Table of Contents  
Year : 2019  |  Volume : 27  |  Issue : 2  |  Page : 62-67

Common Forms of Strabismus in a Tertiary Eye Clinic in Southwest Nigeria

1 Department of Ophthalmology, College of Medicine, University of Ibadan; Department of Ophthalmology, University College Hospital, Ibadan, Nigeria
2 Department of Epidemiology and Medical Statistics, College of Medicine, University of Ibadan, Ibadan, Nigeria

Date of Submission20-Mar-2019
Date of Decision23-Jun-2019
Date of Acceptance23-Jul-2019
Date of Web Publication07-Feb-2020

Correspondence Address:
Mary O Ugalahi
Department of Ophthalmology, College of Medicine, University of Ibadan
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/njo.njo_8_19

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Background: Strabismus occurs worldwide and is associated with undesirable psychosocial impact and negative social prejudice. However, there is a dearth of information on the different subtypes of strabismus in Nigerian patients. The aim of this study is to describe the common forms of strabismus among patients of an eye clinic in southwestern Nigeria. Methods: We retrospectively reviewed all new patients seen at the eye clinic of our hospital between January 1999 and December 2008. Patients with a diagnosis of strabismus were identified from the clinic registers and their case records were reviewed. Information on age, gender, cause, and type of squint as well as associated ocular/systemic disease was retrieved from the case records. Results: A total of 240 patients had strabismus, giving a relative frequency of 1.2%. Mean age of patients with strabismus was 19.8 (±19.7) years while male-to-female ratio was 0.98:1. Esotropia (53.8%) was more common than exotropia (44.2%). The mean age of patients with esotropia was 12.8 years compared with 27.9 years for patients with exotropia (P < 0.001). Sensory strabismus was the most common form of strabismus, occurring in 83 (34.5%) patients, while accommodative esotropia was found in only 5.8% patients. Conclusion: The relative frequency of strabismus in this study is fairly similar to other reports from Nigeria and Africa. Secondary strabismus was quite common whereas primary forms of strabismus such as accommodative esotropia appear to occur less frequently among our patients in comparison to Caucasian populations.

Keywords: Esotropia, exotropia, Nigeria, prevalence, strabismus

How to cite this article:
Olusanya BA, Ugalahi MO, Ayeni O, Fawole OI, Baiyeroju AM. Common Forms of Strabismus in a Tertiary Eye Clinic in Southwest Nigeria. Niger J Ophthalmol 2019;27:62-7

How to cite this URL:
Olusanya BA, Ugalahi MO, Ayeni O, Fawole OI, Baiyeroju AM. Common Forms of Strabismus in a Tertiary Eye Clinic in Southwest Nigeria. Niger J Ophthalmol [serial online] 2019 [cited 2022 Aug 8];27:62-7. Available from:

  Introduction Top

Strabismus is a misalignment of the visual axes resulting from deviation of the eyes. It has a worldwide occurrence and has been studied in various populations.[1],[2] It is a relatively common disorder among Caucasian children and afflicts about 3% to 4% of them.[3],[4] Studies in Asian populations reveal a lower prevalence between 0.7% and 1.9%.[5],[6],[7],[8] The prevalence in some African populations is reported to be 0.2% to 1.3%.[9],[10],[11] In Nigeria, prevalence of strabismus ranges between 0.3% and 1.3% in different populations of school children,[12],[13],[14],[15] while figures from hospital-based studies have ranged between 0.4% and 2.4%.[16],[17],[18]

Notwithstanding its low prevalence, strabismus is associated with significant psychosocial impact and negative social prejudice.[1] Children with strabismus are usually taunted and made fun of by school mates and peers.[19] These negative attitudes toward strabismus emerge at a young age, sometimes, as early as 6 years.[20] This tends to result in the development of low self-esteem and poor interpersonal relationships. In fact, strabismus may adversely affect an individual’s ability to obtain (or hold) employment.[21]

The common clinical subtypes of strabismus, in order of decreasing frequency, as reported by Mohney[4] in a cohort of American children, include accommodative esotropia, intermittent exotropia, strabismus associated with central nervous system abnormalities, acquired nonaccommodative esotropia, sensory strabismus, infantile esotropia, and paralytic strabismus.

Previous studies on strabismus in Nigerians have mainly focused on its prevalence and the distribution of the major types, that is, esotropia and exotropia.[14],[15],[17] Thus, only a few of these reports provided details on the various subtypes of strabismus seen among the populations studied. The aim of this study was to describe the common forms of strabismus among patients of an eye clinic in southwestern Nigeria, to provide data on the frequency of the different subtypes of strabismus among Nigerian patients who seek eye care.

  Methods Top

This was a retrospective study conducted at the eye clinic of our hospital. The study population was made up of all patients with strabismus presenting to the eye clinic for the first time during a 10-year period (1999–2008). Patients with missing and incomplete records were excluded from the study. Ethical approval was obtained from the University and Teaching Hospital Institutional review board.

Patients with a diagnosis of strabismus were identified from the clinic registers for the study period and their case records were retrieved from the Medical Records Unit of the Eye Clinic. With the use of a proforma, information was retrieved from the case notes of each patient with strabismus. This information included age at presentation, sex, clinical history, type of strabismus, visual acuity, associated refractive errors, cause of strabismus, and associated ocular disease.

Clinical information included ocular symptoms, duration of symptoms, history of spectacle wear, history of ocular trauma, significant medical history, and family history of strabismus. Visual acuity was tested objectively in school age children and adults with a Snellen chart or Illiterate E chart placed at a distance of 6 m from the patient. Preverbal and preschool children as well as those who could not cooperate for testing with the Snellen chart were subjectively tested with the use of behavioral methods of assessing vision, for example, ability to fixate or follow light and bright objects, and/or aversion to occlusion of one eye compared with the other.

All patients had been seen either by a consultant ophthalmologist or a senior resident doctor. Ocular examination included anterior segment examination with a pen torch and a slit lamp biomicroscope and a posterior segment examination with an ophthalmoscope. Refraction was performed by a senior optometrist. Ocular deviation was detected using the Hirschberg test in which a pen torch was shone onto the eyes and the corneal reflexes were observed. A deviation of the corneal reflex in an eye indicated a misalignment of that eye. The direction of the deviation was confirmed using cover tests. Deviation was measured in degrees (°) based on the millimeters of deviation observed. One millimeter of deviation corresponds approximately to 7.5° of misalignment. Clinical forms of strabismus were defined as described by previous studies.[22],[23],[24]

Collected data was analyzed with the use of Statistical Package for Social Sciences (SPSS version 20; IBM Corp., New York, NY, USA). Bivariate analysis was considered to show significant association when the P value was less than 0.05.

  Results Top

A total of 20,328 new patients presented to the eye clinic during the study period. Two hundred forty (1.2%) of these patients had strabismus. The mean age of the patients with strabismus was 19.8 (±19.7) years, and their ages ranged from 3 months to 75 years. There were 119 males (49.6%) giving a male-to-female ratio of 0.98:1. The age and sex distribution of the patients is presented in [Figure 1].
Figure 1 Age and sex distribution of patients with strabismus.

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The median age of the males was 16 years [interquartile range (IQR) = 27] whereas the median age of females was 11 years (IQR = 33.5). This difference was not statistically significant (Mann–Whitney test; P value = 0.229). There were more females than males among those aged less than 20 years, whereas there was a male preponderance in the older age groups. This difference in sex distribution was not statistically significant with a P value = 0.087.

Clinical history and ocular examination findings

A total of 102 (42%) patients presented on account of ocular deviation, while 108 (45%) of the patients had poor vision as their presenting complaint. Other symptoms included double vision (12.5%), drooping of upper eyelids (10.0%), headaches (6.3%), eye aches (2.9%), and face turn (0.4%). Thirty-nine (16.3%) patients had a history of ocular trauma, 25 (10.4%) had a history of neurodevelopmental delay, while six (2.5%) had a family history of strabismus.

With respect to ocular examination findings, 105 (43.8%) patients had abnormal findings in the anterior segment of the eye, whereas 51 (21.3%) had abnormal posterior segment findings. The most common abnormal ocular finding in the anterior segment was cataract (50 patients; 20.8%), followed by ptosis (21 patients; 8.8%). Macular scar was the most common abnormal posterior segment finding, occurring in 20 (8.3%) patients. Other abnormal findings in the anterior and posterior segments of the patients are shown in [Table 1] and [Table 2].
Table 1 Abnormal anterior segment findings in the eyes of 240 strabismus patients

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Table 2 Abnormal posterior segment findings in the eyes of 240 strabismus patients

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Characteristics of ocular deviation

Regarding the characteristics of the deviation, 233 (97.1%) of the patients had a horizontal deviation, five patients (2.1%) had a vertical deviation, while two patients (0.8%) had a combination of vertical and horizontal deviations. One hundred twenty-nine patients (53.8%) had esotropia, 106 (44.2%) had exotropia, three (1.3%) patients had hypertropia, and four (1.7%) had hypotropia. The deviation was unilateral in 192 (80%) patients whereas it affected both eyes (i.e., alternating) in 48 (20%) patients. The degree of deviation was recorded in only 153 (63.8%) patients. The mean deviation was 26.2° (±11.6°) and the deviation ranged between 7° and 45°.

The median age for patients with esotropia was 5 years (IQR = 18), whereas that of patients with exotropia was 24 years (IQR = 29.25). This difference was statistically significant (Mann–Whitney test, P value <0.001) Furthermore, 82.5% of patients aged 0 to 19 years (children and adolescents) had esotropia compared with 33.3% of those aged 20 years and above who had esotropia (odds ratio = 9.44; P < 0.001).

Causes of strabismus

In 68 (28.3%) patients, there was no specific cause for the strabismus, thus these patients were classified as having primary strabismus. On the other hand, 172 (71.7%) patients had associated ocular or neurological disease as the cause of strabismus (secondary strabismus), the most common cause being cataract in 50 (29%) patients. The causes of secondary strabismus are presented in [Table 3], while [Table 4] shows the distribution of the main ocular diagnosis that was made in the strabismus patients.
Table 3 Causes of secondary strabismus in 172 patients

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Table 4 Main ocular diagnosis in 240 strabismus patients

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Clinical subtypes of strabismus

Sensory strabismus was the most common subtype, occurring in 83 (34.5%) patients, whereas the least common was congenital exotropia (three patients; 1.3%). The subtype of strabismus was undetermined in three (1.3%) patients. The frequency of the different subtypes of strabismus found in the patients is presented in [Table 5].
Table 5 Clinical subtypes of strabismus in 240 patients

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  Discussion Top

The relative frequency of strabismus among eye clinic patients in this study was found to be 1.2%. This is relatively higher than the figure of 0.4% observed by Baiyeroju-Agbeja and Owoeye[16] in Ibadan, a decade earlier. This disparity may be because their study was on children less than 16 years of age, whereas this study involved patients of all age groups. It could also indicate a true increase in the incidence of strabismus in the population or an increase in the awareness and utilization of eye care services by the population. A prospective population-based study specifically designed to determine the prevalence of strabismus would be useful to confirm or refute this hypothesis. Furthermore, it is relatively difficult to compare the relative frequency of strabismus in this study with other reports from Nigeria and Africa that ranged between 0.2% and 1.3% because the focus of most of these studies were school children.[9],[10],[11],[12],[13],[14],[15],[17],[18]

Esotropia was more common than exotropia in the present study. This finding is similar to the earlier report from Ibadan[16] as well as some other reports from Nigeria.[14],[15],[17] But it differs from the findings of Ebana Mvogo et al.[25] in Cameroon and Kikudi et al.[26] in Zaire, where exotropia was more common. It is, however, important to note that the study population in these two studies were much smaller than the number studied in the present study as well as the other Nigerian studies. It may, thus, be safe to conclude that esotropia is indeed more common than exotropia in Africans, although a prospective population-based study may be necessary to make an assertion on this.

The effect of age on the type of horizontal deviation has been previously documented. Mohney et al.[27] reported that esotropia is more common than exotropia in the first 6 years of life while exotropia becomes predominant thereafter. The present study also found that older age was associated with an increased likelihood of having exotropia compared to esotropia. In this study, gender was not associated with the type of horizontal deviation. This is in concert with the previous reports that gender has no influence on the occurrence of strabismus.[22],[28],[29]

With regard to etiology, the proportion of secondary strabismus in this study (71.7%) was significantly higher than that observed by Berk et al.[30] who reported that 13% of their patients had secondary strabismus. This difference may be as a result of the fact that this study is hospital based and is simply a reflection of referral patterns and eye care utilization behavior of the population, not representing the true picture in the population. A population-based study would elucidate this further.

Cataract was the most common cause of secondary strabismus, closely followed by cranial nerve paralysis. Cataract is a known cause of strabismus especially in children[31],[32],[33]; however, previous studies have not identified it to be a leading cause of secondary strabismus. Berk et al.[30] in their study of secondary strabismus observed that cataract was responsible for 9.7% of the patients, while Ebana Mvogo et al.[34] reported that 16.1% of their patients with secondary strabismus had cataracts. These figures are relatively less than the finding in this study (29%). Also, previous studies revealed that posterior segment abnormalities were a more common cause of strabismus than anterior segment abnormalities. This contrasts with our findings in which anterior segment causes were predominant. Further investigation into the causes of strabismus in Africans is required to clarify these differences.The most common subtype of strabismus identified in this study was sensory strabismus. This finding is at variance with population-based reports, mostly in Caucasians, that have stated that accommodative esotropia is the most common form of strabismus.[4],[35] This finding may stem from the fact that the population studied included all ages, whereas the previous reports were on childhood populations. Second, the high proportion of secondary strabismus in this study may account for this difference.

Paralytic strabismus was the second most common subtype of strabismus in this study but was found to be the fifth common by Mohney.[4] In addition, in the present study, infantile esotropia was more common than intermittent exotropia, whereas intermittent exotropia was observed to rank second after accommodative esotropia in Mohney’s study.[4] This may suggest that infantile esotropia is more common than intermittent exotropia in Africans,[36] but further research is needed to confirm this. On the other hand, in keeping with Mohney’s observation,[4] congenital exotropia was the least common subtype of strabismus in this study.

The main limitation of this study is its retrospective nature. Patients’ information was occasionally incomplete and there were missing records. This could have resulted in the inadvertent omission of eligible patients from this study. Another limitation is the fact that data from hospital-based studies, such as ours, is usually not a true reflection of the epidemiology of diseases in the community.

In conclusion, the relative frequency of strabismus among eye clinic patients in this study is fairly similar to previous reports from Nigeria and Africa. Esotropia was more common than exotropia; however, older age was associated with an increased likelihood of exotropia. Majority of the patients in the study had secondary strabismus. Sensory strabismus was the most common subtype of strabismus followed by paralytic strabismus, while accommodative esotropia appears not to be as common as in Caucasian populations. Further research on strabismus, specifically, a population-based survey across all age-groups, is necessary to better understand the prevalence, causes, risk factors, and types of strabismus in our population.

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Conflicts of interest

There are no conflicts of interest.

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  [Figure 1]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]


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