What causes a lazy eye?
A lazy eye (amblyopia) can develop in a child because one eye does not see an object as well as the other eye. Hence, the nerve pathways between the back of the eye called the retina and to the brain via the optic nerve do not receive as many visual signals. The brain, therefore, suppresses or ignores what it is seeing from the weaker eye and does not develop that eyes supporting visual centre brain function.
If there is a lazy eye there is often nothing actually wrong physically with the eye when we do an eye examination. There may not be an associated squint (strabismus), the eyes can appear externally to be the same.
The paediatric ophthalmologist looks for the presence of undetected cataract, then checks for any defect in the retina. She regularly checks the refraction in case corrective glasses are required and does other visual tests as necessary.
Refraction is a very confusing word to ordinary mums and dads because it is such a medical term. Refraction means testing the child’s eyes to see whether they need glasses. In a child under seven, this has to be done with dilating drops because the child has such a strong ability to alter the focus of their eyes (‘accommodation’) that they can completely alter the findings of the refraction by looking at the person testing them, but this can be corrected for by dilating their eyes first so that they do not have that ability. Accommodation is blocked temporarily by putting in cycloplegic eyedrops. These also dilate the pupil which helps Dr Raoof examine the retina.
The cycloplegic eye drops are put in by the nurse at Clinica London after the child has been assessed by the orthoptist and then Ms Naz Raoof does the cycloplegic refraction herself to see whether there is a refractive error.
A refractive error can include short-sightedness called myopia, or long-sightedness, which is also known as hypermetropia. Astigmatism is another form of refractive error, which essentially means the front of the eye is more oval shaped than round. If there is a significant difference in the prescription between the two eyes, this gives the child a difference in sharpness of vision between the eyes. This is called a refractive anisometropia. The result is one eye can become ‘lazy’ compared with the other. If glasses are given, the difference between the two eyes is accounted for and there is no longer a reason for a child’s brain to prefer one eye over the other.
The most common type of difference of vision between the two eyes is when one eye is long-sighted, meaning they can see very well for distance but no so well for near, whereas the other eye is near normal or less long-sighted. The long-sighted eye is at risk of becoming lazy or amblyopic. A child who is long-sighted may well require glasses and a patch to prevent or treat their squint and amblyopia. Sometimes the difference in sharpness of the vision between the two eyes is due to short-sightedness or imperfections of the cornea leading to astigmatism and again that can also cause a lazy eye, which is going to have to be treated with glasses or contact lenses and patching.
In some children, the lazy eye is caused by a combination of a squint and a refractive problem. All of this can be assessed by Ms Naz Raoof, the paediatric Ophthalmologist, together with her team of orthoptists at Clinica London. If there is a medical problem of the eye stopping the child from seeing well, such as a cataract, they will also be picked up by Ms Raoof and treated accordingly. Factors associated with a lazy eye are the child being born premature or being a small size and weight when they are born, a family history of a lazy eye and other developmental problems.
Not treating a lazy eye can lead to permanently reduced vision so it is important to detect this while the child is young under the age of 4 or 5 so that treatment in the form of glasses and patching can be started.