The loss of alignment of one of the two visual axes towards the fixed object is called strabismus. It is a frequent process, affecting 3-4% of children .
In the non-aligned eye or strabismic eye, the image of the fixed object is not projected in the fovea, but in the retina area more or less away from it, depending on the angle of deviation. The fact of projecting the image of the objects outside the fovea gives rise to the appearance of profound sensorial alterations, the more serious the smaller the child, regardless of the angle of deviation. These sensory alterations affect visual function in both the monocular and the binocular aspect. Here we will only mention the strabismic amblyopia, which originates in the strabismic eye by a suppression reflex from the fovea of the healthy eye, and that is manifested clinically by a decrease in visual acuity. This phenomenon is commonly known as " lazy eye " or " lazy eye ". Only in completely alternating strabismus, in which one eye or another is diverted indiscriminately, strabismic amblyopia does 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 strabic 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 development period or visual plasticity, the results being better as soon as the treatment is established. For this reason, early diagnosis of strabismus is essential.
Any intermittent deviation after the sixth month of life requires ophthalmological examination. If the deviation is constant, the examination is necessary at any age.
Early exploration of any strabismic child is important, because it must be remembered that some strabismus are secondary to serious organic lesions of the eyeball ( chorioretinitis, retinoblastoma, retrolental fibroplasia, transparent media opacities , etc.) and that, although its percentage is low In relation to idiopathic strabismus, it is necessary that the exploration of every strabismic child includes the fundus examination and transparent media, however small the child may be.
During the first six months of life, transient deviation of one or both eyes can 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 observing the corneal luminous reflex; When the child looks at a small light (lantern), the corneal reflexes should be symmetrically centered in both pupils.
The examination will be made in front view and in the different positions of the gaze.
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 reflection is located at the edge of the pupil of the strabismic eye, the angle of deviation will be about 12 to 15º; if it remains in the center of the iris, it will be about 25 to 30º, and if it remains in the corneal limbus, about 45º.
If the child's collaboration is sufficient, the cover test, which is more accurate, will be carried out for greater security.. For this the child is made to fix a light or a small object, and is occluded with a hand or an eye occluder. By covering this eye, it is observed if there is any "fixation" movement in the unobstructed eye. If there is movement of fixation, it means that the eye was previously deviated, that is, that there is squint. Next, the same maneuver is performed on the other eye. If neither of them has made any movement, it is considered that there is no strabismus.
During the realization of the cover test it is essential to ensure that the child keeps the eye unobstructed by fixing the light or object presented without making any movement.