Anisometropia
What is anisometropia?
If your child is long-sighted, short-sighted or has astigmatism and has to wear glasses for this,
they have what is called a refractive error. Sometimes the refractive error is different in one eye
to the other eye.
For example, one eye may be very slightly long or short-sighted and the other eye more
significantly so. Any combination of refractive errors is possible, so one eye may have no
refractive error at all while the other eye is long or short-sighted. It is even possible for one eye
to be long-sighted and the other short-sighted. Where the refractive error differs between the
two eyes, it is called anisometropia.
What causes anisometropia?
Anisometropia occurs when one eye has a different length or shape to the other eye. It seems
that the eyes simply develop at different rates during early childhood, though the reason for this
is unclear. Sometimes it is a family trait so there is probably a hereditary element in some
cases.
What happens to a child’s eyesight if anisometropia is present?
The eye that has the lesser refractive error receives a clearer picture than the other eye. As the
brain is unable to match up a clear picture with a blurred one, it begins to ignore the blurred eye
and concentrate on the clearer one. Over time, the sight in the eye with the greater refractive
error becomes gradually poorer, a condition known as amblyopia (also called lazy eye). The
other eye may not see clearly either, if it has a moderate refractive error itself.
How will I know if my child has anisometropia?
Usually there are no outward signs of a problem, especially if the better-seeing eye has little or
no refractive error itself. Occasionally parents report that their child tends to close or rub one
eye, probably if they have become aware that they can’t see as well out of it. Some children
have a squint (eye turn) as well as anisometropia and this is the first sign that something is not
right.
If you have not noticed anything, the presence of a lazy eye will be detected through routine
vision screening. When your child attends for an eye test, the examiner, usually an orthoptist,
will ensure that each eye is covered in turn so that the sight of each is measured separately.
If unequal eyesight (visual acuity) is found, your child will be examined by an optometrist
(optician) to find out if a refractive error/anisometropia is present.
How is it treated?
The first stage is to prescribe glasses which your child should wear all the time. The glasses
give each eye the opportunity to receive a clear picture. You may notice that the two glasses
lenses look very different from each other. Usually one will be thicker, this being the stronger
lens to help the eye with the greater refractive error.
Your child’s eyesight will be checked after 2 - 3 months, and again after 5 - 6 months, of glasses
wear to see if it is improving. In most cases, there will be a significant improvement in these
early months, provided the glasses are worn full time. Eyesight is always checked with the
glasses on.
If after around 6 months of wear the eye with the greater refractive error is still lagging behind,
your child may be prescribed a period of eye patching (occlusion therapy) to encourage further
improvement in the lazy eye. For a lot of children with anisometropia, this is not necessary if
glasses are worn really well. Eye patching is more likely to be needed if there is also a squint
present. During a period of eye patching, you will need to visit the Orthoptic Department more
often e.g. every 6 - 8 weeks.
Once the lazy eye has improved, can my child stop wearing glasses?
The answer for the majority of children is no. Wearing glasses does not in itself ‘cure’ the
anisometropia. If glasses are stopped, the eyesight in the eye with the greater refractive error
will deteriorate again.
It is important that glasses are worn full time until the period of visual development is complete,
which is normally at around age 7 - 8 years.
After this, the situation will be reviewed, taking into account the strength of the glasses and your
child’s eyesight with and without them. At this stage, it may be possible for your child to wear
the glasses just for close work or even stop wearing them completely. This decision is taken
under the careful supervision of the orthoptist and/or optometrist and only when it is certain that
the vision is stable and will not deteriorate.
Are there any alternatives to glasses and eye patching (occlusion therapy)?
Glasses are used for the vast majority of children with anisometropia. In exceptional
circumstances, where the degree of anisometropia is very marked, contact lenses may be used
instead.
The use of eye patching is the standard treatment for lazy eye (amblyopia).
Your orthoptist will discuss other options with you should the need arise.
If you have any queries, or require further information
please contact the Orthoptic Department on 01332 785659.
Reference Code: P1608/1675/09.2014/VERSION1
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