ACTA 0 P H T H A L M 0 L O G I C A
68 (1990) 559-563
lntraocular pressure in school myopia Olavi Parssinen Department of Ophthalmology (Head:Anneli Klemetti), Central Hospital of Central Finland, Jyvaskyla, Finland
Abstract. The relationship between intraocular pressure and. ocular refraction and axial length were studied in a follow-up of myopic children and in a cross-section sample of school children. In the follow-up group intraocular pressure significantly decreased with age. Mean intraocular pressure at the mean age of 10.9 years was 17.4 mmHg, and three years later 16.1 mmHg. Intraocular pressure above 20 mrnHg was recorded in 7.6% of total measurements in the follow-upgroup and in 8.7% of cases in the cross-sectionsample. There was a significant correlation between mean intraocular pressure and the spherical equivalent both at the beginning and at the end of the study in the follow-up group and in the crosssection sample among the boys only. There was also a significant positive correlation between intraocular pressure and axial length at the end of the follow-up among the boys but not among the girls. Key words: myopia - schoolchildren- refraction - intraocular pressure - glaucoma - axial length.
Intraocular pressure has been proposed as one of the factors responsible for myopic progression. This hypothesis has been supported by many studies which have shown a relationship between myopia and intraocular pressure (Tomlinson & Philips 1970; Perkins & Phelps 1982). It has been suggested that some of the myopic progression could result from an inherited biomechanical weakness of the sclera that allows it to stretch in response to stress (Pruett 1988). Few studies exist concerning intraocular pressure in school myopia and the extend to which intraocular pressure has some prognostic value on myopic progression. The purpose of this study was to investigate in-
traocular pressure (IOP) in school children and determine the correlations between IOP and the spherical equivalent and myopic progression. In addition, the relationships between IOP and axial length were studied.
Materialand Methods The study material comprised two groups of children; a follow-up group and a cross-section group. The follow-upgroup included those selected for a randomized clinical trial of myopia treatment (Hemminki & Pfirssinen 1987). Myopic children from the 3rd and 5th grades of primary school were selected for the follow-up study. Children whose spherical equivalent exceeded -3.OD or was less than -0.25D in either eye or less than -0.5D in the worst eye were excluded. Cases of ocular pathology or general disease were excluded from the study. Abnormal IOP was not an exclusion criterion. The selection procedure and exclusions have been described in more detail in an earlier paper (Hemminki & P5rssinen 1987).The children had no previous myopic spectacle correction and were randomly allocated to one of three Merent treatment groups:fully-correctedspectacles 1)to be used continuously, 2) to be used for distant vision only, and 3) bifocals with 1.75D near add. The mean age of the children at the beginning of study was 10.9 years. IOP was measured once a year for 3 years. Altogether 119 boys and 121 girls participated in the follow-up study. IOP measurements could be done annually in 221-237 cases.
+
559
The cross-section group formed part of a study on the wearing of spectacles and occurrence of myopia (Piirssinen 1986). All the children in the 5th grade of comprehensive school who were residents in the area served by the Central Hospital of Central Finland were sent a questionnaire concerning the wearing of spectacles, symptoms relating to near and distant vision, etc. Of this total of 1494 children, 1384 (93%)responded. Of those children, 156 were called for clinical excamination about one year later. The selection was randomly made from among those who answered as not possessing spectacles. Equal number of boys and girls were randomly selected from them. A further criterion for selection was the respondent's own or parents' opinion that the time the child spent indoors was either a) more than that of hislher age peers or b) less than that of hisfher age peers. Other children were excluded before selection of the sample. The primary reason for these criterion was to compare the refractionsof children with different habits of spending time. One hundred and forty-one (90%)of the children invited attended the ophthalmological examination. It proved possible to measure IOP in 138 of these children. The average age of the children in this group was 12 years. Thirty-eight (27%)of them were myopic. Those with a negative spherical equivalent in the right eye were regarded as myopic. IOP was measured in both groups by HaagStreit applanation tonometry after administering one drop of Oftan-Fluorecah" (oxybuprocain 3% and fluoresceinnatrium 1.25%).In most cases the eyelids were kept open by the assisting nurse while the child fmed his gaze at about 2 m distance. IOP was usually measured first in the right eye and then in the left eye. Refraction in both groups was measured about 45-60 min after administering 2 drops of Oftan- Syclo" (1%cyclopentolate-hydrochloride). Axial length measurements were made using a Storz a-scan with soft probe in the follow-upgroup only. The measurements were taken in 171 cases during the final visit. All the measurements of IOP, refraction and axial length were done by the author. The correlations between refraction, refraction changes, axial length and IOP were analyzed using Pearsson's product-moment correlation coefficients. Two-tailed t-test and analysis of variance were used for analyzing the relationships of these 560
variables in the three different myopia treatment groups and between the boys and the girls. Paired t-test was used when comparing the differences of IOP in the follow-up group at four annual measurements
Results In the follow-up group there were no significant differences in IOP between the three different myopia treatment groups (analysis of variance at beginning of the study: P = 0.518, and at the end of the study: P = 0.885). There were also no significant differences between the IOP of the right eye (17.4f 2.7 mmHg) and that of the left eye (17.2 f 2.7 mmHg) at the beginning of the follow-upor at the end of the study (right eye: 16.1 f 2.8 mmHg; left eye: 15.8 f 2.7 mmHg). There were also no significant dBerences in IOP between the boys and the girls either at the beginning (two-tailed t-test: P = 0.650) or at the end of the follow-up study (twotailed t-test: P = 0.078). The spherical equivalents in the different treatment groups were similar at the beginning of the follow-up (Hemminki & Piirssinen 1987). There were also no differences between the boys and the girls or between the right and left eyes. The spherical equivalent progressed during the follow-up somewhat faster in the distant-use group than in the continuous-use group and also faster among the girls than among the boys (Parssinen et al. 1989). The groups in the follow-up study are yet treated henceforth as a whole and only the values for the right eye are used. In the cross-sectiongroup there was also no sig nificant difference between the IOP of the right eye (17.0 f 2.8 mmHg) and that of the left eye (16.5 f 2.9 mmHg) (two-tailed t-test: P = 0.159). Therefore, only the values for the right eye will be used. The IOP of the girls (17.6 f2.9 mmHg) was higher than that of the boys (16.2 k 2.5 mmHg) (two-tailed t-test: P = 0.002). There was also a significant difference in the spherical equivalent between the boys (+0.72 f 1.03D) and girls (+0.01 f 1.24D) (twotailed t-test: P < 0.001). The distributionof IOP in the four annual measurements of the follow-upgroup and the cross-section group are shown in Table 1. IOP exceeded 20 mmHg in 70 cases (7.6%)of all the annual measurements in the follow-up group.
Table 1. The distribution of intraocular pressure (IOP) in the follow-up group at four annual measurement and in the cross-sectiongroup.
I
IOP mmHg 9-10
11-12
13-14
This IOP level was reached in 10.1%of cases at the time of the first measurement (mean age 10.9 years) and in 3.8%of cases three years later. The average of the four annual measurements exceeded 20 m m H g in 15 cases (7.2%).All the annual measurements were 20 mmHg or more in 5 cases (2.1%).In the cross-sectiongroup IOP exceeded 20 mmHg in 12 cases (8.7%). There was a significant decrease in IOP during the follow-up both among the boys and the girls (Table 2). Relationships between intraocular pressure and refraction and axial length
The negative correlations between IOP and the spherical equivalent among the boys were significant both at the beginning and at the end of the follow-up (Table 3). Among the girls the respective correlations were non-significant. In the cross-sectional group the
16-16
17-18
19-20
21-22
23-24
25
correlation between IOP and refraction was, interestingly, also significant only among the boys, and again reached the range of significancewhen both sexes were treated as a whole. There was a weak correlation between the increase in myopia and IOP at the end of the follow-up among the boys (R = 0.153, N = 116, P = 0.05), but not among the girls. The initial IOP did not have any prognostic value for myopic progression during the follow-up. When IOP and axial length were compared in the follow-upgroup there was a significant correlation in the case of the final control visit (Table 4). This relationship did not hold true with the girls.
Discussion The initial pressure level in the myopia follow-up group was nearly the same as that inJensen’s study (1988)of myopic school children. In their material
Table 2. The means of intraocular pressure (IOP mmHg) at four annual measurements among myopic children in the follow-up group. Age (years)
Boys N
Girls
IOP
P < 0.001 Paired t-test between the first and the final measurements. 36
Acta Ophthal. 68.5
N
Both sexes
IOP
P < 0.001
N
IOP
P < 0.001
561
Follow-up group - at the beginning
-
at the end
Sex
Refraction
IOP
R
N
P
Boys Girls Both
-1.43 f 0.57 -1.43 k 0.62 -1.43f0.59
17.3 k 2.9 17.5 k 2.5 17 . 4 f 2 . 7
-0.180 0.001 -0.090
117 120 237
0.026 0.498 0.083
Boys Girls Both
-2.86 f 1.15 -3.25 k 1.18 -3.06f 1.18
15.8 f 3.0 16.4 k 2.5 16.1 k 2.8
-0.193 0.030 -0.105
116 118 234
0.019 0.374 0.055
Boys Girls Both
f 0 . 7 2 f 1.02 +0.01 k 1.24 +0.33 f 1.20
16.2 f 2.5 17.6k2.9 17.0 f 2.8
-0.352 -0.1 1 1 -0.259
62 73 135
0.003 0.175 0.00 1
Cross-section group
of 10-20 year-olds Abdalla & Hamdi (1970) found the mean IOP of 15.73 mmHg in cases of low myopia and 14.05 mmHg in cases of emmetropia. The limitation in their material was the exclusion from their study of eyes with applanation readings above 20 mmHg or below 10 mmHg. In this study IOP exceeded 20 mmHg in surprisingly many cases. It would be interesting to monitor such children: what is their risk of glaucoma in the future? Barraquer & Varas (1971) found that IOP gradually declined up to the age of twenty. In this study of myopic children IOP also tended to decline with age. It cannot be said how far this change is dependent on growth and change in orbital and lid structures or on changes in scleral rigidity. In the study of Castren & Pohjola (1961) scleral rigidity was highest in hyperopic eyes and lowest in a group of 15-year-oldmyopics. As the IOP measurements were carried out using applanation tonometry, the rigidity differences should not have
562
any significant influence on the IOP readings. Numerous confounding factors have been reported to be connected with intraocular pressure (Sikes 1985). For example fright or a straining of the eyelids could have induced a greater rise in IOP in younger compared to older children. The connection between IOP and myopia found in some studies (Todinson & Phillips 1970; Perkins & Phelps 1982) had led to the suggestion that IOP could be one reason for myopic progression (Kelly 1981; Pruett 1988). It must be remembered, however, that IOP is not a stable variable but is liable to fluctuation caused by many factors (Piltz et al. 1985). For example, head position (Tokoro 1974), or eye movements (Coleman & Trokel 1969) can influence IOP. Accommodation has been thought to cause a rise in intraocular pressure in close work and thus could be the link in the correlation observed between myopia and close work (Kelly 1981). On the other hand Armaly 8c Rubin (1961) found that intraocular pressure drops when
Sex
IOP
Axial length
R
N
Boys Girls Both
15.8 zi 3.0 16.4 k 2.5 16.1 f 2.8
24.79 k 0.92 24.46 f 0.80 24.62 f 0.88
0.350 0.132 0.250
85 86 171
P 0.001 0.111 < 0.00 1
a person accommodatesand remains low if accommodation is maintained. Greene (1980)came to the conclusion that in convergence the tension in the extraocular muscles, especially the oblique muscles, increase vitreous pressure and that this mechanical increase in pressure is a more important reason for myopia than accommodation.However, according to the law of La Place, the stress born by the ocular coats at any given intraocular pressure is the greater the larger the eye (Greene 1980). Thus, both higher IOP and increased axial lenght cause an increase in the basic stress on the sclera, and that stress is modified by many factors which have an influence on IOP. Why a correlation between IOP and the spherical equivalent and axial length existed among the boys but not among the girls in both groups of this study, remains unexplained. The etiology of IOP and myopia is obviously multifactorial and various etiological factors may have different influences among boys and girls. Increased IOP and possible temporary elevations of IOP produced by inclined head position, accommodation, convergence and eye movements in reading could be one of the mediators leading to scleral stretching and myopic progression.
References Abdalla M C & Hamdi M (1970):Applanation ocular tension in myopia and emmetropia. Br J Ophthalmol54: 122-125. Armaly F M & Rubin M L (1961): Accommodation and applanation tonometry. Arch Ophthalmol 65 (3): 415423. BarraquerJ I & VarasJ M (1971):Annotations concerning the relation of forces and pressures in eyes during growth. Ann Ophthalmol3: 425-428. CastrenJ A & Pohjola S (1961):Refraction and scleral rigidity. Acta Ophthalmol (Copenh) 39: 1011-1014. Coleman DJ & Trokel S (1969):Direct-recordedintraocular pressure variations in a human subject. Arch Ophthalmol 82: 637-640. Greene P R (1980):Mechanical considerations in myopia: Relative effects of accommodation, convergence, intraocular pressure, and the extraocular muscles. Am J Optom Physiol Opt 57: 902-914.
Hemminki E & Parssinen T 0 (1987): Prevention of myopic progress by glasses. Study design and the firstyear results of a randomized trial among school-children. Am J Optom Physiol Opt 64: 611-616. Kelly T S-B (1981): The arrest and prophylaxis of expansion glaucoma (myopia).Fledelius H C, Alsbirk P H & Goldschmidt E (eds).Doc Opthalmol Proc Ser 28: 249253. Dr W. Junk Publishers, The Hague. Jensen H (1988): Tim0101 maleate in the control of myopia. Acta Ophthalmol (Copenh) Suppl 185: 128129. Parssinen 0 (1986): The wearing of spectacles and occurence of myopia. Acta Universitatis Tamperensis, Ser A, Vol 207: 1-158. Parssinen 0, Hemminki E, Klemetti A (1989): Effect of spectacle use and accommodation on myopic progression: final results of a three-year randomized clinical trial among schoolchildren. Br J Ophthalmol73: 547751. Perkins E S & Phelps C D (1982): Open angle glaucoma, ocular hypertension, low-tension glaucoma, and refraction. Acta Ophthalmol 100: 1464-1467. Piltz J R, Starita R, Miron M & Hemkind P (1985): Momentary fluctuations of intraocular pressure in normal and glaucomatous eyes. Am J Ophthalmol99: 333-337. Pruett R C (1988): Progressive myopia and intraocular pressure: what is the linkage?A literature review. Acta Ophthalmol (Copenh) Suppl 185: 117-127. Sikes S C (1985): The effect of age and fittness on intraocular pressure at rest and following a maxial exercise bout. Indiana University. University Microfilms International: 30-49. Tokoro T (1974):The mechanism of elongation of the eye axis. Report 11. Relationship between the refraction and intraocular pressure rise following postural changes. Folia Ophthalmol Jpn 25: 70. Tomlinson A & Philips C I (1970): Applanation tension and axial length of the eyeball. Br J Ophthalmol 54: 548-553.
Received on July 21st, 1989. Author’s address:
Olavi Parssinen, MD, Kannaksenk. 5, 40600 Jyvaskyla, Finland.
563
68 (1990) 559-563
lntraocular pressure in school myopia Olavi Parssinen Department of Ophthalmology (Head:Anneli Klemetti), Central Hospital of Central Finland, Jyvaskyla, Finland
Abstract. The relationship between intraocular pressure and. ocular refraction and axial length were studied in a follow-up of myopic children and in a cross-section sample of school children. In the follow-up group intraocular pressure significantly decreased with age. Mean intraocular pressure at the mean age of 10.9 years was 17.4 mmHg, and three years later 16.1 mmHg. Intraocular pressure above 20 mrnHg was recorded in 7.6% of total measurements in the follow-upgroup and in 8.7% of cases in the cross-sectionsample. There was a significant correlation between mean intraocular pressure and the spherical equivalent both at the beginning and at the end of the study in the follow-up group and in the crosssection sample among the boys only. There was also a significant positive correlation between intraocular pressure and axial length at the end of the follow-up among the boys but not among the girls. Key words: myopia - schoolchildren- refraction - intraocular pressure - glaucoma - axial length.
Intraocular pressure has been proposed as one of the factors responsible for myopic progression. This hypothesis has been supported by many studies which have shown a relationship between myopia and intraocular pressure (Tomlinson & Philips 1970; Perkins & Phelps 1982). It has been suggested that some of the myopic progression could result from an inherited biomechanical weakness of the sclera that allows it to stretch in response to stress (Pruett 1988). Few studies exist concerning intraocular pressure in school myopia and the extend to which intraocular pressure has some prognostic value on myopic progression. The purpose of this study was to investigate in-
traocular pressure (IOP) in school children and determine the correlations between IOP and the spherical equivalent and myopic progression. In addition, the relationships between IOP and axial length were studied.
Materialand Methods The study material comprised two groups of children; a follow-up group and a cross-section group. The follow-upgroup included those selected for a randomized clinical trial of myopia treatment (Hemminki & Pfirssinen 1987). Myopic children from the 3rd and 5th grades of primary school were selected for the follow-up study. Children whose spherical equivalent exceeded -3.OD or was less than -0.25D in either eye or less than -0.5D in the worst eye were excluded. Cases of ocular pathology or general disease were excluded from the study. Abnormal IOP was not an exclusion criterion. The selection procedure and exclusions have been described in more detail in an earlier paper (Hemminki & P5rssinen 1987).The children had no previous myopic spectacle correction and were randomly allocated to one of three Merent treatment groups:fully-correctedspectacles 1)to be used continuously, 2) to be used for distant vision only, and 3) bifocals with 1.75D near add. The mean age of the children at the beginning of study was 10.9 years. IOP was measured once a year for 3 years. Altogether 119 boys and 121 girls participated in the follow-up study. IOP measurements could be done annually in 221-237 cases.
+
559
The cross-section group formed part of a study on the wearing of spectacles and occurrence of myopia (Piirssinen 1986). All the children in the 5th grade of comprehensive school who were residents in the area served by the Central Hospital of Central Finland were sent a questionnaire concerning the wearing of spectacles, symptoms relating to near and distant vision, etc. Of this total of 1494 children, 1384 (93%)responded. Of those children, 156 were called for clinical excamination about one year later. The selection was randomly made from among those who answered as not possessing spectacles. Equal number of boys and girls were randomly selected from them. A further criterion for selection was the respondent's own or parents' opinion that the time the child spent indoors was either a) more than that of hislher age peers or b) less than that of hisfher age peers. Other children were excluded before selection of the sample. The primary reason for these criterion was to compare the refractionsof children with different habits of spending time. One hundred and forty-one (90%)of the children invited attended the ophthalmological examination. It proved possible to measure IOP in 138 of these children. The average age of the children in this group was 12 years. Thirty-eight (27%)of them were myopic. Those with a negative spherical equivalent in the right eye were regarded as myopic. IOP was measured in both groups by HaagStreit applanation tonometry after administering one drop of Oftan-Fluorecah" (oxybuprocain 3% and fluoresceinnatrium 1.25%).In most cases the eyelids were kept open by the assisting nurse while the child fmed his gaze at about 2 m distance. IOP was usually measured first in the right eye and then in the left eye. Refraction in both groups was measured about 45-60 min after administering 2 drops of Oftan- Syclo" (1%cyclopentolate-hydrochloride). Axial length measurements were made using a Storz a-scan with soft probe in the follow-upgroup only. The measurements were taken in 171 cases during the final visit. All the measurements of IOP, refraction and axial length were done by the author. The correlations between refraction, refraction changes, axial length and IOP were analyzed using Pearsson's product-moment correlation coefficients. Two-tailed t-test and analysis of variance were used for analyzing the relationships of these 560
variables in the three different myopia treatment groups and between the boys and the girls. Paired t-test was used when comparing the differences of IOP in the follow-up group at four annual measurements
Results In the follow-up group there were no significant differences in IOP between the three different myopia treatment groups (analysis of variance at beginning of the study: P = 0.518, and at the end of the study: P = 0.885). There were also no significant differences between the IOP of the right eye (17.4f 2.7 mmHg) and that of the left eye (17.2 f 2.7 mmHg) at the beginning of the follow-upor at the end of the study (right eye: 16.1 f 2.8 mmHg; left eye: 15.8 f 2.7 mmHg). There were also no significant dBerences in IOP between the boys and the girls either at the beginning (two-tailed t-test: P = 0.650) or at the end of the follow-up study (twotailed t-test: P = 0.078). The spherical equivalents in the different treatment groups were similar at the beginning of the follow-up (Hemminki & Piirssinen 1987). There were also no differences between the boys and the girls or between the right and left eyes. The spherical equivalent progressed during the follow-up somewhat faster in the distant-use group than in the continuous-use group and also faster among the girls than among the boys (Parssinen et al. 1989). The groups in the follow-up study are yet treated henceforth as a whole and only the values for the right eye are used. In the cross-sectiongroup there was also no sig nificant difference between the IOP of the right eye (17.0 f 2.8 mmHg) and that of the left eye (16.5 f 2.9 mmHg) (two-tailed t-test: P = 0.159). Therefore, only the values for the right eye will be used. The IOP of the girls (17.6 f2.9 mmHg) was higher than that of the boys (16.2 k 2.5 mmHg) (two-tailed t-test: P = 0.002). There was also a significant difference in the spherical equivalent between the boys (+0.72 f 1.03D) and girls (+0.01 f 1.24D) (twotailed t-test: P < 0.001). The distributionof IOP in the four annual measurements of the follow-upgroup and the cross-section group are shown in Table 1. IOP exceeded 20 mmHg in 70 cases (7.6%)of all the annual measurements in the follow-up group.
Table 1. The distribution of intraocular pressure (IOP) in the follow-up group at four annual measurement and in the cross-sectiongroup.
I
IOP mmHg 9-10
11-12
13-14
This IOP level was reached in 10.1%of cases at the time of the first measurement (mean age 10.9 years) and in 3.8%of cases three years later. The average of the four annual measurements exceeded 20 m m H g in 15 cases (7.2%).All the annual measurements were 20 mmHg or more in 5 cases (2.1%).In the cross-sectiongroup IOP exceeded 20 mmHg in 12 cases (8.7%). There was a significant decrease in IOP during the follow-up both among the boys and the girls (Table 2). Relationships between intraocular pressure and refraction and axial length
The negative correlations between IOP and the spherical equivalent among the boys were significant both at the beginning and at the end of the follow-up (Table 3). Among the girls the respective correlations were non-significant. In the cross-sectional group the
16-16
17-18
19-20
21-22
23-24
25
correlation between IOP and refraction was, interestingly, also significant only among the boys, and again reached the range of significancewhen both sexes were treated as a whole. There was a weak correlation between the increase in myopia and IOP at the end of the follow-up among the boys (R = 0.153, N = 116, P = 0.05), but not among the girls. The initial IOP did not have any prognostic value for myopic progression during the follow-up. When IOP and axial length were compared in the follow-upgroup there was a significant correlation in the case of the final control visit (Table 4). This relationship did not hold true with the girls.
Discussion The initial pressure level in the myopia follow-up group was nearly the same as that inJensen’s study (1988)of myopic school children. In their material
Table 2. The means of intraocular pressure (IOP mmHg) at four annual measurements among myopic children in the follow-up group. Age (years)
Boys N
Girls
IOP
P < 0.001 Paired t-test between the first and the final measurements. 36
Acta Ophthal. 68.5
N
Both sexes
IOP
P < 0.001
N
IOP
P < 0.001
561
Follow-up group - at the beginning
-
at the end
Sex
Refraction
IOP
R
N
P
Boys Girls Both
-1.43 f 0.57 -1.43 k 0.62 -1.43f0.59
17.3 k 2.9 17.5 k 2.5 17 . 4 f 2 . 7
-0.180 0.001 -0.090
117 120 237
0.026 0.498 0.083
Boys Girls Both
-2.86 f 1.15 -3.25 k 1.18 -3.06f 1.18
15.8 f 3.0 16.4 k 2.5 16.1 k 2.8
-0.193 0.030 -0.105
116 118 234
0.019 0.374 0.055
Boys Girls Both
f 0 . 7 2 f 1.02 +0.01 k 1.24 +0.33 f 1.20
16.2 f 2.5 17.6k2.9 17.0 f 2.8
-0.352 -0.1 1 1 -0.259
62 73 135
0.003 0.175 0.00 1
Cross-section group
of 10-20 year-olds Abdalla & Hamdi (1970) found the mean IOP of 15.73 mmHg in cases of low myopia and 14.05 mmHg in cases of emmetropia. The limitation in their material was the exclusion from their study of eyes with applanation readings above 20 mmHg or below 10 mmHg. In this study IOP exceeded 20 mmHg in surprisingly many cases. It would be interesting to monitor such children: what is their risk of glaucoma in the future? Barraquer & Varas (1971) found that IOP gradually declined up to the age of twenty. In this study of myopic children IOP also tended to decline with age. It cannot be said how far this change is dependent on growth and change in orbital and lid structures or on changes in scleral rigidity. In the study of Castren & Pohjola (1961) scleral rigidity was highest in hyperopic eyes and lowest in a group of 15-year-oldmyopics. As the IOP measurements were carried out using applanation tonometry, the rigidity differences should not have
562
any significant influence on the IOP readings. Numerous confounding factors have been reported to be connected with intraocular pressure (Sikes 1985). For example fright or a straining of the eyelids could have induced a greater rise in IOP in younger compared to older children. The connection between IOP and myopia found in some studies (Todinson & Phillips 1970; Perkins & Phelps 1982) had led to the suggestion that IOP could be one reason for myopic progression (Kelly 1981; Pruett 1988). It must be remembered, however, that IOP is not a stable variable but is liable to fluctuation caused by many factors (Piltz et al. 1985). For example, head position (Tokoro 1974), or eye movements (Coleman & Trokel 1969) can influence IOP. Accommodation has been thought to cause a rise in intraocular pressure in close work and thus could be the link in the correlation observed between myopia and close work (Kelly 1981). On the other hand Armaly 8c Rubin (1961) found that intraocular pressure drops when
Sex
IOP
Axial length
R
N
Boys Girls Both
15.8 zi 3.0 16.4 k 2.5 16.1 f 2.8
24.79 k 0.92 24.46 f 0.80 24.62 f 0.88
0.350 0.132 0.250
85 86 171
P 0.001 0.111 < 0.00 1
a person accommodatesand remains low if accommodation is maintained. Greene (1980)came to the conclusion that in convergence the tension in the extraocular muscles, especially the oblique muscles, increase vitreous pressure and that this mechanical increase in pressure is a more important reason for myopia than accommodation.However, according to the law of La Place, the stress born by the ocular coats at any given intraocular pressure is the greater the larger the eye (Greene 1980). Thus, both higher IOP and increased axial lenght cause an increase in the basic stress on the sclera, and that stress is modified by many factors which have an influence on IOP. Why a correlation between IOP and the spherical equivalent and axial length existed among the boys but not among the girls in both groups of this study, remains unexplained. The etiology of IOP and myopia is obviously multifactorial and various etiological factors may have different influences among boys and girls. Increased IOP and possible temporary elevations of IOP produced by inclined head position, accommodation, convergence and eye movements in reading could be one of the mediators leading to scleral stretching and myopic progression.
References Abdalla M C & Hamdi M (1970):Applanation ocular tension in myopia and emmetropia. Br J Ophthalmol54: 122-125. Armaly F M & Rubin M L (1961): Accommodation and applanation tonometry. Arch Ophthalmol 65 (3): 415423. BarraquerJ I & VarasJ M (1971):Annotations concerning the relation of forces and pressures in eyes during growth. Ann Ophthalmol3: 425-428. CastrenJ A & Pohjola S (1961):Refraction and scleral rigidity. Acta Ophthalmol (Copenh) 39: 1011-1014. Coleman DJ & Trokel S (1969):Direct-recordedintraocular pressure variations in a human subject. Arch Ophthalmol 82: 637-640. Greene P R (1980):Mechanical considerations in myopia: Relative effects of accommodation, convergence, intraocular pressure, and the extraocular muscles. Am J Optom Physiol Opt 57: 902-914.
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Received on July 21st, 1989. Author’s address:
Olavi Parssinen, MD, Kannaksenk. 5, 40600 Jyvaskyla, Finland.
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