What is amblyopia?
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.
More important than this definition is the idea that amblyopia is a cortical phenomenon.
The cause may be in the eye, but the alterations associated with amblyopia concern the cerebral areas related to sight. The normal development of these areas of the brain requires that the visual information (images) received by the visual cortex, during the first years of life, is of high quality. This period of life, that comprises the first 6 or 7 years, is named critical period, and it corresponds to the lapse of time during which the brain has enough plasticity to develop visual capability.
When does amblyopia develop?
Amblyopia may develop after birth. It depends on the cause. At birth, the visual phenomenon is essentially sub-cortical: all the cortical structure is morphologically present, but its functionality only develops afterwards by action of the luminous stimuli.
If there exists a congenital anomaly (for example, cataract) that prevents light from entering the eye, amblyopia develops immediately. This is the most serious form of amblyopia, because in this case the period of cerebral plasticity ends after the first months of life and so the treatment is extremely urgent. Congenital cataract requires surgery to be performed between 6 weeks and 2 or 3 months of life. In other cases, amblyopia may develop later, when its causes (such as strabismus or refractive errors) arise; the critical period is longer, going until the age of 7 or 8 years.
Why is amblyopia named “vacant eye” or “lazy eye”?
The designation “lazy eye” is very common, meaning that the functionality of the eye is below normal values.
Which are the main 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 important than the symptoms.
Usually, children over 3 or 4 years old are already able to cooperate in some vision tests, and an abnormal response may arouse the suspicion of amblyopia. In younger children, it is necessary to search for other signs, such as the reaction to the occlusion of each eye or the pattern of fixation of objects or images with each eye.
More secure is the diagnosis of the existence of alterations that can cause amblyopia.
Which are the causes of amblyopia?
Amblyopia has several causes.
The most serious causes, but rarer, requiring urgent treatment, are those that obstruct the entrance of light and images into the eye. They are characterized by alterations of the normal red reflection of the eye fundus. Cataract, cornea and vitreous humour alterations are examples of these causes.
The most frequent causes are refractive errors and strabismus, representing about 99% of all the causes. Among refractive errors, the situations of a very high refractive error or an error difference between the two eyes (anisometropia) are especially important.
Is prevention possible?
Indeed it is.
The obstructions of the visual axis should be treated before amblyopia becomes persistent. Also, amblyogenic refractive errors should be corrected at an early stage, in order to prevent amblyopia. Strabismus has mostly a refractive cause, and so it can be prevented by the use in due time of corrective glasses. In cases of strabismus with other origins, its early treatment can also prevent amblyopia.
How is a correct diagnosis made?
Evaluating visual acuity: in case of amblyopia, the result 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.
But 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.
How can the parents help to treat this problem? Is the monitoring of an ophthalmologist essential?
The ideal is to prevent the persistence of amblyopia.
So, visual screening should be performed in due time, as previously explained. The first screening, for the causes of amblyopia already mentioned, should be carried out in the 2nd year of life. 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.
If the presence of amblyopia is diagnosed, the treatment should be individualized according to its cause, its severity and the child’s age. It should be taken into account that even in cases treated with success a relapse of amblyopia is possible until the age of 7 – 8 years.
Leads for the parents
When should your child have the first eye examination?
1. A paediatrician knows that a child’s eyes should be examined since the first days of its life. Therefore he checks up any morphologic alteration or unnatural pupillary reflexes, in order to immediately refer the baby to a specialist in paediatric ophthalmology.
2. A paediatric ophthalmologist should always be consulted in case of:
- History of ocular diseases in the family;
- Increased risk of ocular diseases: premature babies, children with neurological alterations, facial dysmorphies or genetic diseases;
- Suspicion of strabismus, as soon as it is detected.
3. The factors of risk of amblyopia, such as strabismus, refractive errors and opacity of transparent structures, should be screened as earlier as possible. The detection is very easy to make when the child is over 1 year old. The 2nd year of life is the ideal time for the first visual screening.
4. Visual acuity can be measured at any age, but the method can be more or less complex according to the patient’s age. Still, over 3 years of age, it is possible to evaluate it with very simple methods available to all eye care professionals.
5. The child’s visual acuity should be measured as earlier as possible. If it is not the case, it is obligatory to evaluate it before going to school for the first time.