Documenta Ophthalmologica 43,1 : 147-150, 1977 ULTRASONOGRAPH1C REFRACTION OF APHAKIC INFANTS AND CHILDREN M. BELKIN & A. LEVINSON
(Jerusalem, lsrael) ABSTRACT A simple method of aphakic refraction using ultrasonographic axial length measurement and corneal curvature determination is presented. This method was compared with skiascopy in refraction eleven eyes of aphakic infants. No significant differences were found between the relative errors measured by these two methods. Cataracts of any etiology in infants, children and young people are generally dealt with by the aspiration technique (Ryan & yon Noorden, 1971). This technique, while superior to other methods in many respects, has one disadvantage in that the pupillary opening produced is occasionally small and irregular. This tends to make post-operative skiascopic refraction difficult if not impossible, since sometimes no skiascopie reflex can be seen. In this presentation, an alternative method of refraction is suggested, in which the refractive error is calculated from ultrasonographic measurements. MATERIAL AND METHODS Eleven aphakic eyes of 7 infants aged 4 to 36 months were refracted under general anesthesia using both ultrasonography and skiascopy. The axial lenght of the eye was measured by Time Amplitude Ultrasonography. From the value obtained in millimeters (f) the total dioptric power of the eye (F) was calculated using the formule (Emsley, 1952): F = n x 1000, f where n is the global refractive index (1.33). The refractive error was determined by subtracting from the total dioptric power of the eye (F) the dioptric equivalent of the radius of curvature of the cornea. The latter was determined by trial contact lens fitting technique with which it is possible to measure the corneal curvature within an error of 0.05 mm. For control purposes, refraction was also performed by skiascopy with a trial contact lens of the approximate refractive error of the eye in place.
147
RESULTS The refractive errors of the 11 eyes are given in Table 1. The ultrasonographically measured anteroposterior diameters were b e t w e e n 20 and 22 mm. The total dioptric p o w e r was b e t w e e n + 6 0 . 5 0 D and +66.75 D. In ten eyes, the radius of corneal curvature was found to be b e t w e e n 7.29 m m ( + 4 6 . 2 5 D) and 7.89 m m (+42.75 D). In one eye (case 5) it was 6.36 m m
(+ 53.00 D). The refractive errors calculated b y subtracting the corneal dioptric p o w e r f r o m the total dioptric p o w e r o f the eye was b e t w e e n + 14.25 D and 23.00 D in 10 eyes and 7.50 D in the eye of case 5.
Table 1. Refractive error determined by ultrasonographic method. Case no.
1 2 3 4 5 6 7
Eye
Ultrasonographically measured anteroposterior diameter (to near. 0.Smm)
Total diopCorneal tric power curvature calculated diopters from formula (to near. (to near. 0.25D) mm. 0.25mm)
Refractive error (to near. 0.25D)
R L R R R L R R L R L
22.0 22.0 21.0 22.0 20.5 20.0 22.0 20.0 20.0 22.0 22.0
+60.50 +60.50 +63.50 +60.50 +65.00 +66.75 +60.50 +66.75 +66.75 +60.50 +60.50
+18.25 +18.25 +20.50 +17.75 +20.00 +22.25 +7.50 +23.00 +23.00 +14.25 +14.50
7.45 7.45 7.84 7.89 7.50 7.58 6.36 7.71 7.71 7.29 7.33
+42.25 +42.25 +43.00 +42.75 +45.00 +44.50 +53.00 +43.75 +43.75 +46.25 +46.00
The comparison b e t w e e n the refractive errors thus calculated and these values as determined by skiascopy is given in Table 2. T w o eyes (cases 2 and 5) show identical results and four eyes a difference of half diopter or less. There was a difference of 0.75 to 1.00 diopter in three eyes and a difference o f 2.25 diopters in b o t h eyes o f case one. The Wilcoxon matched pairs signed ranks test p e r f o r m e d on this data established that the difference b e t w e e n the values obtained by t w o m e t h o d s of refraction was n o t staffstically significant ( a = 0 . 0 1 ; n = 9; T = 18). DISCUSSION Aspiration is the m e t h o d o f choice for the removal of cataracts in y o u n g
148
Table 2. Comparison of refractive errors as determined by ultrasonography and by skiascopyo Case Eye no.
Refractiveerror (to nearest 0.25 D) determined by Ultrasonography
1 2 3 4 5 6 7
R L R R R L R R L R L
+18.25 +18.25 +20.50 +17.75 +20.00 +22.25 +7.50 +23,00 +23.00 +14.25 +14.50
Skiascopy +16.00 +16.00 +20.50 +18.00 +20.50 +21.50 +7.50 +24.00 +24.00 +14.00 +14.00
patients. Skiascopic refraction of eyes after cataract aspiration is sometimes difficult or impossible because of a small or irregular opening in an opaque posterior lens capsule. This makes prescription of spectacles or contact lenses in such cases largely guesswork, especially in infants and children in ivhich subjective refraction is impossible. A method is presented by which this difficulty can be overcome. Since the aphakic eye is a simple optical instrument in which the cornea is the only refractive surface, it is possible to calculate the refractive error by subtracting the corneal dioptric power from the total dioptric power of the eye. The corneal curvature can be determined using either keratometry or contact lens fitting technique. The latter was used in the refractions presented here since contact lenses were prescribed and astigmatism was irrelevant. If keratometry is used this method can serve for spectacle prescription as well. The total dioptric power of the eye was calculated from the ultrasonographically measured anteroposterior diameter of the eye. The use of ultrasound is sufficiently reliable for this purpose in aphakia since the main source of error in ultrasonographic ocular biometry is the lens, in which the velocity of sound differs from the velocity in the other ocular components. A comparison of the refractive error thus calculated and that obtained by skiascopy did not show significant differences. This simple and rapid method of aphakic refraction seems to be adequate for use in prescribing contact lenses for aphakic eyes, especially those of infants and children. Further proof is required to determine whether it can be used for spectacle prescription.
149
REFERENCES Emsley H.H. Visual Optics. VoL 1, 5th ed., p. 360. London, Hatton (1952). Ryan, S.J. & yon Noorden G.K. Further Observations on the Aspiration Technique in Cataract Surgery. Am. J. Ophth. 7 : 6 2 6 - 6 3 0 (1971). Authors' address: Department of Ophthalmology Hadassah University Hospital P.O. Box 499 Jerusalem Israel
150
(Jerusalem, lsrael) ABSTRACT A simple method of aphakic refraction using ultrasonographic axial length measurement and corneal curvature determination is presented. This method was compared with skiascopy in refraction eleven eyes of aphakic infants. No significant differences were found between the relative errors measured by these two methods. Cataracts of any etiology in infants, children and young people are generally dealt with by the aspiration technique (Ryan & yon Noorden, 1971). This technique, while superior to other methods in many respects, has one disadvantage in that the pupillary opening produced is occasionally small and irregular. This tends to make post-operative skiascopic refraction difficult if not impossible, since sometimes no skiascopie reflex can be seen. In this presentation, an alternative method of refraction is suggested, in which the refractive error is calculated from ultrasonographic measurements. MATERIAL AND METHODS Eleven aphakic eyes of 7 infants aged 4 to 36 months were refracted under general anesthesia using both ultrasonography and skiascopy. The axial lenght of the eye was measured by Time Amplitude Ultrasonography. From the value obtained in millimeters (f) the total dioptric power of the eye (F) was calculated using the formule (Emsley, 1952): F = n x 1000, f where n is the global refractive index (1.33). The refractive error was determined by subtracting from the total dioptric power of the eye (F) the dioptric equivalent of the radius of curvature of the cornea. The latter was determined by trial contact lens fitting technique with which it is possible to measure the corneal curvature within an error of 0.05 mm. For control purposes, refraction was also performed by skiascopy with a trial contact lens of the approximate refractive error of the eye in place.
147
RESULTS The refractive errors of the 11 eyes are given in Table 1. The ultrasonographically measured anteroposterior diameters were b e t w e e n 20 and 22 mm. The total dioptric p o w e r was b e t w e e n + 6 0 . 5 0 D and +66.75 D. In ten eyes, the radius of corneal curvature was found to be b e t w e e n 7.29 m m ( + 4 6 . 2 5 D) and 7.89 m m (+42.75 D). In one eye (case 5) it was 6.36 m m
(+ 53.00 D). The refractive errors calculated b y subtracting the corneal dioptric p o w e r f r o m the total dioptric p o w e r o f the eye was b e t w e e n + 14.25 D and 23.00 D in 10 eyes and 7.50 D in the eye of case 5.
Table 1. Refractive error determined by ultrasonographic method. Case no.
1 2 3 4 5 6 7
Eye
Ultrasonographically measured anteroposterior diameter (to near. 0.Smm)
Total diopCorneal tric power curvature calculated diopters from formula (to near. (to near. 0.25D) mm. 0.25mm)
Refractive error (to near. 0.25D)
R L R R R L R R L R L
22.0 22.0 21.0 22.0 20.5 20.0 22.0 20.0 20.0 22.0 22.0
+60.50 +60.50 +63.50 +60.50 +65.00 +66.75 +60.50 +66.75 +66.75 +60.50 +60.50
+18.25 +18.25 +20.50 +17.75 +20.00 +22.25 +7.50 +23.00 +23.00 +14.25 +14.50
7.45 7.45 7.84 7.89 7.50 7.58 6.36 7.71 7.71 7.29 7.33
+42.25 +42.25 +43.00 +42.75 +45.00 +44.50 +53.00 +43.75 +43.75 +46.25 +46.00
The comparison b e t w e e n the refractive errors thus calculated and these values as determined by skiascopy is given in Table 2. T w o eyes (cases 2 and 5) show identical results and four eyes a difference of half diopter or less. There was a difference of 0.75 to 1.00 diopter in three eyes and a difference o f 2.25 diopters in b o t h eyes o f case one. The Wilcoxon matched pairs signed ranks test p e r f o r m e d on this data established that the difference b e t w e e n the values obtained by t w o m e t h o d s of refraction was n o t staffstically significant ( a = 0 . 0 1 ; n = 9; T = 18). DISCUSSION Aspiration is the m e t h o d o f choice for the removal of cataracts in y o u n g
148
Table 2. Comparison of refractive errors as determined by ultrasonography and by skiascopyo Case Eye no.
Refractiveerror (to nearest 0.25 D) determined by Ultrasonography
1 2 3 4 5 6 7
R L R R R L R R L R L
+18.25 +18.25 +20.50 +17.75 +20.00 +22.25 +7.50 +23,00 +23.00 +14.25 +14.50
Skiascopy +16.00 +16.00 +20.50 +18.00 +20.50 +21.50 +7.50 +24.00 +24.00 +14.00 +14.00
patients. Skiascopic refraction of eyes after cataract aspiration is sometimes difficult or impossible because of a small or irregular opening in an opaque posterior lens capsule. This makes prescription of spectacles or contact lenses in such cases largely guesswork, especially in infants and children in ivhich subjective refraction is impossible. A method is presented by which this difficulty can be overcome. Since the aphakic eye is a simple optical instrument in which the cornea is the only refractive surface, it is possible to calculate the refractive error by subtracting the corneal dioptric power from the total dioptric power of the eye. The corneal curvature can be determined using either keratometry or contact lens fitting technique. The latter was used in the refractions presented here since contact lenses were prescribed and astigmatism was irrelevant. If keratometry is used this method can serve for spectacle prescription as well. The total dioptric power of the eye was calculated from the ultrasonographically measured anteroposterior diameter of the eye. The use of ultrasound is sufficiently reliable for this purpose in aphakia since the main source of error in ultrasonographic ocular biometry is the lens, in which the velocity of sound differs from the velocity in the other ocular components. A comparison of the refractive error thus calculated and that obtained by skiascopy did not show significant differences. This simple and rapid method of aphakic refraction seems to be adequate for use in prescribing contact lenses for aphakic eyes, especially those of infants and children. Further proof is required to determine whether it can be used for spectacle prescription.
149
REFERENCES Emsley H.H. Visual Optics. VoL 1, 5th ed., p. 360. London, Hatton (1952). Ryan, S.J. & yon Noorden G.K. Further Observations on the Aspiration Technique in Cataract Surgery. Am. J. Ophth. 7 : 6 2 6 - 6 3 0 (1971). Authors' address: Department of Ophthalmology Hadassah University Hospital P.O. Box 499 Jerusalem Israel
150
