The word "squint" refers to a failure of the visual axes to meet at the point of regard. For normal vision, each eye must be focused on and lined up with the object of regard. The fact that we have two eyes positioned some 60 mm apart means that we can accumulate considerably more data about our environment than would be possible with one eye alone. This can best be exemplified by considering what happens when one eye is suddenly lost as the result of injury or disease. Apart from the obvious loss of visual field, which necessitates turning the head to the blind side, the patient experiences impaired distance judgement. The skilled worker notices a deterioration in the ability to perform fine tasks and the elderly notice that they pour tea into the saucer rather than the cup. In time, depth perception might improve and the patient adapts to the defect to some extent; children can adapt to one-eyed vision in a remarkable way. But, it seems that modern civilised living does not have such great demands for binocular vision now that many tasks are carried out by machines. It is no coincidence that those animals whose survival depends on catching their food by means of accurate distance judgement have their eyes placed in front of their head, enabling the two eyes to be focused together on their prey.
Investigation of a normal human population reveals that although the eyes are situated on the front of the face, they do not always work together, and it will be seen that there are a number of reasons why the mechanism might fail. The ability to use the eyes together is called binocular vision. It can be measured and graded by presenting each eye separately, but simultaneously, with a series of images. The instrument used to do this is called a synoptophore (Figure 14.1).
1. Simultaneous macular perception is said to be present if the subject can see two dissimilar images that are presented simultaneously to each eye, for example a triangle to one eye and a circle to the other.
2. Fusion is present if the subject can see two parts of a whole image as one whole when each half is presented to a separate eye, for example a picture of a house to one eye and a picture of a chimney to the other, and the whole picture is maintained as one as the eyes converge. The range of fusion can be measured in degrees.
3. Stereopsis, the third grade of binocular vision, is present if, when slightly dissimilar views of an object are presented to each eye separately, a single three-dimensional view of the whole is seen. Stereopsis itself can also be graded if fine degrees of impairment of binocular function need to be measured.
This ability of ours to put together the images from each eye and make a single picture in our minds seems to develop during the early years of life and furthermore, its development seems to depend on visual input. Below the age of eight
years, any misalignment of the eyes that disturbs binocular vision can permanently damage this function.
If the alignment of the eyes is disturbed for any reason during childhood, the child might at first, as one might expect, notice double vision but quickly learns to suppress the image from one eye, thereby eliminating the annoyance of diplopia at the expense of binocular vision. In fact, most, but not all, children learn to suppress when using monocular instruments, switching the other eye on again when the instrument is not being used. Prolonged suppression seems to lead to a more permanent state of visual loss called amblyopia of disuse. The word "amblyopia" simply means blindness. Suppression is a temporary switching off of one eye when the other is in use, whereas amblyopia of disuse is a permanent impairment of vision, which could affect the career prospects of the patient. Amblyopia of disuse can also occur if the sight of one eye is defective as the result of opacities in the media, even though the alignment of the eyes has not been disturbed. Again, this only occurs in children under the age of eight years. Covering one eye of a baby could lead to permanent impairment of the vision of that eye, as well as impairment of the ability to use the eyes together. An adult can have one eye covered for many months or even years without suffering visual loss.
Before considering the causes and effects of squint in children and adults, it is necessary to know something of the different kinds of squint.
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