|Year : 1977 | Volume
| Issue : 4 | Page : 6-16
Measurement of the anterior segment of the eyeball in emmetropia, ametropia and primary glaucoma
Medical College, Gwalior, India
Medical College, Gwalior
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Shukla B. Measurement of the anterior segment of the eyeball in emmetropia, ametropia and primary glaucoma. Indian J Ophthalmol 1977;25:6-16
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Shukla B. Measurement of the anterior segment of the eyeball in emmetropia, ametropia and primary glaucoma. Indian J Ophthalmol [serial online] 1977 [cited 2020 Nov 24];25:6-16. Available from: https://www.ijo.in/text.asp?1977/25/4/6/34606
President, Members of the All India Ophthalmological Society, Ladies and Gentlemen,
At the outset I express my deep gratitude to the members of the scientific and managing committees of this society for having bestowed on me the honour of delivering Maj. S.C. Dutta Oration. In the presence of my worthy teachers and distinguished ophthalmologists of the country I feel very humble in shouldering this responsibility. Whatever little I have achieved is because of their blessings and inspiration. Appreciation of my humble contribution has given me much faith and confidence.
In the fitness of things I would first pay my homage to Maj. S.C. Dutta after whom this award has been instituted from 1971. Maj. Dutta was born on 3rd October 1890 in Calcutta and spent his childhood in Bihar, Madhya Pradesh and Punjab. He graduated from Calcutta Medical College in 1913 and had his postgraduate training in Moorfields Eye Hospital, London. He was the President of the Opthalmological Society of Bengal and was elected .President of the All India Opthalmological Society for the year 1953-54. Of his many contributions his work on Epidemic Dropsy Glaucoma was outstanding. He worked in the Mayo Hospital for a long time and became the Chairman of the Board of Governors of that hospital during his last few years. Dr. Dutta was a distinguished Rotarian and was elected as Rotary Governor of District 53 in 1.953-54, incidently the same year when he was the president of our society.
Now I come to the subject of my oration which concerns with the measurement of the anterior segment of the eyeball in emmetropia, ametropia and primary glaucoma of both types. Quantitative estimation has always been the basis of scientific knowledge. In medical science, measurements of the structural patterns and physiological variations have provided valuable clues to disease processes. In the field of opthalmology, measurement of the anterior segment of the eyeball has evoked great interest because variations in the anatomical relationships between the structures forming the anterior chamber are known to have a close association with glaucoma.
In the present work, the depth, volume, shape and the angle of the anterior chamber and the diameter, thickness and the radius of curvature of cornea have been measured in emmetropia, hypermetropia, myopia, closed angle glaucoma and open angle glaucoma. The effect of age, sex and refraction and the correlation of these factors with each other has been studied. The role of various anatomical factors in the pathogenesis of closed angle glaucoma has been evaluated. From the results of the present study and the observations of other workers a new concept of the aetiology of primary closed angle glaucoma is suggested.
| Material and Method|| |
In all, 290 cases were studied and divided into five groups including emmetropia, hypermetropia, myopia, closed angle glaucoma and open angle glaucoma [Figure - 1]. After a routine eye examination the depth of the anterior chamber was measured at the centre of the pupil by a special attachment to Haag Streit 900 slit lamp [Figure - 2].
The mean value was calculated from three readings on each eye. First the depth was measured from anterior surface of the cornea to the anterior surface of the lens. Corneal thickness was subsequently measured at centre by a similar attachment and the value subtracted from the previous reading giving the correct depth of the anterior chamber. The patient was instructed to look at a fixation light to relax accommodation by the eye not under observation. These attachments are based on Jaeger's principle. Viewing the subject's eye through a special eyepiece, a narrow vertical section of the anterior chamber is seen and the field of view is split into an upper and a lower half. These two halves can be separated horizon ally and the degree of separation measured accurately by a stale. The points to be measured lying in the optical slit image can be brought to exact coincidence. The angle between the slit lamp and the microscope is fixed at 40° [Figure - 3],[Figure - 4].
The radius of curvature of cornea was measured by Javal-Schiotz' keratometer and the mean of the horizontal and vertical radii was noted. The corneal diameter was measured by Wesseley's keratometer. It is a one foot long hollow tube with a small aperture at one end and a plastic scale at the other end marked in millimeters.
The volume of the anterior chamber was measured by a photographic method devised by Jones and Maurice. A simple 35 mm camera is mounted on the central pillar of the Haag-Str. it slit lamp pointing vertically upward [Figure - 6]. To correct the outline of anterior chamber for the distortions introduced by corneal curvature grids were prepared. They represent a system of millimeter squares distorted under identical conditions. An appropriate grid was placed on the outline obtained and retraced onto graph paper by reading off the coordinates. The undistorted outline was thus obtained on a graph paper. It was cut out and folded at the optic axis and attached to one arm of a specially constructed balance. A strip 1 cmx 8 cm, cut from the same graph paper, served as a rider on the other arm of the balance and its distance along the arm was a direct measure of the volume of the anterior chamber in cubic millimeters. The basis of this method is that the volume of the anterior chamber is equal to that swept through by half rotation of its cross section as shown by Heim. 
The shape of the anterior chamber was arbitrarily divided into three types viz. A, B and C. In type A, the line joining the diametrically opposite ends of the root of iris passed above the anterior surface of the lens: in type B it touched or nearly touched the anterior surface whereas in type C it passed below it [Figure - 7]. A simple plastic instrument was constructed to measure the width of the angle of the anterior chamber in degrees. It had one straight arm with parallel sides and another arm with a radius of curvature of 7.33 mm. The angle of separation between the two arms could be measured in degrees by a protractcr attached to the straight arm [Figure - 8]. This simple goniometer was placed on the ou line, of the anterior chamber obtained and the angle read off in degrees. The data obtained from clinical observations eras analysed using bicmedical computer programmes. Four computor programmes were run. The first gave the mean, standard deviation, standard error of mean and range of each variable in each group. In second the above data was separately analysed for males and femals. The third gave separate analysis for right and left eyes with difference. The fourth programme gave correlation of different variables in each group with cross tabulation plots. Further statistical analysis was done to find the level of significance of different values.
| Observations|| |
In 40 out of 290 cases it was not possible to study the volume, shape and the angle of the anterior chamber. 3 [Table - 1] shows the diagnostic classification of cases and their sex distribution. There was a fair distribution of cases except in hypermetropia where the number was slightly less.
[Table - 2] shows the mean depth of the anterior chamber in each group. The range, standard deviation and standard error of mean was also worked out. In closed angle glaucoma the depth is very significantly less (p<.001) than all the other groups. In open angle glaucoma also the depth is significantly less than that in emmetropia. The depth in myopia is significantly greater than that in emmetropia but there is no significant difference between hypermetropia and emmetropia. There is a progressive decrease of the depth of the anterior chamber with advancing age, though there was a tendency for a rise at the extreme end. There was no significant difference in depth in the two sexes and the decrease with age was also similar.
[Table - 3] shows the volume of the anterior chamber in cubic millimeters. The standard deviation is very large in all the groups (28-60 cmm). Again the volume of the anterior chamber is very significantly smaller in closed angle glaucoma than in all other groups (p.. .001). Conversely in myopia it is significantly greater than rest of the groups (p <.001). In open angle glaucoma the volume is significantly lower than in emmetropia but no significant difference was observed between hypermetropia and emmetropia.
The angle of the anterior chamber was measured quantitatively by a procedure called goniometry. For comparison the angle was studied qualitatively also by gonioscopy in 113 cases. [Table - 4] shows the angle of the anterior chamber in degrees. Here also in closed angle glaucoma the value is significantly lower than that inother groups (p <.00 1). Similarly in myopia the angle is significantly wider than that in other groups (p<.001). The angle with in open angle glaucoma and hypermetropia did not significantly differ from emmetropia.
[Table - 5] shows the shape of the anterior chamber. Type A was extremely rare in closed angle glaucoma (3.15%). In other groups there was no significant difference in distribution of this shape. Type B was seen most frequently in myopia (69.88%) and least frequently in closed angle glaucoma (24.21%). Type G was very significantly commoner in closed angle glaucoma (p<.001). In open angle glaucoma also this shape was common more than in emmetropia. In myopia it was significantly rare than in emmetropia. There was no significant difference between emmetropia and hypermetropia.
[Table - 6] shows various corneal measurements in the five groups. No significant difference was observed in the radius of curvature of cornea in any of the groups. In closed angle glaucoma the diameter of the cornea is very significantly smaller than rest of the groups (p ....001). In open angle glaucoma also it was less than emmetropia. In myopia the diameter was significantly greater than rest.
So far the dimensions in closed angle glaucoma have been found to be significantly smaller than in other groups but the corneal thickness in this disease is significantly greater than other groups (P ....001). In open angle glaucoma also the corneal thickness was greater than in emmetropia. Although the cornea was thinnest in myopia the difference was not statistically significant from other groups.
[Table - 7] shows age distribution of the depth. There is a progressive decrease of depth with the increase in age.
[Table - 8] shows the distribution of refractive error in primary glaucomas. In closed angle gloucoma the incidence of hypermetropia is significantly higher than myopia (p.. .001). There is no such difference in open angle glaucoma and the two refractive errors are almost equally distributed.
[Table - 9] shows the chamber depth in bilateral closed angle glaucoma. The difference of depth in two eyes is not of significance.
[Table - 10] shows the chamber depth in 12 cases of unilateral closed angle glaucoma. Although shallower depth is found in the affected eye the difference is not statistically significant.
[Table - 11] shows positive correlation between the qualitative estimation and quantitative measurement of the angle of the anterior chamber in 113 eyes. With each successive higher grade of angle, the mean quantitative value of the angle is also greater. In [Table - 12],[Table - 13] various diagnostic groups are compared with each other in all the parameters and their differences are statistically evaluated.
| Discussion|| |
Although there is significant difference in various ocular measurements in the five groups studied, highly positive inter-correlation was between chamber depth, volume, angle and corneal diameter. It shows that the growth of the anterior segment is usually uniform in all dimensions.
A good correlation was found between goniometric and gonioscopic findings in 113 eyes  . Correlation of depth of the anterior chamber with volume, shape and angle has not been reported previously. Only Sugar  has reported in 10 cases of glaucoma a comparison between the depth, volume and angle width but without a control group or statistical analysis.
The present study has confirmed the association of a shallow chamber with closed angle glaucoma. It was first noted by Salzmann  in 1914 and confirmed subsequently by Barkan  , Sugar  etc. It has also confirmed that in primary closed angle glaucoma the unaffected eye also has shallow depth as observed by Rosengren  and Tornquist  .
The shape of the anterior chamber in profile has probably been studied for the first time. In closed angle glaucoma type C is very common (72.6%) and type A is very rare (3.1%) [Table - 5]. As even in emmetropia type C shape is present in about 25% cases it cannot be assumed that this shape necessarily predisposes to closed angle glaucoma. Conversely, the probability of an eye with type A shape developing closed angle glaucoma is very rare.
Hypermetropia has been regarded as an important predisposing factor in the pathogenesis of closed angle glaucoma (Hird  . Posner and Schlossman  and Perkins and Jay  ) However in the present study if we compare hypermetropia with emmetropia no significant difference is observed in any of the parameters.
On the other hand there is a significant difference in all the measurements between hypermetropia and closed angle glaucoma [Table 15]. In cases of closed angle glaucoma only 31.13% had hypermetropia whereas emmetropia was observed in 62.27% cases [Table - 11]. Grieten and Weekerss have also found that in closed angle glaucoma all the dimensions of the eye are smaller than the nonglaucomatous eyes of the same age and with the same degree of hypermetropia. Thus the role of hypermetropia in this disease does not seem to be so significant as so far thought to be. It would be more appropriate to presume that in a myopic eye the possibility of developing closed angle glaucoma is remote.
There is no doubt that the eye in closed angle glaucoma presents distinct variations from other groups including open angle glaucoma. However if one concentrates on the type of variation rather than the degree it would be obvious that in all the parameters analysed the variations in open angle glaucoma are of the same nature as in closed angle glaucoma. In other words all dimensions in open angle glaucoma are smaller than emmetropia though to a lesser extent than in closed angle glaucoma.
Smallness of the anterior segment of the eye in closed angle glaucoma is almost conclusively proved from the present study. Luyckx-Bacus and Weekers  have carried out ultrasonic measurements in normal and glaucomatous patients. In 43 cases of closed angle they have found vitreous length to be 15.52 mm (S.D. 1.21). This does not differ much from the value in 41 hypermetropic eyes but on comparing the measurements with 43 emmetropic eyes in whom the vitreous length was 16.32 mm (S.D. .84), a statistically significant difference was found. Thus it seems that the posterior segment is also smaller in closed angle glaucoma.
Smith  has commented that the size of the cornea is a fairly reliable indication of the overall size of the eyeball and found it to be smaller in closed angle glaucoma. In the present study also the corneal diameter was found to be very significantly smaller (p<.001) in closed angle glaucoma than in all other groups. Smith has actually shown in two cases after enucleation the small size of the eyeball. Hence it is quite justifitable to assume that the whole eyeball in closed angle glaucoma it much smaller than the normal eye.
On the basis of the measurements of the anterior segment of the eyeball and presuming that the corneal diameter is a fairly reliable index of the overall size of the eyeball it has been possible to grade the eyes in emmetropia, hypermetropia, myopia, closed angle glaucoma and open angle glaucoma according to their overall sizes. It has also been determined whether the overall difference in the two categories is statistically significant or not [Table 15],[Table 16]. The eyes can be placed in the following descending order [Figure - 9]
1. Myopia (biggest eye)
4. Open angle glaucoma
5. Closed angle glaucoma (smallest eye)
It will be noticed that while the overall size is decreasing from myopia to closed angle glaucoma the corneal thickness is increasing in the same order. The difference between myopia and emmetropia is significant but the difference between emmetropia and hypermetropia is not significant. The difference between open angle glaucoma and closed angle glaucoma is significant but the difference between open angle glaucoma and hypermetropia is not significant. The difference between emmetropia and open angle glaucoma is significant. This relationship can be readily understood by a simple diagram.
In the new born the normal eye is hypermetropia and as the growth proceeds this tends to diminish till emmetropia is reached. In some cases the tendency progresses further resulting in myopia. If the hypermetropic eye can be considered an undeveloped eye, the myopic one might be considered in a limited sense, overgrown'. Although this view is not now generally held and axial length is regarded as only one of the determining factors in ametropia, on a similar analogy one might consider the eye in closed angle glaucoma as undergrown. The axial length is certainly a very important parameter for the overall growth of the eye.
Shallow chamber, narrow depth, small volume and short corneal diameter can reasonably be expected in a small eye, but a significant increase in the thickness of the cornea in closed angle glaucoma was an unexpected finding (p<.001). It may be argued that increased corneal thickness may be due to corneal oedema. However Ytteborg  has shown that only when the intra-ocular pressure exceeds 40 mm Hg. the corneal thickness increases due to oedema. As most of the cases studied in this series were normotensive at the time of examination the possibility of increased thickness due to corneal oedema is negligible. A compensatory hypertrophy of the corneal stroma due to raised pressure was thought of as an alternative explanation but microscopic examination in 30 cases ruled out this possibility.
If a balloon is inflated the thickness of the wall, within certain limits, will go on decreasing as the size of the balloon increases. The growth of the eyeball is somewhat similar and one would expect a thinner coat (cornea and sclera) as the eyeball grows and increases in size. It is interesting to note that as the overall size of the eyeball decreases from myopia to closed angle glaucoma the corneal thickness increases in the same order supporting the above view. Although the sclera has not been measured in the present work it appears that the thickness of sclera also increases in the same order as the cornea.
According to Friedenwald's equation, the coefficient of ocular rigidity K, is inversely proportional to the total ocular volume; we would expect larger eyes to have lower values for K than smaller eyes. Clinically this is seen most markedly in highly myopic eyes in which the coefficient of rigidity is low; the converse, an increase in ocular rigidity in hypermetropic eyes is less striking  . The ocular glaucoma might well offer an explanation.
Abnormally small eyes due to retarded growth are seen in microphthalmos, a condition usually associated with other congenital deformities. Pure micropthalmos or nanophtalmos is a realtively rare condition which represents an arrested development of the globe in all the dimensions after the embryonic fissure is closed  . As main features of pure microphthalmos simulate those of primary closed angle glaucoma it is conceivable that the latter may be a former's variation.
In the intra-uterine life the lens is much larger than the eyeball and the anterior chamber is merely a chink at 10 mm stage. As the growth proceeds the size of eyeball in proportion to lens becomes much larger and the anterior chamber goes on deepening till puberty when the adult size is reached. A small eyeball and a relatively large lens (as indicated by a shallow chamber) again suggests the possibility of retarded growth in this condition.
| Conclusion|| |
In closed angle glaucoma the crux of the matter appears to be a disparity between the sizes of eyeball and lens the latter being proportionately larger. The eyeball attains the adult size soon after birth but the lens goes on increasing with age and thus increasing the disparity. Ultimately a critical level is reached where the pressure regulatory mechanism of the eye fails and the clinical picture of closed angle glaucoma sets in. The disparity can be clinically judged by the corneal diameter indicating the size of eyeball and depth of the anterior chamber indicating the size of lens and the possibility of onset of closed angle glaucoma can be assessed. [Table - 14].
| Acknowledgement|| |
I wish to express my deep gratitude to Professor E.S. Perkins for his kind help, guidance and supervision. The Medical Research Council of United Kingdom kindly awarded the A.P.W. Fellowship to undertake this research project. I am gratefull to Dr. D.M. Maurice for explaining his technique for measuring volume of the anterior chamber and to Mr.C. Downing for constructing the plastic goniometer. The paper is based on the thes is for Ph. D. from London university.
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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9]
[Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5], [Table - 6], [Table - 7], [Table - 8], [Table - 9], [Table - 10], [Table - 11], [Table - 12], [Table - 13], [Table - 14]