Documenta Ophthalmologica 82: 249-255, 1992. 9 1992 Kluwer Academic Publishers. Printed in the Netherlands.
Myopia progression in young school children and intraocular pressure H A N N E JENSEN Gentofie Hospital, Department of Ophthalmology, Division of Paediatric Ophthalmology and Handicaps, Faculty of Medicine, University of Copenhagen, Denmark Accepted 2 November 1992
Key words: Axial length, Intraocular pressure, Myopia, Myopia progression Abstract. Studies of the literature have given the impression that intraocular pressure (IOP) may influence the refractive error (RE); the reason being that the IOP in some studies is higher among myopic subjects than among emmetropes and hypermetropes. The progression of myopia was studied in 49 children, aged 9-12 years, over a period of 2 years. The rate of progression was 1.14 D/2 years and the IOP 16.6 mmHg. Comparison of the rate of progression between children with an IOP above 16 mmHg and those with an IOP of 16 mmHg or less showed a statistically significant difference: 1.32D/2 years as compared to 0.86D/2 years. These results are supported by ultrasound measurement of the axial length. The author recommends that measurement of lOP should be included in the design of studies of myopia progression. The reason for this is, that it would appear that those persons most at risk of developing higher degrees of myopia have a high IOP.
Introduction Considerable attention has been given for quite a number of years to the problems of myopia and myopia progression. In particular, much research has been carried out in order to determine the etiology of the condition in the hope that it would be possible to treat myopia or perhaps avoid the condition developing. A number of hereditary disorders (Stickler syndrome, Ehlers-Danloss syndrome and Marfan syndrome) have been shown to be associated with myopia, and studies of twins have also demonstrated the importance of heredity. On the other hand, the latter studies have also shown that environmental factors can influence or induce myopia, and the present opinion is that myopia has a multifactorial background. As the net result in all cases of myopia is a distorted sclera, it is possible that there is a common mechanism, so that a number of various developmental disorders join forces to form one common pathway. A factor that may be of importance to the etiology is the intraocular pressure (IOP), and the present article reviews the available literature regarding this hypothesis and briefly describes an investigation of myopia progression in young school children where the IOP was one of the main factors studied.
250 Review of the literature
Axial myopia is characterized by the posterior sclera having been stretched out of shape over a period of years. It is highly probable that the myopic scleral wall differs biochemically from normal. The sclera consists of 32% dry weight and 75% of this is collagen, the remaining 25% being noncollagenous protein and mucopolysaccharides. In the normal eye, there are mature, large diameter collagen fibres in the outer scleral layer, but in the inner layer the fibres are immature and of small diameter. It is probable, that the collagen is produced by fibroblasts in the inner layer, and that the fibres gradually move outward. In the myopic eye there are abnormalities in the size of the fibres and their arrangement [1, 2]. Further, there is increasing evidence that the collagen chemistry is also abnormal [3]. The sclera shows viscoelastic creep under stress and it is assumed that the elasticity and creep are related to the progression of myopia, when provoked by IOP or other forces [4]. It has been claimed throughout the years, that open angle glaucoma is more common among myopes and that myopia is present in a higher percentage of patients with glaucoma than in the population as a whole. Curtin [5] found among myopic patients above the age of 40 years, that 11.2% of eyes with an axial length of more than 26.5 mm suffered from glaucoma, while in eyes with an axial length of more than 30.5 mm the percentage rose to 23.1%. Perkins & Phelps [6] analyzed the refraction of patients with primary open angle glaucoma, ocular hypertension and low tension glaucoma and demonstrated, that myopia occurred more frequently in each of these groups than would be expected in a normal population of similar age. They also found that 6.9% of normal subjects between the ages of 55 and 74 years were myopic while no less than 27.4% of patients with glaucoma were myopic. The same association has been found by others [7, 8]. It has also been demonstrated, that the response to tension is similar in highly myopic eyes and eyes with glaucoma [9]. In a study of 20 highly myopic patients with no detectable glaucoma 19 were positive steroid responders and showed a reduced facility of outflow during steroid provocation [10]. A number of studies have compared the IOP in groups of patients with different refractive errors, and some of these have shown that the IOP is higher in myopic eyes. Tomlinson & Philips [11] studied the lOP in subjects between the age of 18 and 27 years. There were 32 myopes, 27 emmetropes and 16 hypermetropes. The lOP in these subjects was 15.49mmHg, 14.74mmHg and 13.91mmHg, respectively. The difference between myopes and hypermetropes was statistically significant. There was also a statistically significant positive correlation between the IOP and axial length, in other words, the higher the IOP, t h e greater the axial length. Hamdi found [12] a significant relationship between refractive error (RE)
251 and IOP, as well as a relationship between age and IOP in 760 subjects. In addition, he found that young emmetropes (age: 11-20 years) (N = 75) had an lOP of 14.05 mmHg while corresponding myopes with a RE < - 6 D ( N = 3 9 ) had an IOP of 15.74mmHg; the difference being statistically significant. The IOP among high myopes (RE > - 6 D) was 14.61 mmHg, which was not significantly different from the lOP of emmetropes. Barraquer [13] found that 95% of 94 hypermetropes had an IOP below or equal to 16 mmHg and that 5% had an IOP above 16 mmHg, while 32% of 445 myopes had an IOP below or equal to 16mmHg and 68% above 16 mmHg. Finally, David [14] examined 2.403 persons above 40 years of age; their mean IOP was 14.86 mmHg. He found that the IOP increased with the RE from a mean of 14.19 among hypermetropes to 16mmHg among high myopes (the difference being statistically significant). The trend to increasing IOP from hypermetropia to myopia was also observed in each age group, but was weakest among persons above the age of 70 years. Some authors were unable to demonstrate any such relationship between refraction and IOP. Bengtsson [15] found that the IOP was affected by four factors (sex, season, time of day and systemic blood pressure) while RE did not seem to influence the IOR In his investigation there was an over-all rise in ocular tension with age, but as the study population was not divided into age groups and in respect of RE, one cannot exclude the possibility that a difference is present in the various RE groups. The same criticism can be employed with regard to the work of Kragha [16], where the IOP was measured in 151 women and 233 men aged between 8 and 75 years. There were 166 myopic, 69 emmetropic and 149 hypermetropic patients in the study. The mean IOP was found to be 16.5 mmHg, 16.9 mmHg and 16.1 mmHg, respectively. The differences were not statistically significant. A significant trend was observed between age and IOP and as the age distribution is not described, it is difficult to accept the given conclusion that IOP has no influence on RE. On the other hand, Bonomi et al. [17] studied the IOP of 137 anisometropic subjects with unilateral high myopia. The contralateral eye was emmetropic, hyperopic or myopic below - 5 D. The IOP was 15.98 mmHg among those below the age of 40 years and with highly myopic eyes, while the contralateral eye had an lOP of 16.56 mmHg, this difference was not statistically significant. These results are comparable to those of Hamdi, where it was found that the IOP in the high myopic eyes was lower than the IOP in the myopic eyes, when the RE was below - 6 D.
Own investigation
Subjects. The material reported here is part of a longitudinal study of myopia progression in young school children. A screening investigation of 8769 children attending the 2nd to 5th grade in the schools of the
252 municipality of Odense formed the basis for the selection of children who had bilateral myopia, at least - 1 . 2 5 D in the most myopic eye, Danish parents and no general illness. A total of 159 children were available for the longitudinal study and 51 children (26 boys, 25 girls) were randomly selected from this group. After the initial examination, the children were given glasses for constant use. Two children dropped out during the study as they bought contact lenses and therefore did not use the prescribed glasses. The mean age was 11 years with a range from 9-12 years. Methods. An eye examination was performed initially and every 6 months for a two-year period. The RE was among the parameters measured. R E was determined subjectively for each eye separately and then t o g e t h e r before and after cyclopentolate 1%. The RE was also measured by means of an autorefractor (Topcon RM A-5000) before and after cyclopentolate. The subjectively determined refraction was found to be in good agreement with that using the autorefractor after cyclopentolate. The results achieved by use of the autorefractor after cyclopentolate were therefore employed as reference values in the following as they are regarded as being unbiased [18]. The IOP was measured with the Goldmann applanation tonometer. The axial length was measured by means of ultrasound. The time amplitude method (A-mode) was employed and an Ocuscan 400 Digital Biometric Ruler used. The children were prepared according to the instructions, the pupil being dilated so that reflections from the iris were avoided. The children looked directly at the fixation light, and with correct contact and axial directions all the echoes (corneal front face, the two lens surfaces, the vitreoretinal interface) were equally high on the display unit. Results. The intraocular pressure at each of the bi-annual examinations is shown in the Table 1. The results are from the right eye. Some children are missing from the table, because of refusal, unsuccessful measurement or due to drop out. There was no statistically significant difference (Wilcoxon paired test) between the right and left eyes. The IOP had a mean value of 16.6 mmHg at the initial examination, while the following mean values were lower. The differences were statistically significant at the 6 and 18 month examination but not after 12 and 24 months. These examinations were performed in the winter season, the other two in the summer, which might
Table 1. IOP based on biannual examinations (mmHg) Time
No.
IOP
SD
Range
Baseline 6 months 12 months 18 months 24 months
49 44 44 42 43
16.6 15.6 16.1 16.0 16.0
2.8 2.4 2.4 2.3 2.4
10-22 10-21 10-22 10-21 12-22
253
z
_o 3 1/1 o9
inn n~ (.9 O
!
n- 2 I:1_ n..r
LLI
>" 1 I
,e
9
9
(M O
-1
101 ,
,
'
' I/5 '
'
'
' 201 ,
,
,
I 251'mm Hg
I0P Fig. 1. Scattergram of the two-year progression of myopia as a function of the baseline IOP.
have influenced the results. The scattergram (Fig. 1) is showing the 2-year progression of myopia as a function of the baseline IOP. Children having a baseline pressure ~ 1 6 m m H g (Table 2). The results showed that there was a significantly higher progression rate among those with a high baseline IOP as compared to those with a low baseline IOP. The baseline IOP, the AL and the change in A L were compared. A mean A L of 24.67 mm was found in children with a pressure > 1 6 mmHg while children with a IOP ~ 16 mmHg
20 0.86 D 0.55 D
27 1.32 D 0.70 D
254 Table 3. Axial elongation over two years, sub-divided according to baseline intraocular pressure (mean value)
No. Mean* SD
IOP ~ 16 mmHg
IOP > 16 mmHg
19 0.40 mm 0.28 mm
23 0.65 mm 0.34 mm
* statistically significant (p < 0.05)
present material. The axial elongation has been sub-divided according to baseline IOP in Table 3. The changes were greatest among the children with a high IOP and the elongation was significantly greater as compared to those with a low IOP.
Discussion
From the above it can be concluded, that some connection between IOP and RE probably does exist. In this group of myopic children, those with the highest IOP had the highest progression rate, as underlined by an excessive growth in AL. Therefore, it is necessary to be sure that IOP is of the same magnitude in groups that are to be compared in respect of RE and progression, as otherwise incorrect conclusions may be drawn. It seems reasonable to suggest that the measurement of IOP is essential in all studies of myopia progression, and that variations in the IOP should be taken into account.
References i. Avetisov ES, Khoroshilova-Maslova IP, Andreeva LD. Ultrastructural changes of the medium in myopia. Vestn Oftalmol 1980; 97: 36. 2. Curtin BJ, Iwamoto T, Renaldo DP. Normal and staphylomatous sclera of high myopia: An electron microscopic study. Arch Ophthalmol 1979; 97: 912. 3. Iomdina EN, Vinetskaya MI. Biomechanical and biochemical properties of myopic sclera. Proceedings of the International Society for Eye Research 1990; 6: 425. 4. Greene PR. Mechanical considerations in myopia: Relative effects of accommodation, convergence, intraocular pressure and the extraocular muscles. Am J Optom Physiol Opt 1980; 57: 902. 5. Curtin BJ. Myopia: A review of its etiology, patogenesis and treatment. Surv Ophthalmol 1970; 15: 1. 6. Perkins ES, Phelps CD. Open angle glaucoma, ocular hypertension, low-tension glaucoma and refraction. Arch Ophthalmol 1982; 100: 1464. 7. Diaz-Dominguez D. Sur le rapport de la grande myopie et de l'hypertension oculaire. Ann Oculist 1961; 194: 597. 8. Huet JF, Massin M. Le glaucomc du jeune myope. Arch Ophthalmol 1977; 37: 33. 9. Pruett RC. Progressive myopia and intraocular pressure: What is the linkage? Acta Ophthalmol 1988; 66; Suppl 185: 117.
255 10. Thomas Jg~ Pruett RC. Steroid provocation testing in high myopia. Proc. 3rd Int Conf Myopia (May 1986, Rome, Italy). 1987. 11. Tomlinson A, Phillips CI. Applanation tension and axial length of the eyeball. Br J Ophthalmol 1970; 54: 548. 12. Hamdi M. Statistical analysis of applanation tension in myopia and emmetropia. Bull Ophthalmol Soc Egypt 1973; 66: 271. 13. Barraquer JI. Patogenia de la miopia. Archivas Soc Am Ophthalmol Optom 1974; 10: 107. 14. David R, Zangwill LM, Tessler Z, Yassur Y. The correlation between intraocular pressure and refractive status. Arch Ophthalmol 1985; 103: 1812. 15. Bengtsson B. Some factors affecting the distribution of intraocular pressures in a population. Acta Ophthalmol 1972; 50: 33. 16. Kragha IKOK. Normal intraocular pressures. Glaucoma 1987; 9: 89. 17. Bonomi L, Mecca E, Massa F. Intraocular pressure in myopic anisometropia. Intern Ophthalmol 1982; 5: 145. 18. Jensen H. Myopia progression in young school children. Acta Ophthalmol 1991; 69 (Suppl): 200.
Address for correspondence: Dr. H. Jensen, Ojenklinikken, Vangedehuse, Sognevej 40, DK2820 Gentofte, Denmark. Phone: +45 39 573536; Fax: +45 31 654186
Myopia progression in young school children and intraocular pressure H A N N E JENSEN Gentofie Hospital, Department of Ophthalmology, Division of Paediatric Ophthalmology and Handicaps, Faculty of Medicine, University of Copenhagen, Denmark Accepted 2 November 1992
Key words: Axial length, Intraocular pressure, Myopia, Myopia progression Abstract. Studies of the literature have given the impression that intraocular pressure (IOP) may influence the refractive error (RE); the reason being that the IOP in some studies is higher among myopic subjects than among emmetropes and hypermetropes. The progression of myopia was studied in 49 children, aged 9-12 years, over a period of 2 years. The rate of progression was 1.14 D/2 years and the IOP 16.6 mmHg. Comparison of the rate of progression between children with an IOP above 16 mmHg and those with an IOP of 16 mmHg or less showed a statistically significant difference: 1.32D/2 years as compared to 0.86D/2 years. These results are supported by ultrasound measurement of the axial length. The author recommends that measurement of lOP should be included in the design of studies of myopia progression. The reason for this is, that it would appear that those persons most at risk of developing higher degrees of myopia have a high IOP.
Introduction Considerable attention has been given for quite a number of years to the problems of myopia and myopia progression. In particular, much research has been carried out in order to determine the etiology of the condition in the hope that it would be possible to treat myopia or perhaps avoid the condition developing. A number of hereditary disorders (Stickler syndrome, Ehlers-Danloss syndrome and Marfan syndrome) have been shown to be associated with myopia, and studies of twins have also demonstrated the importance of heredity. On the other hand, the latter studies have also shown that environmental factors can influence or induce myopia, and the present opinion is that myopia has a multifactorial background. As the net result in all cases of myopia is a distorted sclera, it is possible that there is a common mechanism, so that a number of various developmental disorders join forces to form one common pathway. A factor that may be of importance to the etiology is the intraocular pressure (IOP), and the present article reviews the available literature regarding this hypothesis and briefly describes an investigation of myopia progression in young school children where the IOP was one of the main factors studied.
250 Review of the literature
Axial myopia is characterized by the posterior sclera having been stretched out of shape over a period of years. It is highly probable that the myopic scleral wall differs biochemically from normal. The sclera consists of 32% dry weight and 75% of this is collagen, the remaining 25% being noncollagenous protein and mucopolysaccharides. In the normal eye, there are mature, large diameter collagen fibres in the outer scleral layer, but in the inner layer the fibres are immature and of small diameter. It is probable, that the collagen is produced by fibroblasts in the inner layer, and that the fibres gradually move outward. In the myopic eye there are abnormalities in the size of the fibres and their arrangement [1, 2]. Further, there is increasing evidence that the collagen chemistry is also abnormal [3]. The sclera shows viscoelastic creep under stress and it is assumed that the elasticity and creep are related to the progression of myopia, when provoked by IOP or other forces [4]. It has been claimed throughout the years, that open angle glaucoma is more common among myopes and that myopia is present in a higher percentage of patients with glaucoma than in the population as a whole. Curtin [5] found among myopic patients above the age of 40 years, that 11.2% of eyes with an axial length of more than 26.5 mm suffered from glaucoma, while in eyes with an axial length of more than 30.5 mm the percentage rose to 23.1%. Perkins & Phelps [6] analyzed the refraction of patients with primary open angle glaucoma, ocular hypertension and low tension glaucoma and demonstrated, that myopia occurred more frequently in each of these groups than would be expected in a normal population of similar age. They also found that 6.9% of normal subjects between the ages of 55 and 74 years were myopic while no less than 27.4% of patients with glaucoma were myopic. The same association has been found by others [7, 8]. It has also been demonstrated, that the response to tension is similar in highly myopic eyes and eyes with glaucoma [9]. In a study of 20 highly myopic patients with no detectable glaucoma 19 were positive steroid responders and showed a reduced facility of outflow during steroid provocation [10]. A number of studies have compared the IOP in groups of patients with different refractive errors, and some of these have shown that the IOP is higher in myopic eyes. Tomlinson & Philips [11] studied the lOP in subjects between the age of 18 and 27 years. There were 32 myopes, 27 emmetropes and 16 hypermetropes. The lOP in these subjects was 15.49mmHg, 14.74mmHg and 13.91mmHg, respectively. The difference between myopes and hypermetropes was statistically significant. There was also a statistically significant positive correlation between the IOP and axial length, in other words, the higher the IOP, t h e greater the axial length. Hamdi found [12] a significant relationship between refractive error (RE)
251 and IOP, as well as a relationship between age and IOP in 760 subjects. In addition, he found that young emmetropes (age: 11-20 years) (N = 75) had an lOP of 14.05 mmHg while corresponding myopes with a RE < - 6 D ( N = 3 9 ) had an IOP of 15.74mmHg; the difference being statistically significant. The IOP among high myopes (RE > - 6 D) was 14.61 mmHg, which was not significantly different from the lOP of emmetropes. Barraquer [13] found that 95% of 94 hypermetropes had an IOP below or equal to 16 mmHg and that 5% had an IOP above 16 mmHg, while 32% of 445 myopes had an IOP below or equal to 16mmHg and 68% above 16 mmHg. Finally, David [14] examined 2.403 persons above 40 years of age; their mean IOP was 14.86 mmHg. He found that the IOP increased with the RE from a mean of 14.19 among hypermetropes to 16mmHg among high myopes (the difference being statistically significant). The trend to increasing IOP from hypermetropia to myopia was also observed in each age group, but was weakest among persons above the age of 70 years. Some authors were unable to demonstrate any such relationship between refraction and IOP. Bengtsson [15] found that the IOP was affected by four factors (sex, season, time of day and systemic blood pressure) while RE did not seem to influence the IOR In his investigation there was an over-all rise in ocular tension with age, but as the study population was not divided into age groups and in respect of RE, one cannot exclude the possibility that a difference is present in the various RE groups. The same criticism can be employed with regard to the work of Kragha [16], where the IOP was measured in 151 women and 233 men aged between 8 and 75 years. There were 166 myopic, 69 emmetropic and 149 hypermetropic patients in the study. The mean IOP was found to be 16.5 mmHg, 16.9 mmHg and 16.1 mmHg, respectively. The differences were not statistically significant. A significant trend was observed between age and IOP and as the age distribution is not described, it is difficult to accept the given conclusion that IOP has no influence on RE. On the other hand, Bonomi et al. [17] studied the IOP of 137 anisometropic subjects with unilateral high myopia. The contralateral eye was emmetropic, hyperopic or myopic below - 5 D. The IOP was 15.98 mmHg among those below the age of 40 years and with highly myopic eyes, while the contralateral eye had an lOP of 16.56 mmHg, this difference was not statistically significant. These results are comparable to those of Hamdi, where it was found that the IOP in the high myopic eyes was lower than the IOP in the myopic eyes, when the RE was below - 6 D.
Own investigation
Subjects. The material reported here is part of a longitudinal study of myopia progression in young school children. A screening investigation of 8769 children attending the 2nd to 5th grade in the schools of the
252 municipality of Odense formed the basis for the selection of children who had bilateral myopia, at least - 1 . 2 5 D in the most myopic eye, Danish parents and no general illness. A total of 159 children were available for the longitudinal study and 51 children (26 boys, 25 girls) were randomly selected from this group. After the initial examination, the children were given glasses for constant use. Two children dropped out during the study as they bought contact lenses and therefore did not use the prescribed glasses. The mean age was 11 years with a range from 9-12 years. Methods. An eye examination was performed initially and every 6 months for a two-year period. The RE was among the parameters measured. R E was determined subjectively for each eye separately and then t o g e t h e r before and after cyclopentolate 1%. The RE was also measured by means of an autorefractor (Topcon RM A-5000) before and after cyclopentolate. The subjectively determined refraction was found to be in good agreement with that using the autorefractor after cyclopentolate. The results achieved by use of the autorefractor after cyclopentolate were therefore employed as reference values in the following as they are regarded as being unbiased [18]. The IOP was measured with the Goldmann applanation tonometer. The axial length was measured by means of ultrasound. The time amplitude method (A-mode) was employed and an Ocuscan 400 Digital Biometric Ruler used. The children were prepared according to the instructions, the pupil being dilated so that reflections from the iris were avoided. The children looked directly at the fixation light, and with correct contact and axial directions all the echoes (corneal front face, the two lens surfaces, the vitreoretinal interface) were equally high on the display unit. Results. The intraocular pressure at each of the bi-annual examinations is shown in the Table 1. The results are from the right eye. Some children are missing from the table, because of refusal, unsuccessful measurement or due to drop out. There was no statistically significant difference (Wilcoxon paired test) between the right and left eyes. The IOP had a mean value of 16.6 mmHg at the initial examination, while the following mean values were lower. The differences were statistically significant at the 6 and 18 month examination but not after 12 and 24 months. These examinations were performed in the winter season, the other two in the summer, which might
Table 1. IOP based on biannual examinations (mmHg) Time
No.
IOP
SD
Range
Baseline 6 months 12 months 18 months 24 months
49 44 44 42 43
16.6 15.6 16.1 16.0 16.0
2.8 2.4 2.4 2.3 2.4
10-22 10-21 10-22 10-21 12-22
253
z
_o 3 1/1 o9
inn n~ (.9 O
!
n- 2 I:1_ n..r
LLI
>" 1 I
,e
9
9
(M O
-1
101 ,
,
'
' I/5 '
'
'
' 201 ,
,
,
I 251'mm Hg
I0P Fig. 1. Scattergram of the two-year progression of myopia as a function of the baseline IOP.
have influenced the results. The scattergram (Fig. 1) is showing the 2-year progression of myopia as a function of the baseline IOP. Children having a baseline pressure ~ 1 6 m m H g (Table 2). The results showed that there was a significantly higher progression rate among those with a high baseline IOP as compared to those with a low baseline IOP. The baseline IOP, the AL and the change in A L were compared. A mean A L of 24.67 mm was found in children with a pressure > 1 6 mmHg while children with a IOP ~ 16 mmHg
20 0.86 D 0.55 D
27 1.32 D 0.70 D
254 Table 3. Axial elongation over two years, sub-divided according to baseline intraocular pressure (mean value)
No. Mean* SD
IOP ~ 16 mmHg
IOP > 16 mmHg
19 0.40 mm 0.28 mm
23 0.65 mm 0.34 mm
* statistically significant (p < 0.05)
present material. The axial elongation has been sub-divided according to baseline IOP in Table 3. The changes were greatest among the children with a high IOP and the elongation was significantly greater as compared to those with a low IOP.
Discussion
From the above it can be concluded, that some connection between IOP and RE probably does exist. In this group of myopic children, those with the highest IOP had the highest progression rate, as underlined by an excessive growth in AL. Therefore, it is necessary to be sure that IOP is of the same magnitude in groups that are to be compared in respect of RE and progression, as otherwise incorrect conclusions may be drawn. It seems reasonable to suggest that the measurement of IOP is essential in all studies of myopia progression, and that variations in the IOP should be taken into account.
References i. Avetisov ES, Khoroshilova-Maslova IP, Andreeva LD. Ultrastructural changes of the medium in myopia. Vestn Oftalmol 1980; 97: 36. 2. Curtin BJ, Iwamoto T, Renaldo DP. Normal and staphylomatous sclera of high myopia: An electron microscopic study. Arch Ophthalmol 1979; 97: 912. 3. Iomdina EN, Vinetskaya MI. Biomechanical and biochemical properties of myopic sclera. Proceedings of the International Society for Eye Research 1990; 6: 425. 4. Greene PR. Mechanical considerations in myopia: Relative effects of accommodation, convergence, intraocular pressure and the extraocular muscles. Am J Optom Physiol Opt 1980; 57: 902. 5. Curtin BJ. Myopia: A review of its etiology, patogenesis and treatment. Surv Ophthalmol 1970; 15: 1. 6. Perkins ES, Phelps CD. Open angle glaucoma, ocular hypertension, low-tension glaucoma and refraction. Arch Ophthalmol 1982; 100: 1464. 7. Diaz-Dominguez D. Sur le rapport de la grande myopie et de l'hypertension oculaire. Ann Oculist 1961; 194: 597. 8. Huet JF, Massin M. Le glaucomc du jeune myope. Arch Ophthalmol 1977; 37: 33. 9. Pruett RC. Progressive myopia and intraocular pressure: What is the linkage? Acta Ophthalmol 1988; 66; Suppl 185: 117.
255 10. Thomas Jg~ Pruett RC. Steroid provocation testing in high myopia. Proc. 3rd Int Conf Myopia (May 1986, Rome, Italy). 1987. 11. Tomlinson A, Phillips CI. Applanation tension and axial length of the eyeball. Br J Ophthalmol 1970; 54: 548. 12. Hamdi M. Statistical analysis of applanation tension in myopia and emmetropia. Bull Ophthalmol Soc Egypt 1973; 66: 271. 13. Barraquer JI. Patogenia de la miopia. Archivas Soc Am Ophthalmol Optom 1974; 10: 107. 14. David R, Zangwill LM, Tessler Z, Yassur Y. The correlation between intraocular pressure and refractive status. Arch Ophthalmol 1985; 103: 1812. 15. Bengtsson B. Some factors affecting the distribution of intraocular pressures in a population. Acta Ophthalmol 1972; 50: 33. 16. Kragha IKOK. Normal intraocular pressures. Glaucoma 1987; 9: 89. 17. Bonomi L, Mecca E, Massa F. Intraocular pressure in myopic anisometropia. Intern Ophthalmol 1982; 5: 145. 18. Jensen H. Myopia progression in young school children. Acta Ophthalmol 1991; 69 (Suppl): 200.
Address for correspondence: Dr. H. Jensen, Ojenklinikken, Vangedehuse, Sognevej 40, DK2820 Gentofte, Denmark. Phone: +45 39 573536; Fax: +45 31 654186
