The loss of alignment of one of the two visual axes towards the fixed object is called strabismus. It is a common condition affecting 3-4% of children.
In the non-aligned eye or strabismic eye, the image of the fixed object is not projected on the fovea, but on the retinal area more or less distant from it, depending on the angle of deviation. The fact that the image of the objects is projected outside the fovea leads to the appearance of profound sensory disturbances, more severe the younger the child is, regardless of the angle of deviation. These sensory disturbances affect both monocular and binocular visual function. Here we will only mention strabismic amblyopia, which originates in the strabismic eye by a suppression reflex from the fovea of the healthy eye, and is clinically manifested by a decrease in visual acuity. This phenomenon is commonly known as “lazy eye” or “lazy eye”. Only in totally alternating strabismus, in which one eye or the other is deviated indistinctly, does strabismic amblyopia not appear.
Therefore, we have seen that in strabismus there is a double aspect, the anatomical one, of deviated eye, and the functional one, of strabismic amblyopia and binocular sensory alterations. The anatomical alteration could be corrected, in principle, by means of a surgical intervention at any age, but the sensory alterations can only be treated during the period of development or visual plasticity, with better results the earlier the treatment is established. For this reason, early diagnosis of strabismus is essential.
Any intermittent deviation after the sixth month of life requires ophthalmologic examination. If the deviation is constant, examination is necessary at any age.
Early examination of any strabismic child is important, because it should be remembered that some strabismus is secondary to serious organic lesions of the eyeball (chorioretinitis, retinoblastoma, retrolental fibroplasia, opacities of transparent media, etc.) and that, although their percentage is low in relation to idiopathic strabismus, it is necessary that the examination of any strabismic child includes fundus and transparent media examination, no matter how young the child may be.
Diagnosis of strabismus
During the first six months of life, transient deviation of one or both eyes may be considered normal, since binocular vision is not perfectly established. Before that age, only constant deviation will be considered pathological.
The simplest method for the diagnosis of strabismus is the Hirschbert test, which consists in the observation of the corneal light reflex; when the child looks at a small light (flashlight), the corneal reflexes should be symmetrically centered in both pupils.
The examination should be carried out in frontal gaze and in the different gaze positions.
This method allows not only the qualitative diagnosis of strabismus, but also the quantitative one. As an approximation, it can be said that if the reflex is located at the edge of the pupil of the strabismic eye, the angle of deviation will be about 12 to 15º; if it is in the center of the iris, it will be about 25 to 30º, and if it is in the corneal limbus, it will be about 45º.
If the child’s cooperation is sufficient, the cover test, which is more accurate, will be performed for greater safety. To do this, the child is made to fixate a light or a small object, and one eye is occluded with a hand or an occluder. When this eye is covered, it is observed if there is any “fixation” movement in the non-occluded eye. If there is fixation movement, it means that the eye was previously deviated, i.e., strabismus is present. The same maneuver is then performed on the other eye. If neither eye has made any movement, it is considered that there is no strabismus.
During the cover test it is essential to ensure that the child keeps the non-occluded eye fixating the light or object presented without making any movement.