What is anisometropia?
Strabismus is the loss of parallelism of the eyes in such a way that they are not aligned in the same direction as the object of visual interest. It affects an estimated 4% of children in the population and is one of the most common diseases found in children.
In normal conditions, the eyes look at the same object and the brain is able to integrate the two images, one in each eye, to obtain a single image with 3D characteristics.
For this coordination to work, there is a complex system in charge of sending the orders from the brain to each of the extraocular muscles. When one of these circuits fails, strabismus appears.
Depending on the direction, the strabismuses may be horizontal, vertical, torsional or mixed. Strabismus may affect just one eye or it might altenate, i.e. either one eye or the other deviates.
The deviation might be constant or intermittent and appear only when an individual is tired, has a fever or is distracted, etc. Furthermore, it may develop at any age: it is congenital if it appears before the age of 6 months, acquired in childhood if it starts between the age of 2 and 3, or an adult strabismus if it develops in adulthood.
The origin of strabismus is mutifactorial. Despite the fact that in certain types the cause is indeed known, in the majority of cases it is unknown or not fully known.
What we do know, however, is that there are different genetic factors (a family history of strabismus) and environmental factors (prematurity, low birth weight, the mother smoking and drinking during the pregnancy, neonatal problems...) and optical factors (moderate to severe hypermetropia, a different prescription in one eye to the other...) that may determine whether the patient develops strabismus.
Stabismus cannot be prevented, but it can be detected early. For this reason, it is fundamental to have a full eye exam by a specialist at the age of 2-3 even though the child does not have any symptoms, and another at the age of 5-6 before the visual system has fully matured.
Strabismus treatment includes different therapeutic options that must be considered in a personalised way by a specialist ophthalmologist. For treatment to be effective, it must be administered as early as possible.
Refractive defect correction
First, we must correct the refractive defect, if there is one, using glasses or contact lenses. In some cases, such as accommodative strabismus, optical correction alone may compensate the deviation.
Second, if amblyopia (lazy eye) develops in the deviated eye, we must restore the vision to that eye. To do so, the most effective measure is occlusion of the healthy eye using a patch.
The alternatives to occlusion which fall under the term 'penalisation' involve inducing blurred vision in the good eye. This may be optical using a lens with a filter or incorrect prescription, or pharmacological by putting drops binto the eye that dilate the pupil.
In general, penalisation is usually reserved to cases where it is impossible to use a patch, such as maintaining the vision obtained after the occlusion or in certain cases of nystagmus.
Once the optical defect and amblyopia have been corrected, surgical treatment can go ahead. In addition to the surgery, there are other therapeutic options available such as visual rehabilitation, prisms and botulinum toxin.
The botulinum toxin injection is an alternative in certain cases such as oculomotor paralysis. Orthoptic exercises may be useful as a complement to surgery and in selected cases such as phorias and intermittent divergent strabismus. Prisms are indicated in small strabismuses that cause diplopia.
There are many types of strabismus. The most common in clinical practice are the following:
Convergent strabismus or esotropia
Divergent strabismus or exotropia
Restrictive strabismus with a "mechanical" element that stops the eye from rotating and therefore causes limited movement in a certain direction. Children may get restrictive strabismuses, but they are more common in adulthood (acquired). We can categorise them into two large groups:
Frequently asked questions
As it is an innervation problem (the nerve sending information to the muscle to get it to make the movement fails), we cannot restore the eye's external rotation ability. Therefore, if the patient does not have a deviation in the primary gaze position (looking straightforward) and/or torticollis, then this operation is not indicated.
Restrictive strabismuses are incomitant strabismuses (the magnitude of the deviation varies in the different positions of the gaze) which is why the aim of the surgery should be to obtain maximum alignment and eliminate diplopia in the primary gaze position and in the reading position. Therefore, it is normal that double vision persists in another position after the procedure.
Although in the majority of cases the cause is unknown, the hereditary factor plays an important role in its pathogenesis. Different studies suggest that a patient with a family history of strabismus is four times more likely to get it.
Treatment for accommodative strabismus involves full correction with glasses or contact lenses. It is fundamental that the child wears their glasses all day to prevent the strabismus from becoming unbalanced. If optical correction neutralises the whole deviation, strabismus surgery will not be required. At the age of 21, you can look at correcting hypermetropia by undergoing refractive surgery, as this way we can eliminate the optical error and, in turn, its consequences, i.e. the deviation.