Definition
Amblyopia is insufficient visual acuity of one or both eyes. In technical terms, the eye is considered amblyopic when its visual acuity is inferior to normal acuity at least on two lines of a subjective scale of vision measurement.
Why does amblyopia persist?
It is always provoked by an incorrect visual experience during the first months or years of life. The speed of its development, its persistence and seriousness depend on its cause.
At birth, the cerebral function of sight is very limited: the cortical centres of sight are morphologically developed but functioning still very rudimentarily. Initially, the visual phenomenon is essentially sub-cortical: the visual cortex and the other cortical areas related to sight only develop functionally after birth. Their normal development, structured and sequential, requires that the visual cortex receives high quality images.
Causes
Amblyopia has several causes. The most frequent are refractive errors and strabismus, representing about 99% of all the causes. Among refractive errors, the situations of a high refractive error or an error difference between the two eyes (anisometropia) are especially important. These causes of amblyopia are particularly significant in children over 12 – 15 months old. Although the treatment is usually possible in patients under 7 or 8 years of age, the sooner it begins the easier and quicker will be the recovery.
But there are other causes, more serious and fortunately rarer, requiring urgent treatment. These include all alterations obstructing the entrance of light and images into the eye. In these cases amblyopia may be persistent, and in the most serious situations it may become irreversible, if its cause is not treated during the first months of life. Congenital cataract is a good example: when its obstruction to light entrance is significant, amblyopia develops immediately. This is one of the most serious causes of this disease, because in this case the period of cerebral plasticity ends after the first months of life and the treatment is thus extremely urgent: congenital cataract requires surgery to be performed between 6 weeks and 2 or 3 months of life.
Which are the symptoms of amblyopia?
The only symptom of amblyopia is reduced vision! However, young children seldom complain of poor sight, and so the signs of dim vision are more relevant than the symptoms.
It is important to consider the fact that a child with a very dim vision on just one eye has no visual limitation!
Even with low visual acuity of both eyes, many children do not appear to have a deficiency limiting their visual function!
Diagnosis
The diagnosis is made through the evaluation of visual acuity: in case of amblyopia, it is inferior to normal acuity. This measurement is only possible in children over 3 or 4 years old, when they already cooperate.
In younger children, not yet cooperative, we can infer the existence of amblyopia of one eye, when there is a reaction to the occlusion of the other eye.
In case of strabismus, we can conclude that one eye is amblyopic, when the child gazes at the objects only with the other eye.
However, the most secure way to diagnose and to prevent amblyopia is detecting ocular alterations that can cause it. The screening for these alterations should be done since birth and during the 1st year of life, by paediatricians and family physicians. During the 2nd and the 3rd year of life, all the children should be screened by an ophthalmologist for any signs of important refractive errors, which can cause amblyopia, or strabismus and consequently amblyopia.
Prevention
Better than to treat a persistent amblyopia is to prevent its development.
In case of obstruction of the visual axis, its cause should be immediately treated, before the existence of a severe amblyopia and the arising of other problems such as nystagmus.
Refractive errors should be corrected at an early stage, in order to prevent the persistence of amblyopia.
The majority of the cases of strabismus has a refractive cause, and so it can be prevented by the use in due time of corrective glasses. In case of strabismus with other origins, its early treatment can also prevent amblyopia.
Screening
It is possible to prevent amblyopia by detecting, at an early stage, the ocular alterations that can cause it. Therefore, visual screening is very important in children and it should be performed in due time.
The causes of obstruction of the visual axis should be detected within the first days: paediatricians are usually very careful and do it very easily.
As for the most frequent causes of amblyopia, the first screening should be carried out during the 2nd year of life, around 14 – 15 months of age. A second screening should be performed at 4 years old, when the child already cooperates, to measure visual acuity through suitable vision tests. It is also important to check the visual function before going to primary school, in order to ensure the existence of sensory conditions adequate for a correct learning.
Treatment
If the presence of amblyopia is diagnosed, the treatment should be individualized according to its cause, its severity and the child’s age.
Recurrence
The recurrence of amblyopia is very frequent! Even in cases treated with success, a relapse is possible until the age of 7 – 8 years. Thus, all the children who had amblyopia should be carefully monitored until that age.