Emic concern

Ultrasonography was used in this study to ascertain the biometry of the eye due to the following reasons. The eye is located at the body surface and during ultrasonography it normally reflects only a few echoes that can easily be identified.^1-5 Ultrasonography is cheap, readily available, easy to perform, and can be carried out easily in the clinical setting. Ultrasonography is able to demonstrate the existence and composition of the eyeball, presence of intra-ocular and extra-intra-ocular masses, as well as monitor changes in dimensions or appearance of such masses.

Two hundred subjects were assessed in the study, made up of 75 males (38%) and 125 females (62%) giving a M: F ratio of 1:1.6 as shown in Table 1.

The higher preponderance of females in this study may be attributable to chance. Another reason may be that there is generally a lower life expectancy for the males compared to the females which accounts for an overall higher female to male ratio. Irabor⁶⁶ showed a similar pattern, where the mean age of normal subjects was 29.18 years with more females (n=92, or 92%) than males (n=8, or 8%). A different observation was however made in the study of Alaskan Eskimos by Van Rens et al ⁶⁰ in which there was a prevalence of males (n = 218 or 59.3%) compared to females (n = 149, or 40.7%). The reason for this sex variation was not however provided by the authors.

The eyeball diameters measured in this study are the vertical or

cranio-while others have demonstrated that the vertical or cranio-caudal diameter is equal to the axial or antero-posterior diameter.^28,33,36 The mean vertical or cranio-caudal, transverse and antero-posterior diameters for males showed consistently higher values than females throughout the entire study, this was not statistically significant. Differential ocular growth is a still incompletely understood complex process governed by a potentially large number of genes, hormonal and environmental factors. The finding of slightly higher male eyeball diameters correlated with works done by Stenstrom ²⁸, Scammon and Wilmer ²⁶, Sorsby et al ²⁷, and Guthoff ¹, as shown in Table 8.

There was also consistently no statistically significant difference in all the reports.

The finding was also corroborated by Balwin ³⁷, and explained that the eyeball dimension was affected by the corneal curve or bulge. The females in the study were observed to have showed a more significant tendency towards corneal steepening and therefore had a lower value for eyeball dimensions, while the males only showed a very slight tendency towards corneal flattening, and therefore a higher eyeball dimension. These findings are similar to the study by Osuobeni et al⁶, Irabor⁶⁵ and Koraszewska – Matuszewska et al⁴ who reported that males had longer axial lengths than females. The finding however was supported by earlier work on dimensions of the orbit and eye which reported a slightly larger diameter in females than males (Schulz ³⁵ and later by Krogan³⁸). The explanation for this difference was not however stated by the authors.

The study showed that the values for right eyeball diameter were consistently

right and left eye in both males and females. The variability in the size of the eyeball from one person to another is large, but differences between the two sides in a normal population are small. Krogan ³⁸ showed that the left eye in males was slightly smaller that the right, but similar findings was not present in females. There was no explanation offered by the author for such an observation. Irabor⁶⁵ showed that in all groups studied (emmetropes, myopes and hyperopes), the difference in axial lengths in the right and left eyes were statistically not significant. These findings was stated to be probably due to the fact that most paired organs within the human body are not symmetrical in size, for example, the kidneys, thyroid, and ovaries amongst others.⁵⁵

This study showed that there was a gradual increase in eyeball diameter with age, with the older age group having the highest values in both males and females. The finding was more prominent in the males. It contradicted the study by Irabor⁶⁵ in which the younger age group showed higher eyeball diameters than the older age group. The reason given by the author is the generalized reduction in organ sizes as people grew older. Of important note is that although both studies were carried out in the same environment, the factors evaluated, sample size and objectives were different for both studies.

The reduction in eyeball diameters and volume occurring at an older age group was further supported by a study of ocular dimensions with aging in normal eyes done by Lim et al⁶¹. The study showed a gradual decrease in

Scammon and Wilmer.²⁶ They explained this finding using the growth curve of the eyeball reported by Hirsh et al³⁴, which stated that the growth of the eyeball was similar to the growth curve of the brain and central nervous system rather that to the general body growth, and not related to height, diet, skeletal, genital or lymphoidal growth. The finding further stated that during the period of active growth, the eye shares with the brain the perculiarity of having a precocious growth.

The evaluation of eyeball volume was done in this study electronically using the measurements from the three eyeball diameters measured. There was a gradual increase in eyeball diameters and volume with relation to the age of the subjects. The study showed that the males had consistently higher values and the right was slightly larger than the left. (M: R= 10.64 ccm³, L=

10.19ccm³; F: R= 10.62 ccm³, L= 10.03 ccm³). Eyeball volume is substantially influenced by age and sex. A rapid growth in eyeball volume occurs in-utero and until 18 months of age, followed by a phase of slower growth. At birth, the eyeball volume is approximately 3 ccm³ and gradually increases to about 9-10 ccm³ by 18-30 years of age.⁷⁰ A possible explanation for this finding in the study is that the eyeball is a pressurized chamber with the corneo-scleral coat of the eyeball confining the intraocular pressure. This preserves the eye’s dimensions with the uveal circulation as the source of the intraocular fluid.

This results in a steady ocular pressure throughout life, even in the elderly.⁷⁰ The overall effect was a consistent preservation of adult eyeball diameter once the maximum value has been attained throughout life or even a gradual increase of these values in the elderly subjects. Again, a number of structural changes are encountered in the elderly subjects, which includes the rigidity of

leads to an accumulation of intraocular fluid, explaining the apparent increased eyeball diameter found in advancing age group.

However the measurement of eyeball volume in all the studies has not been done using a universal method. The difference being that the eyeball has been assumed to be either slightly ellipsoidal or spherical for eyeball volume estimation. Furthermore, the eyeball volume has generally been estimated with measurement of only one or two or all three eyeball diameters. This explains the different mathematical formulae used to calculate eyeball volume in different studies and the resultant difficulty to corroborate the findings in these studies. Studies done have showed that the calculation of all three diameters have a better result in estimation of eyeball volumetry.42-43,68-70

The values in this study were lower than the dimensions previously reported in Caucasians in work done by Scammon and Wilmer ²⁶. The variability in the size of orbital structures from one person to another is large, but such differences between the both sexes in a normal population are not statistically significant. Variation in eyeball diameters maybe due to racial differences in eyeball sizes, method of eyeball diameter measurement (contact or non-contact A- or B- mode ultrasonography), age group studied or a combination of all these factors. The values in this study were similar to findings in

corroborated the works of previously higher values reported in Caucasians by Stenstrom ²⁸Scammon et al ²⁶ Sorsby et al ²⁷, and Guthoff ¹.

The eyeball diameters forms the basis for the calculation of the intraocular lens power,^58-6 and diagnosis of refractive hypermetropia and myopia due to eyeball size. Irabor⁶⁵ discussed extensively on the findings of eyeball diameters in relation to age and refractive errors as found in our environment.

The accurate diameter of the eyeball is useful to calculate the intraocular power (IOL) and prevent any refractive error from occurring after surgery. A measurement error of 0.2 mm in the measurement of the axial length when using this formula will cause a refractive error of about 0.6 Dioptres.

In view of the above findings, it is therefore safe to conclude that this new through the eyelid technique of ocular biometry is reliable and justifiable.