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ORIGINAL ARTICLE
Year : 2023 | Volume
: 37
| Issue : 1 | Page : 48-54
Stereopsis following delayed strabismus surgery in early-onset strabismus
JT Chinmayee, K Kshama, Vidhya Eswaran
Department of Ophthalmology, Bangalore Medical College and Research Institute, Bengaluru, Karnataka, India
Correspondence Address:
K Kshama
Department of Ophthalmology, Bangalore Medical College and Research Institute, Bengaluru, Karnataka
India
Source of Support: None, Conflict of Interest: None
DOI: 10.4103/sjopt.sjopt_9_22
PURPOSE: Improved stereoacuity following delayed strabismus surgery is associated with long-term alignment of eyes and reduced severity of amblyopia. The objective of this study is to evaluate stereopsis following delayed strabismus surgery in early-onset strabismus.
METHODS: It is a hospital-based prospective, nonrandomized, interventional case study. Thirty patients with early-onset strabismus (before 3 years of age), presenting after 6 years of age were included. History taking and full orthoptic workup was done. Stereopsis was tested with Titmus Fly Chart test. The patients were treated surgically to correct strabismus and were followed up for 3 months. Quantitative parameters were compared between preoperative and postoperative follow-up at 3 months using McNemar test, Chi-square test, and independent t-test.
RESULTS: Of the 21 patients who had gross stereopsis preoperatively, nine of them (42.86%) attained fine stereopsis at 3 months postoperatively (P = 0.003, McNemar test). Ten (33.33%) of 30 patients had amblyopia and three out of 30 patients (10%) had preoperative “nil” stereopsis. Of the patients who showed improvement (16 of 30 patients – 53.33%), most of the improvement was observed in (a) exodeviations (56.25%, n = 9), (b) earlier age of presentation-before 10 years of age (47.36%, n = 9).
CONCLUSION: The benefits of surgical correction of early-onset strabismus include improvement in stereopsis. Therefore, even in an early-onset squint with a delayed presentation rehabilitation of stereopsis should be the goal of treatment.
Keywords: Delayed strabismus surgery, early onset strabismus, stereopsis
The frontally located eyes in humans give binocular single vision and stereoscopic depth perception. Stereopsis is the paramount consequence of binocular single vision. A highly co-ordinated motor and sensory process is responsible for binocular single vision and hence stereopsis.[1]
Bi-foveal fixation is absent at birth and binocular single vision starts to develop after birth. The onset of development of stereopsis is at 3–5 months of age, and there is quite a rapid development of stereopsis during 1st year of life. Maximum stereoacuity is attained by the age of 6 years but is still three times poorer than that of adult values.[1] Hence birth till 6 years of age is considered as critical period for binocular single vision. One of the prerequisites for development of binocular single vision is central fixation with co-ordinated and well-functioning extra ocular muscles and good fusional reserves. Any insult during this critical period, leads to underdevelopment of retino-cortical pathways and visual centres for development of binocular single vision and stereopsis and thus leads to amblyopia and impaired stereopsis.[2]
Binocular single vision is the hallmark of retinal correspondence while diplopia is the hallmark of retinal disparity. This horizontal retinal disparity (i.e., diplopia) within the Panum's area confers depth perception or stereopsis. The horizontal extent of Panum's fusional area at the central 2°–4° is 6–10 min and the retinal receptive fields in the centre are closely spaced. Fine stereopsis in central visual fields can be explained by specific and closely spaced retinal receptive fields in the central visual fields. Whereas the horizontal extent at 12° peripheral to fovea is 30–40 min of arc, indicating gross stereopsis.[3]
The neurophysiologic basis of stereopsis was explained by Hubel and Wiesel in terms of behaviour of a visual neuron in striate cortex in response to its stimulation in its receptive field. These visual neurons are arranged in the form of ocular dominance columns in the visual cortex. Ocular dominance columns are stripes of neurons in the visual cortex that respond preferentially to input from one eye or the other. Mono-ocular deprivation as in strabismus during critical period of visual development causes the columns to degrade and the normal eye assumes control of more of the cortical cells. Hence ocular dominance columns are important for stereopsis.[4]
Strabismus of any type at this critical period leads to subnormal stereoacuity.[5] Various studies have stressed upon the requirement of early alignment for restoring stereoacuity. But patients with strabismus seldom approach for treatment and present late; owing to superstitious beliefs and ignorance and difficult access to tertiary eye care institutes. Hence early alignment is not usually possible. Therefore, this study evaluates the stereopsis following a delayed strabismus surgery in early onset strabismus. Since improvement of stereopsis post operatively helps to maintain the alignment and prevents the development of severe amblyopia,[6] this study evaluates the significance of considering improvement of stereopsis as a goal for strabismus surgery even in delayed presentation.
MethodsThe objective of this study is to evaluate stereopsis following delayed strabismus surgery in early onset strabismus.
This is a hospital based prospective, nonrandomised, interventional case series. Patients attending squint clinic at a tertiary referral hospital in South India, presenting with early onset strabismus were considered for this study.
A pilot study was done from June– September 2017, and it was found that two patients with early onset strabismus attended the clinic during this period. Extrapolating this data sample size of 30 is considered. A total of 30 patients with early onset strabismus (onset before 3 years of age) presenting after 6 years, to our hospital from the period of November 2017 to May 2019, were included in this study. Onset of squint was ascertained by patient information and confirmed by old photographs. The study was approved by the Scientific Research Committee and Institutional Ethics Committee of our institute. A detailed history taking, ocular examination and full orthoptic evaluation including sensory tests such as Bagolini's striated glass test and Worth's four dot test and refraction was done. Stereopsis was tested with Titmus Fly Chart test.
After evaluation patients were treated as per standard intervention after obtaining an informed written consent and assent. The patients were followed up at our hospital for a period of 3 months and vision, ocular alignments, binocular single vision, stereopsis and refraction was assessed at all postoperative follow ups. Postoperative deviation in all subjects was <10PD. Improvement in stereopsis was defined as any improvement from the preoperative stereopsis to postoperative stereopsis at every follow up. For the purpose of statistical analysis, stereopsis was classified as fine stereopsis (100 s of arc and below) and gross stereopsis (as 160 s of arc and above).[7] The number of patients attaining fine stereopsis at the last follow up and the significance of change from gross to fine stereopsis was analysed statistically. Quantitative parameters are compared between preoperative and postoperative follow up at three months using McNemar test, Chi-square test, Independent t-test.
Inclusion criteria
Patients with early-onset (before 3 years of age) strabismusPatients with early-onset strabismus with delayed presentation i.e., beyond 6 years of agePatients fit for surgery and giving informed written consent and assent for both study and surgery.Exclusion criteria
Restrictive squintsParalytic squintPatients not giving informed written consent.Statistical analysis
The study was analyzed using Statistical Package for the Social Sciences (SPSS version 23.0) statistical analysis software, IBM Corp, released on 2015, IBM SPSS Statistics for Windows , version 23.0, Armonk, NY: IBM Corp. statistical analysis software. The improvement in stereopsis was tested for statistical significance using McNemar test. The threshold for statistical significance was set to P < 0.05 according to conventional threshold.
ResultsOf the 30 patients of this study, 16 patients had esotropia (53.33%) and 14 patients had exotropia (46.66%) [Graph 1].
Esotropes marginally outnumbered patients with exotropia. Three out 14 exotropes (21.42%) showed poor control, nine out of 14 exotropes (64.28%) showed fair control and only two out of 14 exotropes (14.28%) showed good control. There were 19 male patients out of 30 patients (63.33%). Eight out of 16 patients with esotropia (50%) had an onset before 6 months of age; while onset of squint was later at 6 months to 3 years in nine out of 14 cases of exotropia (64.3%) [Graph 2].
About nine out of 16 esotropes presented at 10 or <10 years of age (56.25%) and seven out of 14 patients of exotropes (50%) presented later after 21 years of age.
A total of 10 patients out of 30 (33.33%) had amblyopia. Amblyopia was present in only two out of 14 patients of exotropia (14.3%); whereas, as expected, out of 16 patients with esotropia, eight of them had amblyopia (50%). Patients presenting with esotropia had higher incidence of amblyopia.
Three out of 30 patients (10%) had preoperative “nil” stereopsis of whom all three of them had esotropia. Twenty one of the 30 subjects (70%) had gross stereopsis (160–3000 s of arc and six of those 30 patients (20%) had fine stereopsis (less than or equal to 100 s of arc. All those three subjects who had nil stereopsis preoperatively (10%) and those who had fine stereopsis preoperatively (20%) continued to remain under the same group. Twenty one out of 30 patients (70%) had gross stereopsis preoperatively. Nine out of these 21 patients with gross stereopsis (42.86%) attained fine stereopsis at a three month postoperative period (P = 0.003, McNemar test). The rest 12 of those 21 subjects (57.14%) continued to remain in the category of gross stereopsis [Table 1] and [Graph 3]. And this shift was seen to be statistically significant by McNemar test (P = 0.003)
Table 1: Cross tabulation of change of category of pre- and post-operative stereopsis (n=30)
Overall 16 out of 30 patients (53.33%) showed improvement in stereopsis while 14 out of 30 patients (46.67%) of them stayed in the unchanged category, that is they did not show any improvement. Nine of the 16 patients with improvement were exotropes (56.25%) as compared to 43.75% (n = 7) were exotropes who showed improvement [Graph 4].
Of the patients, who showed improvement (16 of 30 patients– 53.33%) most of improvement was observed in a) exodeviations-nine out of 16 patients (56.25%) who showed improvement were exotropes b) earlier age of presentation-before 10 years of age– nine out of 19 patients (47.36%) who showed improved stereopsis presented before 10 years of age.
Pre operatively, of the 12 patients with hyperopia seven (58.3%) patients had gross stereopsis as compared to patients with astigmatism (77.8%, n = 7) and anisometropia (75.0%, n = 6) who had higher percentages of patients with gross stereopsis. As much as 33.3% (n = 4) of the patients with hyperopia had fine stereopsis as compared to only 22.2%, (n = 2) of patients with astigmatism having fine stereopsis, suggesting that fine grade of stereopsis was mostly present in patients of hyperopia rather than anisometropia or astigmatism in this study. There was only one patient with myopia and he had gross stereopsis [Graph 5].
This changed postoperatively at 3 months. Of the 12 patients with hyperopia nine (75%) showed fine stereopsis, four out of nine patients with astigmatism (44.4%) showed fine stereopsis, the one person who had myopia now showed fine stereopsis and all those who had anisometropia did not show any improvement to fine stereopsis (P = 0.032) [Graph 6].
The average deviation noted in patients with esotropia was 61.68 ± 19.37 for near and 60.31 ± 18.47 for distance. The average deviation noted in patients with exotropia was 63.92 ± 12.80 for near and 64.07 ± 14.76 for distance. All patients had a postoperative deviation of < 10PD.
The most common reason for delay of presentation to the hospital for treatment was superstition (53.33%, n = 16) because they thought having squint is good fortune. The next common reason was ignorance (26.6%, n = 8) because patients were unaware of the deleterious effects of squint and of the available treatment options [Graph 7]. 20% (n = 6) of them had other reasons for delay like difficult access to health facility, other emergencies.
Previously many researchers have studied the importance of stereoacuity and its outcomes following treatment of strabismus. Immense work on importance of stereoacuity has been done by Eileen E. Birch. She stated that, even a subnormal achievement of stereoacuity following correction of strabismus is beneficial by giving a better long-term stability of alignment, lesser requirement of additional surgery, long-term quality of life and reduced risk of severe amblyopia.
Various studies have stressed upon the requirement of early alignment for restoring stereoacuity. But patients with strabismus seldom approach for treatment and hence present late. Thus early alignment is not usually possible. Hence in this study stereoacuity outcomes following delayed strabismus surgery are evaluated.
In this study patients with esotropia marginally outnumbered (53.33%, n = 16) the patients with exotropia (46.6%, n = 14). Of the 16 patients who had esotropia eight patients (50%) had an earlier onset of squint at < 6 months of age; while onset of squint was relatively later at 6 months to 3 years in nine cases out of 14 cases of exotropia (64.3%). Early onset of strabismus in patients with esotropia can be explained by poor fusional divergence and hence breakdown of fusion and early appearance of squint. The study showed relative early presentation in most of the patients with esotropia (56.25%, n = 9) i.e., 56.25% of esotropes presented at or before 10 years of age, as compared to patients with exotropia; most of whom (50%, n = 7) presented later after 21 years. Esotropia is usually associated with more visual morbidity and asthenopic symptoms leading to early presentation.[8] As exotropia is usually intermittent in nature; patients with exotropia tend to present late. Esotropia patients had higher incidence of amblyopia (50%, n = 8); whereas, only two patients out of 14 (14.3%) had amblyopia. This can be explained by the large angle deviations and constant nature of esotropia and most of the times esotropia tend to be associated with high refractive errors.
In this study fine stereopsis was defined as less than or equal to 100 s of arc and gross stereopsis being 160–3000 s of arc.[8] Out of the 30 patients, 21 patients (70%) had gross stereopsis, six patients (20%) had fine stereopsis and three patients (10%) had nil stereopsis. Out of the 30 patients, all most seven (23.3%) of them had a preoperative stereopsis of 200 s of arc. Of these seven patients who had 200 s of arc stereopsis, six of them had exotropia (85.71%). This indicates better stereopsis in cases of exotropia. The number of patients with nil preoperative stereopsis in this study was as low as three patients (10%, n = 3), when compared to other studies in [Table 2].[9],[10],[11]
All these three patients with nil stereopsis were esotropic. Overall, most patients of the study had 200 s of arc stereopsis, patients with exotropia had better stereopsis which can be explained by mostly intermittent, small angle and alternating nature of exotropia.
Fifty per cent of patients in the study by Mets, et al.[10] had nil stereopsis preoperatively. In the study by O'Neal TD, et al. none of the study subjects had nil stereopsis.[10] Whereas, 61% and 22% of patients had nil distance and near stereoacuity respectively in the study by Adams WE, et al.[11]
Those patients who had 800 s of arc, 3000 s of arc and nil preoperative stereopsis remained in the same category postsurgery. A greater percentage of change was seen in patients who had 200 s of arc of preoperative stereopsis. Six out of 7 patients who had 200 s of arc stereopsis showed improvement (85.7%). Whereas; two out of three patients (66.66%) with 63 s of arc, two out of three patients with (66.66%) with 100 s of arc and three out of four patients (75%) with 160 s of arc showed a change in stereopsis. Therefore most of the improvements was seen in patients having 200 s of arc and 160 s of arc stereopsis. Patients with dense amblyopia having 800 and 3000 s of arc stereopsis did not show any change whereas those with moderate amblyopia having 200 s of arc and 160 s of arc stereopsis showed a greater improvement.
Overall 16 subjects of 30 subjects of the study showed postoperative improvement (53.33%). This study showed that nine out of 21 patients (42.86%) who fell in the category of gross stereopsis preoperatively came to fall into the category of fine stereopsis 3 months postoperatively. This change showed a statistical significance by McNemar test (P = 0.003). The rest 12 of those 21 subjects (57.14%) continued to remain in the category of gross stereopsis. In a retrospective study of patients who underwent surgery for strabismus by Mets, et al., a 42% improvement in stereoacuity, measured using Titmus stereo acuity test, was observed.[9] The major limitation of this study is that two different binocular function was tested i.e., stereopsis and fusion. The study includes patients with paralytic strabismus in whom stereoacuity is well developed but fusion is lost; which is restored following correction of strabismus. Thus the present study conducted by us excludes paralytic and restrictive strabismus and tests only one component of binocular single vision i.e., stereopsis.
In the study by O'Neal, et al. both distance and near stereoacuity was measured in patients with intermittent exotropia. Patients with completely absent stereopsis were excluded from their study. It was shown that distance stereoacuity improved by 45% and near stereoacuity improved by 75%.[10] Patients with intermittent exotropia usually have better stereoacuity even before treatment. Our study measured only near stereoacuity but both esotropia and exotropia patients were considered and showed a statistically significant improvement of stereopsis postoperatively by 42.86%.
Again in the study by Adams, et al., where stereopsis was evaluated in patients with intermittent exotropia, improvement in stereopsis observed was 61% (P = 0.04) for distance and no change was observed for near. A higher percentage of improvement in the study by Adams, et al. could be explained by the fact that the study considered only patients with intermittent exotropia and esotropia was not considered.[11]
Achievement of even subnormal stereoacuity postoperatively may have benefits. Presence of even crude stereopsis aids in the development of sensorimotor milestones in early childhood. Improved stereoacuity outcomes are associated with better long-term alignment, reduced risk of severe amblyopia, improved sensorimotor developmental milestones, better reading ability, and improved quality of life.[6] Hence, rehabilitation of stereopsis should be a goal for treatment of strabismus regardless of age of presentation and alignment.
Favourable characteristics for improved stereoacuity outcomes were exotropia, earlier presentation, and absence of amblyopia and history of previous treatment.
The most common reason for delay was superstition (53.33%, n = 16) as most of them thought that having strabismus; especially esotropia, conferred them a good fortune. The next common reason was ignorance (27%, n = 8) because patients were unaware of the deleterious effects of squint. Twenty per cent (n = 6) of them had other reason for delay like difficult access to health facility, other emergencies. Hence the myth should be busted, general public should be made aware of the deleterious effects of squint and early referral to a strabismologist should be the norm.
ConclusionThe benefits of surgical correction of early onset strabismus include improvement in stereopsis. Therefore, even in an early onset squint with a delayed presentation rehabilitation of stereopsis should be the goal of treatment.
Limitations of the study
The relatively smaller sample size and short duration of the study period limits the scope of this study. Distance stereoacuity was not tested. The long-term alignment after improved stereopsis can be evaluated.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References
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