If you think that your infant can see pretty much the way you do, you are mistaken. Every child has to ‘learn’ to see, and children usually attain full visual maturity when they are around 8 years.
The eyes are like two digital cameras connected to the visual centre found in the back part of the brain. This centre acts much like the central processing unit of a computer, processing data inputs from each of the ‘cameras’ or eyes, and producing a single cohesive output – which becomes the object we get to finally see. In an infant, this visual centre in the brain is like an unformatted hard disc. The centre develops by constantly receiving visual messages from either eye.
The period leading up to the age of 8 years is described as a critical period in the development of the visual centre. If there is any impediment to vision during this period, the visual centre in the brain will fail to reach full development and the child is likely to be left with sub-normal sight for the rest of his life. This condition is called Amblyopia or in layman’s terms – a ‘lazy eye’. (Amblyos in Greek means lazy or dull).
The ground-breaking evidence of this condition came from Hubel and Weisel, who in their experiments with kittens, showed how eyes that were covered during the early months of visual development ended up not making connections with the visual centre in the brain. In recognition of their work, they were awarded the Nobel prize for Physiology/Medicine in 1981.
What brings about a ‘lazy eye’?
If a child needs glasses to see clearly, do give him one. You cannot wish his visual impairment away. One of the very common causes of a ‘lazy eye’ is an uncorrected refractive error in a child, leading to a sub-optimal vision. This usually happens if the error becomes very pronounced in one eye compared to the other or if the nature of the error in one eye is totally different from the error in the other - for example, one eye having near vision and the other eye having far vision. The visual information from the eye that sees better is processed by the brain while the eye that does not see well gets ignored by the brain. The eye that does not see well therefore connects poorly with the visual centre and if the visual impediment is not removed in good time (8 years in this case ) the child will be left with a permanent handicap.
If both eyes do not see well, then the condition of Amblyopia could result in both eyes. The longer the affected child is left untreated for Amblyopia, the more permanent the damage to the eyes.
There are several other causes for Amblyopia and the development of a squint is one of them. We cannot pinpoint whether the squint is the cause or the result of Amblyopia. The development of cataracts (opacity in the lens) in infancy or childhood can cause severe Amblyopia, especially when only one eye is affected.
How do we know if the child has reduced vision or amblyopia?
Children are not likely to complain about difficulties with their vision because they do not know what normal vision is like. Remember, they are still learning to see - so they have no way of benchmarking the normal against the abnormal. Parents need to be alert on their behalf and watch for signs which indicate that vision development, even if not complete, is not in proper order.
Do your children prefer to sit close to the television all the time? Do they read from books kept very close to their faces? Do their noses almost touch their desks when they are writing or drawing? Some children would squint especially when tired or when they are making an excessive attempt to focus accurately on something they are interested in. This type of squint may be relieved by wearing the appropriate glasses. The ophthalmologist will be better placed to come out with an accurate diagnosis of an exhibited symptom.
So what if one eye is amblyopic or lazy?
Children with only one good eye will never be able to use their two eyes together and therefore, cannot develop depth perception and stereopsis (three dimensional vision). This is a great handicap as it will not allow them to take up certain professions later in life where these functions are essential. They will also have difficulties in playing games like cricket, where accurate depth perception is critical to striking a ball. The importance of having a spare eye in the event of losing one eye in an accident or disease cannot be over emphasised. It is worth knowing that in individuals using only one eye, the lifetime risk of injury to that good eye is close to 50%.
Monitoring children’s vision
As the chances of success in curing the ‘lazy eye’ plummets after the age of 8 years, the condition needs to be diagnosed and treated at the earliest. Therefore, the newborn should be examined for vision before being discharged from the nursery. A paediatrician can ascertain if the child has a normal red reflex in either eye. This is a good method to detect problems like congenital cataract straight away.
A preschool check at age 2 will help detect squints and major refractive errors. However, checks prior to this may be essential in families with history of squints or defective vision. 5-year-olds will need a routine check to rule out refractive errors and other problems.
In the future, medical research may come out with other ideas for treating adolescents and adults having Amblyopia. But for now, take your child to those annual check-ups even if the result is normal each time.
Dr V C Parthasarathy FRCOPHTH (London), DO, is a consultant paediatric ophthalmologist at Apollo Children’s Hospital, Chennai.