Documenta Ophthalmolgica 46,2: 305-308, 1979 SQUINT IN MONOZYGOTIC TWINS B. DE VRIES & W.A. HOUTMAN
(Groningen) Key words: Strabismus, Heredity, Monozygotic twins ABSTRACT In a group of monozygofic twins, in which at least one of the pair squinted, strabismus was observed in both twins in only about half of the cases. Within the concordant group there was variation in the way in which the anomaly became manifest. INTRODUCTION With regard to the heredity of souint no generally accepted opinion exists, in spite of its obvious, often familial, occurrence (Richter, 1967; Schlossman, t 952, Waardenburg, 1963, etc.). Waardenburg (1963) favoured the existence of one main autosomal gene. Czellitzer (1922) considered that two recessive genes were concernea. Crone and Velzeboer (1956) concluded on the basis of their investigation that one common hereditary mechanism must exist. A more commonly accepted theory is multiconditional heredity of mainly recessive factors (Richter, 1967; Burian & yon Noorden, 1974; Clausen, 1923; Gri~tzner, 1970). It is thus understandable that the occurrence of squint within pairs of monozygotic twins is of particulor interest. Not only because this fact could support the idea of hereditary predisposition, but also because it could appear that endogenous and exogenous factors influence the development of the squint. There is some discrepancy in the literature on the concordance in the occurrence of squint within pairs of monozygotic twins. Waardenburg (1963) found complete concordance in l l cases, Schlossman ( 1 9 5 2 ) i n 3 cases and Richter (1967) in 11 of 12 cases. In contrast to these are the findings of Weekers (1956), Kondo (1975), Franqois (1958) and Lanthony (1977). Weekers' investigation covered 7 pair.~ of monozygotic twins; the monozygosity was established on the grounds of complete concordance in the blood groups. In only 3 pairs there was concordance in the occurrence of squint. The concordance with regard to the refraction in groups of monozygotic twins is great according to both Waardenburg, Weekers and Richter.
305
PRESENTSTUDY In the last 4 years 17 pairs of monozygotic twins were examined, of which at least one of each pair had a squint. The criteria for the selection were: the colour and implantation of the hair, the colour and structure of the iris, the height, the general resemblance, and inability of people in his direct surroundings and the examiner to tell the two apart in early youth. According to Nichols (1966) the fulfilment of these criteria establishes monozygosity with 90% reliability, if the reliability given by an extensive blood group examination is taken to be
100%. The age at which the first examination was performed varied in the 17 pairs from 10 months to 43 years; 3 pairs were older than 6 years. The data from the last examination, especially with reference to the binocular functions, were used for this study, unless amblyopia or squint had been treated. In all cases the squint had been noticed in at least one of the children in the first year of life. The distribution of the refraction was as follows. Fourteen pairs had 2 identical refractions. Of these seven had a normal refraction, i.e., a spherical correction of less than 2 diopters and/or a cylindrical correction of less than 1 diopter. The other 7 pairs had the same abnormnl refraction, i.e., a spherical correction of 2 diopters or more and/or a cylindrical correction of 1 diopter or more. In 3 pairs there was a difference in the refraction. In one myopic pair both children had the same degree of anisometropia but one had a higher degree of myopia. In another pair both children had the same oblique astigmatism of 1.5 diopters but different in the spherical value of the hypermetropia. In one pair the refraction was different. In only one pair both twins had a large convergent angle of squint. Their refraction was identical, they were both amblyopic and neither had a disorder of ocular motility. In 3 pairs both children had the same degree of high hypermetropia, more than 5 diopters. In two of these pairs one twin had a manifest convergent squint with a large angle, with or without correction, while the other twin had an intermittent esotropia without correction. In the third pair one child had a manifest convergent angle of squint of 25 ~ while the position of the eyes of the other child was normal and the binocular functions were positive, including a positive 4" PT, indicating the presence of single binocular vision. Two of the 3 squinting children were amblyopic in this group of high hypermetropes. In all 3 cases the strabismus was associated with an obvious Vpattern.
306
We could subdivide the other pairs into 2 groups. In 5 pairs a manifest angle of squint greater than 5 ~ was found in one twin and a microstrabismus in the other. In 4 pairs the refraction was identical and in the fifth pair the same degree of anisometropia was found. Eight of the 10 children were amblyopic and all eight retained a slight degree of amblyopia after treatment. Examination of the ocular motility revealed nothing unusual. The second group consisted of 8 pairs of twins in which only one of the pair had a convergent squint. The angle of squint varied up to 15 degrees, the other twin did not squint. In none of these cases could incidents during childbirth or later be considered responsible for the appearance of the squint. In 6 pairs the refraction was identical. One pair had the same oblique astigmatism of 1.5 diopters and! 2 diopters' difference in spherical value. In the last pair one child had astigmatism in both eyes; this is the only pair in which the monozygosity may be questioned on the grounds of the difference in refraction. The non-squinting twin reacted positively to tests for single binocular vision, including the 4-diopter prism test. Six of the 8 squinting children were amblyopic, one showed an A-syndrome and one a V-syndrome. If we now consider the concordance of the whole group with regard to strabismus, without differentiating between large angles of squint, accommodative squint and microstrabismus, eight of the 17 pairs of twins may be called concordant. This figure is more in agreement with Weekers and Francois than with Waardenburg and Richter. This concordance in 8 pairs at least supports the theory that hereditary factors are involved. In addition to the absence of concordance in 9 cases, the cases in which only one child squints, the variation between the members of a pair which is concordant is remarkable. This is also true for the manifestation of the V-syndrome in the group o:f high-hypermetropes. It thus appears that, with the same hereditary structure, there is a gradual transition from a straight position of the eyes to microstrabismus and large angles of squint. These findings are in agreement with those of Crone and Velzeboer, who came to the conclusion, on the grounds of their data, that a common hereditary pathogenetic factor is involved, irrespective of the manifestation of the strabismus. Our findings seem to support this theory, as our material is based on an identical hereditary structure. Crone and Velzeboer thought it possible that the pathogenetic element was defined in Worth's theory of fusion; the ocular motor reflexes then explain how the final abnormal position of the eye arises. Worths ideas about fusion, however, are not in the first place concerned with the regulation of the straight position of the eyes. In their place it is tempting to place the
307
m o t o r fusion m e c h a n i s m as u n d e r s t o o d by H o f m a n n : an adaptation mechanism w h i c h tries to maintain the straight position. This can explain the gradual transitions, while in some circumstances small changes in the influence e x e r t e d by one or m o r e factors can lead to large differences in manifestation. REFERENCES Burian, H.M. & G.K. von Noorden. Binocular vision and ocular motility. -Theory and management of strabismus-. C.V. Mosby Comp., Saint Louis (1974). Clausen, W. & J. Bauer. Beitr~ge und Gedanken zur Lehre yon der Vererbung des Strabismus concomitans. Zeitschrift ffArAugenheillc. 50:313-334 (1923). Crone, R.A. & C.M.J. Velzeboer. Statistics on strabismus in the Amsterdam youth. Arch. Ophthalmol. 55:455-470 (1956). CzeUitzer, A. Wie vererbt slch Schielen? Klin. Mbl. Augenheilk. 79:519 (1922). Francois, J. l'H~r~dit~ en Ophtalmologie, IV Strabisme concomitant, 291-299, Masson et Cie, Paris (1958). Griitzner, P., K. Yazawa & B.E. Spivey. Heredity and strabismus, b~rvey of Ophthalmol. 14:441-456 (1970). Hofmann, F.B. & A. Bielschowsky. Uber die der Willkiir entzogenen Fusionsbewegungen der Augen. lfflugers Archly ges. PhysioL 8 0 : 1 - 4 0 (1900). Kondo, K., E. Mori & K. Adachi. A study on squint in 21 cases of monozygotic twins. Folia OphthalrnoL Jap. 26:166-171 (1975). Lanthony, Ph., N. Ithier & Mp.P. Picard. Les jumeaux strabiques. Journl. Fran~ais d'Orthoptique 9 : 9 3 - 1 0 0 (1977). Nichols, R.C. & W.C. Bilbro Jr. The diagnosis of twin zygosity. Acta genet. 16:265-275 (1966). Richter, S. Untersuchungen tiber die Heredit~t des Strabismus concomitans. Georg Thieme, Leipzig (1967). Schlossman, A. & B.S. Priestley. Role of heredity in etiology and treatment of strabismus. Arch. OphthalrnoL 47:1-20 (1952). Waardenburg, P.J. Genetics and Ophthalmology. Volume II, Neuro-Ophthalmology, van Gorcum, Assen (1963). Weekers, R., P. Moureau, J.Hacourt & A.Andre. Contribution ~ l'~tiologie du strabisme concomitant et de l'amblyonie pzr l'~tude de jumeaux uniter bivitellins. Bull de la Soc. Ralge d'Ophtalmol. 112:146-173. (1956). Author's address: Ophthalmological Clinic Academic Hospital Oostersingel 59 Groningen The Netherlands
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(Groningen) Key words: Strabismus, Heredity, Monozygotic twins ABSTRACT In a group of monozygofic twins, in which at least one of the pair squinted, strabismus was observed in both twins in only about half of the cases. Within the concordant group there was variation in the way in which the anomaly became manifest. INTRODUCTION With regard to the heredity of souint no generally accepted opinion exists, in spite of its obvious, often familial, occurrence (Richter, 1967; Schlossman, t 952, Waardenburg, 1963, etc.). Waardenburg (1963) favoured the existence of one main autosomal gene. Czellitzer (1922) considered that two recessive genes were concernea. Crone and Velzeboer (1956) concluded on the basis of their investigation that one common hereditary mechanism must exist. A more commonly accepted theory is multiconditional heredity of mainly recessive factors (Richter, 1967; Burian & yon Noorden, 1974; Clausen, 1923; Gri~tzner, 1970). It is thus understandable that the occurrence of squint within pairs of monozygotic twins is of particulor interest. Not only because this fact could support the idea of hereditary predisposition, but also because it could appear that endogenous and exogenous factors influence the development of the squint. There is some discrepancy in the literature on the concordance in the occurrence of squint within pairs of monozygotic twins. Waardenburg (1963) found complete concordance in l l cases, Schlossman ( 1 9 5 2 ) i n 3 cases and Richter (1967) in 11 of 12 cases. In contrast to these are the findings of Weekers (1956), Kondo (1975), Franqois (1958) and Lanthony (1977). Weekers' investigation covered 7 pair.~ of monozygotic twins; the monozygosity was established on the grounds of complete concordance in the blood groups. In only 3 pairs there was concordance in the occurrence of squint. The concordance with regard to the refraction in groups of monozygotic twins is great according to both Waardenburg, Weekers and Richter.
305
PRESENTSTUDY In the last 4 years 17 pairs of monozygotic twins were examined, of which at least one of each pair had a squint. The criteria for the selection were: the colour and implantation of the hair, the colour and structure of the iris, the height, the general resemblance, and inability of people in his direct surroundings and the examiner to tell the two apart in early youth. According to Nichols (1966) the fulfilment of these criteria establishes monozygosity with 90% reliability, if the reliability given by an extensive blood group examination is taken to be
100%. The age at which the first examination was performed varied in the 17 pairs from 10 months to 43 years; 3 pairs were older than 6 years. The data from the last examination, especially with reference to the binocular functions, were used for this study, unless amblyopia or squint had been treated. In all cases the squint had been noticed in at least one of the children in the first year of life. The distribution of the refraction was as follows. Fourteen pairs had 2 identical refractions. Of these seven had a normal refraction, i.e., a spherical correction of less than 2 diopters and/or a cylindrical correction of less than 1 diopter. The other 7 pairs had the same abnormnl refraction, i.e., a spherical correction of 2 diopters or more and/or a cylindrical correction of 1 diopter or more. In 3 pairs there was a difference in the refraction. In one myopic pair both children had the same degree of anisometropia but one had a higher degree of myopia. In another pair both children had the same oblique astigmatism of 1.5 diopters but different in the spherical value of the hypermetropia. In one pair the refraction was different. In only one pair both twins had a large convergent angle of squint. Their refraction was identical, they were both amblyopic and neither had a disorder of ocular motility. In 3 pairs both children had the same degree of high hypermetropia, more than 5 diopters. In two of these pairs one twin had a manifest convergent squint with a large angle, with or without correction, while the other twin had an intermittent esotropia without correction. In the third pair one child had a manifest convergent angle of squint of 25 ~ while the position of the eyes of the other child was normal and the binocular functions were positive, including a positive 4" PT, indicating the presence of single binocular vision. Two of the 3 squinting children were amblyopic in this group of high hypermetropes. In all 3 cases the strabismus was associated with an obvious Vpattern.
306
We could subdivide the other pairs into 2 groups. In 5 pairs a manifest angle of squint greater than 5 ~ was found in one twin and a microstrabismus in the other. In 4 pairs the refraction was identical and in the fifth pair the same degree of anisometropia was found. Eight of the 10 children were amblyopic and all eight retained a slight degree of amblyopia after treatment. Examination of the ocular motility revealed nothing unusual. The second group consisted of 8 pairs of twins in which only one of the pair had a convergent squint. The angle of squint varied up to 15 degrees, the other twin did not squint. In none of these cases could incidents during childbirth or later be considered responsible for the appearance of the squint. In 6 pairs the refraction was identical. One pair had the same oblique astigmatism of 1.5 diopters and! 2 diopters' difference in spherical value. In the last pair one child had astigmatism in both eyes; this is the only pair in which the monozygosity may be questioned on the grounds of the difference in refraction. The non-squinting twin reacted positively to tests for single binocular vision, including the 4-diopter prism test. Six of the 8 squinting children were amblyopic, one showed an A-syndrome and one a V-syndrome. If we now consider the concordance of the whole group with regard to strabismus, without differentiating between large angles of squint, accommodative squint and microstrabismus, eight of the 17 pairs of twins may be called concordant. This figure is more in agreement with Weekers and Francois than with Waardenburg and Richter. This concordance in 8 pairs at least supports the theory that hereditary factors are involved. In addition to the absence of concordance in 9 cases, the cases in which only one child squints, the variation between the members of a pair which is concordant is remarkable. This is also true for the manifestation of the V-syndrome in the group o:f high-hypermetropes. It thus appears that, with the same hereditary structure, there is a gradual transition from a straight position of the eyes to microstrabismus and large angles of squint. These findings are in agreement with those of Crone and Velzeboer, who came to the conclusion, on the grounds of their data, that a common hereditary pathogenetic factor is involved, irrespective of the manifestation of the strabismus. Our findings seem to support this theory, as our material is based on an identical hereditary structure. Crone and Velzeboer thought it possible that the pathogenetic element was defined in Worth's theory of fusion; the ocular motor reflexes then explain how the final abnormal position of the eye arises. Worths ideas about fusion, however, are not in the first place concerned with the regulation of the straight position of the eyes. In their place it is tempting to place the
307
m o t o r fusion m e c h a n i s m as u n d e r s t o o d by H o f m a n n : an adaptation mechanism w h i c h tries to maintain the straight position. This can explain the gradual transitions, while in some circumstances small changes in the influence e x e r t e d by one or m o r e factors can lead to large differences in manifestation. REFERENCES Burian, H.M. & G.K. von Noorden. Binocular vision and ocular motility. -Theory and management of strabismus-. C.V. Mosby Comp., Saint Louis (1974). Clausen, W. & J. Bauer. Beitr~ge und Gedanken zur Lehre yon der Vererbung des Strabismus concomitans. Zeitschrift ffArAugenheillc. 50:313-334 (1923). Crone, R.A. & C.M.J. Velzeboer. Statistics on strabismus in the Amsterdam youth. Arch. Ophthalmol. 55:455-470 (1956). CzeUitzer, A. Wie vererbt slch Schielen? Klin. Mbl. Augenheilk. 79:519 (1922). Francois, J. l'H~r~dit~ en Ophtalmologie, IV Strabisme concomitant, 291-299, Masson et Cie, Paris (1958). Griitzner, P., K. Yazawa & B.E. Spivey. Heredity and strabismus, b~rvey of Ophthalmol. 14:441-456 (1970). Hofmann, F.B. & A. Bielschowsky. Uber die der Willkiir entzogenen Fusionsbewegungen der Augen. lfflugers Archly ges. PhysioL 8 0 : 1 - 4 0 (1900). Kondo, K., E. Mori & K. Adachi. A study on squint in 21 cases of monozygotic twins. Folia OphthalrnoL Jap. 26:166-171 (1975). Lanthony, Ph., N. Ithier & Mp.P. Picard. Les jumeaux strabiques. Journl. Fran~ais d'Orthoptique 9 : 9 3 - 1 0 0 (1977). Nichols, R.C. & W.C. Bilbro Jr. The diagnosis of twin zygosity. Acta genet. 16:265-275 (1966). Richter, S. Untersuchungen tiber die Heredit~t des Strabismus concomitans. Georg Thieme, Leipzig (1967). Schlossman, A. & B.S. Priestley. Role of heredity in etiology and treatment of strabismus. Arch. OphthalrnoL 47:1-20 (1952). Waardenburg, P.J. Genetics and Ophthalmology. Volume II, Neuro-Ophthalmology, van Gorcum, Assen (1963). Weekers, R., P. Moureau, J.Hacourt & A.Andre. Contribution ~ l'~tiologie du strabisme concomitant et de l'amblyonie pzr l'~tude de jumeaux uniter bivitellins. Bull de la Soc. Ralge d'Ophtalmol. 112:146-173. (1956). Author's address: Ophthalmological Clinic Academic Hospital Oostersingel 59 Groningen The Netherlands
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