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Accommodative Convergence in Hypermetropia
Accommodative Convergence in Hypermetropia Gunter K. von Noorden, M.D., and Cynthia W. Avilla, B.S.
We compared the clinical characteristics of esotropic, hypermetropic children whose strabismus was fully corrected with spectacles (refractive accommodative esotropia) with those who remained orthotropic (that is, had no manifest strabismus on the cover test) in the presence of uncorrected hypermetropia. In addition to a standard ophthalmologic and orthoptic examination, we determined the stimulus accommodative convergence/accommodation (AC/ A) ratio by using the gradient method over a range of 6 diopters, the near point of accommodation, and random dot stereopsis. Hypermetropic patients without esotropia or significant esophoria were found to have a low AC/ A ratio in contrast to those patients with refractive accommodative esotropia. This finding explains why esodeviations may be absent in some hypermetropic patients with uncorrected vision. We found a high prevalence of abnormally low near points of accommodation and defective or absent stereopsis in both groups of patients.
T HE RELATIONSHIP between esotropia and hypermetropia has been established ever since Donders made his classic contribution in 1864. 1 In his treatise, Donders asserted that strabismus may be absent in those with severe hypermetropia and speculated that such patients do not make the effort to accommodate sufficiently to clear their blurred retinal images. Donders suggested that if such patients would exert accommodation in accordance with their severe hypermetropia, esotropia would develop. Accepted for publication April 19, 1990. From the Cullen Eye Institute, Baylor College of Medicine and the Ophthalmology Service, Texas Children's Hospital, Houston, Texas. This study was supported in part by grant EY 07001 from the National Institutes of Health. Reprint requests to Gunter K. von Noorden, M.D., Ophthalmology Service, Texas Children's Hospital, Box 20269, Houston, TX 77225.
©AMERICAN JOURNAL OF OPHTHALMOLOGY
110:287-292,
We had on several occasions, however, encountered patients with uncorrected hypermetropia who had to make a great accommodative effort to have a visual acuity of 20/20. These patients, however, remained essentially orthotropic at near and distance fixation or developed only insignificant amounts of esophoria. This observation, which is in apparent contrast with Donders' teachings, led us to study this unusual response.
Subjects and Methods Between 1973 and 1989, we studied nine consecutive, alert, and cooperative patients (Group A) who fulfilled the following criteria: no history of esotropia or any other form of strabismus, no previous optical correction of a hypermetropia of +4.00 diopters or more, absence of anisometropia of more than 1.50 diopters, and orthotropia or esophoria of less than 10 prism diopters at near or distance fixation without spectacles on an accommodative fixation target. None of the patients had worn spectacles previously, and uncorrected visual acuity at distance was 20/30 or better with each eye in all but one patient (Case 2) whose visual acuity was R.E.: 20/50 and L.E.: 20/40. The ages of these patients ranged from 5 to 16 years at the time of the first examination (mean, 7.8 years). The mean refractive error was + 7.52 diopters (range, +4.25 to + 15.00 diopters) in the right eye and +7.51 diopters (range, +4.00 to + 15.00 diopters) in the left eye. In addition to a complete ophthalmologic examination, which included an orthoptic work-up, data on stimulus accommodation convergence/accommodation (AC/ A) ratio, near point of accommodation, and stereoacuity were obtained. A cycloplegic refraction was performed, and the full refractive error was prescribed. After the spectacles had been worn for at least six weeks, the stimulus AC/ A ratio was measured with the gradient method" by clipping spherical lenses ranging from -3.00 to +3.00 diopters in SEPTEMBER,
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288
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I-diopter steps on the patient's spectacles. The heterophoria or heterotropia induced by the lenses was measured with the prism cover test while the patient fixated on an accommodative target at a viewing distance of 33 cm. The AC/ A ratio was computed by dividing the change in horizontal heterophoria in prism diopters by the change in stimulus to accommodation in diopters (prism diopters Zdiopters [p.d./D]). The near point of accommodation was determined with the Prince rule while the patient was wearing the full hypermetropic correction. An accommodative target (20/30 letter) was moved toward the patient along the ruler until the patient reported blurring. Several determinations were made, and the average distance at which blurring occurred was recorded in diopters. Stereopsis was measured with the TNO random dot stereo test. A control group (Group B) consisted of 30 consecutive patients with refractive accommodative esotropia, that is, the patients' esotropia was completely corrected by spectacles at near and distance fixation. Of these children, 29 had a corrected visual acuity of 20/40 or better. One patient (Case 1) had a visual acuity of R.E.: 20/50 and L.E.: 20/40. The patients' ages ranged from 6 to 13 years (mean, 11.6 years). The mean refractive error was +5.64 diopters (range, +3.50 to +9.25 diopters) in the right eye and +5.75 diopters (range, +3.75 to +9.50 diopters) in the left eye. All patients in Group B had a constant esotropia without their spectacles at near and distance fixation and had been esotropic for varying periods before the prescription of their first spectacles. One patient (Case 15) had a micro tropia, and the other
patients were orthotropic or had a well-controlled esophoria with spectacles.
Results The AC/ A ratio in patients from Group A (Table 1) was found to be either reduced or its slope to be completely flat (Cases 5 and 9), in which case no significant phoria response could be evoked by adding plus and minus spherical lenses to the patient's spectacles. In other words, there was no apparent synkinesis between accommodation and convergence. The mean AC/ A ratio in Group A was 1.53 p.d.jD (range, 0 to 2.6 vs 4.19 p.d./D (range, 1.1 to 9.0 p.d./D in Group B (Table 2). This difference was statistically significant (r-test, P < .001). The results of measurements of the near point of accommodation in Group A and Group B are compared in the Figure with the upper and lower limits of normal near points of accommodation as established by Duane." Perhaps coincidentally, abnormally low near points were more frequently encountered in the right eye than in the left eye. The near points of all right eyes were chosen to display the variations from the normal range. According to Duane," a near point of accommodation of less than 10 diopters in the age group under consideration (5 to 14 years) must be considered abnormally low. Two of eight patients from Group A had an abnormally low near point in one eye (Cases 1 and 2), and two had a reduced near point in both eyes (Cases 5 and 9) (Table 1). Of 30 children from Group B, ten had
TABLE 1 CLINICAL CHARACTERISTICS OF PATIENTS IN GROUP A REFRACTION
CASE NO.,
VISUAL ACUITY
AGE (YRS)
R.E.
L.E.
R.E.
L.E.
1.7 2.6 3. 5 4. 8 5. 9 6. 5 7. 16 8. 7 9.8
+15.00 +13.00 + 1.00 x 95 +5.50 +4.75 +7.00 +4.25 +4.00 + 0.50 x 105 +6.50 +7.00
+15.00 +13.00 + 1,00 x 80 +5.75 + 0,25 x 30 +4.75 +6.00 +4.00 +3.75 + 0.50 x 40 +7.50 +7.00
20/30 20/50 20/30 20/20 20/30 20/20 20/20 20/25 20/20
20/30 20/40 20/25 20/20 20/20 20/20 20/20 20/30 20/20
*NPA indicates near point of accommodation.
NPA (0)·
STEREOPSIS
R.E.
L.E.
ACtA
(SEC)
6 8
10 10
2.3 1.3 2.3 2.6
240 1,980
Not tested
13 3 14 10 14 9
13 8 15 10 14 9
Absent
2.1 1.6 1.6 Absent
Nil Nil Not tested
60 15 60 240
Accommodative Convergence in Hypermetropia
Vol. 110, No.3
TABLE 2 CLINICAL CHARACTERISTICS OF PATIENTS IN GROUP B NPA (0)'
CASE NO .• AGE (YRS)
1,7 2, 22 3, 6 4,8 5, 6
6,8 7, 10 8, 10 9, 8
10,9 11,6 12, 9 13,13 14,7 15,8 16,12 17,6
18, 8 19,8 20, 6 21,12 22, 13 23, 9 24, 7 25, 6
26,8
27, 9
28, 9 29, 12 30, 10
STEREOPSIS
R.E.
L.E.
AC/A
(SEC)
8 9 11 9 15 8
9
1.1 6.8
1,980
7
12 12 11 18 12 11 11 12 11 15 12 9
10 8 8 8
12 12 11 10 13 11 10
9 9
10 15 9 9
12 14 10 18 12 12 11 13 12 15 12 8
10 9 9 9
10 12 9
12 14 12 11
4.5
4.1 4.1 3.8 3.1 3.5
2.1 4.3 3.0 5.6
2.3 8.0 3.0 6.3 4.6 3.0 9.0 4.3 4.8 3.8 4.8 1.3 1.5 3.3 5.0 5.6 5.3
4.0
Nil
120 120 240 30 60 60 Nil
Nil Nil
Nil
Nil 240 Nil 30 240 Nil 240 Nil
Nil 240 1,980 1,980 1,980 60 240 1,980 60 120
°NPA indicates near point of accommodation.
abnormally low near points of accommodation in one (Cases 3, 4, and 26) or both eyes (Cases 1,6,7,19,21,22, and 23). Four patients (Cases 1, 2, 5, and 9) from Group A and one patient (Case 1) from Group B had reduction of both AC/ A ratio and near point of accommodation, but no correlation between these two measurements existed in the remainder of the patients from Groups A and B. Stereopsis was considered to be decreased when it was measured to be less than 240 seconds of arc.' Reduced stereopsis or stereoblindness was recorded in five of eight patients from Group A whose stereopsis was tested and in 21 of 30 patients from Group B. Thus, reduced or absent stereopsis was a feature that occurred frequently in patients with severe hy-
289
permetropia with and without refractive accommodative esotropia. We found no relationship between reduced stereopsis and a reduced near point of accommodation because we studied stereoblind patients with a normal near point of accommodation (Group A, Case 4; Group B, Cases 9 to 13, 15, 18, and 20) and those with reduced near points of accommodation and normal stereopsis (Group B, Cases 6 and 7). Likewise, no relationship was seen between visual acuity at near and reduced stereopsis because visual acuity at near, though not tested in all patients, was normal (11) in patients with reduced stereopsis (Group B, Cases 15, 18, 20, and 21).
Discussion According to Donders,' uncorrected hypermetropia may cause esotropia because of excessive accommodation. Donders also realized, however, that only a comparatively small number of esotropic patients develop hypermetropia and suggested that fusion preserves most hypermetropic individuals from strabismus. If accommodative convergence is compensated for by fusional divergence, one would expect such patients to have a significant degree of esophoria at near and distance fixation. The patients from Group A, however, were orthotropic or had esophorias of less than 10 prism diopters. The data from Group A show that a low AC/A ratio must be added to excessive accommodative convergence and the range of motor fusion amplitudes as a factor that determines whether a hypermetropic individual becomes esophoric or esotropic or remains orthotropic. These patients were orthotropic or had only insignificant amounts of esophoria without spectacles despite excessive accommodation required to recognize the fixation target used during measurement of the near deviation. It has been determined that the normal values for the AC/A ratio range from 2.76 p.d.yD to 4.50 p.d.j/D, depending on the methods used." The mean AC/A ratios obtained in Group A must be considered abnormally low and those from Group B as within the normal range. The stimulus method" to determine the AC/A ratio is used widely in clinical work.v" As suggested by Alpern, Kincaid, and Lubeck," however, one disadvantage of this method is the
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AMERICAN JOURNAL OF OPHTHALMOLOGY
September, 1990
18r-------------------..., •
16 14
s-
12
~
8
o
Figure (von Noorden and Avilla). Near point of accommodation in relation to age. Open circles indicate patients in Group A; solid circles, patients in Group B; stippled area, normal range; A, lowest values; B, average values; and C, highest values.
10
6
•
o
4
o
2
oL..-........ 5
--I._...I.-.......-.I""'-..a-....._
6
7
8
9
........ I--'--I t--"'-_....
10 11 12 Age (Years)
difficulty in determining the state of the patient's accommodation when plus or minus lenses are placed before the eyes. It is planned to measure the response AC/ A ratio, that is, the AC/ A ratio related to the accommodative response in a suitable subject to corroborate our findings even though a predictable increase of the response AC/ A ratio of 8%6 would not change the significance of our data. We must ask whether the reduced AC/ A ratio in Group A is a primary, innate, or a secondary phenomenon. This question becomes moot if one shares the view of many investigators who consider the stimulus AC/A ratio as a stable, genetically determined relationship between accommodation and convergence, based on a fixed central nervous system arrangement that remains immutable until the presbyopic age or throughout life."" If one agrees with Helmholtz," however, that the association between accommodation and convergence is a learned process, the possibility must be considered that certain individuals with uncorrected severe hypermetropia fail to make the sustained accommodative effort necessary to focus and consequently develop a low AC/ A ratio. In that case, we would expect a good correlation between a low AC/ A ratio and a reduced near point of accommodation. With the exception of Cases 1, 2, 5, and 9 from Group A and Case 1 from Group B, however, such a correlation did not exist in the remainder of our patients. For this reason and because of the studies cited, we are inclined to favor the first theory and suggest that an intrinsically low AC/ A ratio may pro-
16
22
tect certain patients with uncorrected hypermetropia from developing esotropia or esophoria. The finding of a reduced near point of accommodation in hypermetropic patients with and without strabismus is of interest. Low near points of accommodation have been described in connection with ametropic amblyopia in patients with severe hypermetropia.P:" It has even been suggested that one of the possible causes of ametropic amblyopia is a reduced near point of accommodation." Reduced near vision in the presence of normal distance vision, however, is not a recognizable arnblyopiogenic factor, and with the exception of Case 2, none of our patients with a reduced near point was amblyopic. Rather, we suggest the possibility that patients with uncorrected hypermetropia do not fully accommodate at all times during infancy. The resulting hypoaccommodative state may eventually cause a reduced accommodative range, perhaps even a disuse atrophy of the ciliary muscle. Although this hypothesis remains unsupported, we have no better explanation for a reduced near point of accommodation in more than one third of our patients. We did not determine the pupillary near response in those with reduced near points to explore whether there are other anomalies of the near reflex in such patients. The finding of reduced or absent stereopsis as a common feature in patients with moderate or severe hypermetropia was unexpected. Reduced or absent stereopsis was not consistently associated with a reduced near vision from a reduced near point of accommodation. We were
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Accommodative Convergence in Hypermetropia
also unable to associate the age at which the hypermetropia was optically corrected and the esodeviation was controlled with spectacles with the sensitivity of stereopsis. We believe, however, that the reduced stereopsis in Group B might be explained on the basis of varying periods of esotropia that were present before the wearing of corrective lenses. It is well known from clinical experience" and animal experiments'S" that disruption of binocular vision during visual immaturity will cause a permanent defect of stereopsis. It will be of inter'est to explore whether the defect of stereopsis in patients with refractive accommodative esotropia can be correlated with optokinetic asymmetry, which is another sign of early disruption of binocular vision.":" Reduced stereopsis in five of the eight patients tested in Group A must be caused by a different mechanism because none of the children we studied had manifest strabismus or a history of such. What these patients may have had in common, however, was a period of blurred form vision in both eyes before optical correction of their hypermetropia. Defective stereopsis has been reported after bilateral form vision deprivation from patching of infants. 21•23 We have also learned from experiments in infant monkeys that a reduction in the number of cortical binocular neurons (which are necessary for stereopsis") is not only caused by artificial strabisrnus.P:" anisometropia.F and unilateral eyelid closure," but occurs also after bilateral and symmetric deprivation of form vision." It is reasonable to assume, therefore, that the blurring of vision during infancy before the wearing of spectacles caused deprivation of form vision, loss of cortical binocular function, and thus a reduction or loss of stereopsis in most patients from Group A. It is of interest that a mild ametropic amblyopia was present in only one of these patients (Case 2), which suggests that stereopsis is a visual function more sensitive to the effects of bilateral visual deprivation than visual acuity. This observation is in accord with recent data from our animal laboratory that show that only brief periods of bilateral form vision deprivation in visually immature monkeys suffice to cause a decrease of stereopsis without significantly affecting monocular visual acuity." To avoid the visual deprivation effect of uncorrected hypermetropia in excess of four diopters on the immature visual system, to improve near vision in those with a reduced near point of accommodation, and to preserve stereopsis,
291
we recommend optical correction in children under the age of 4 years, even in the absence of a manifest or latent esodeviation.
References 1. Donders, F. c.: On the Anomalies of Accommodation and Refraction of the Eye With a Preliminary Essay on Physiological Dioptrics. London, The New Sydenham Society, 1864, p. 292. 2. Sloan, L., Sears, M., and Jablonski, M.: Convergence-accommodation relationship. Arch. Ophthalmol. 63:283, 1960. 3. Duane, A.: Studies in monocular and binocular accommodation with their clinical applications. Am. J. Ophthalmol. 5:865, 1922. 4. Reinecke, R. D. In Discussion: March, W. R., Rowlings, S. c.. and Mumma, J. V.: Evaluation of clinical stereoacuity tests. Ophthalmology 87:1265, 1980. 5. Franceschetti, A., and Burian, H. M.: Gradient accommodative convergence/accommodative ratio in families with and without esotropia. Am. J. Ophthalmol. 70:558, 1970. 6. Alpern, M., Kincaid, W. M., and Lubeck, M. J.: Vergence and accommodation. III. Proposed definitions of AC/A ratios. Am. J. Ophthalmol. 48:141, 1959. 7. Morgan, M. W., [r.: Relationship between accommodation and convergence. Arch. Ophthalmol. 47:745, 1952. 8. Fincham, E. F.: The proportion of ciliary muscular force required for accommodation. ]. Physiol. (Lond.) 128:99, 1955. 9. Alpern, M., and Hirsch, M. J. Cited in Alpern, M., and Larson, B. F.: Vergence and accommodation. IV. Effect of luminance quantity on the AC/ A. Am. J. Ophthalmol. 49:1140,1960. 10. Alpern, M.: Types of movement. In Davson, H. (ed.): The Eye, vol. 3. New York, Academic Press, 1962, p, 123. 11. Breinin, G. M., and Chin, N. B.: Accommodation, convergence and aging. Doc. Ophthalmol. 34:109, 1973. 12. von Helmholtz, H.: Helmholtz's Treatise on Physiological Optics. In Southall, J. P. C. (ed.): vol. 3. Translated from the 3rd German edition. New York, Dover Publications, 1962, p. 55. 13. Werner, D. B., and Scott, W. E.: Amblyopia case reports. Bilateral hypermetropic ametropic amblyopia. J. Pediatr. Ophthalmol. Strabismus 22:203, 1985. 14. Schoenleber, D. B., and Crouch, E. R.: Bilateral hypermetropic amblyopia. ]. Pediatr. Ophthalmol. Strabismus 24:75,1987. 15. von Noorden, G. K.: Current concepts of infantile esotropia. Eye 2:343, 1988. 16. Crawford, M. L. J" von Noorden, G. K., Meharg, L. S., Rhodes, J. W., Harwerth, R. S., Smith,
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E. L., III, and Miller, D. D.: Binocular neurons and binocular function in monkeys and children. Invest. Ophthalmol. Vis. Sci. 24:491, 1983. 17. Crawford, M. L. J., Smith, E. L., III, Harwerth, R. S., and von Noorden, G. K.: Stereoblind monkeys have few binocular neurons. Invest. Ophthalmol. Vis. Sci. 25:779, 1984. 18. Tychsen, L., Hurtig, R. R., and Scott, W. E.: Pursuit is impaired but the vestibulo-ocular reflex is normal in infantile strabismus. Arch. Ophthalmol. 103:536, 1985. 19. Flynn, J. T.: Vestibulo-optokinetic interaction in strabismus. Am. Orthopt. J. 32:36, 1982. 20. van Hof-van Duin, j., and Mohn, G.: Monocular and binocular optokinetic nystagmus in humans with defective stereopsis. Invest. Ophthalmol. Vis. Sci. 27:574, 1986. 21. Hoyt, C. S.: The long-term visual effects of short-term binocular occlusion of at-risk neonates. Arch. Ophthalmol. 98:1967, 1980. 22. Glass, P.: Another look at long-term visual effects of binocular occlusion in neonates (letter to the editor). Arch. Ophthalmol. 102:968, 1984. 23. Wright, K. W., Wehrle, M. J., and Urrea, P. T.: Bilateral total occlusion during the critical period of
September, 1990
visual development. Arch. Ophthalmol. 105:321, 1987. 24. Crawford, M. L. J., Smith, E. L., III, Harwerth, R. S., and von Noorden, G. K.: Stereoblind monkeys have few binocular neurons. Invest. Ophthalmol. Vis. Sci. 25:779, 1984. 25. Baker, F. H., Grigg, P., and von Noorden, G. K.: Effects of visual deprivation and strabismus on the response of neurons in the visual cortex of the monkey, including studies on the striate and prestriate cortex in the normal animal. Brain Res. 66:185, 1974. 26. Crawford, M. L. J., and von Noorden, G. K.: Optically induced comitant strabismus in monkeys. Invest. Ophthalmol. Vis. Sci. 19:1105, 1980. 27. von Noorden, G. K., and Crawford, M. L. J.: Form deprivation without light deprivation produces the visual deprivation syndrome in Macaca mulatta. Brain Res. 129:37, 1977. 28. Crawford, M. L. J., de Faber, J. T., Pesch, T. W., and von Noorden, G. K.: Binocular cells and stereopsis in monkeys. ARVO abstracts. Supplement to Invest. Ophthalmol. Vis. Sci. Philadelphia, J. B. Lippincott, 1989, p. 315.
OPHTHALMIC MINIATURE
"Stand still and shut your eyes for a moment," commanded Miss Brandon's voice from the bed, "and then you'Il be able to see. I can't have the blinds up. My eyes are bad." Mrs. Brandon obediently halted, shut her eyes, and presently opened them again. The gloom was now less dense to her sight and without difficulty she reached the chair placed by the bedside. Angela Thirkell, The Brandons London, The Hogarth Press, 1988, p. 37
We compared the clinical characteristics of esotropic, hypermetropic children whose strabismus was fully corrected with spectacles (refractive accommodative esotropia) with those who remained orthotropic (that is, had no manifest strabismus on the cover test) in the presence of uncorrected hypermetropia. In addition to a standard ophthalmologic and orthoptic examination, we determined the stimulus accommodative convergence/accommodation (AC/ A) ratio by using the gradient method over a range of 6 diopters, the near point of accommodation, and random dot stereopsis. Hypermetropic patients without esotropia or significant esophoria were found to have a low AC/ A ratio in contrast to those patients with refractive accommodative esotropia. This finding explains why esodeviations may be absent in some hypermetropic patients with uncorrected vision. We found a high prevalence of abnormally low near points of accommodation and defective or absent stereopsis in both groups of patients.
T HE RELATIONSHIP between esotropia and hypermetropia has been established ever since Donders made his classic contribution in 1864. 1 In his treatise, Donders asserted that strabismus may be absent in those with severe hypermetropia and speculated that such patients do not make the effort to accommodate sufficiently to clear their blurred retinal images. Donders suggested that if such patients would exert accommodation in accordance with their severe hypermetropia, esotropia would develop. Accepted for publication April 19, 1990. From the Cullen Eye Institute, Baylor College of Medicine and the Ophthalmology Service, Texas Children's Hospital, Houston, Texas. This study was supported in part by grant EY 07001 from the National Institutes of Health. Reprint requests to Gunter K. von Noorden, M.D., Ophthalmology Service, Texas Children's Hospital, Box 20269, Houston, TX 77225.
©AMERICAN JOURNAL OF OPHTHALMOLOGY
110:287-292,
We had on several occasions, however, encountered patients with uncorrected hypermetropia who had to make a great accommodative effort to have a visual acuity of 20/20. These patients, however, remained essentially orthotropic at near and distance fixation or developed only insignificant amounts of esophoria. This observation, which is in apparent contrast with Donders' teachings, led us to study this unusual response.
Subjects and Methods Between 1973 and 1989, we studied nine consecutive, alert, and cooperative patients (Group A) who fulfilled the following criteria: no history of esotropia or any other form of strabismus, no previous optical correction of a hypermetropia of +4.00 diopters or more, absence of anisometropia of more than 1.50 diopters, and orthotropia or esophoria of less than 10 prism diopters at near or distance fixation without spectacles on an accommodative fixation target. None of the patients had worn spectacles previously, and uncorrected visual acuity at distance was 20/30 or better with each eye in all but one patient (Case 2) whose visual acuity was R.E.: 20/50 and L.E.: 20/40. The ages of these patients ranged from 5 to 16 years at the time of the first examination (mean, 7.8 years). The mean refractive error was + 7.52 diopters (range, +4.25 to + 15.00 diopters) in the right eye and +7.51 diopters (range, +4.00 to + 15.00 diopters) in the left eye. In addition to a complete ophthalmologic examination, which included an orthoptic work-up, data on stimulus accommodation convergence/accommodation (AC/ A) ratio, near point of accommodation, and stereoacuity were obtained. A cycloplegic refraction was performed, and the full refractive error was prescribed. After the spectacles had been worn for at least six weeks, the stimulus AC/ A ratio was measured with the gradient method" by clipping spherical lenses ranging from -3.00 to +3.00 diopters in SEPTEMBER,
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287
288
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AMERICAN JOURNAL OF OPHTHALMOLOGY
I-diopter steps on the patient's spectacles. The heterophoria or heterotropia induced by the lenses was measured with the prism cover test while the patient fixated on an accommodative target at a viewing distance of 33 cm. The AC/ A ratio was computed by dividing the change in horizontal heterophoria in prism diopters by the change in stimulus to accommodation in diopters (prism diopters Zdiopters [p.d./D]). The near point of accommodation was determined with the Prince rule while the patient was wearing the full hypermetropic correction. An accommodative target (20/30 letter) was moved toward the patient along the ruler until the patient reported blurring. Several determinations were made, and the average distance at which blurring occurred was recorded in diopters. Stereopsis was measured with the TNO random dot stereo test. A control group (Group B) consisted of 30 consecutive patients with refractive accommodative esotropia, that is, the patients' esotropia was completely corrected by spectacles at near and distance fixation. Of these children, 29 had a corrected visual acuity of 20/40 or better. One patient (Case 1) had a visual acuity of R.E.: 20/50 and L.E.: 20/40. The patients' ages ranged from 6 to 13 years (mean, 11.6 years). The mean refractive error was +5.64 diopters (range, +3.50 to +9.25 diopters) in the right eye and +5.75 diopters (range, +3.75 to +9.50 diopters) in the left eye. All patients in Group B had a constant esotropia without their spectacles at near and distance fixation and had been esotropic for varying periods before the prescription of their first spectacles. One patient (Case 15) had a micro tropia, and the other
patients were orthotropic or had a well-controlled esophoria with spectacles.
Results The AC/ A ratio in patients from Group A (Table 1) was found to be either reduced or its slope to be completely flat (Cases 5 and 9), in which case no significant phoria response could be evoked by adding plus and minus spherical lenses to the patient's spectacles. In other words, there was no apparent synkinesis between accommodation and convergence. The mean AC/ A ratio in Group A was 1.53 p.d.jD (range, 0 to 2.6 vs 4.19 p.d./D (range, 1.1 to 9.0 p.d./D in Group B (Table 2). This difference was statistically significant (r-test, P < .001). The results of measurements of the near point of accommodation in Group A and Group B are compared in the Figure with the upper and lower limits of normal near points of accommodation as established by Duane." Perhaps coincidentally, abnormally low near points were more frequently encountered in the right eye than in the left eye. The near points of all right eyes were chosen to display the variations from the normal range. According to Duane," a near point of accommodation of less than 10 diopters in the age group under consideration (5 to 14 years) must be considered abnormally low. Two of eight patients from Group A had an abnormally low near point in one eye (Cases 1 and 2), and two had a reduced near point in both eyes (Cases 5 and 9) (Table 1). Of 30 children from Group B, ten had
TABLE 1 CLINICAL CHARACTERISTICS OF PATIENTS IN GROUP A REFRACTION
CASE NO.,
VISUAL ACUITY
AGE (YRS)
R.E.
L.E.
R.E.
L.E.
1.7 2.6 3. 5 4. 8 5. 9 6. 5 7. 16 8. 7 9.8
+15.00 +13.00 + 1.00 x 95 +5.50 +4.75 +7.00 +4.25 +4.00 + 0.50 x 105 +6.50 +7.00
+15.00 +13.00 + 1,00 x 80 +5.75 + 0,25 x 30 +4.75 +6.00 +4.00 +3.75 + 0.50 x 40 +7.50 +7.00
20/30 20/50 20/30 20/20 20/30 20/20 20/20 20/25 20/20
20/30 20/40 20/25 20/20 20/20 20/20 20/20 20/30 20/20
*NPA indicates near point of accommodation.
NPA (0)·
STEREOPSIS
R.E.
L.E.
ACtA
(SEC)
6 8
10 10
2.3 1.3 2.3 2.6
240 1,980
Not tested
13 3 14 10 14 9
13 8 15 10 14 9
Absent
2.1 1.6 1.6 Absent
Nil Nil Not tested
60 15 60 240
Accommodative Convergence in Hypermetropia
Vol. 110, No.3
TABLE 2 CLINICAL CHARACTERISTICS OF PATIENTS IN GROUP B NPA (0)'
CASE NO .• AGE (YRS)
1,7 2, 22 3, 6 4,8 5, 6
6,8 7, 10 8, 10 9, 8
10,9 11,6 12, 9 13,13 14,7 15,8 16,12 17,6
18, 8 19,8 20, 6 21,12 22, 13 23, 9 24, 7 25, 6
26,8
27, 9
28, 9 29, 12 30, 10
STEREOPSIS
R.E.
L.E.
AC/A
(SEC)
8 9 11 9 15 8
9
1.1 6.8
1,980
7
12 12 11 18 12 11 11 12 11 15 12 9
10 8 8 8
12 12 11 10 13 11 10
9 9
10 15 9 9
12 14 10 18 12 12 11 13 12 15 12 8
10 9 9 9
10 12 9
12 14 12 11
4.5
4.1 4.1 3.8 3.1 3.5
2.1 4.3 3.0 5.6
2.3 8.0 3.0 6.3 4.6 3.0 9.0 4.3 4.8 3.8 4.8 1.3 1.5 3.3 5.0 5.6 5.3
4.0
Nil
120 120 240 30 60 60 Nil
Nil Nil
Nil
Nil 240 Nil 30 240 Nil 240 Nil
Nil 240 1,980 1,980 1,980 60 240 1,980 60 120
°NPA indicates near point of accommodation.
abnormally low near points of accommodation in one (Cases 3, 4, and 26) or both eyes (Cases 1,6,7,19,21,22, and 23). Four patients (Cases 1, 2, 5, and 9) from Group A and one patient (Case 1) from Group B had reduction of both AC/ A ratio and near point of accommodation, but no correlation between these two measurements existed in the remainder of the patients from Groups A and B. Stereopsis was considered to be decreased when it was measured to be less than 240 seconds of arc.' Reduced stereopsis or stereoblindness was recorded in five of eight patients from Group A whose stereopsis was tested and in 21 of 30 patients from Group B. Thus, reduced or absent stereopsis was a feature that occurred frequently in patients with severe hy-
289
permetropia with and without refractive accommodative esotropia. We found no relationship between reduced stereopsis and a reduced near point of accommodation because we studied stereoblind patients with a normal near point of accommodation (Group A, Case 4; Group B, Cases 9 to 13, 15, 18, and 20) and those with reduced near points of accommodation and normal stereopsis (Group B, Cases 6 and 7). Likewise, no relationship was seen between visual acuity at near and reduced stereopsis because visual acuity at near, though not tested in all patients, was normal (11) in patients with reduced stereopsis (Group B, Cases 15, 18, 20, and 21).
Discussion According to Donders,' uncorrected hypermetropia may cause esotropia because of excessive accommodation. Donders also realized, however, that only a comparatively small number of esotropic patients develop hypermetropia and suggested that fusion preserves most hypermetropic individuals from strabismus. If accommodative convergence is compensated for by fusional divergence, one would expect such patients to have a significant degree of esophoria at near and distance fixation. The patients from Group A, however, were orthotropic or had esophorias of less than 10 prism diopters. The data from Group A show that a low AC/A ratio must be added to excessive accommodative convergence and the range of motor fusion amplitudes as a factor that determines whether a hypermetropic individual becomes esophoric or esotropic or remains orthotropic. These patients were orthotropic or had only insignificant amounts of esophoria without spectacles despite excessive accommodation required to recognize the fixation target used during measurement of the near deviation. It has been determined that the normal values for the AC/A ratio range from 2.76 p.d.yD to 4.50 p.d.j/D, depending on the methods used." The mean AC/A ratios obtained in Group A must be considered abnormally low and those from Group B as within the normal range. The stimulus method" to determine the AC/A ratio is used widely in clinical work.v" As suggested by Alpern, Kincaid, and Lubeck," however, one disadvantage of this method is the
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AMERICAN JOURNAL OF OPHTHALMOLOGY
September, 1990
18r-------------------..., •
16 14
s-
12
~
8
o
Figure (von Noorden and Avilla). Near point of accommodation in relation to age. Open circles indicate patients in Group A; solid circles, patients in Group B; stippled area, normal range; A, lowest values; B, average values; and C, highest values.
10
6
•
o
4
o
2
oL..-........ 5
--I._...I.-.......-.I""'-..a-....._
6
7
8
9
........ I--'--I t--"'-_....
10 11 12 Age (Years)
difficulty in determining the state of the patient's accommodation when plus or minus lenses are placed before the eyes. It is planned to measure the response AC/ A ratio, that is, the AC/ A ratio related to the accommodative response in a suitable subject to corroborate our findings even though a predictable increase of the response AC/ A ratio of 8%6 would not change the significance of our data. We must ask whether the reduced AC/ A ratio in Group A is a primary, innate, or a secondary phenomenon. This question becomes moot if one shares the view of many investigators who consider the stimulus AC/A ratio as a stable, genetically determined relationship between accommodation and convergence, based on a fixed central nervous system arrangement that remains immutable until the presbyopic age or throughout life."" If one agrees with Helmholtz," however, that the association between accommodation and convergence is a learned process, the possibility must be considered that certain individuals with uncorrected severe hypermetropia fail to make the sustained accommodative effort necessary to focus and consequently develop a low AC/ A ratio. In that case, we would expect a good correlation between a low AC/ A ratio and a reduced near point of accommodation. With the exception of Cases 1, 2, 5, and 9 from Group A and Case 1 from Group B, however, such a correlation did not exist in the remainder of our patients. For this reason and because of the studies cited, we are inclined to favor the first theory and suggest that an intrinsically low AC/ A ratio may pro-
16
22
tect certain patients with uncorrected hypermetropia from developing esotropia or esophoria. The finding of a reduced near point of accommodation in hypermetropic patients with and without strabismus is of interest. Low near points of accommodation have been described in connection with ametropic amblyopia in patients with severe hypermetropia.P:" It has even been suggested that one of the possible causes of ametropic amblyopia is a reduced near point of accommodation." Reduced near vision in the presence of normal distance vision, however, is not a recognizable arnblyopiogenic factor, and with the exception of Case 2, none of our patients with a reduced near point was amblyopic. Rather, we suggest the possibility that patients with uncorrected hypermetropia do not fully accommodate at all times during infancy. The resulting hypoaccommodative state may eventually cause a reduced accommodative range, perhaps even a disuse atrophy of the ciliary muscle. Although this hypothesis remains unsupported, we have no better explanation for a reduced near point of accommodation in more than one third of our patients. We did not determine the pupillary near response in those with reduced near points to explore whether there are other anomalies of the near reflex in such patients. The finding of reduced or absent stereopsis as a common feature in patients with moderate or severe hypermetropia was unexpected. Reduced or absent stereopsis was not consistently associated with a reduced near vision from a reduced near point of accommodation. We were
Vol. 110, No.3
Accommodative Convergence in Hypermetropia
also unable to associate the age at which the hypermetropia was optically corrected and the esodeviation was controlled with spectacles with the sensitivity of stereopsis. We believe, however, that the reduced stereopsis in Group B might be explained on the basis of varying periods of esotropia that were present before the wearing of corrective lenses. It is well known from clinical experience" and animal experiments'S" that disruption of binocular vision during visual immaturity will cause a permanent defect of stereopsis. It will be of inter'est to explore whether the defect of stereopsis in patients with refractive accommodative esotropia can be correlated with optokinetic asymmetry, which is another sign of early disruption of binocular vision.":" Reduced stereopsis in five of the eight patients tested in Group A must be caused by a different mechanism because none of the children we studied had manifest strabismus or a history of such. What these patients may have had in common, however, was a period of blurred form vision in both eyes before optical correction of their hypermetropia. Defective stereopsis has been reported after bilateral form vision deprivation from patching of infants. 21•23 We have also learned from experiments in infant monkeys that a reduction in the number of cortical binocular neurons (which are necessary for stereopsis") is not only caused by artificial strabisrnus.P:" anisometropia.F and unilateral eyelid closure," but occurs also after bilateral and symmetric deprivation of form vision." It is reasonable to assume, therefore, that the blurring of vision during infancy before the wearing of spectacles caused deprivation of form vision, loss of cortical binocular function, and thus a reduction or loss of stereopsis in most patients from Group A. It is of interest that a mild ametropic amblyopia was present in only one of these patients (Case 2), which suggests that stereopsis is a visual function more sensitive to the effects of bilateral visual deprivation than visual acuity. This observation is in accord with recent data from our animal laboratory that show that only brief periods of bilateral form vision deprivation in visually immature monkeys suffice to cause a decrease of stereopsis without significantly affecting monocular visual acuity." To avoid the visual deprivation effect of uncorrected hypermetropia in excess of four diopters on the immature visual system, to improve near vision in those with a reduced near point of accommodation, and to preserve stereopsis,
291
we recommend optical correction in children under the age of 4 years, even in the absence of a manifest or latent esodeviation.
References 1. Donders, F. c.: On the Anomalies of Accommodation and Refraction of the Eye With a Preliminary Essay on Physiological Dioptrics. London, The New Sydenham Society, 1864, p. 292. 2. Sloan, L., Sears, M., and Jablonski, M.: Convergence-accommodation relationship. Arch. Ophthalmol. 63:283, 1960. 3. Duane, A.: Studies in monocular and binocular accommodation with their clinical applications. Am. J. Ophthalmol. 5:865, 1922. 4. Reinecke, R. D. In Discussion: March, W. R., Rowlings, S. c.. and Mumma, J. V.: Evaluation of clinical stereoacuity tests. Ophthalmology 87:1265, 1980. 5. Franceschetti, A., and Burian, H. M.: Gradient accommodative convergence/accommodative ratio in families with and without esotropia. Am. J. Ophthalmol. 70:558, 1970. 6. Alpern, M., Kincaid, W. M., and Lubeck, M. J.: Vergence and accommodation. III. Proposed definitions of AC/A ratios. Am. J. Ophthalmol. 48:141, 1959. 7. Morgan, M. W., [r.: Relationship between accommodation and convergence. Arch. Ophthalmol. 47:745, 1952. 8. Fincham, E. F.: The proportion of ciliary muscular force required for accommodation. ]. Physiol. (Lond.) 128:99, 1955. 9. Alpern, M., and Hirsch, M. J. Cited in Alpern, M., and Larson, B. F.: Vergence and accommodation. IV. Effect of luminance quantity on the AC/ A. Am. J. Ophthalmol. 49:1140,1960. 10. Alpern, M.: Types of movement. In Davson, H. (ed.): The Eye, vol. 3. New York, Academic Press, 1962, p, 123. 11. Breinin, G. M., and Chin, N. B.: Accommodation, convergence and aging. Doc. Ophthalmol. 34:109, 1973. 12. von Helmholtz, H.: Helmholtz's Treatise on Physiological Optics. In Southall, J. P. C. (ed.): vol. 3. Translated from the 3rd German edition. New York, Dover Publications, 1962, p. 55. 13. Werner, D. B., and Scott, W. E.: Amblyopia case reports. Bilateral hypermetropic ametropic amblyopia. J. Pediatr. Ophthalmol. Strabismus 22:203, 1985. 14. Schoenleber, D. B., and Crouch, E. R.: Bilateral hypermetropic amblyopia. ]. Pediatr. Ophthalmol. Strabismus 24:75,1987. 15. von Noorden, G. K.: Current concepts of infantile esotropia. Eye 2:343, 1988. 16. Crawford, M. L. J" von Noorden, G. K., Meharg, L. S., Rhodes, J. W., Harwerth, R. S., Smith,
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E. L., III, and Miller, D. D.: Binocular neurons and binocular function in monkeys and children. Invest. Ophthalmol. Vis. Sci. 24:491, 1983. 17. Crawford, M. L. J., Smith, E. L., III, Harwerth, R. S., and von Noorden, G. K.: Stereoblind monkeys have few binocular neurons. Invest. Ophthalmol. Vis. Sci. 25:779, 1984. 18. Tychsen, L., Hurtig, R. R., and Scott, W. E.: Pursuit is impaired but the vestibulo-ocular reflex is normal in infantile strabismus. Arch. Ophthalmol. 103:536, 1985. 19. Flynn, J. T.: Vestibulo-optokinetic interaction in strabismus. Am. Orthopt. J. 32:36, 1982. 20. van Hof-van Duin, j., and Mohn, G.: Monocular and binocular optokinetic nystagmus in humans with defective stereopsis. Invest. Ophthalmol. Vis. Sci. 27:574, 1986. 21. Hoyt, C. S.: The long-term visual effects of short-term binocular occlusion of at-risk neonates. Arch. Ophthalmol. 98:1967, 1980. 22. Glass, P.: Another look at long-term visual effects of binocular occlusion in neonates (letter to the editor). Arch. Ophthalmol. 102:968, 1984. 23. Wright, K. W., Wehrle, M. J., and Urrea, P. T.: Bilateral total occlusion during the critical period of
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visual development. Arch. Ophthalmol. 105:321, 1987. 24. Crawford, M. L. J., Smith, E. L., III, Harwerth, R. S., and von Noorden, G. K.: Stereoblind monkeys have few binocular neurons. Invest. Ophthalmol. Vis. Sci. 25:779, 1984. 25. Baker, F. H., Grigg, P., and von Noorden, G. K.: Effects of visual deprivation and strabismus on the response of neurons in the visual cortex of the monkey, including studies on the striate and prestriate cortex in the normal animal. Brain Res. 66:185, 1974. 26. Crawford, M. L. J., and von Noorden, G. K.: Optically induced comitant strabismus in monkeys. Invest. Ophthalmol. Vis. Sci. 19:1105, 1980. 27. von Noorden, G. K., and Crawford, M. L. J.: Form deprivation without light deprivation produces the visual deprivation syndrome in Macaca mulatta. Brain Res. 129:37, 1977. 28. Crawford, M. L. J., de Faber, J. T., Pesch, T. W., and von Noorden, G. K.: Binocular cells and stereopsis in monkeys. ARVO abstracts. Supplement to Invest. Ophthalmol. Vis. Sci. Philadelphia, J. B. Lippincott, 1989, p. 315.
OPHTHALMIC MINIATURE
"Stand still and shut your eyes for a moment," commanded Miss Brandon's voice from the bed, "and then you'Il be able to see. I can't have the blinds up. My eyes are bad." Mrs. Brandon obediently halted, shut her eyes, and presently opened them again. The gloom was now less dense to her sight and without difficulty she reached the chair placed by the bedside. Angela Thirkell, The Brandons London, The Hogarth Press, 1988, p. 37