Ametropia and orthophoria of both eyes is mandatory for the foundation of Binocular Single Vision (BSV). BSV encompasses a point to point normal correspondence of the two retinae with foveae being the principle corresponding points. In spite of normal anatomical development of both eyes, interruption of physiological development in visual acuity of one eye can lead to amblyopia. This definition is based solely on visual acuity; however, this definition does not take into account the qualitative differences in vision that amblyopic often experience resulting in poorer vision than that measured by Snellen’s chart. Amblyopia till date remains a diagnosis of exclusion1.
Children constitute 35 - 40% of the general population.School going children3 therefore, form an important large target group and school vision screening plays an important part in early detection of amblyopia and institution of appropriate therapy, which is of immense value towards preventing the development of lifelong visual morbidity The time necessary for amblyopia to occur during the critical period is shorter for stimulus deprivation amblyopia than for strabismic amblyopia or anisometropic amblyopia. Around the age of two to four weeks, a period of dramatic developmental plasticity6 is found to occur when the developing functional and anatomic organization of the visual system is influenced markedly by visual experiences, which are both uniocular and or binocular. Visually significant conditions like monocular or binocular visual deprivation, anisometropia, strabismus or abnormal visual environment during this period results in significant electrophysiological and anatomic abnormalities in striate cortex and in the lateral geniculate nucleus (LGN).
There occur losses in the number of responsive cells to the deprived eye, a loss of binocularly responsive cells, shrinkage of cells in the LGN laminae5 serving the deprived eye and significant abnormalities in the response qualities of the cells that remain throughout life. Primate models of amblyopic have repeatedly shown the primary visual cortex (area VI) to be dysfunctional. Functional imaging confirms the presence of processing abnormalities in the area VI of the human cortex and also hints at deficits within the higher cortical areas.
In the context of amblyopia, it has often been stated that treatment is ineffective after the age of 8 years. While this may be true for some types of amblyopia, for example, complete unilateral congenital cataract, where treatment must be commenced within a few weeks of birth to be effective, for other causes of amblyopia there is increasing evidence that treatment may be effective beyond this age.
A cross sectional and time bound study was conducted among 50 children aged 5 -15 years attending the OP at ophthalmology department, GRH Madurai as well as the school health program screening camps who had decreased visual acuity, complaints of asthenia and deviation of eye were thoroughly examined for their refractive status, anterior and posterior segment examination was done to determine the prevalence of amblyopia in school going children and to study the types of amblyopia occurring during school children in our area – their improvement after partial occlusion therapy. Patients aged 5-15 years with refractive errors were included in the study. Criteria for exclusion were patients with known cause of reduced visual acuity ,patients with myopia greater than spherical equivalent of - 6.00 D ,history of previous amblyopia treatment within one year ,those who had underwent intraocular surgery, known skin reaction to patch or bandage adhesive and dropouts and non-compliant patients were excluded from final analysis
ETHICAL COMMITTEE CLEARENCE:
Ethical committee clearance was obtained from Institutional Review Board / Independent Ethics Committee of Govt. Rajaji Hospital.
A diagnosis of Amblyopia was made and they were further grouped into moderate and severe amblyopia according to their degree of visual loss. A few numbers of the patients who presented with temporal pallor on dilated fundus examination were further subjected to CT scan to rule out a possibility of organic lesion. A thorough refractive correction was done. Patients with high myopia and hypermetropia were excluded from the study. Treatment options like refractive error correction and occlusion were discussed with the patients. The duration for occlusion therapy was three hours where the patients were emphasized to do near activity tasks. Compliance to treatment for more effective results was stressed to both the patient and his / her parents. Depending on the severity of amblyopia2, children with moderate amblyopia were advised occlusion for three hours per day and children with severe amblyopia were advised occlusion for six hours per day. The importance of near vision tasks was instructed to the patients. Parents and teachers were asked to motivate the child to wear the occlusion patches.
These patients were explained the importance of treatment of amblyopia where stress was given on early intervention, self-motivation and dedicated follow up. They were asked to come for follow up after three months of occlusion. Significant improvement in visual acuity was seen in younger patients with good compliance which was tested with Snellen’s visual acuity chart that showed an improvement in visual acuity by one or two lines. Patients whose visual acuity failed to improve after three months of occlusion therapy were counselled for better compliance and motivation was given to increase the duration of occlusion to six hours a day. However, a very limited number of patients who failed to improve even after six months of age were referred for alternate method of treatment and explained about use of low visual aids etc.
Patients with significant amount of ptosis or congenital cataract were explained about the possibility of stimulus deprivation amblyopia and were referred for surgical correction. In each case after analysis of the type of amblyopia proper counselling was given and the prognosis for each case was discussed with both the parents of the patient and the teachers also.
OBSERVATION AND ANALYSIS:
The number of patients in the younger age group (5-10 years of age) was 32, and those in the older age group (11-15 years of age) were 18. There were totally 50 patients of whom 26 were boys and 24 were girls. A higher incidence of amblyopia was detected among the younger age group 64% as compared to the older age group 36%. A proper understanding of complaints revealed that the patients who came to our OPD mainly complained of visual discomfort 50% which was followed by deviation of the eye 30% and a few patients had complaints of asthenia. A higher incidence of amblyopia was seen in the rural population 58% as compared to the urban population 42 %. Our study shows moderate amblyopia 72% is more prevalent than severe amblyopia 28%. The response to occlusion therapy in severe amblyopia is less than moderate amblyopia with some reversibility (Table: 3).
A careful analysis of anterior segment in these patients revealed a normal anterior segment in nearly 70% of the patients and tropias in the remaining 30%. Posterior segment examination revealed normal fundus in majority of the patients 94% and with mild temporal pallor in 4% of the cases. The patients with temporal pallor were further evaluated with non-invasive imaging studies like CT scan to rule out any brain pathology. CT scan was found to be normal in these patients helped us in the exclusion of amblyopia of organic aetiology.
Among the refractive errors prevalent in these patients (Table: 1), simple myopia 20%, and simple hypermetropia 40% were commonly seen. Myopic astigmatism was 8% more common than hyperopic astigmatism was 2%. The prevalence of myopic among tropias10%, exotropia was more commonly seen than esotropia 5%. The duration for partial occlusion therapy was three to six hours per day.
Studies have shown that early screening for amblyopia results in better outcomes. Screening for amblyopia begins from infancy. Countries that have instituted early vision screening programs have reduced rates of amblyopia. The American Academy of Paediatrics recommends routine eye screening in new born period as well in subsequent child visits. High risk patients, for example premature babies, children with family history of congenital cataract, malignant tumours like retinoblastoma, those with delayed milestones and genetic illness should be also screened for comprehensive eye examination. Presence of family history for strabismus4 and ptosis should alert the paediatrician to refer the patient for a thorough ophthalmic evaluation.
Amblyopia and ambylogenic factors are the commonest target conditions for preschool vision screening and previous reports have stressed that the treatment of amblyopia is not effective after 8 years of age. However, recent reports have supported the view that age is not a barrier for the success of amblyopia therapy. Amblyopia treatment studies (ATS) recently reported improvement in visual acuity in older children also. Hence the study also includes children of the older age group who were treated with partial occlusion therapy7 with adequate response in visual potential.
Even after proper treatment for occlusion therapy, there is a risk of recurrence of amblyopia which is around 17% to 95%. In a recent study by the PEDIG group2, the recurrence rate of amblyopia following therapy was seen in nearly 24% of the children who were analysed after a year of their treatment. Recurrence rate can be monitored as early as the thirteenth week following discontinuation of therapy. Hence it is mandatory that proper weaning of therapy is done, like children who were given patching treatment for six hours per day were treated with a weaning therapy of two hours patching per day before discontinuing the treatment to decrease the recurrence rate of amblyopia. The chances of recurrence are always high till the child becomes visually mature i.e. around ten years of age. Hence a careful monitoring should be done up to the age of 1 year, every 2 months up to the age of 2 years and then every 4 to 6months up to the age of visual maturity is obtained.
LIMITATIONS OF THE STUDY
Since the study was time bound, follow up of the patients after six months was difficult and hence the analysis of maintenance of the improved visual acuity was not feasible.
The incidence and the progression of occlusion amblyopia, an important and a threatening complication of this therapy could not be assessed to the lack of compliance and time.
Role of critical periods for sensitivity in development of amblyopia has to be mentioned. CRITICAL PERIOD is the duration of time at which the child’s visual system is sensitive to the abnormal input caused by stimulus deprivation, strabismus or significant refractive errors. Another important issue to be assessed is the prognostic significance and variations in different types of amblyopia.
A statistical analysis of the outcome of treatment following three months of occlusion therapy revealed ‘p’ value of greater than 0.05 which concludes the fact that age is a significant factor (Table: 2) in analyzing the outcome of treatment of amblyopia. The reasons for success following treatment were due to good compliance to treatment, proper motivation by the parents and most importantly the age of the patient.
We express our sincere thanks and gratitude the Dean, Government Rajaji Hospital and Madurai Medical College for permitting us to conduct this study. I express my deep sense of gratitude HOD of Neurology, and HOD of Paediatrics for evaluation of their patients and their support to this study. I am extremely grateful to all our Assistant Professors and PG Residents for their constant source of cheer and encouragement throughout the study. I thank all my patients who have formed the back bone of my study without them this work would not have been possible. I am also thankful to paramedical staffs of all departments for their concern.
There is no financial interest in this study.
1. Singh A, Dangda S, Kumar B (2015) Recent trends in amblyopia therapy: a review. Science Postprint 1(2): e00048. doi:10.14340/spp.2015.04R0003.
2. Pediatric eye disease investigator group. Treatment of Anisometropia amblyopia in children with refractive correction. Ophthalmology 2006; 113:895-903
3. Desai S, Desai R, Desai N, Lohia S, Bhargava G, Kumar K. School Health Appraisal. Indian Journal of Ophthalmology1989, 37:173-175
4. American Academy of Ophthalmology; Amblyopia preferred Practice Pattern. San Francisco, American Academy of Ophthalmology 2007
5. Kiorpes L, Mckees SP. Neural mechanism underlying amblyopia. Curr.Opin. Neurobiol.1999;9: 480-485.
6. Daw NW.Mechanisms of plasticity in the visual cortex.invest Ophthalmol Vis Sci.1994; 35: 4168
7.Sana Al-Zuhaibi, Iman Al-Harthi, Pascale Cooymans, Aisha Al-Busaidi, Yahya Al-Farsi,1 and Anuradha Ganesh.A Compliance of amblyopic patients with occlusion therapy: A pilot study Oman J Ophthalmol. 2009 May-Aug; 2(2): 67–72