

ORIGINAL ARTICLE 

Year : 2021  Volume
: 114
 Issue : 1  Page : 3033 

Prevalence of different types of intermittent exotropia among exotropic children at Ain Shams University ophthalmology outpatient clinic
Rehab A.M Sallam^{1}, Noha M Samak^{2}, Ahmed T Ismail^{2}, Mohammad A Rashad^{2}
^{1} Department of Medicine & Surgery, Faculty of Medicine, Ain Shams University, Cairo, Egypt ^{2} Department of Ophthalmology Department, Faculty of Medicine, Ain Shams University, Cairo, Egypt
Date of Submission  21Oct2020 
Date of Acceptance  18Nov2020 
Date of Web Publication  31Mar2021 
Correspondence Address: BSc Rehab A.M Sallam Department of Medicine & Surgery, Faculty of Medicine, Ain Shams University, Postal Code 13766 Egypt
Source of Support: None, Conflict of Interest: None  Check 
DOI: 10.4103/ejos.ejos_62_20
Introduction Intermittent exotropia ‘X(T)’ is a disorder of binocular eye movement control, in which one eye intermittently moves outward. It is the commonest type of exodeviation and is usually detected by the parents in early childhood. Patients with X(T) tend to manifest their deviation when they are tired, stressed, or have cold. Purpose To manifest the prevalence of different types of X(T) in exotropic children aged 6–12 years attending Ain Shams University ophthalmology outpatient clinic. Patients and methods This is a crosssectional study including 139 exotropic children aged 6–12 years attending Ain Shams University ophthalmology outpatient clinic. All patients underwent full medical and ophthalmic history. The ophthalmic examinations included visual acuity, ocular movements, and the angles of deviation. Further ocular examinations included slitlamp examination for anterior segment, fundus examination, and Worth 4dot test. Results A total of 139 exotropic children (6–12 years old) attending the ophthalmology outpatient clinic of Ain Shams University were included. We found that X(T) is the commonest type of exodeviation, in 86.3%. The ‘basic type’ was the commonest subtype of X(T), in 84.2%. The true divergence excess was the second most common type, in 7.5%, pseudodivergence was seen in 5.8%, and the convergence insufficiency was the least common type seen in 2.5%. Conclusions Intermittent exotropia was the commonest type of exodeviation among children aged 6–12 years. The basic type was the commonest subtype of X(T).
Keywords: basic, convergence insufficiency, divergence excess, intermittent exotropia, types of intermittent exotropia
How to cite this article: Sallam RA, Samak NM, Ismail AT, Rashad MA. Prevalence of different types of intermittent exotropia among exotropic children at Ain Shams University ophthalmology outpatient clinic. J Egypt Ophthalmol Soc 2021;114:303 
How to cite this URL: Sallam RA, Samak NM, Ismail AT, Rashad MA. Prevalence of different types of intermittent exotropia among exotropic children at Ain Shams University ophthalmology outpatient clinic. J Egypt Ophthalmol Soc [serial online] 2021 [cited 2021 Jul 30];114:303. Available from: http://www.jeos.eg.net/text.asp?2021/114/1/30/313081 
Introduction   
Intermittent exotropia ‘X(T)’ is the main childhood divergent strabismus, accounting for 47.7% of exotropia among different populations in childhood [1].
Intermittent exotropia is a disorder of binocular eye movement control, where one eye intermittently turns outward. The outward deviation is greatest and likely occurs at far distances viewing, when the oculomotor convergence effort is weakest, and occurs frequently when the patient is under stress, tired, ill, or in particular test situations. X(T) can also occur at near as convergence insufficiency [2].
The signs of X(T) include asthenopia, blurred vision, visual fatigue, photophobia, and rarely, diplopia in older children and adults. Through the phoric time of X(T), the two eyes are completely aligned and also the patient has bifoveal fusion and perfect stereoacuity. This perfect bifoveal fusion develops because of the well alignment of both eyes at early time of infancy, when the cortical connections of binocular vision are being created. A lesser section of the patients are basic monofixators and do not get normal bifoveal fusion with good stereopsis. Rarely a patient will develop significant amblyopia. The poor fusion in these patients is associated with a majority of the tropic phase. During the tropic phase of X(T), patients exhibit large hemiretinal or regional inhibition of the temporal retina. Abnormal retinal correspondence in the tropic phase and normal retinal correspondence in the phoric time have been demonstrated in some patients with X(T) [2].
Progression of X(T) can be explained as a more increase in the incidence or the amount of the exotropia (at near or far), with intensifying suppression and deficit of stereopsis. This progression relies upon the patient’s fusional reserve and is advanced by the progress of abnormal sensory patterns [3].
Intermittent exotropia has been divided into four groups according to the Burian classification system. This classification is established on the concept of fusional convergence, divergence, and the amounts of the distance and near outward deviations. Based on the examination after occlusion and with a +3.00 D add at near, X(T) is classified into basic X(T), which is defined as the deviation at distance within 10 prism diopters (PD) of the near deviation; divergence excess, which is defined as the distance deviation is 10 PD or more than the near deviation; simulated or pseudodivergence excess, which is termed as a higher deviation at distance than near deviation but the near deviation increases within 10 PD of the distance deviation after 30–60 min of monocular patching; true divergence excess, which is described as the divergence excess persists even after prolonged binocular dissociation by monocular patching; and convergence insufficiency, which is defined as the near deviation is 10 PD or more than the distance deviation [4].
Aim   
The aim was to assess the prevalence of X(T) and each type among exotropic children attending Ain Shams University ophthalmology outpatient clinic.
Patients and methods   
This is a crosssectional study aiming to estimate the prevalence of X(T) and its types among exotropic patients aged 6–12 years from the ophthalmology outpatient clinic of Ain Shams University Hospital. A total of 139 patients of both sexes who have true exodeviation were enrolled in the study. We excluded patients with previous strabismus surgery, ocular trauma, paralytic strabismus, reasons of sensory exotropia, and restrictive strabismus.
The patients were subjected to full medical and ophthalmic history with special attention to onset, duration, variability, and associations such as diplopia or anomalous head position, history of fever, ocular trauma, surgery, orthoptic treatment, and family history. Examination included external appearance, globes (e.g. proptosis), lids (e.g. ptosis), slitlamp examination for anterior segment, and fundus examination using indirect ophthalmoscope with the pupil dilated at the completion of examination.
Visual acuity (VA) of all children was measured, with and without spectacles by Landlot C chart at a distance of 6 m. VA data were recorded in a LogMAR format, and refraction was subsequently performed to obtain the bestcorrected VA.
Ocular movements were examined at nine positions of gaze. Duction and version movements were examined.
The angles of deviations were measured for children with refractive correction (if present), and ocular alignment was assessed using the cover–uncover test and the alternate cover test with fixation targets at both 33 cm and 6 m. Angles of aberration were assessed with prism alternate cover test including distance with correction and near with correction. If angle of X(T) was more at distant than near (divergence excess), one eye was covered with thick gauze to break tenacious proximal fusion between the two eyes for at least 30 min, and then prism alternate cover test was done to value the degree of X(T) at near.
Further ocular examinations included Worth 4dot test for testing sensory status.
The refractive status of children was evaluated with and without cycloplegic eye drops. For cycloplegic refraction, one drop of topical 1.0% cyclopentolate was administered to each eye twice with a 5min interval. Cycloplegic refraction was performed using autorefractometer 30–45 min following the second drop.
Results   
Our study included 139 exotropic children from Ain Shams University ophthalmology outpatient clinic. Their age ranged from 6 to 12 years old. The mean age of studied patients was 8.4 years, and the median age for patients of exotropia was 8 years with SD 2.3. The sample included 44 (31.7%) male and 95 (68.3%) female patients.
Prevalence of X(T) was 120 (86.3%) of all exotropic patients, where the ‘basic type’ was the commonest subtype of X(T) (84.2%), the ‘true divergence excess’ was the second most common subtype in nine (7.5%), seven (5.8%) children had ‘pseudodivergence excess’, and the ‘convergence insufficiency’ was the rarest type in three (2.5%), as shown in [Table 1].
Patients with X(T) comprised 40 (33.3%) males and 80 (66.7%)females. A higher percentage of basic subtype was found among females (86.3%) compared with among males (80%), and a higher percentage of true divergence excess was found among males (15%) compared with among females (3.8%). Convergence insufficiency was equal in both sexes (2.5%), and a higher percentage of pseudodivergence excess was found among females (7.5%) compared with among males (2.5%).
Distribution of family history among X(T) was positive in 18 (15%) patients and negative in 102 (85%) patients.
The angles of deviation for distance with glasses (distance with correction) were assessed in 72 patients. The minimum angle was 10 PD, the maximum angle was 60 PD, and the mean angle was 26.52 PD. The angles for near with glasses (near with correction) were measured in 72 patients. The minimum angle was 10 PD, the maximum angle was 65 PD, and the mean was 25.62 PD.
The angles of deviation for distance without glasses were assessed in 67 patients. The minimum angle was 6 PD, the maximum angle was 60 PD, and the mean was 26.50 PD. The angles for near without glasses were measured in 67 patients. The minimum angle was 10 PD, the maximum angle was 55 PD, and the mean was 25.62 PD, as shown in [Table 2].
Regarding the mean of unaided VA between the types of X(T), the lower mean (better vision) of right eye VA was found among the patients with pseudodivergence excess (0.17 logMAR) compared with other groups. A lower mean of left eye VA was found among the patients with true and pseudodivergence excess (0.25 logMAR) compared with other groups. A lower mean of bestcorrected VA in right eye (0.10 logMAR) and left eye (0.06 logMAR) was found among patients with true divergence excess compared with other groups.
The result of Worth 4dot test between X(T) patients, for testing the sensory status showed 137 patients had fusion with a percentage of 98.3% and two patients had suppression of one eye, with a percentage of 1.6%.
Discussion   
Our study included randomly 139 exotropic children from Ain Shams University ophthalmology outpatient clinic. Regarding the sex, patients who have X(T) comprised 40 (33.3%) males and 80 (66.7%) females. This was similar to the study of all Olmsted County about the differences of sex among children diagnosed with X(T). A total of 184 children with X(T), were diagnosed in Olmsted County Minnesota during the 20year review time. Overall, 118 (64.1%) of the 184 patients were females and 66 (35.9%) patients were males. Intermittent exotropia was almost double in girls compared with boys in this definite population [5].
According to prevalence of types of X(T), basic type was the commonest type, in 101 (84.2%); true divergence excess was the second type, in nine (7.5%); pseudodivergence excess was seen in seven (5.8%), and convergence insufficiency was seen in three (2.5%). This was similar to a China study to define the prevalence and subtypes for X(T) in preschool children aged 36 years in eastern China among 166 children with X(T); the ‘basic type’ was the commonest type of X(T) (74.7%), the ‘divergence excess’ was the second (19.9%), whereas the ‘convergence weakness’ was the rarest (5.4%) [6].
In a previous study in Singapore [7], it was reported that ‘divergence excess’ was the major subtype, which accounted for 59.5% of all X(T). However, in that study, children were not routinely patched to eliminate tenacious proximal fusion. Thus, we believed that some ‘basic type’ might had been misclassified as ‘divergent excess’ in the Singapore study. The ‘convergence insufficiency’ type was discovered to be least common in both studies.
Mohney and Huffaker determined the relative proportion of the several kinds of childhood exotropia. They studied 235 consecutive children for exotropia, in a mainly rural Appalachian region, without past surgical treatment. The kinds of exotropia analyzed among the 235 children and records were as follows: X(T) was 112 (47.7%); convergence insufficiency was 27 (11.5%); sensory exotropia was 24 (10.2%); paralytic exotropia was five (2.1%); congenital exotropia was four (1.7%); neonatal exotropia, which resolved after 4 months of age, was three (1.3%); and the remaining 10 (4.3%) had other forms of exodeviation. X(T) was the commonest kind of divergent strabismus in this study population [1]. This is similar to our study, where X(T) was the commonest kind of exotropia (86.3%) of all studied children. However, that study included sensory and paralytic exotropia, which were excepted also in our study.
A retrospective analysis was guided using the medical files of patients who visited the Department of Ophthalmology outpatient clinic in Korea during the period ranging from January 2004 to December 2007. The study included 360 pediatric patients ranging in age from 1 to 14 years, all of whom were recognized with X(T). The basic subtype was the commonest form of X(T) seen in 310 (86.2%) children [8], which was similar to our study. In our study, the basic type was the commonest form of X(T), seen in 101 (84.2%) children.
Conclusion   
We found that X(T) is the commonest type of exodeviation, and the basic subtype was the commonest subtype of X(T).
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References   
1.  Mohney BG, Huffaker RK. Common forms of childhood exotropia. Ophthalmology 2003; 110:2093–2096. 
2.  Von Noorden GK, Campus EC. Principles of surgical treatment. In: Lampert R, Cox K, Burke D, (editors). Binocular vision and ocular motility: theory and management of strabismus. 6th ed. St. Louis, MO, USA: Mosby Inc.; 2002; 356–365. 
3.  Zhang KK, Koklanis K, Georgievski Z. Intermittent exotropia: A review of the natural history and nonsurgical treatment outcomes. Aust Orthopt J 2007; 39:31. 
4.  Burian HM, Franceschett AT. Evaluation of diagnostic methods for the classification of exodeviation. Am J Ophthalmol 1971; 71:34–41. 
5.  Nusz KJ, Mohney BG, Diehl NN. Female predominance in intermittent exotropia. Am J Ophthalmol 2005; 140:546–547. 
6.  Pan CW, Zhu H, Yu JJ, Ding H, Bai J, Chen J et al. Epidemiology of intermittent exotropia in preschool children in China. Optom Vis Sci 2016; 93:57–62. 
7.  Chia A, Dirani M, Chan YH, Gazzard G, Au Eong KG, Selvaraj P et al. Prevalence of amblyopia and strabismus in young Singaporean Chinese children. Invest Ophthalmol Vis Sci 2010; 51:3411–3417. 
8.  Jung JW, Lee SY. Comparison of the clinical characteristics of intermittent exotropia in children and adults. Korean J Opthalmol 2010; 24:96–100. 
[Table 1], [Table 2]
