B M F B M F B M F B M F B M F B
SPH (D)
ÿ2.13 ÿ2.20 ÿ2.16 43.20 43.66 43.39 3.38 3.26 3.33 3.72 3.61 3.68 17.13 16.97 17.06 24.18 23.85 24.04 3.09 3.08 3.09
0.00 0.01 0.01 42.95 43.32 43.17 3.30 2.98 3.11 3.68 3.78 3.74 16.42 15.98 16.16 23.40 22.75 23.01 2.97 2.91 2.94
+0.83 +1.09 +0.94 42.40 43.19 42.75 3.00 2.87 2.94 3.90 3.93 3.91 16.28 15.82 16.08 23.18 22.53 22.89 2.91 2.88 2.89
KAV (D) ACD (mm) LT (mm) VCD (mm) AL (mm) AL/CR
M = males; F = females; B = males and females; SPH = spherical equivalent of the refractive error, KV and KH corneal radius of curvature along the vertical and horizontal meridians; KAV = average corneal radius of curvature; ACD = anterior chamber depth; LT = lens thickness; VCD = vitreous chamber depth; AL = axial length; AL/CR = axial length/corneal radius ratio
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alent. However, the type of refractive error had a signi®cant eect on the anterior chamber depth: males, F = 3.18, df = 2, p = 0.047; females, F = 5.77, df = 2, p = 0.005; for all subjects, F = 8.895, df = 2, p = 0.002. Generally, myopes had the deepest anterior chamber, followed by the emmetropes and then the hyperopes. For the male subjects, a two-sample t-test demonstrated that emmetropes had deeper anterior chamber than hyperopes (t = 2.198, df = 32, p = 0.035). Also, myopes had signi®cantly deeper anterior chamber than hyperopes (t = 2.265, df = 44, p = 0.028). There was no signi®cant dierence between the anterior chamber depths of myopes and emmetropes (t = ÿ0.969, df = 68, p = 0.336). For the female subjects, only the anterior chamber depth of myopes was signi®cantly deeper than that of emmetropes (t = ÿ3.38, df = 71, p = 0.0000). The results of similar tests for all subjects are shown in Table 3. Again, myopes had the deepest anterior chamber followed by emmetropes and hyperopes. Lens thickness Females had slightly but insigni®cantly thicker lenses than males (t = 0.163, df = 150, p = 0.871). The lens thickness varied signi®cantly with the spherical equivalent of the refractive error for the male subjects (F = 1.979, df = 49, p = 0.034) although this was not the case with either the female subjects (F = 1.242, df = 54, p = 0.294) or all subjects (F = 1.009, df = 74, p = 0.0484).
For the female subjects, the type of refractive error signi®cantly aected the lens thickness (F = 4.62, df = 2, p = 0.01). Emmetropes had signi®cantly thicker lenses than myopes (t = 2.792, df = 71, p = 0.006). Similarly, hyperopes had signi®cantly thicker lenses than myopes (t = ÿ2.553, df = 32, p = 0.015). There was no signi®cant dierence between the lens thickness of emmetropes and hyperopes (t = ÿ0.844, df = 45, p = 0.402). For the male subjects, an analysis of variance test demonstrated that there were no signi®cant dierences in lens thickness between the three refractive error groups (F = 1.597, df = 2, p = 0.209). For all subjects, the type of refractive error signi®cantly aected the lens thickness (F = 3.675, df = 2, p = 0.027). The results of subsequent two-samples ttests (Table 3) indicated that only the hyperopes had signi®cantly thicker lenses than myopes. Vitreous chamber depth Males had signi®cantly deeper vitreous chamber depth than females (t = ÿ2.778, df = 150, p = 0.006). When all subjects were considered together the vitreous chamber depth varied signi®cantly with the magnitude of the spherical equivalent of the refractive error (F = 1.708, df = 122, p = 0.048). The vitreous chamber depth was signi®cantly dierent between the three refractive error groups: males, F = 5.480, df = 2, p = 0.006; females, F = 14.10, df = 2, p = 0.000). For all subjects, the depth of the vitreous chamber was also signi®cantly dierent
Table 3. Results of two-sample t-tests between emmetropic (E), myopic (M) and hyperopic (H) subjects for combined male and female subjects Mean variables compared KAV (E)±KAV (M) KAV (E)±KAV (H) KAV (M)±KAV (H) ACD (E)±ACD (M) ACD (E)±ACD (H) ACD (M)±ACD (H) LT (E)±LT (M) LT (E)±LT (H) LT (M)±LT (H) VCD (E)±VCD (M) VCD (E)±VCD (H) VCD (M)±VCD (H) AL (E)±AL (M) AL (E)±AL (H) AL (M)±AL (H) AL/CR (E)±AL/CR (M) AL/CR (E)±AL/CR (H) AL/CR (M)±AL/CR (H)
Difference between means
t-statistic
df
p-value
Decision
ÿ0.23 0.42 0.65 ÿ0.22 0.16 0.38 0.07 ÿ0.16 ÿ0.23 ÿ0.90 0.08 0.99 ÿ1.03 0.12 1.14 ÿ0.15 0.04 0.19
ÿ0.8953 0.6924 1.2955 ÿ3.7179 1.2029 2.9987 1.5988 ÿ1.6555 ÿ2.6393 ÿ6.0550 0.2916 2.8912 ÿ6.6946 0.3791 3.2572 ÿ8.3604 1.0991 4.8684
141 79 78 141 79 78 141 79 78 141 79 78 141 79 78 141 79 78
0.3721 0.4907 0.1989 0.0003 0.2326 0.0036 0.1121 0.1018 0.0100 0.0000 0.7713 0.0049 0.0000 0.7056 0.0017 0.0000 0.2751 0.0000
N/S NS NS S N/S S N/S N/S S S NS S S N/S S S N/S S
KAV = average corneal radius of curvature; ACD = anterior chamber depth; LT = lens thickness; VCD = vitreous chamber depth; AL = axial length; AL/CR = axial length/corneal radius ratio; NS = not significantly different; S = significantly different
Ocular components values in Saudis: E. P. Osuobeni between the three refractive error groups (F = 19.908, df = 2, p = 0.000). Two sample t-tests (Table 3) indicated that myopes had signi®cantly deeper vitreous chamber than emmetropes and hyperopes. The vitreous chamber depth of emmetropes was not signi®cantly dierent from that of hyperopes. Axial length Males had signi®cantly longer axial length than females (t = ÿ3.937, df = 150, p = 0.000). For the combined male and female subjects, the axial length varied signi®cantly with the magnitude of the refractive error (F = 3.142, df = 125, p = 0.001). The axial length was signi®cantly dierent between the three refractive groups: males, F = 7.324, df = 2, p = 0.001; females, F = 14.97, df = 2, p = 0.000); all subjects, F = 23.893, df = 2, p = 0.000). Myopic male subjects had signi®cantly longer axial length than emmetropes (t = ÿ3.451, df = 68, p = 0.000) and hyperopes (t = 2.117, df = 44, p = 0.039). There was no signi®cant dierence between the axial length of emmetropes and hyperopes (t = 0.576, df = 32, p = 0.569). Similarly, myopic female subjects had signi®cantly longer axial length than emmetropes (t = ÿ5.46, df = 71, p = 0.000) and hyperopes (t = 2.52, df = 32, p = 0.01). The results of similar analyses for all subjects are shown in Table 3. The decisions are similar to those of the males and females. Axial length/corneal radius ratio (AL/CR) The average axial length/corneal radius ratio was signi®cantly dierent between genders (t = ÿ2.808, df = 150, p = 0.006) being higher in males than females. The AL/CR ratio varied signi®cantly as a function of the magnitude of the spherical equivalent of the refractive error: males, F = 13.783, df = 35, p = 0.000; females, F = 5.00, df = 70, p = 0.02. Analysis of variance tests demonstrated that the AL/ CR ratio varied signi®cantly with the type of refractive error for both the male (F = 17.181, df = 2, p = 0.000) and female (F = 20.949, df = 2, p = 0.000) subjects. Myopic male subjects had signi®cantly higher AL/ CR ratio than their emmetropic (t = ÿ4.943, df = 68, p = 0.000) and hyperopic (t = 3.393, df = 44, p = 0.001) counterparts. Also, male emmetropes had signi®cantly higher AL/CR ratio than hyperopes (t = 2.224, df = 32, p = 0.033). Similarly, myopic female subjects had signi®cantly higher AL/CR ratio than emmetropes (t = ÿ6.30, df = 71, p = 0.000) and hyperopes (t = 3.42, df = 32,
493
p = 0.000). However, there was no signi®cant dierence between the AL/CR ratio of female emmetropes and hyperopes (t = 0.52, df = 45, p = 0.61). Analysis of the combined male and female data showed that the AL/CR ratio varied signi®cantly with the type of the refractive error (F = 40.191, df = 2, p = 0.000). In general, myopic subjects had signi®cantly higher AL/CR ratio than emmetropic (t = ÿ8.360, df = 141, p = 0.000) and hyperopic subjects (t = 4.868, df = 78, p = 0.000). There was no signi®cant dierence between the AL/CR ratio of emmetropes and hyperopes (t = 1.099, df = 79, p = 0.275). Correlation coecients Table 4 is a display of the correlation coecients between the variables measured in the present study. In general, the sign of the correlation coecient between two variables is similar for the male and female subjects although their magnitude occasionally dier. A negative correlation coecient between the spherical equivalent of the refractive error and another variable, e.g. axial length, indicates that as the magnitude of the variable increases, the spherical equivalent of the refractive error becomes more myopic. The opposite is true for a positive correlation coecient. Table 4 shows that the spherical equivalent of the refractive error was negatively correlated with all the optical components dimensions except lens thickness and corneal radius of curvature with which positive correlations were obtained. There is a negative correlation between average corneal radius of curvature and anterior chamber depth. This indicates that ¯at corneas are associated with shallow anterior chamber depths. As would be expected, there is a very high positive correlation between vitreous chamber depth and axial length.
Discussion The criteria used to classify subjects into the three refractive error types was the same or very similar to that used in some previous studies (Sorsby et al., 1957; Goss et al., 1990; Grosvenor and Scott, 1991; Lam and Goh, 1991; Van Rens and Arkel, 1991; Grosvenor and Scott, 1993; Goh and Lam, 1994; Lam et al., 1994; McBrien and Adams, 1997). According to Zadnik et al. (1992) only changes greater than ÿ0.69 and +0.56 D are outside the range of measurement error for repeated measures of non-cycloplegic subjective refraction. Consequently, measurement of refractive error e2 0.75 D is required to establish the
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Ophthal. Physiol. Opt. 1999 19: No 6
Table 4. Correlation coefficients between refractive error and ocular components
SPH (D) KAV (D) ACD (mm) LT (mm) VCD (mm)
Gender
KAV (D)
ACD (mm)
LT (mm)
VCD (mm)
AL (mm)
AL/CR
M F B M F B M F B M F B M F B
ÿ0.06$ ÿ0.09$ ÿ0.06$
ÿ0.28* ÿ0.36* ÿ0.33* +0.10$ +0.05$ +0.02$
+0.14* +0.27* +0.20* ÿ0.09$ ÿ0.09$ ÿ0.09$ ÿ0.51* ÿ0.67* ÿ0.57*
ÿ0.67* ÿ0.64* ÿ0.65* ÿ0.52* ÿ0.46* ÿ0.50* +0.23* +0.44* +0.39* ÿ0.27* ÿ0.50* ÿ0.38*
ÿ0.71* ÿ0.66* ÿ0.68* ÿ0.50* ÿ0.43* ÿ0.47* +0.39* +0.62* +0.56* ÿ0.20$ ÿ0.45* ÿ0.32* +0.95* +0.95* +0.95*
ÿ0.84* ÿ0.69* ÿ0.74* N/A N/A N/A +0.51* +0.65* +0.62* ÿ0.30* ÿ0.52* ÿ0.42* +0.60* +0.52* +0.57*
SPH = spherical equivalent of the refractive error; KAV = average corneal radius of curvature; ACD = anterior chamber depth; LT = lens thickness; VCD = vitreous chamber depth; AL = axial length; AL/CR = axial length/corneal radius ratio; M = males; F = females; B = males and females.N/A = not applicable because the corneal radius of curvature goes into the AL/CR ratio.*Significant at 0.05 level.$Not significant
presence of a refractive anomaly or a change in the magnitude of a refractive error. The criterion of refractive error e2 0.75 D used to classify subjects in the present experiment and in some previous studies is therefore consistent with the ®ndings of Zadnik et al. (1992). Myopes and emmetropes make up almost equal numbers of all subjects, 71 (46.71%) and 72 (47.37%), respectively. However, there were very few hyperopes (9, 5.92%). The distribution of the refractive error type was not entirely unexpected considering the fact that most of the subjects were university students of young adult age. Similar low number of hyperopes had been reported for subjects of similar age range to those currently investigated (Lam and Goh, 1991; Goh and Lam, 1994; Goss et al., 1997). The results of the present study show that, as a group, females tend to have steeper corneas (lower radius of curvature), thicker lenses, shorter vitreous chamber depth and axial length than males. Similar observations had been previously reported for subjects of various racial groups (Stenstrom, 1948; Sorsby et al., 1961; Fledelius and Stubgaard, 1986; Garner et al., 1990; Lam and Goh, 1991; Goss and Jackson, 1993; Goh and Lam, 1994; Lam et al., 1994; Goss et al., 1997). The dierences in the average corneal radius of curvature between genders and across refractive error types are all within the range of 95% limits of agreement (ÿ1.03 to +0.84 D) for repeated measures of corneal radius of curvature (Zadnik et al., 1992). The dierences obtained in the present experiment may therefore be partly explained by experimental errors of measurement.
Grosvenor and Goss (1998) wrote that the mean corneal power was greater (smaller radius of curvature) for girls than for boys by approximately 0.50 to 1.00 D. They observed that this was consistent with the fact that the mean axial length was somewhat less for the girls than for the boys. A similar explanation could be advanced for the present results. Table 4 shows that on the average corneal radius of curvature is positively correlated with axial length. Consequently, eyes with long axial length will be associated with corneas with long radius of curvature or low corneal powers. The present results have shown that on the average males have signi®cantly deeper anterior chamber depth than females. Also, myopes have deeper anterior chamber depth than non-myopes. The dierence in anterior chamber depth between the genders (0.24 mm) is within the 95% range (ÿ0.32 to +0.26 mm) of experimental error reported by Zadnik et al. (1992) for repeated measures of this optical component. The dierence in anterior chamber depth between males and females in this study could therefore be due to experimental error. However, other studies have reported deeper anterior chamber depth for males (Alsbirk, 1977; Garner et al., 1990; Goss et al., 1997) and myopes (McBrien and Millodot, 1987; Grosvenor and Scott, 1991; Bullimore et al., 1992; Carney et al., 1997). The dierence in lens thickness between males and females found in this study is very small (0.01 mm) and non-signi®cant. This dierence is well within the 95% range of the limits of agreement for repeated measures of lens thickness (ÿ0.18 to +0.21 mm) according to Zadnik et al. (1992). Although emme-
Ocular components values in Saudis: E. P. Osuobeni tropes and hyperopes tend to have thicker lenses than myopes, only the hyperopes had signi®cantly thicker lenses. The dierence in lens thickness between myopes and hyperopes (ÿ0.23 mm) is outside the limits of experimental error found by Zadnik et al. (1992). Females had signi®cantly shallower vitreous chamber depth than males. The dierence, ÿ0.44 mm, is outside the range of experimental error (ÿ0.37 to +0.37 mm) for repeated measures of vitreous chamber depth reported by Zadnik et al. (1992). Other investigators have reported signi®cantly deeper vitreous chamber depth in males than females (Alsbirk, 1977; Fledelius, 1982b; Garner et al., 1990; Lam and Goh, 1991; Goh and Lam, 1994; Lam et al., 1994; Goss et al., 1997). Also, the present results collaborate previous ®ndings that myopes have the deepest vitreous chamber (McBrien and Millodot, 1987; Grosvenor and Scott, 1991; Bullimore et al., 1992; Carney et al., 1997). The present results have demonstrated that males have signi®cantly longer axial length than females. Also, myopes have the longest axial length followed by emmetropes and then hyperopes. Similar results have been reported previously (Alsbirk, 1977; Fledelius, 1982b; McBrien and Millodot, 1987; Garner et al., 1990; Grosvenor and Scott, 1991; Lam and Goh, 1991; Bullimore et al., 1992; Goh and Lam, 1994; Lam et al., 1994; Carney et al., 1997). The mean AL/CR ratios obtained for Saudis in the present experiment are smaller than those reported for Hong Kong Chinese subjects (Goh and Lam, 1994; Lam et al., 1994). The dierence could be explained by the fact that the Chinese subjects had higher myopic spherical equivalent of the refractive error. Calculations based on data presented by Alsbirk (1977) show that Greenland Eskimos of mean age 20 years have AL/CR ratio of 3.02 (males) and 3.01 (females). These values are quite similar to those obtained in the present experiment. According to Sorsby et al. (1957), British Caucasian emmetropes (ÿ0.50 to +0.50 D) have AL/CR ratio of 3.06, myopes (ÿ0.51 to ÿ4.00 D) have AL/CR ratio of 3.24 and hyperopes (+0.51 to +4.00 D) a ratio of 2.98. The corresponding values for the Saudis are emmetropes (2.94), myopes (3.09) and hyperopes (2.89). The AL/CR ratio has the highest correlation with refractive error (males = ÿ0.84; females = ÿ0.69). Stenstrom (1948) also reported correlation coecient of ÿ0.84 between refractive error and AL/CR, although other investigators had reported correlation coecient between the two as high as ÿ0.92 (Grosvenor and Scott, 1994). The dierence in the correlation coecient between the male and female subjects in the present study could be partly due to the fact that myopes made up 54.67% of the male popu-
495
lation and only 38.96% of the female population. The fact that the AL/CR ratio has the highest correlation coecient with refractive error supports the conclusion of Grosvenor and Scott (1994) that the AL/CR ratio is the most signi®cant determinant of the refractive state of the eye. As shown in Table 4, there is a very signi®cant positive correlation (+0.95) between the vitreous chamber depth and axial length. Similar results had been previously reported (Goh and Lam, 1994; Lam et al., 1994). Grosvenor and Goss (1998) observed that the refractive component change that bears the major responsibility for the development of myopia is axial elongation, although a steep cornea may also play an important role. Hyperopia is also thought to be predominantly axial in nature (Strang et al., 1998). The correlation coecient between the axial length of the present Saudi male subjects and the spherical equivalent of their refractive error was ÿ0.71, the corresponding value for the females subjects was ÿ0.66. This dierence could also be explained by the fact that there were more myopic male subjects than females. Lam et al. (1994) reported correlation coecient of ÿ0.66 between axial length and refractive error for male Hong Kong Chinese over 40 years, and ÿ0.62 for their female counterparts. Goh and Lam (1994) gave the correlation coecient between axial length and refractive error as ÿ0.61 for male Hong Kong Chinese between the ages of 19 and 39 years and ÿ0.64 for female Hong Kong Chinese subjects between the same age range. Garner et al. (1990) reported that the correlation coecient between refractive error and axial length for Malay children was ÿ0.66 while the corresponding value for Melanesian children was ÿ0.17. They explained that the dierence between the two racial groups was due to the fact that the Melanesian children were mainly emmetropic. Without implying a time sequence in the process of emmetropization, an eye with excessively elongated axial length and a correspondingly deep vitreous chamber will require less refractive power than average to achieve perfect focus on the retina. This demand could be met by a ¯atter cornea or a thinner lens or a combination of both. An emmetropic eye could be regarded as an eye in which these adjustments have been successful. Conversely, in ametropic eyes, the need for adjustment in corneal and/or lenticular power, in response to axial length, had not been ecient or successful. The correlation coecient quanti®es the direction and rate of change in the magnitude of one variable as a result of changes in another variable. Following the reasoning above, the rate of change of corneal power and/or lenticular thickness, subsequent to changes in axial length, or correlation coecient, should be greater for emmetropic than
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ametropic subjects. Goss et al. (1997) cited a number of references in which higher correlation coecients have been reported in emmetropes than for groups with a wide range of refractive errors. The anterior central corneal radius of curvature is positively correlated with refractive error. The correlation coecients found in this study, +0.06 and +0.09, are in general agreement with previously reported cases (Fledelius, 1982a; Goh and Lam, 1994; Grosvenor and Scott, 1994; Lam et al., 1994; Carney et al., 1997; Goss et al., 1997). The positive sign indicates that a ¯at central cornea (long radius of curvature) tends to be associated with hyperopic eyes. The correlation coecient is quite similar between gender. The present results also proved that lens thickness is positively correlated with spherical equivalent of refractive error indicating that thicker lenses are associated with less myopic eyes. The correlation coecient for the female subjects is approximately two times that for the males. This is thought to be due to the higher number of emmetropic female subjects. Corneal radius of curvature is positively correlated with vitreous chamber depth and axial length. This implies that as the globe increases in length the cornea becomes ¯atter. Previous studies have also reported positive correlations between corneal radius of curvature and vitreous chamber depth, and between corneal radius of curvature and axial length (Garner et al., 1990; Lam and Goh, 1991; Grosvenor and Scott, 1993; Lam et al., 1994). The results of the present and previous studies have demonstrated that myopic eyes, which are long, have steeper (shorter radius of curvature) corneas. Scott and Grosvenor (1993) explained this apparent contradiction by suggesting that together with increase in axial length, corneal steepening also occurs, during the development of myopia. The lens thickness is negatively correlated with vitreous chamber depth and axial length. This indicates that as part of the process of emmetropization, the lens must become thinner as the vitreous chamber depth increases. Table 4 shows that the correlation coecient is higher for the female subjects than the males. This may also be attributed to the larger number of emmetropic female subjects.
Acknowledgements I gratefully acknowledge the role played by Badria Mo'aned Al-Enzey and Majeed Sadoun AlAbdulmunem in making arrangements to secure subjects for this study and in helping with some of the measurements.
References Alsbirk, P. H. (1977). Variation and heritability of ocular dimensions. A population study among adult Greenland Eskimos. Acta Ophthalmol. 55, 443±456. Bullimore, M. A., Gilmartin, B. and Royston, J. M. (1992). Steady-state accommodation and ocular biometry in lateonset myopia. Doc. Ophthalmol. 80, 143±155. Carney, L. G., Mainstone, J. C. and Henderson, B. A. (1997). Corneal topography and myopia. A cross-sectional study. Invest. Ophthalmol. 38, 311±320. Fledelius, H. C. (1982a). Ophthalmic changes from age of 10 to 18 years. A longitudinal study of sequels to low birth weight. III. Ultrasound oculometry and keratometry of anterior eye segment. Acta Ophthalmol. 60, 393±402. Fledelius, H. C. (1982b). Ophthalmic changes from age of 10 to 18 years. A longitudinal study of sequels to low birth weight. IV. Ultrasound oculometry of vitreous and axial length. Acta Ophthalmol. 60, 403±411. Fledelius, H. C. and Stubgaard, M. (1986). Changes in refraction and corneal curvature during growth and adult life. A cross-sectional study. Acta Ophthalmol. 64, 487±491. Garner, L. F., Meng, C. K., Grosvenor, T. P. and Mohidin, N. (1990). Oculars dimensions and refractive power in Malay and Melanesian children. Ophthal. Physiol. Opt. 10, 234±238. Goh, W. S. H. and Lam, C. S. Y. (1994). Changes in refractive trends and optical components of Hong Kong Chinese aged 19±39 years. Ophthal. Physiol. Opt. 14, 378± 382. Goss, D. A. and Jackson, T. W. (1993). Cross-sectional study of changes in the ocular components in school children. Appl. Opt. 32, 4169±4173. Goss, D. A., Cox, V. D., Herrin-Lawson, G. A., Nielson, E. D. and Dolton, W. A. (1990). Refractive error, axial length, and height as a function of age in young myopes. Optom. Vis. Sci. 67, 332±338. Goss, D. A., Van Veen, H. G., Rainey, B. B. and Feng, B. (1997). Ocular optical components measured by keratometry, phakometry, and ultrasonography in emmetropic and myopic optometry students. Optom. Vis. Sci. 74, 489±495. Grosvenor, T. and Goss, D. A. (1998). Role of the cornea in emmetropia and myopia. Optom. Vis. Sci. 75, 132±145. Grosvenor, T. and Scott, R. (1991). Comparison of refractive components in youth±onset and early±adultonset myopia. Optom. Vis. Sci. 68, 204±209. Grosvenor, T. and Scott, R. (1993). Three-year changes in refraction and its components in youth±onset and early± adult-onset myopia. Optom. Vis. Sci. 70, 677±683. Grosvenor, T. and Scott, R. (1994). Role of the axial length/ corneal radius ratio in determining the refractive state of the eye. Optom. Vis. Sci. 71, 573±579. Lam, C. S. Y. and Goh, W. S. H. (1991). The incidence of refractive errors among school children in Hong Kong and its relationship with the optical components. Clin. Expt. Optom. 74, 97±103. Lam, C. S. Y., Goh, W. S. H., Tang, Y. K., Tsui, K. K., Wong, W. C. and Man, T. C. (1994). Changes in refractive trends and optical components of Hong Kong Chinese aged over 40 years. Ophthal. Physiol. Opt. 14, 383±388. Larsen, J. S. (1971a). The sagittal growth of the eye. I. Ultrasonic measurement of the depth of the anterior chamber from birth to puberty. Acta Ophthalmol. 49, 239± 262.
Ocular components values in Saudis: E. P. Osuobeni Larsen, J. S. (1971b). The sagittal growth of the eye. II. Ultrasonic measurement of the axial diameter of the lens and the anterior segment from birth to puberty. Acta Ophthalmol. 49, 427±440. Larsen, J. S. (1971c). The sagittal growth of the eye. III. Ultrasonic measurement of the posterior segment (axial length of the vitreous) from birth to puberty. Acta Ophthalmol. 49, 441±453. Larsen, J. S. (1971d). The sagittal growth of the eye. IV. Ultrasonic measurement of the axial length of the eye from birth to puberty. Acta Ophthalmol. 49, 873±886. McBrien, N. A. and Adams, D. W. (1997). A longitudinal investigation of adult-onset and adult-progression of myopia in an occupational group. Refractive and biometric ®ndings. Invest. Ophthalmol. Vis. Sci. 38, 321±333. McBrien, N. A. and Millodot, M. (1987). A biometric investigation of late onset myopic eyes. Acta Ophthalmol. 65, 461±468. Scott, R. and Grosvenor, T. (1993). Structural model for emmetropic and myopic eyes. Ophthal. Physiol. Opt. 13, 41±47. Sorsby, A., Benjamin, B., Davey, J. B., Sheridan, M. and Tanner, J. M. (1957). Emmetropia and its aberrations. A
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study in the correlations of the optical components of the eye, Medical Research Council Special Report Series No. 293. Her Majesty's Stationery Oce. Sorsby, A., Benjamin, B. and Sheridan, M. (1961). Refraction and its components during the growth of the eye from the age of three, Medical Research Council Special Report Series No. 301. Her Majesty's Stationary Oce. Stenstrom, S. (1948). Investigation of the variation and the correlation of the optical components of human eyes. Am. J. Optom. Arch. Am. Acad. Optom. 25, 218±232, 286±299, 340±350, 388±397, 438±449, 496±505. Strang, N. C., Schmid, K. L. and Carney, L. G. (1998). Hyperopia is predominantly axial in nature. Curr. Eye Res. 17, 380±383. Taw®k, A., Farag, A. A., Mansour, S. Z., Helal, N. F. and Abdel-Azim, E. E. (1993). Ocular biometry in fullterm newborns. Saudi J. Ophthalmol. 7, 101±105. Van Rens, G. H. M. B. and Arkell, S. M. (1991). Refractive errors and axial length among Alaskan Eskimos. Acta Ophtalmol. 69, 27±32. Zadnik, K., Mutti, D. O. and Adams, A. J. (1992). The repeatability of measurement of the ocular components. Invest. Ophthalmol. Vis. Sci. 33, 2325±2333.
PII: S0275-5408(99)00008-3
Ocular components values and their intercorrelations in Saudi Arabians Ebi Peter Osuobeni1 Department of Optometry, College of Applied Medical Sciences, King Saud University, P.O. Box 10219, Riyadh 11433, Saudi Arabia Summary There are no previous studies on the dimensions of ocular components and their intercorrelations in adult Saudi Arabians. In this study ocular components were measured by ultrasonography and keratometry in 152 adult Saudis. Their ages ranged from 16 to 50 years. The males had significantly longer anterior chamber depth (ACD), vitreous chamber depth (VCD), axial length (AL) and axial length/corneal radius ratio (AL/CR) than females. However, females had non-significantly steeper corneas than males. The AL/CR ratio had the highest correlation with refractive error. Corneal radius of curvature was positively correlated with VCD and AL. Lens thickness was negatively correlated with the VCD, AL and AL/CR ratio. Myopes had significantly deeper ACD, VCD, and higher AL/CR ratio than nonmyopes. Myopes had significantly thinner lenses than hyperopes. The average values of the optical components and their intercorrelations are similar to reported values obtained from subjects of other races but with comparable refractive errors. # 1999 The College of Optometrists. Published by Elsevier Science Ltd. All rights reserved
Introduction
that the refractive state of the eye tends towards emmetropia. A breakdown in this regulated growth pattern, or emmetropization, leads to the development of refractive error. There are extensive studies on the dimensions of the optical components in Caucasian populations (Sorsby et al., 1957; Larsen, 1971a±d) and Asian subjects (Garner et al., 1990; Lam and Goh, 1991; Goh and Lam, 1994; Lam et al., 1994). Taw®k et al. (1993) observed that there were no prior studies to determine the average values of the optical components in fullterm newborns in the Arabian region. Consequently, they went on to study 200 Saudi Arabian infants. Ocular dimensions measured include axial length, anterior chamber depth and lens thickness. They concluded that the average values that were obtained for the measured variables were not signi®cantly dierent from those of surveys performed on other populations in the world. The present study was carried out to determine the average values of the dimensions of the optical components and their intercorrelations in adult Saudi Arabians. Currently, such values are not available. The results of this study will be useful in understand-
The prevalence of refractive error is dependent on various factors including age and race. Asians as a group tend to have a higher prevalence of myopia than nonAsians (Lam and Goh, 1991; Goh and Lam, 1994; Lam et al., 1994). The type and magnitude of the refractive error of an eye is determined by the relationships between the dimensions of its optical components. The relevant optical components include the corneal power and radius of curvature, anterior chamber depth, lens thickness and power, vitreous chamber depth and the axial length. The average values of the dimensions of the optical components are also dependent on race, age and gender. The high prevalence of emmetropia in the general population implies that the growth of the optical components must be coordinated such that an excess in the dimension of one component is oset by a reduction adjustment in the dimension of another so 1
Tel./fax: 00966-1-4191077; e-mail:[email protected].
Received: 21 July 1998 Revised form: 1 February 1999
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ing ocular growth patterns and the bases of the development of refractive errors among Saudi Arabians of Arab descent.
Method A total of 152 subjects (75 males and 77 females) took part in this study. All subjects were Saudi Arabian citizens of Arab origin. Their ages ranged from 16 to 50 years with an average of 22.68 years. The subjects were recruited from the student and sta populations of the College of Applied Medical Sciences, King Saud University, Riyadh, Saudi Arabia. Informed consent was obtained from all participating subjects. Each subject was objectively refracted using a Nidek autorefractor (ARK-900) followed by a subjective refraction. Both objective and subjective refractions were done without cycloplegia. The objective refractions were performed by two student assistants while the subjective re®nements were done by the author. Subjects were categorized into refractive groups based on the following subjective refraction ®ndings: emmetropes were those with spherical equivalent of the refractive error E20.50 D, hyperopes had refractive error > + 0.50 D, while myopes were those subjects with spherical equivalent of the refractive error >ÿ0.50 D. The corneal radius of curvature of each eye was measured by using a Bausch & Lomb type manual keratometer. The keratometer calibration was routinely veri®ed by using a steel ball of known radius (6.75 mm, 50.0 D). The keratometer readings on the female subjects were all made by a female student assistant while a male student assistant measured the central corneal radius of all the male subjects. However, both assistants followed exactly the same
procedure. The corneal radius of curvature along the two principal meridians was determined. The average radius of curvature was calculated and used for all subsequent analyses. All ocular components dimensions were measured by the author using a Nidek Echoscan (Model US3300, Japan) equipped with a 10 MHz probe. The subjects corneas were anesthetized, each with one drop of 0.4% Novesin (Benoxinate hydrochloride 4 mg, CIBA Vision, Switzerland). Measurements were made with the subject lying in a supine position while viewing a distant target. The lids were held apart with the ®ngers, care being taken not to apply excessive pressure on the globe. The probe was then lightly placed on the cornea. The waveform was viewed to ensure that it had four clean peaks each from the cornea, anterior and posterior lens surfaces and the retina. A minimum of six acceptable measurements were made. The instrument automatically determined and displayed the average value and standard deviation of each ocular component.
Results and analyses Table 1 shows a summary of some descriptive statistics for each of the measured variable in this study. Initial analyses (one-sample t-tests) showed that for all subjects, there were no signi®cant dierences between the right and left eye values of all measured variables. Only the right eye values are therefore shown in Table 1. Age distribution Most of the male subjects (N = 20, 26.7%) had ages between 18.74 and 20.32 years with a midpoint at 19.53 years. The overwhelming majority of the male
Table 1. Descriptive statistics of the measured variables Gender Age (years) SPH (D) KV (D) KH (D) KAV (D) ACD (mm) LT (mm) VCD (mm) AL (mm) AL/CR Average Standard deviation Minimum Maximum
M F B M F B M F B M F B
22.71 22.66 22.68 4.80 5.39 5.09 16.00 19.00 16.00 42.00 50.00 50.00
ÿ1.11 ÿ0.79 ÿ0.95 1.73 1.41 1.58 ÿ7.63 ÿ5.00 ÿ7.63 1.38 1.75 1.75
43.30 43.81 43.56 1.48 1.71 1.62 39.68 37.11 37.11 46.85 46.75 46.85
42.78 43.08 42.93 1.41 1.91 1.69 39.00 34.27 34.27 45.75 46.62 46.62
43.05 43.44 43.25 1.40 1.67 1.55 39.56 39.15 39.15 46.25 46.21 46.25
3.32 3.08 3.20 0.33 0.40 0.38 2.45 2.07 2.07 3.88 4.08 4.08
3.72 3.73 3.72 0.25 0.28 0.27 3.04 3.30 3.04 4.33 4.70 4.70
16.80 16.36 16.58 1.01 0.94 0.99 15.19 14.54 14.54 19.59 18.92 19.59
23.81 23.16 23.48 0.99 1.03 1.06 21.94 20.91 20.91 26.68 25.89 26.68
3.03 2.97 3.00 0.12 0.14 0.13 2.84 2.42 2.42 3.49 3.30 3.49
SPH = spherical equivalent of the refractive error, KV and KH corneal radius of curvature along the vertical and horizontal meridians; KAV = average corneal radius of curvature; ACD = anterior chamber depth; LT = lens thickness; VCD = vitreous chamber depth; AL = axial length; AL/CR = axial length/corneal radius ratio; M = Males; F = females; B = males and females
Ocular components values in Saudis: E. P. Osuobeni subjects (N = 74, 98.67%) were aged less than 40 years. Only one male subject (1.33%) was aged over 40 years. Thirty-®ve female subjects (45.45%) had ages between 19.11 and 21.21 years with a midpoint at 20.16 years. Seventy-six of the female subjects (98.7%) were aged less than 40 years. Only one female subject (1.33%) was over 40 years. A two-sample t-test showed that there was no signi®cant dierence between the average male and female ages (t = ÿ0.053, df = 150, p = 0.957).
Spherical equivalent of the refractive error The majority of the male subjects (N = 41, 54.67%) were myopic, with an average spherical equivalent of refractive error of ÿ2.13 D. Twenty-nine (38.67%) were emmetropic, with an average spherical equivalent of 0.00 D. Only ®ve of the male subjects (6.67%) were hyperopic with an average spherical equivalent error of +0.83 D. A majority of the female subjects (N = 43, 55.84%) were emmetropic, with an average spherical equivalent of the refractive error of 0.01 D. Thirty were myopic (38.96%) with an average spherical equivalent of ÿ2.20 D, while, only four (5.20%) were hyperopic, with an average spherical equivalent error of +1.09 D. A twosample t-test demonstrated that the average spherical equivalent errors of the male and female subjects were
not signi®cantly p = 0.215).
dierent
(t = 1.245,
491
df = 150,
Corneal curvature The female cornea was slightly more curved (smaller radius of curvature) along the major meridians (Table 1). Although the mean corneal radius of curvature was also slightly smaller than that of the males, the dierence was not signi®cant (t = ÿ1.465, df = 150, p = 0.145). For all subjects, the average corneal radius of curvature varied non-signi®cantly with the spherical equivalent of the refractive error (F = 0.878, df = 44, p = 0.6809) and the type of refractive error (F = 0.991, df = 2, p = 0.374). However, as shown in Table 2, myopes tended to have the steepest corneas, followed by the emmetropes and, lastly, the hyperopes. Anterior chamber depth Males had deeper average anterior chamber depth than females (Table 1). The dierence in the average anterior chamber depths was signi®cant (t = ÿ4.137, df = 150, p = 0.000). For the male (F = 1.02, df = 55, p = 0.50), females (F = 0.68, df = 60, p = 0.86) and all subjects (F = 1.213, df = 90, p = 0.2118), the anterior chamber depth did not vary signi®cantly with the magnitude of the spherical equiv-
Table 2. Average values of measured variables of myopic, emmetropic and hyperopic subjects Gender
Variable
Myopes (<ÿ0.50 DS)
Emmetropes (ÿ0.50 to +0.50 DS)
Hyperopes (> + 0.50 DS)
M F B M F B M F B M F B M F B M F B M F B
SPH (D)
ÿ2.13 ÿ2.20 ÿ2.16 43.20 43.66 43.39 3.38 3.26 3.33 3.72 3.61 3.68 17.13 16.97 17.06 24.18 23.85 24.04 3.09 3.08 3.09
0.00 0.01 0.01 42.95 43.32 43.17 3.30 2.98 3.11 3.68 3.78 3.74 16.42 15.98 16.16 23.40 22.75 23.01 2.97 2.91 2.94
+0.83 +1.09 +0.94 42.40 43.19 42.75 3.00 2.87 2.94 3.90 3.93 3.91 16.28 15.82 16.08 23.18 22.53 22.89 2.91 2.88 2.89
KAV (D) ACD (mm) LT (mm) VCD (mm) AL (mm) AL/CR
M = males; F = females; B = males and females; SPH = spherical equivalent of the refractive error, KV and KH corneal radius of curvature along the vertical and horizontal meridians; KAV = average corneal radius of curvature; ACD = anterior chamber depth; LT = lens thickness; VCD = vitreous chamber depth; AL = axial length; AL/CR = axial length/corneal radius ratio
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alent. However, the type of refractive error had a signi®cant eect on the anterior chamber depth: males, F = 3.18, df = 2, p = 0.047; females, F = 5.77, df = 2, p = 0.005; for all subjects, F = 8.895, df = 2, p = 0.002. Generally, myopes had the deepest anterior chamber, followed by the emmetropes and then the hyperopes. For the male subjects, a two-sample t-test demonstrated that emmetropes had deeper anterior chamber than hyperopes (t = 2.198, df = 32, p = 0.035). Also, myopes had signi®cantly deeper anterior chamber than hyperopes (t = 2.265, df = 44, p = 0.028). There was no signi®cant dierence between the anterior chamber depths of myopes and emmetropes (t = ÿ0.969, df = 68, p = 0.336). For the female subjects, only the anterior chamber depth of myopes was signi®cantly deeper than that of emmetropes (t = ÿ3.38, df = 71, p = 0.0000). The results of similar tests for all subjects are shown in Table 3. Again, myopes had the deepest anterior chamber followed by emmetropes and hyperopes. Lens thickness Females had slightly but insigni®cantly thicker lenses than males (t = 0.163, df = 150, p = 0.871). The lens thickness varied signi®cantly with the spherical equivalent of the refractive error for the male subjects (F = 1.979, df = 49, p = 0.034) although this was not the case with either the female subjects (F = 1.242, df = 54, p = 0.294) or all subjects (F = 1.009, df = 74, p = 0.0484).
For the female subjects, the type of refractive error signi®cantly aected the lens thickness (F = 4.62, df = 2, p = 0.01). Emmetropes had signi®cantly thicker lenses than myopes (t = 2.792, df = 71, p = 0.006). Similarly, hyperopes had signi®cantly thicker lenses than myopes (t = ÿ2.553, df = 32, p = 0.015). There was no signi®cant dierence between the lens thickness of emmetropes and hyperopes (t = ÿ0.844, df = 45, p = 0.402). For the male subjects, an analysis of variance test demonstrated that there were no signi®cant dierences in lens thickness between the three refractive error groups (F = 1.597, df = 2, p = 0.209). For all subjects, the type of refractive error signi®cantly aected the lens thickness (F = 3.675, df = 2, p = 0.027). The results of subsequent two-samples ttests (Table 3) indicated that only the hyperopes had signi®cantly thicker lenses than myopes. Vitreous chamber depth Males had signi®cantly deeper vitreous chamber depth than females (t = ÿ2.778, df = 150, p = 0.006). When all subjects were considered together the vitreous chamber depth varied signi®cantly with the magnitude of the spherical equivalent of the refractive error (F = 1.708, df = 122, p = 0.048). The vitreous chamber depth was signi®cantly dierent between the three refractive error groups: males, F = 5.480, df = 2, p = 0.006; females, F = 14.10, df = 2, p = 0.000). For all subjects, the depth of the vitreous chamber was also signi®cantly dierent
Table 3. Results of two-sample t-tests between emmetropic (E), myopic (M) and hyperopic (H) subjects for combined male and female subjects Mean variables compared KAV (E)±KAV (M) KAV (E)±KAV (H) KAV (M)±KAV (H) ACD (E)±ACD (M) ACD (E)±ACD (H) ACD (M)±ACD (H) LT (E)±LT (M) LT (E)±LT (H) LT (M)±LT (H) VCD (E)±VCD (M) VCD (E)±VCD (H) VCD (M)±VCD (H) AL (E)±AL (M) AL (E)±AL (H) AL (M)±AL (H) AL/CR (E)±AL/CR (M) AL/CR (E)±AL/CR (H) AL/CR (M)±AL/CR (H)
Difference between means
t-statistic
df
p-value
Decision
ÿ0.23 0.42 0.65 ÿ0.22 0.16 0.38 0.07 ÿ0.16 ÿ0.23 ÿ0.90 0.08 0.99 ÿ1.03 0.12 1.14 ÿ0.15 0.04 0.19
ÿ0.8953 0.6924 1.2955 ÿ3.7179 1.2029 2.9987 1.5988 ÿ1.6555 ÿ2.6393 ÿ6.0550 0.2916 2.8912 ÿ6.6946 0.3791 3.2572 ÿ8.3604 1.0991 4.8684
141 79 78 141 79 78 141 79 78 141 79 78 141 79 78 141 79 78
0.3721 0.4907 0.1989 0.0003 0.2326 0.0036 0.1121 0.1018 0.0100 0.0000 0.7713 0.0049 0.0000 0.7056 0.0017 0.0000 0.2751 0.0000
N/S NS NS S N/S S N/S N/S S S NS S S N/S S S N/S S
KAV = average corneal radius of curvature; ACD = anterior chamber depth; LT = lens thickness; VCD = vitreous chamber depth; AL = axial length; AL/CR = axial length/corneal radius ratio; NS = not significantly different; S = significantly different
Ocular components values in Saudis: E. P. Osuobeni between the three refractive error groups (F = 19.908, df = 2, p = 0.000). Two sample t-tests (Table 3) indicated that myopes had signi®cantly deeper vitreous chamber than emmetropes and hyperopes. The vitreous chamber depth of emmetropes was not signi®cantly dierent from that of hyperopes. Axial length Males had signi®cantly longer axial length than females (t = ÿ3.937, df = 150, p = 0.000). For the combined male and female subjects, the axial length varied signi®cantly with the magnitude of the refractive error (F = 3.142, df = 125, p = 0.001). The axial length was signi®cantly dierent between the three refractive groups: males, F = 7.324, df = 2, p = 0.001; females, F = 14.97, df = 2, p = 0.000); all subjects, F = 23.893, df = 2, p = 0.000). Myopic male subjects had signi®cantly longer axial length than emmetropes (t = ÿ3.451, df = 68, p = 0.000) and hyperopes (t = 2.117, df = 44, p = 0.039). There was no signi®cant dierence between the axial length of emmetropes and hyperopes (t = 0.576, df = 32, p = 0.569). Similarly, myopic female subjects had signi®cantly longer axial length than emmetropes (t = ÿ5.46, df = 71, p = 0.000) and hyperopes (t = 2.52, df = 32, p = 0.01). The results of similar analyses for all subjects are shown in Table 3. The decisions are similar to those of the males and females. Axial length/corneal radius ratio (AL/CR) The average axial length/corneal radius ratio was signi®cantly dierent between genders (t = ÿ2.808, df = 150, p = 0.006) being higher in males than females. The AL/CR ratio varied signi®cantly as a function of the magnitude of the spherical equivalent of the refractive error: males, F = 13.783, df = 35, p = 0.000; females, F = 5.00, df = 70, p = 0.02. Analysis of variance tests demonstrated that the AL/ CR ratio varied signi®cantly with the type of refractive error for both the male (F = 17.181, df = 2, p = 0.000) and female (F = 20.949, df = 2, p = 0.000) subjects. Myopic male subjects had signi®cantly higher AL/ CR ratio than their emmetropic (t = ÿ4.943, df = 68, p = 0.000) and hyperopic (t = 3.393, df = 44, p = 0.001) counterparts. Also, male emmetropes had signi®cantly higher AL/CR ratio than hyperopes (t = 2.224, df = 32, p = 0.033). Similarly, myopic female subjects had signi®cantly higher AL/CR ratio than emmetropes (t = ÿ6.30, df = 71, p = 0.000) and hyperopes (t = 3.42, df = 32,
493
p = 0.000). However, there was no signi®cant dierence between the AL/CR ratio of female emmetropes and hyperopes (t = 0.52, df = 45, p = 0.61). Analysis of the combined male and female data showed that the AL/CR ratio varied signi®cantly with the type of the refractive error (F = 40.191, df = 2, p = 0.000). In general, myopic subjects had signi®cantly higher AL/CR ratio than emmetropic (t = ÿ8.360, df = 141, p = 0.000) and hyperopic subjects (t = 4.868, df = 78, p = 0.000). There was no signi®cant dierence between the AL/CR ratio of emmetropes and hyperopes (t = 1.099, df = 79, p = 0.275). Correlation coecients Table 4 is a display of the correlation coecients between the variables measured in the present study. In general, the sign of the correlation coecient between two variables is similar for the male and female subjects although their magnitude occasionally dier. A negative correlation coecient between the spherical equivalent of the refractive error and another variable, e.g. axial length, indicates that as the magnitude of the variable increases, the spherical equivalent of the refractive error becomes more myopic. The opposite is true for a positive correlation coecient. Table 4 shows that the spherical equivalent of the refractive error was negatively correlated with all the optical components dimensions except lens thickness and corneal radius of curvature with which positive correlations were obtained. There is a negative correlation between average corneal radius of curvature and anterior chamber depth. This indicates that ¯at corneas are associated with shallow anterior chamber depths. As would be expected, there is a very high positive correlation between vitreous chamber depth and axial length.
Discussion The criteria used to classify subjects into the three refractive error types was the same or very similar to that used in some previous studies (Sorsby et al., 1957; Goss et al., 1990; Grosvenor and Scott, 1991; Lam and Goh, 1991; Van Rens and Arkel, 1991; Grosvenor and Scott, 1993; Goh and Lam, 1994; Lam et al., 1994; McBrien and Adams, 1997). According to Zadnik et al. (1992) only changes greater than ÿ0.69 and +0.56 D are outside the range of measurement error for repeated measures of non-cycloplegic subjective refraction. Consequently, measurement of refractive error e2 0.75 D is required to establish the
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Table 4. Correlation coefficients between refractive error and ocular components
SPH (D) KAV (D) ACD (mm) LT (mm) VCD (mm)
Gender
KAV (D)
ACD (mm)
LT (mm)
VCD (mm)
AL (mm)
AL/CR
M F B M F B M F B M F B M F B
ÿ0.06$ ÿ0.09$ ÿ0.06$
ÿ0.28* ÿ0.36* ÿ0.33* +0.10$ +0.05$ +0.02$
+0.14* +0.27* +0.20* ÿ0.09$ ÿ0.09$ ÿ0.09$ ÿ0.51* ÿ0.67* ÿ0.57*
ÿ0.67* ÿ0.64* ÿ0.65* ÿ0.52* ÿ0.46* ÿ0.50* +0.23* +0.44* +0.39* ÿ0.27* ÿ0.50* ÿ0.38*
ÿ0.71* ÿ0.66* ÿ0.68* ÿ0.50* ÿ0.43* ÿ0.47* +0.39* +0.62* +0.56* ÿ0.20$ ÿ0.45* ÿ0.32* +0.95* +0.95* +0.95*
ÿ0.84* ÿ0.69* ÿ0.74* N/A N/A N/A +0.51* +0.65* +0.62* ÿ0.30* ÿ0.52* ÿ0.42* +0.60* +0.52* +0.57*
SPH = spherical equivalent of the refractive error; KAV = average corneal radius of curvature; ACD = anterior chamber depth; LT = lens thickness; VCD = vitreous chamber depth; AL = axial length; AL/CR = axial length/corneal radius ratio; M = males; F = females; B = males and females.N/A = not applicable because the corneal radius of curvature goes into the AL/CR ratio.*Significant at 0.05 level.$Not significant
presence of a refractive anomaly or a change in the magnitude of a refractive error. The criterion of refractive error e2 0.75 D used to classify subjects in the present experiment and in some previous studies is therefore consistent with the ®ndings of Zadnik et al. (1992). Myopes and emmetropes make up almost equal numbers of all subjects, 71 (46.71%) and 72 (47.37%), respectively. However, there were very few hyperopes (9, 5.92%). The distribution of the refractive error type was not entirely unexpected considering the fact that most of the subjects were university students of young adult age. Similar low number of hyperopes had been reported for subjects of similar age range to those currently investigated (Lam and Goh, 1991; Goh and Lam, 1994; Goss et al., 1997). The results of the present study show that, as a group, females tend to have steeper corneas (lower radius of curvature), thicker lenses, shorter vitreous chamber depth and axial length than males. Similar observations had been previously reported for subjects of various racial groups (Stenstrom, 1948; Sorsby et al., 1961; Fledelius and Stubgaard, 1986; Garner et al., 1990; Lam and Goh, 1991; Goss and Jackson, 1993; Goh and Lam, 1994; Lam et al., 1994; Goss et al., 1997). The dierences in the average corneal radius of curvature between genders and across refractive error types are all within the range of 95% limits of agreement (ÿ1.03 to +0.84 D) for repeated measures of corneal radius of curvature (Zadnik et al., 1992). The dierences obtained in the present experiment may therefore be partly explained by experimental errors of measurement.
Grosvenor and Goss (1998) wrote that the mean corneal power was greater (smaller radius of curvature) for girls than for boys by approximately 0.50 to 1.00 D. They observed that this was consistent with the fact that the mean axial length was somewhat less for the girls than for the boys. A similar explanation could be advanced for the present results. Table 4 shows that on the average corneal radius of curvature is positively correlated with axial length. Consequently, eyes with long axial length will be associated with corneas with long radius of curvature or low corneal powers. The present results have shown that on the average males have signi®cantly deeper anterior chamber depth than females. Also, myopes have deeper anterior chamber depth than non-myopes. The dierence in anterior chamber depth between the genders (0.24 mm) is within the 95% range (ÿ0.32 to +0.26 mm) of experimental error reported by Zadnik et al. (1992) for repeated measures of this optical component. The dierence in anterior chamber depth between males and females in this study could therefore be due to experimental error. However, other studies have reported deeper anterior chamber depth for males (Alsbirk, 1977; Garner et al., 1990; Goss et al., 1997) and myopes (McBrien and Millodot, 1987; Grosvenor and Scott, 1991; Bullimore et al., 1992; Carney et al., 1997). The dierence in lens thickness between males and females found in this study is very small (0.01 mm) and non-signi®cant. This dierence is well within the 95% range of the limits of agreement for repeated measures of lens thickness (ÿ0.18 to +0.21 mm) according to Zadnik et al. (1992). Although emme-
Ocular components values in Saudis: E. P. Osuobeni tropes and hyperopes tend to have thicker lenses than myopes, only the hyperopes had signi®cantly thicker lenses. The dierence in lens thickness between myopes and hyperopes (ÿ0.23 mm) is outside the limits of experimental error found by Zadnik et al. (1992). Females had signi®cantly shallower vitreous chamber depth than males. The dierence, ÿ0.44 mm, is outside the range of experimental error (ÿ0.37 to +0.37 mm) for repeated measures of vitreous chamber depth reported by Zadnik et al. (1992). Other investigators have reported signi®cantly deeper vitreous chamber depth in males than females (Alsbirk, 1977; Fledelius, 1982b; Garner et al., 1990; Lam and Goh, 1991; Goh and Lam, 1994; Lam et al., 1994; Goss et al., 1997). Also, the present results collaborate previous ®ndings that myopes have the deepest vitreous chamber (McBrien and Millodot, 1987; Grosvenor and Scott, 1991; Bullimore et al., 1992; Carney et al., 1997). The present results have demonstrated that males have signi®cantly longer axial length than females. Also, myopes have the longest axial length followed by emmetropes and then hyperopes. Similar results have been reported previously (Alsbirk, 1977; Fledelius, 1982b; McBrien and Millodot, 1987; Garner et al., 1990; Grosvenor and Scott, 1991; Lam and Goh, 1991; Bullimore et al., 1992; Goh and Lam, 1994; Lam et al., 1994; Carney et al., 1997). The mean AL/CR ratios obtained for Saudis in the present experiment are smaller than those reported for Hong Kong Chinese subjects (Goh and Lam, 1994; Lam et al., 1994). The dierence could be explained by the fact that the Chinese subjects had higher myopic spherical equivalent of the refractive error. Calculations based on data presented by Alsbirk (1977) show that Greenland Eskimos of mean age 20 years have AL/CR ratio of 3.02 (males) and 3.01 (females). These values are quite similar to those obtained in the present experiment. According to Sorsby et al. (1957), British Caucasian emmetropes (ÿ0.50 to +0.50 D) have AL/CR ratio of 3.06, myopes (ÿ0.51 to ÿ4.00 D) have AL/CR ratio of 3.24 and hyperopes (+0.51 to +4.00 D) a ratio of 2.98. The corresponding values for the Saudis are emmetropes (2.94), myopes (3.09) and hyperopes (2.89). The AL/CR ratio has the highest correlation with refractive error (males = ÿ0.84; females = ÿ0.69). Stenstrom (1948) also reported correlation coecient of ÿ0.84 between refractive error and AL/CR, although other investigators had reported correlation coecient between the two as high as ÿ0.92 (Grosvenor and Scott, 1994). The dierence in the correlation coecient between the male and female subjects in the present study could be partly due to the fact that myopes made up 54.67% of the male popu-
495
lation and only 38.96% of the female population. The fact that the AL/CR ratio has the highest correlation coecient with refractive error supports the conclusion of Grosvenor and Scott (1994) that the AL/CR ratio is the most signi®cant determinant of the refractive state of the eye. As shown in Table 4, there is a very signi®cant positive correlation (+0.95) between the vitreous chamber depth and axial length. Similar results had been previously reported (Goh and Lam, 1994; Lam et al., 1994). Grosvenor and Goss (1998) observed that the refractive component change that bears the major responsibility for the development of myopia is axial elongation, although a steep cornea may also play an important role. Hyperopia is also thought to be predominantly axial in nature (Strang et al., 1998). The correlation coecient between the axial length of the present Saudi male subjects and the spherical equivalent of their refractive error was ÿ0.71, the corresponding value for the females subjects was ÿ0.66. This dierence could also be explained by the fact that there were more myopic male subjects than females. Lam et al. (1994) reported correlation coecient of ÿ0.66 between axial length and refractive error for male Hong Kong Chinese over 40 years, and ÿ0.62 for their female counterparts. Goh and Lam (1994) gave the correlation coecient between axial length and refractive error as ÿ0.61 for male Hong Kong Chinese between the ages of 19 and 39 years and ÿ0.64 for female Hong Kong Chinese subjects between the same age range. Garner et al. (1990) reported that the correlation coecient between refractive error and axial length for Malay children was ÿ0.66 while the corresponding value for Melanesian children was ÿ0.17. They explained that the dierence between the two racial groups was due to the fact that the Melanesian children were mainly emmetropic. Without implying a time sequence in the process of emmetropization, an eye with excessively elongated axial length and a correspondingly deep vitreous chamber will require less refractive power than average to achieve perfect focus on the retina. This demand could be met by a ¯atter cornea or a thinner lens or a combination of both. An emmetropic eye could be regarded as an eye in which these adjustments have been successful. Conversely, in ametropic eyes, the need for adjustment in corneal and/or lenticular power, in response to axial length, had not been ecient or successful. The correlation coecient quanti®es the direction and rate of change in the magnitude of one variable as a result of changes in another variable. Following the reasoning above, the rate of change of corneal power and/or lenticular thickness, subsequent to changes in axial length, or correlation coecient, should be greater for emmetropic than
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ametropic subjects. Goss et al. (1997) cited a number of references in which higher correlation coecients have been reported in emmetropes than for groups with a wide range of refractive errors. The anterior central corneal radius of curvature is positively correlated with refractive error. The correlation coecients found in this study, +0.06 and +0.09, are in general agreement with previously reported cases (Fledelius, 1982a; Goh and Lam, 1994; Grosvenor and Scott, 1994; Lam et al., 1994; Carney et al., 1997; Goss et al., 1997). The positive sign indicates that a ¯at central cornea (long radius of curvature) tends to be associated with hyperopic eyes. The correlation coecient is quite similar between gender. The present results also proved that lens thickness is positively correlated with spherical equivalent of refractive error indicating that thicker lenses are associated with less myopic eyes. The correlation coecient for the female subjects is approximately two times that for the males. This is thought to be due to the higher number of emmetropic female subjects. Corneal radius of curvature is positively correlated with vitreous chamber depth and axial length. This implies that as the globe increases in length the cornea becomes ¯atter. Previous studies have also reported positive correlations between corneal radius of curvature and vitreous chamber depth, and between corneal radius of curvature and axial length (Garner et al., 1990; Lam and Goh, 1991; Grosvenor and Scott, 1993; Lam et al., 1994). The results of the present and previous studies have demonstrated that myopic eyes, which are long, have steeper (shorter radius of curvature) corneas. Scott and Grosvenor (1993) explained this apparent contradiction by suggesting that together with increase in axial length, corneal steepening also occurs, during the development of myopia. The lens thickness is negatively correlated with vitreous chamber depth and axial length. This indicates that as part of the process of emmetropization, the lens must become thinner as the vitreous chamber depth increases. Table 4 shows that the correlation coecient is higher for the female subjects than the males. This may also be attributed to the larger number of emmetropic female subjects.
Acknowledgements I gratefully acknowledge the role played by Badria Mo'aned Al-Enzey and Majeed Sadoun AlAbdulmunem in making arrangements to secure subjects for this study and in helping with some of the measurements.
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