Strabismus has been defined as an intermittent or constant misalignment of the visual axes of the eyes, disrupting binocular single vision. The nature of the misalignment is most commonly horizontal, but vertical and oblique deviations are also encountered. The deviation may be found in one, several, or all fields of gaze and may be latent, due to fusional mechanisms, or manifest. In this chapter, the heredity of primary concomitant strabismus will be discussed. Strabismus associated with traumatic paralysis or injury to the extraocular muscles will not be considered due to its obvious acquired nature. The inheritance of strabismus as part of a recognized ophthalmic or systemic syndrome will also be discussed.

In the multifactorial model of inheritance, several gene loci contribute to the expression of a deviation of the extraocular muscles. Multiple loci can be affected by multiple environmental factors, thus leading to variations in the expression of the phenotypic strabismus. It is postulated that a certain degree of interaction may be necessary among the various factors involved to produce a strabismus—the “threshold effect.” When the threshold is reached, the strabismus becomes manifest.

Several characteristics of the multifactorial model of inheritance distinguish it from single gene modes of inheritance¹¹:

1. The percentage of affected siblings is similar to the percentage of other affected first-degree relatives.
   
2. There is a sharp decline in prevalence from first-degree to second-degree relatives.
   
3. The risk of recurrence varies from family to family, increasing when there are greater numbers of affected near relatives.
   
4. Differences in frequency of strabismus by gender are encountered.
   

Thus, the risk of recurrence of strabismus in an affected family with a multifactorial mode of inheritance is based on the frequency of that strabismus in the general population and the number of family members affected in the pedigree under study. Various systems for calculating this risk have been proposed. The reader is referred to Curnow and Smith¹² for a discussion of one such system of calculating risk of recurrence in a condition with a multifactorial pattern of inheritance.

The study of a large population with multiple affected families is therefore of utmost importance in investigating conditions with a multifactorial mode of genetic transmission. The risk of recurrence of strabismus in a sibling or offspring of an affected individual can be more correctly calculated using a large population. Also, the effect of environmental factors on the transmission of the trait can be more accurately assessed.

To eliminate some of the possible environmental factors encountered when determining rates of strabismus in different racial groups, Eustace¹⁵ examined second-generation West Indian children born in Birmingham, England. The prevalence of strabismus and refractive error in this population was then compared with previous data collected on white children from the same region. Exotropia was found to be four times more frequent in the West Indian population of children. In addition, myopia was significantly more frequent in the West Indian children, especially among the exotropes.

Ing and Pang¹⁶ studied the incidence of strabismus in individuals of white, Asian, and mixed descent from the Honolulu area. Esotropia was noted to be more common in whites, whereas Asians more frequently manifested exotropia. Mixed-descent individuals were fairly evenly divided between divergent and convergent strabismus. In examining the interracial incidences of nonaccommodative and accommodative esotropia, it was found that Asians and individuals of mixed descent manifested equal incidences of both subtypes. However, the incidence of accommodative esotropia found in whites was nearly four times that of nonaccommodative esotropia.

Several studies have investigated the incidence of strabismus in families with strabismic members. Considerable differences have been encountered in these studies. Schlossman and Priestley¹⁷ found that 47.5% of the patients with strabismus in their study belonged to families with two or more affected members. They further broke this down to show an incidence of 48.9% in esotropes and 36.8% in exotropes. Worth¹⁸ had similar findings, whereas Scobee¹⁹ reported an incidence of 41%. Several studies have reported the familial incidence of strabismus to be as high as 65%.^20–22

Such wide variations in reported findings are attributable to several factors. Many studies dealt with strabismus as the phenotype and did not distinguish specific subtypes of strabismus, such as accommodative or congenital esotropia, exotropia, or the different phorias. Sample sizes varied, as did target populations, thus adding to the heterogeneity of the studies. The underlying incidence of strabismus in the population under study was not always determined. Data-gathering methods also varied. Some studies relied on patient histories; others examined all subjects. The extent of investigation of different pedigrees varied considerably.

The difficulties encountered when conducting a population study of strabismus were underscored by Kornder and colleagues²³ in their study of the prevalence of strabismus in school-aged children in British Columbia. Screening personnel with varied training backgrounds were employed. In addition, a self-administered questionnaire was used to determine familial history of strabismus. The results obtained via examination varied significantly with the level of training of the examiner. In addition, subjective methods of evaluation were found to be inferior to evaluation by both subjective and objective methods. The most frequently “missed” group was children with intermittent exotropia.

In a carefully studied population from Greece, Chimonidou and coworkers²⁴ found the overall familial incidence of a history of strabismus to be 55%. In an effort to refine the data, the authors limited their subsequent investigation to the 345 patients who were longstanding patients of the Athens University Eye Clinic. In this group, they found that 97% of affected siblings had the same type of strabismus as the proband. Eighty-three percent of the patients had a significant refractive error. Of the 148 patients with congenital strabismus (defined as onset within the first year of life), 42% had siblings who developed strabismus at a more advanced age. The remaining 58% had siblings who developed strabismus at the same age, within one year. In the five pairs of twins encountered, strabismus appeared at the same age in both twins. Type of strabismus and refractive error were also noted to be the same for each pair. One pair of monozygous twins was studied. This set of twins was separated at 3 months of age and raised under vastly different socioeconomic conditions. Both developed esotropia at 3 years of age.

Concordance rates in both groups are analyzed for the trait under study. If the concordance rate of a trait in monozygotic twins approaches 100%, while the same rate in dizygotic twins is significantly less, then genetic transmission is likely. When the concordance rates between monozygotic and dizygotic twins do not differ significantly, environmental influences and other random factors assume greater importance in the etiology of the trait under study.

Rubin and associates²⁵ surveyed ophthalmologists from 50 countries for data on twins with ocular motor anomalies and calculated the heritability of various eye conditions and factors involved in the development of fixation and fusion reflexes. Ophthalmic conditions rated high for genetic influence were exotropia, hyperopia, and myopia. Moderate genetic influences were seen in astigmatism, eccentric fixation, amblyopia, and esotropia.

Waardenburg⁵ analyzed the presence of strabismus in 69 pairs of monozygotic twins, 58 pairs from the literature and 11 pairs from his own practice. Concordance was found in 81.2%, thus demonstrating some environmental effect. Seven pairs of monozygotic twins were identified with 100% concordance for exotropia. Fifty-one pairs of dizygotic twins were then considered. This included 15 pairs from his own practice. Concordance for strabismus was found in 8.9%. He postulated that this eliminated the possibility that environmental factors alone were responsible for strabismus, since the concordance of strabismus in dizygotic twins was significantly higher than that of the general population and nearly equal to that of siblings from unaffected parents.^7,26

Richter²⁶ corroborated Waardenburg's findings when she found concordance for strabismus in 11 of 12 monozygotic twin pairs (92%) and in only 7 of 27 dizygotic twin pairs (26%). Concordance in amblyopia and retinal correspondence was found to be about 44% in monozygous twins but only 17% in dizygous twins.

Somewhat different results were found in two smaller studies. Weekers and colleagues²⁷ found only three of seven pairs (43%) of monozygotic twins concordant for strabismus. However, six of seven pairs (86%) showed identical ametropia. From this, they concluded that strabismus is not hereditary but only secondary to other abnormalities. DeVries and Houtman²⁸ found 8 of 17 pairs (47%) of monozygotic twins concordant for strabismus. Several of the concordant pairs were noted to have large variations in the expression of their strabismus. This was thought to represent a common predisposition to strabismus, modified by heredity.

Kvapilikova²⁹ compared the sensory status of 34 pairs of monozygous twins to that of 34 pairs of dizygous twins. A high degree of concordance (81%) was found for foveolar fusion in the monozygous twins. The dizygous twins demonstrated moderate concordance (58%) for this characteristic. The author concluded that variability of fusion did have some hereditary component.

The study of differences encountered in monozygotic twins has also yielded other interesting insights. Shippman and coworkers³⁰ reported a pair of esotropic monozygotic twins. One twin demonstrated a V pattern esotropia; the other twin demonstrated an A pattern esotropia. At the time of surgery, twin 2 was found to have anomalous insertions of both medial rectus muscles. The medial rectus insertions of twin 1 were entirely normal. The authors believed that the abnormal medial rectus insertions in one twin were responsible for the difference in pattern of strabismus.

Bucci and associates³¹ reported on two sets of monozygotic twins, each raised in the same environment. One twin of each set had accommodative esotropia. Both esotropic twins manifested greater hyperopia on cycloplegic refractions. Spectacle correction was not given to the orthophoric twins, and neither manifested a subsequent strabismus. The authors concluded, “The greater hyperopia in the strabismic twins advanced these predisposed children beyond the threshold required to express this particular defect.”

Some twin studies have suggested that refractive errors are inherited.³⁵ A multigenic mode of inheritance has been demonstrated in the refractive range between + 6 and -4 diopters,¹¹ but the extremes of the refractive curve show a strong monogenic influence.³⁶ Keiner,³⁷ who believed that a patient's refraction was determined by the presence and degree of amblyopia, proposed a contrasting theory. He based this theory on the observation that amblyopia has been noted to prevent emmetropization of the affected eye, and pointed to the large percentage of hypermetropes found among strabismic patients as proof of this relationship.

Francois,⁴ on the other hand, observed that not all family members with strabismus demonstrate amblyopia. Only 67% to 73% of strabismic patients have some degree of amblyopia. Francois went on to find similar percentages of amblyopia in monozygotic twins (73.6%), dizygotic twins (63.1%), and siblings of strabismic persons (68.3% to 71.3%). Using the same data, he concluded that impairment of binocular vision, anomalous retinal correspondence, and false projection are secondary, nonhereditary characteristics. Finally, no genetic difference was observed between alternating and monocular strabismus.

Using quantitative genetic methods, Spivey³⁸ identified spherical refractive error as a key variable in strabismus. He further noted vergence ability to be a primary biologic difference between strabismic and nonstrabismic families with refractive errors. Spivey identified three conditions highly correlated with esotropia in offspring or younger siblings: (1) when a parent has esotropia, (2) when the parents are normal, but there is a family history of esotropia, and (3) when the parents are normal but there is, between them, very low vergence ability and a significant hyperopia. The accommodative convergence to accommodation (AC/A) ratio was also noted to be a variable in this process. Its exact role, however, was not delineated.

The role of the AC/A ratio in the inheritance of esotropia was explored by Maumenee and colleagues.³⁹ They found that members of a family with an esotropic propositus have a higher ratio than a random population. This suggests that the AC/A ratio may be a factor in the inheritance of esotropia. Hofstetter⁴⁰ found a high correlation of AC/A ratio in monozygotic twins, suggesting a genetic basis.

Chew and coworkers⁴¹ followed a large cohort of children from gestation to age 7 years. They identified several risk factors for esotropia and exotropia that confound the hereditary study of strabismus. Maternal cigarette smoking and low birth weight were both found to be important and independent risk factors for both forms of horizontal strabismus. The strabismus risk was noted to increase with increasing number of cigarettes smoked by the mother. Other risk factors for both forms of horizontal strabismus were nonspecific uterine bleeding during the third trimester, gestational age, and duration of the second stage of labor. Race, mother's age, and number of prior pregnancies were significant risk factors for esotropia. The 1-minute Apgar score was noted to be a significant factor for exotropia.

Podgor and associates⁴² subsequently estimated familial aggregation of horizontal strabismus in this population, adjusting for the previously noted environmental risk factors. They found the risk of esotropia in a child doubled if a sibling had esotropia. The association was found to be even stronger in multiple births, especially monozygotic twins. In exotropia, a strong association was found in multiple-birth siblings. No significant associations were noted between siblings from separate single births.

Several studies have examined the risk factors and hereditary patterns of monofixation syndrome. Lang⁴⁴ examined the fixation patterns of 22 patients with monofixation syndrome from 10 families. He concluded that the hereditary factor involved in these cases was independent of anisometropia, heterophoria, or fixation patterns. He went on to state that anomalous retinal correspondence in microtropia is an inherited primary congenital defect rather than an acquired anomaly.

Cantolino and von Noorden⁴⁵ studied family members of 20 patients with monofixation syndrome, comparing them with 16 control patients and their families. Fifty-four percent of the family members of monofixators manifested sensory and motor anomalies that included heterotropias, diminished fusional amplitudes, eccentric fixation, anomalous retinal correspondence, and deficient stereopsis. Four siblings were found to have “decompensated microtropia”—that is, an accommodative or nonaccommodative esotropia that, after appropriate treatment, reverted to a pre-existing microtropia. Only 26% of the control families were found to have any sensory or motor anomalies. The families with microtropic probands also demonstrated more anisometropia than control families. These authors concluded that the hereditary pattern of microtropia was most consistent with multifactorial inheritance, modified by several risk factors.

FAMILIAL BLEPHAROPHIMOSIS

Familial blepharophimosis is a diverse group of congenital disorders with varying degrees of ptosis, epicanthus inversus, and shortened palpebral fissures accompanied by temporal displacement of the lower puncta. The most common form is the blepharophimosis, ptosis, epicanthus inversus, telecanthus syndrome that is dominantly inherited.⁴⁶ Several authors have noted that the condition appears to be transmitted more frequently through males than females. This finding may be related to the association of primary amenorrhea in some affected females.^47,48

Individuals with blepharophimosis syndromes often demonstrate a pseudoesotropic appearance. At times, there is an associated manifest deviation. Frydman and associates⁴⁹ described a recessively inherited variant of familial blepharophimosis associated with a V-esotropia and upper gaze paralysis in several patients.

BROWN SYNDROME

Brown syndrome is characterized by an inability to elevate the affected eye in adduction. The syndrome has been subdivided into true and simulated varieties. The true syndrome usually occurs in childhood and is thought to arise from a congenital abnormality of the anterior tendon of the superior oblique muscle. The simulated syndrome includes cases that are acquired. The acquired syndrome may be intermittent and has been observed to undergo spontaneous resolution in some instances.⁵⁰

The congenital form of Brown syndrome is usually sporadic, but familial forms have been described.^50–55 In two reports,^52,53 the involved individuals were monozygotic twins, whereas in all reports, the number of affected individuals was too small to comment on the mode of inheritance.

DUANE'S RETRACTION SYNDROME

Duane's retraction syndrome is a neurogenic brain stem ocular motor dysfunction.^56,57 It is characterized by narrowing of the palpebral fissure and retraction of the globe on attempted adduction of the eye. Adduction, abduction, or both may be limited. Frequently, the involved eye shows an upshoot or downshoot on adduction. Although birth trauma has been proposed as a likely cause,⁵⁸ no consistent anatomic or neurologic abnormality has been found to account for all cases.⁵⁹ Smith and Cibis have reported monozygous twins with discordant Duane's syndrome thought to be secondary to twin-to-twin transfusion.⁶⁰

Several unusual aspects of this condition have been identified. Bilateral involvement occurs in only 15% to 20% of cases. In unilateral cases, the left eye is involved 57.5% of the time. Gender distribution is unequal—57% of the cases involve females. Thirty percent to 50% of patients have associated congenital defects involving ocular, skeletal, and neural structures.⁵⁹

Most cases of Duane's syndrome are sporadic, but familial cases that are unilateral or, more commonly, bilateral have been reported,^34,61 constituting approximately 10% of all cases.⁶² When Duane's syndrome is noted to be familial, reported series show an autosomal dominant pattern of inheritance.^63,64 The association of Duane's syndrome, deafness, and the Klippel-Feil syndrome has been reported to be inherited in an autosomal dominant manner.⁶² Sevel and Kassar⁶⁵ have reported on the occurrence of bilateral Duane's syndrome in three successive generations of one family. Mehrdorn and Kommerell⁶⁶ have reported monozygous twins with a mirror-like localization of their Duane's Syndrome, born to an affected mother. Cytogenetic studies have yielded varying chromosomal abnormalities.^67,68

EXTERNAL OPHTHALMOPLEGIA

External ophthalmoplegia is a family of conditions characterized by varying degrees of bilateral ptosis and partial to complete external ophthalmoplegia. Congenital external ophthalmoplegia has been noted to have an autosomal dominant mode of transmission with variability within affected families as to onset, severity, and associated anomalies. However, some pedigrees have suggested a recessive mode of transmission.⁶⁹ Consanguinity of parents has sometimes been noted.⁴ The latedeveloping form of external ophthalmoplegia is transmitted as a simple dominant gene.³

Chronic progressive external ophthalmoplegia is one of the components of the Kearns-Sayre syndrome. Both autosomal dominant and autosomal recessive pedigrees have been noted in familial cases of the Kearns-Sayre syndrome. However, most cases appear to occur sporadically.⁷⁰

Sporadic progressive external ophthalmoplegia and Kearns-Sayre syndrome has been associated with single large-scale mitochondrial DNA deletions in muscle. Progressive external ophthalmoplegia with an autosomal dominant inheritance pattern has been reported with multiple mitochondrial DNA deletions.⁷¹

The range of muscle involvement in oculopharyngeal muscular dystrophy can include ophthalmoplegia. Autosomal dominant transmission of this condition has been suggested by most, but not all, pedigree studies.⁷² Many patients with this condition are of French descent, living in Canada or New England.⁷³ The disorder has been linked to chromosome 14q11.2-q13 in French-Canadian pedigrees. Linkage has also been established in several non-French-Canadian pedigrees.⁷⁴ However, a different chromosomal haplotype was identified in other non-French-Canadian families.^75,76 This may represent a second, possibly independent mutation.

CONGENITAL FIBROSIS SYNDROMES

The term congenital fibrosis comprises a group of disorders characterized by the variable replacement of normal contractile muscle tissue by fibrous tissue or bands. The number of muscles affected and the degree of fibrosis⁷⁷ determine its clinical presentation. The cause of the syndrome is unknown. It can be monocular or binocular, and marked familial variation has been noted.¹¹ Hereditary tendencies have been noted in some pedigrees.⁷⁷ Many cases of generalized fibrosis are transmitted in an autosomal dominant manner and are found in a number of succeeding generations.⁷⁸ However, autosomal recessive transmission cannot be ruled out when only siblings manifest the disease.⁷⁹ Congenital fibrosis of the inferior rectus is seldom noted to be familial, but autosomal dominant patterns have been noted in familial cases.⁷⁹ Cibis⁸⁰ reported two families with dominantly inherited congenital familial fibrosis in which there was evidence of a nuclear or supranuclear etiology. In these families, it was thought that the fibrosis may be secondary to an inherited neurogenic lesion. Engle and colleagues⁸¹ performed linkage studies of seven unrelated families with completely penetrant autosomal dominant congenital fibrosis. In each of these families, the disease gene linked to the pericentromeric region of chromosome 12. They subsequently described intracranial, orbital, and muscle pathology in three affected family members of this population.⁸² There was absence of the superior division of the oculomotor nerve and its corresponding alpha motor neurons. The external levator and superior rectus muscles were found to be abnormal. Also, increased numbers of internal nuclei and central mitochondrial clumping were found in other extraocular muscles not innervated by the superior division of the third cranial nerve. This suggests a dominantly inherited abnormality in the development of the extraocular muscle lower motor neuron.

MOEBIUS SYNDROME

Moebius syndrome is characterized by congenital facial nerve palsy associated with paralysis of other cranial motor nerves, most commonly a bilateral abducens nerve palsy. The facial nerve palsy is usually bilateral and incomplete, involving the lower part of the face.⁸³ Limb and orofacial anomalies have been reported in 50% of cases.⁸⁴ The orofacial anomalies include micrognathia, cleft palate, tongue anomalies, ear malformations, and bifid uvula.^84,85 A wide spectrum of limb anomalies has also been noted. Syndactyly, club feet, toe deformities, and symbrachydactyly are seen, in addition to the more severe absence of digits or amputation defects of the limbs.^84–86

Most reported cases of Moebius syndrome are sporadic, with both genders equally affected.⁸⁵ However, familial cases have been reported with autosomal dominant, autosomal recessive, and multifactorial modes of transmission.^87–90 Autosomal dominant inheritance has been most frequently observed.^91–96 Autosomal and X-linked recessive patterns have been observed in affected siblings or cousins with parental consanguinity in some cases.^97–100

Henderson⁸³ noted that familial cases were usually of the pure facial diplegia type. However, Moebius-Poland syndrome has been noted in a male whose mother had Poland syndrome.¹⁰¹ The association of Moebius syndrome with arthrogryposis multiplex congenita has been reported in a patient whose sibling had only arthrogryposis.¹⁰² Moebius syndrome with arthrogryposis multiplex congenita has also been reported in male twins thought to be monozygotic.¹⁰³

Reported cases of Moebius syndrome associated with digital anomalies appear to be sporadic without etiologic evidence of genetic or environmental factors.^{83,100,104–107} However, other cases have been reported with evidence for teratogenicity or environmental influence.^108–112 Bersu and coworkers¹¹³ have postulated that the associated oral and limb anomalies result from disruptions in development occurring at about the fourth week of gestation. Miller and colleagues,¹¹⁴ as well as other investigators,^85,86 have proposed a formal genesis syndrome wherein a similar mechanism produces diverse but related anomalies. This is supported by Lipson and associates,¹¹⁵ who reported significant events of pregnancy in 8 of 15 cases of Moebius syndrome, including hyperthermia, previous uterine surgery, electric shock, failed abortion, prolonged rupture of the membranes, and alcohol abuse. The association with hyperthermia was confirmed in a second study.¹¹⁶ Bouwes-Bavinck and Weaver¹¹⁷ proposed an interruption of embryonic blood supply as the etiology of Moebius syndrome, with the extent and nature of associated defects dependent on the location of vascular insufficiency.

Several cytogenetic studies have identified an abnormality of chromosome 13 in inherited Moebius syndrome.¹¹⁸ A balanced translocation between chromosomes 1 and 13 was observed in conjunction with a variant of the Moebius-Poland syndrome in seven members of one family over three generations.¹¹⁹ This variant did not include abducens palsy. A balanced translocation between chromosome 1 and 13 was also noted by Donahue and associates¹²⁰ in a child with Moebius-Poland syndrome and cardiac and central nervous system anomalies. The translocation was also found to be present in the phenotypically normal father and brother.

CONGENITAL SUPERIOR OBLIQUE PALSY

Congenital superior oblique palsy is characterized by variable hypertropia, head tilt in the direction opposite the vertical deviation, and overaction of the antagonist inferior oblique muscle. It can be distinguished from the acquired form by larger-than-normal vertical fusional amplitudes and contralateral facial hypoplasia. This fourth cranial nerve palsy is usually sporadic, but familial forms have been described with autosomal dominant transmission.^121,122

TABLE ONE. Single Gene Disorders Frequently Associated with Strabismus

TABLE TWO. Inherited Ataxias Associated With Strabismus

  Friedrich's spinal ataxia
  Marie's cerebellar ataxia
  Krabbe's syndrome
  Louis-Bar syndrome
  Marinesco-Sjögren's syndrome

AARSKOG SYNDROME

Aarskog (facial-digital-genital) syndrome is an X-linked disorder characterized by short stature, digital anomalies, shawl scrotum, hypertelorism, and blepharoptosis. Brodsky and coworkers¹²⁴ reported three family members with Aarskog syndrome and a V-pattern esotropia, accompanied by latent nystagmus, inferior oblique overaction, and amblyopia.

CRANIOFACIAL DISORDERS

The craniofacial synostoses are a family of disorders characterized by premature closure of one or more bony sutures in the skull. Crouzon disease (dysostosis craniofacialis) and Apert syndrome (acrocephalosyndactyly) are two such syndromes with an autosomal dominant mode of inheritance. Affected individuals present with shallow orbits due to anterior displacement of the sphenoid bone and the orbital process of the frontal bone. The orbital axes are subsequently divergent. The most characteristic finding on motility examination is a V-pattern exotropia or esotropia with markedly overacting inferior oblique muscles.¹²⁵

Waardenburg syndrome, which is also transmitted as an autosomal dominant condition, is associated with both esotropia and pseudoesotropia. The characteristic lid configuration of the syndrome is noted to impart a pseudoesotropic appearance. However, 20% of patients with this condition also manifest a true esotropia.¹²⁶

CHROMOSOMAL ANOMALIES

Strabismus is commonly found in several chromosomal anomalies. Esotropia is the most common form of strabismus seen in individuals with chromosomal disease, occurring in 59% of patients with 49 XXXXY anomaly and in 33% of individuals with trisomy 21.¹²⁷ Exotropia is commonly seen in trisomy 21, Wolf syndrome (4p-), and cri-du-chat syndrome (5p-).⁵² Howard¹²⁷ has detailed the presence and type of strabismus seen in an array of chromosomal aberrations (Table 3).

TABLE THREE. Chromosomal Aberrations Associated With Strabismus

The practical significance of the study of inheritance patterns in strabismus lies in the ability to predict its occurrence and plan intervention. Identification of risk factors in a multifactorial model of inheritance aids in early detection and, therefore, treatment of strabismus. Toward this goal, further studies in the heredity of strabismus are warranted.

1. Hippocrates: Airs, waters and places. In: The Genuine Works of Hippocrates, translated by Francis Adams, p 171. New York, William Wood and Company, 1886

2. Bohm L: Das Schielen und der Schnenschnitt in seinen Wirkingen auf Stelling und Sehrkraft der Augen. Berlin, 1845

3. Mash AJ, Grutzner P, Hegmann JP, Spivey BE: Strabismus. In: Goldberg MF (ed): Genetic and Metabolic Eye Disease. Boston, Little, Brown and Co., 1974

4. Francois J: Heredity in Ophthalmology, pp 225–269. St Louis, CV Mosby, 1961

5. Waardenburg PJ: Squint and heredity. Doc Ophth 7–8:422, 1954

6. Schlossman A: Heredity in ophthalmology. Eye Ear Nose Throat Mon 36:237, 1957

7. Czellitzer A: Wie verebt sich schielen? Arch Rassen Gesellsch Biol 14:377, 1923

8. Dahlberg G, Nordlow W: Genetics of convergent strabismus. Acta Genet Statist Med 2:1, 1951

9. Richter S: Zur Hereditat des strabismus concomitans. Humangenetik 3:235, 1947

10. Cross HE: The Heredity of strabismus. American Orthoptic J 25:11, 1975

11. Miller MT, Folk ER: Strabismus. In: Renie WA (ed): Goldberg's Genetic and Metabolic Eye Disease, 2nd ed. Boston/Toronto, Little, Brown & Co, 1986

12. Curnow RN, Smith C: Multifactorial models for familial diseases in man. J Roy Stat Soc A 2:131, 1975

13. Duke-Elder S, Wybar K. Ocular motility and strabismus. In: Duke-Elder S (ed): System of Ophthalmology. St. Louis, CV Mosby, 1973

14. Hohm S: Le strabisme concomitant chez les palenegrides au Gabon, Afrique Equatoriale Française. Acta Ophthalmol (Kbh) 17:367, 1939

15. Eustace P: Myopia and divergent squint in West Indian children. Br J Ophthalmol 56:559, 1972

16. Ing MR, Pang SW: The racial distribution of strabismus. In: Reinecke RD (ed): Strabismus: Proceedings of the 3rd Meeting of International Strabismological Association, May 1978, Japan. New York, Grune and Stratton, 1978

17. Schlossman A, Priestley BS: Role of heredity in etiology and treatment of strabismus. Arch Ophthalmol 47:1, 1952

18. Worth CA: Squint: Its Causes, Pathology and Treatment (5th ed), p 61. London, Bailliere, Tindall & Cox, 1923

19. Scobee RG: Esotropia. Am J Ophthalmol 34:817, 1951

20. Pratt-Johnson JA, Lunn CT: Early case findings and the hereditary factor in strabismus. Can J Ophthalmol 2:50, 1967

21. Dufier JL, Briard ML, Bonaiti C, Frezal J, Saraux H: Inheritance in the etiology of convergent squint. Ophthalmologica 179:225, 1979

22. Cohn H: Uber Vererbrung und behandlung des einwarts-schielen. Berl Klin Wochenschr 41:1047, 1904

23. Kornder LD, Nursey JN, Pratt-Johnson A, Beattie A: Detection of manifest strabismus in young children. Am J Ophthalmol 77:211, 1974

24. Chimonidou E, Palimeris G, Koliopoulas J, Velissaropoulous P: Family distribution of concomitant squint in Greece. Br J Ophthalmol 61:27, 1977

25. Rubin W, Helm C, McCormack MK: Ocular motor anomalies in monozygotic and dizygotic twins. In: Reinecke R (ed): Strabismus: Proceedings of the 3rd Meeting of the International Strabismological Association, Asilomar, California, p 89. New York, Grune and Stratton, 1978

26. Richter S: Untersuchungen Uber die Hereditat des Strabismus Concomitans (Abhandlungen ans dem Gebiete der Augen heilkunde, Sammliung von monographien. Band 35). Leipzig, Georg Thieme, 1967. Quoted by Mash AJ, Grutzner P, Hegmann JP, et al: Strabismus. In Goldberg MF (ed): Genetic and Metabolic Eye Disease, p 261. Boston, Little, Brown and Co, 1974

27. Weekers R, Moureau P, Hacourt J, Andre A: Contribution a l'etude du strabisme concomitant et de l'amblyopie par l'etude de jumeaux uni- et bivitellins. Ophthalmologica (Basel) 132:209, 1956

28. DeVries B, Houtman WA: Squint in monozygotic twins. Doc Ophth 6:305, 1979.

29. Kvapilikova K: Heredity of the fusion capacity. Cesk Oftalmol 25:332, 1969

30. Shippman S, Schudel S, Millman A, Weseley AC: Unusual ocular findings in identical twins. J Pediatr Ophth Strab 25:298, 1998

31. Bucci FA, Catalano RA, Simon JW: Discordance of accommodative esotropia in monozygotic twins. Am J Ophthalmol 107:84, 1989

32. Crone RA, Velzeboer CM: Statistics in strabismus in the Amsterdam youth. Arch Ophthalmol 55:455, 1956

33. Massin M, Drouard E: Contribution a l'etude des strabismes concomitants familiaux. Ann Ocul (Paris) 198:323, 1965

34. Waardenburg PJ, Franceschetti A, Klein D: Genetics and Ophthalmology, vol. 1, p 423. Oxford, Blackwell Scientific Publications, 1961

35. Law FW: The refractive error of twins. Br J Ophthalmol 19:99, 1935

36. Sorsby A: Biology of the eye as an optical system. In Duane T (ed): Clinical Ophthalmology. Hagerstown, Maryland, Harper & Row, 1979

37. Keiner GBJ: Physiology and pathology of the optomotor reflexes. Am J Ophthalmol 42:233, 1956

38. Spivey BE: Strabismus: Factors in anticipating its occurrence. Australian J Ophthalmol 8:5, 1980

39. Maumenee IH, Alston A, Mets MB, Flynn JT, Mitchell TN, Beaty TH: Inheritance of congenital esotropia. Transactions of the Am Ophth Soc 84:85, 1986

40. Hofstetter HW: Accommodative convergence in identical twins. Am J Optometry 25:480, 1948

41. Chew E, Remaley NA, Tamboli A, Zhao J, Podgor M, Klebanoff M: Risk factors for esotropia and exotropia. Arch Ophthalmol 112:1349, 1994

42. Podgor M, Remaley NA, Chew E: Associations between siblings for esotropia and exotropia. Arch Ophthalmol 114:739, 1996

43. Parks MM: Monofixation syndrome. In: Duane TD (ed): Clinical Ophthalmology. Philadelphia: Harper & Row, 1981

44. Lang J: Microtropia. Arch Ophthalmol 81:758, 1969

45. Cantolino SJ, von Noorden GK: Heredity in microtropia. Arch Ophthalmol 81:753, 1969

46. Kohn R, Romano PE: Blepharoptosis, blepharophimosis, epicanthus inversus and telecanthus: A syndrome with no name. Am J Ophthalmol 72:625, 1971

47. Townes PL, Menchler EL: Blepharophimosis, ptosis, epicanthus inversus and primary amenorrhea. Arch Ophthalmol 97:1664, 1979

48. Zlotogora J, Sagi M, Cohen T: The blepharophimosis, ptosis and epicanthus inversus syndrome: Delineation of two types. Am J Hum Genet 35:1020, 1983

49. Frydman M, Cohen HA, Karmon G, Savir H: Autosomal recessive blepharophimosis, ptosis, V-esotropia, syndactyly and short stature. Clin Genet 41(2):57, 1992

50. Moore AT, Walker J, Taylor D: Familial Brown's syndrome. J Pediatr Ophthalmol Strabismus 25:202, 1988

51. Gowan M, Levy J: Heredity in the superior oblique tendon sheath syndrome. Br Orthoptic J 25:91, 1968

52. Katz NN, Whitmore PU, Beauchoup GR: Brown's syndrome in twins. J Pediatr Ophthalmol Strabismus 18:32, 1981

53. Finlay A, Powell S: Brown's syndrome in identical twins. Br Orthoptic J 39:73, 1982

54. Brown HW: True and simulated superior tendon sheath Syndromes. Doc Ophth 34:123, 1973

55. Hamed L: Bilateral Brown syndrome in three siblings. J Pediatr Ophthalmol Strabismus 28:306, 1991

56. Hotchkiss MG, Miller NR, Clark AW, Green WR: Bilateral Duane's retraction syndrome: A clinical-pathologic case report. Arch Ophthalmol 98:870, 1980

57. Miller NR, Kiel SM, Green WR, Clark AW: Unilateral Duane's retraction syndrome (type 1). Arch Ophthalmol 100:1468, 1982

58. Miller NR: Walsh & Hoyt's Clinical Neuro-Ophthalmology, p 691. Baltimore, Williams & Wilkins, 1985

59. Pfaffenback DD, Cross HE, Kearns TP: Congenital anomalies in Duane's retraction syndrome. Arch Ophthalmol 88:635, 1972

60. Smith DE, Cibis GW: Discordant Duane's retraction syndrome in monozygotic twins. Am J Ophthalmol 122:749, 1996

61. Merritt JC, Grimson BS, Timmons C, Pantell JP, Caldwell R: Head tilt test in Duane's syndrome. Ann Ophthalmol. 13:1019, 1981

62. Kirkham TH: Inheritance of Duane's syndrome. Br J Ophthalmol 54:323, 1970

63. Cooper H: A series of cases of congenital ophthalmoplegia externa (nuclear paralysis) in the same family. Br Med J 1:917, 1910

64. Laughlin RC: Hereditary paralysis of the abducens nerve. Am J Ophthalmol 20:396, 1937

65. Sevel D, Kassar BS: Bilateral Duane's syndrome: Occurrence in three successive generations. Arch Ophthalmol 91:492, 1974

66. Mehrdorn E, Kommerell G: Inherited Duane's syndrome: Mirror-like localization of oculomotor disturbances in monozygotic twins. J Pediatr Ophthalmol Strabismus 16:152, 1979

67. Chew CKS, Foster P, Hurst JA, Salmon JF: Duane's retraction syndrome associated with chromosome 4q27- segment deletion. Am J Ophthalmol 119:807, 1993

68. Tibiletti MG, Sala E, Colombo D, Arlati S, Varisco T, LaPlaca G: Chromosome 22 marker in a child with Duane syndrome and urogenital abnormalities. Ann Genet 39:168, 1996

69. Francois J: Congenital ophthalmoplegias. In: Brunette J-R, Barbeau A (eds): Progress in Neuro-Ophthalmology, vol. 2. Amsterdam, Excerpta Medica, 1969

70. Schnitzler ER, Robertson WC Jr: Familial Kearns-Sayre syndrome. Neurology (NY) 29:1172, 1979

71. Moslemi A, Melberg A, Holme E, Oldfors A: Clonal expansion of mitochondrial DNA with multiple deletions in autosomal dominant progressive external ophthalmoplegia. Ann Neurol 40:707, 1996

72. Lewis I: Oculopharyngeal muscular dystrophy. A family study. In Brunette J-R, Barbeau A (eds): Progress in Neuro-Ophthalmology, vol. 2. Amsterdam: Excerpta Medica, 1969

73. Victor M, Hayes R, Adams RD: Oculopharyngeal muscular dystrophy. N Engl J Med 267:1269, 1962

74. Grewal RP, Cantor R, Turner G, Greal RK, Detera-Wadleigh SD: Genetic mapping and haplotype analysis of oculopharyngeal muscular dystrophy. Neuroreport 9(6):961, 1998

75. Creel GB, Guiliani MJ, Lacomis D, Holbach SM: Oculopharyngeal muscular dystrophy: Non-French-Canadian pedigrees. Muscle Nerve 21:816, 1998

76. Stajich JM, Gilchrist JM, Lennon F et al: Confirmation of linkage of oculopharyngeal muscular dystrophy to chromosome 14q11.2-q13 in American families suggests the existence of a second causal mutation. Neuromuscular Disorders 7 (Suppl 1):S75, 1997

77. Harley RD, Rodrigues MM,, Crawford JS: Congenital fibrosis of the extraocular muscles. Trans Am Ophthalmol Soc 76:197, 1978

78. Gillies WE, Harris AJ, Brooks AMV, Rivers MR: Congenital fibrosis of the vertically acting extraocular muscles. Ophthalmology 102:607, 1995

79. Isenberg S: Genetic aspects of strabismus. In: Emery AH, Rimoin DL (eds): Principles and Practice of Medical Genetics, p 555. New York, Churchill Livingstone, 1983

80. Cibis GW: Electromyography in congenital familial ophthalmoplegia. In Reinecke R (ed): Strabismus II. Proceedings of the Fourth Meeting of the International Strabismological Association, Asilomar, California, 1982. New York, Grune & Stratton, 1984

81. Engle EC, Marondel I, Houtman WA et al: Congenital fibrosis of the extraocular muscles (autosomal dominant congenital external ophthalmoplegia): Genetic homogeneity, linkage refinement, and physical mapping on chromosome 12. Am J Hum Genet 57:1086, 1995

82. Engle EC, Goumnerov BC, McKeown CA et al: Oculomotor nerve and muscle abnormalities in congenital fibrosis of the extraocular muscles. Ann Neurol 41:314, 1997

83. Henderson JL: The congenital facial diplegia syndrome: Clinical features, pathology and etiology. Brain 62:381, 1939

84. Gorlin RG, Pindborg J, Cohen MD: Syndromes of the Head and Neck, 2d ed. New York, McGraw-Hill, 1976

85. Temtamy SA, McKusick VA: The genetics of hand malformations. Birth Defects 14:73, 1978

86. Herrmann J, Pallister PD, Gilbert EF, et al: Studies of malformation syndromes of man XXXXIB: Nosologic studies in the Hanhart and Moebius syndrome. Eur J Pediatr 122:19, 1976

87. Collins DL, Schimke RN: Moebius syndrome in a child and extremity defect in her father. Clin Genet 22:312, 1982

88. Legum C, Godel V, Nemet P: Heterogeneity and pleiotropism in the Moebius syndrome. Clin Genet 20:254, 1981

89. Wishnick MD, Nelson L, Huppert L et al: Moebius syndrome and limb abnormalities with dominant inheritance. Ophthalmic Paediatr Genet 2:77, 1983

90. Baraitser M: Genetics of Moebius syndrome. J Med Genet 13:415, 1977

91. Wilbrand H, Saenger A: Die Kongenitalen augenmuskel ahmungen (kernaplasie). In Neurologie des Auges, vol 8, p 179. Munchen, JF Bergmann, 1921

92. Fortanier AH, Speijer N: Eine Erblichkeitsforschung bei einer familie mit ange borenen beweglichkeitsstorungen der hinnerven. Genetica 17:471, 1935

93. Hicks AM: Congenital paralysis of lateral rotators of eyes with paralysis of muscles of face. Arch Ophthalmol 30:38, 1943

94. Van der Weil HJ: Hereditary congenital facial paralysis. Acta Genet (Basel) 7:348, 1957

95. Kruger KE, Freidrich D: Familiare kongenitale motilitatsstorungen der augen. Klin Monatsbl Augenheilkd 142:101, 1963

96. Becker-Christensen F, Lund HT: A family with Moebius syndrome. J Pediatr 84:115, 1974

97. Thomas HM: Congenital facial paralysis. J Nerv Ment Dis 25:571, 1898

98. Cadwalader WB: A clinical report of two cases of agenesis (congenital paralysis) of the cranial nerves. Am J Med Sci 163:744, 1922

99. Stark T: Uber Kongenitale und progressive ophthalmoplegien (unter beruckstigung des infantlen Moebiuschen kernschwund's). Zbl Ges Ophthal 43:148, 1940

100. Harrison M, Parker N: Congenital facial diplegia. Med J Aust 1:650, 1960

101. Rojas-Martinez A, Garcia-Cruz D, Rodriguez Garcia A, Sanchez-Corona J, Rivas F: Poland-Moebius syndrome in a boy and Poland syndrome in his mother. Clinical Genetics 40:225, 1991

102. Sprofkin BE, Hillman IW: Moebius syndrome: Congenital orofacial paralysis. Neurology 6:50, 1956

103. Hanissian AS et al: Moebius syndrome in twins. Am J Dis Child 120:472, 1970

104. Richards RN: The Moebius syndrome. J Bone Joint Surg 35A:437, 1953

105. Gorlin RJ: Some facial syndromes. In: Malformation Syndromes. BD:OAS, White Plains, The National Foundation, V(2):65, 1969

106. Walsh FB, Hoyt WF: Clinical Neuro-Ophthalmology, 3rd ed. Baltimore, Williams & Wilkins, 1969

107. Sugarman GI, Stark HH: Moebius syndrome with Poland's anomaly. J Med Genet 10:192, 1973

108. Merz M, Wojtowicz S: The Moebius syndrome. Am J Ophthalmol 63:837, 1967

109. Nevin NC, Burrows D, Allen G et al: Aglossia-adactylia syndrome. J Med Genet 12:89, 1975

110. Elsahy NI: Moebius syndrome associated with the mother taking thalidomide during gestation. Plast Reconst Surg 51:93, 1973

111. Graham JM, Edwards MJ, Lipson AH et al: Gestational hyperthermia as a cause for Moebius syndrome. Teratology 37:461, 1988

112. Lipson I, Webster WS, Brown-Woodman PDC: Animal model-human correlation for the Moebius syndrome. Teratology 37:474, 1988

113. Bersu ET, Pettersen JC, Charboneau WJ et al: Studies of malformation syndromes of man XXXXIA: Anatomical studies in the Hanhart syndrome. Eur J Pediatr 122:1, 1976

114. Miller MT, Ray V, Owens P, Chen F: Moebius and Moebius-like syndromes (TTV-OFM, OMLH). J Pediatr Ophthalmol Strab 26:176, 1989

115. Lipson AH, Webster WS, Brown-Woodman PDC, Osborn RA: Moebius syndrome: animal model-human correlations and evidence for a brain stem vascular etiology. Teratology 40:339, 1989

116. Graham JM, Edwards MS, Lipson AH, Webster WS: Gestational hyperthermia as a cause for Moebius syndrome. Clin Res 37:195A, 1989

117. Bouwes-Bavinck JN, Weaver DD: Subclavian artery disruption sequence: Hypothesis of a vascular etiology for Poland, Klippel-Feil and Moebius anomalies. Am J Med Genet 23:903, 1986

118. Sll JJ, Smart RD, Viljoen DL: Deletion of chromosome 13 in Moebius syndrome. J Med Genet 28:413, 1991

119. Ziter FA, Wiser WC, Robinson A: Three-generation pedigree of a Moebius syndrome variant with chromosome translocation. Arch Neurol 34:437, 1977

120. Donahue SP, Wenger SL, Steele MW, Gorin MB: Broad-spectrum Moebius syndrome associated with a 1;11 chromosome translocation. Ophthalmol Paediatr Genet 14:17, 1993

121. Astle WF, Rosenbaum AL: Familial congenital fourth cranial nerve palsy. Arch Ophthalmol 103:532, 1985

122. Harris DJ, Memmen JE, Katz NNK, Parks MM: Familial congenital superior oblique palsy. Ophthalmology 93:88, 1986

123. Paul TO, Hardage LK: The heritability of strabismus. Ophthalmic Genetics 15:1, 1994

124. Brodsky MC, Keppen LD, Rice CD, Ranells JD: Ocular and systemic findings in the Aarskog (facial-digital-genital) syndrome. Am J Ophthalmol 109:450, 1990

125. Miller MT,, Folk ER: Strabismus associated with craniofacial anomalies. Am Orthoptic J 25:27, 1975

126. Delleman JW, Hageman MJ: Ophthalmologic findings in 34 patients with Waardenburg's syndrome. J Pediatr Ophthalmol Strabismus 15:341, 1978

127. Howard R: Chromosomal disease and strabismus. Am Orthoptic J 27:138, 1977