Eye Care Blog

Squint, or strabismus, is a general term used to describe any one of several conditions in which the two eyes are not properly co-ordinated. One eye focuses on an object, but the other eye fails to align itself to the same fixation. Most commonly the errant eye is aligned inwards (cross-eye or esotropia) or outwards (wall-eye or exotropia); less commonly it looks upwards (hypertropia) or downwards (hypotropia). Other words to describe these conditions are ‘cast’ and ‘turn’. The phrase ‘evil eye‘ has also doubtless been used in association with squint. Where a squint remains uncorrected the person who has it is often forced to adopt an unusual, even grotesque, head- posture, adding to a general impression of unsightliness. But because the squint sufferer has the potential for normal stereoscopic vision the treatment of squint is an important ophthalmic activity.

The reasons why the two eyes do not work together, either temporarily or permanently, can be several. Normally the two eyes move together, maintaining fixation on the same object or objects in the distance. As the object comes closer, the eyes converge somewhat to maintain fixation. However, when an object is placed very close to the eyes such dual fixation is no longer possible, and it is normal for one eye to wander. What is abnormal is for one eye to wander when the object is still in the middle or far distance. This state of affairs can occur to anyone in special circumstances, when they are very tired, or ill, or under the influence of drugs and/or alcohol. But a permanent deviation by one eye, either inwards, outwards, upwards or downwards, is an abnormality which, if allowed to go uncorrected, can destroy stereoscopic vision and lead to monocular blindness. In children the deviation is most commonly inwards; in adults, when a squint develops afresh, it tends to be outwards.

When such a deviation occurs, there are two broad categories of cause. Either it may be sensory, some malfunction in the image-making processes of the optic system, or it may be motor-related. The abnormality may involve the eye itself, or it may involve any part of the nerves, muscles and brain concerned with visual functions. Thus almost anything that affects the stimulus of sight may be held responsible for the fact that one eye squints. For example, one eye may from birth only be able to form a blurred image so that even when this is optically corrected later on, the brain is unable to fuse its picture with that of the other normal eye. Or the two eyes may be normal in every way, but the brain itself is unable to fuse their pictures.

The commonest explanation of squint among children is excessive long sight. Such children attempt to correct their vision by accommodation, causing the muscles that converge the eyes to contract reflexly. To maintain fusion, to avoid seeing double, the brain subconsciously chooses one eye to fixate whilst the other strays into a corner. Sometimes the eyes alternate in this way, so that first one and then the other is used. Whilst most squints of this type are treatable and preventable by the early use of optical correction, the young age of the child often hinders diagnosis. If an eye stays in a corner and is not used then, in the first place, its visual stimulation is inhibited or suppressed, and, eventually, it may become deprived of sight. If the condition remains untreated for several years during the child’s development then critical good vision may never be regained. When this happens the result is called amblyopia.

Diagnosis at a very early age is difficult, but, with the help of electro-diagnostic methods, it is sometimes possible to find out which part of the visual system is at fault and hence to decide whether treatment will be of any value. Treatment itself may take many forms, but essentially involves stimulation of the weak eye while the good eye is occluded. After a child has reached the age of seven or eight the likelihood of a successful treatment diminishes.

Among adults the development of a squint is most commonly associated with the loss of vision in one eye. Provided that the patient’s binocular vision was normal beforehand it may become possible again once sight is restored to the affected eye. The most usual cause of this type of squint is injury to the eye with resultant cataract formation. Once the cataract is removed and vision is restored, using either a contact lens or intra-ocular implant, binocular vision becomes possible once more. However, the same thing happening to a young child, with a traumatic cataract in one eye while the other stays normal, will rarely result in good vision, even where the cataract is expertly removed and a contact lens worn. The reason for this is the much greater plasticity of the young person’s brain. Even when amblyopia is the result of only a few weeks’ lost vision in one eye, the effect, in a young child, can be decisive. The brain refuses to confuse the good image with the bad one, and so, even during treatment, the strong eye remains dominant, and the weaker one is suppressed.

Apart from injury to the eye, however, squint may be caused by abnormalities in the brain itself, in the nerves, or even in the muscles of the eye. As with sensory malfunction, motor malfunction may be subject to any one of a number of explanations. For example, that part of the brain which co-ordinated eye- movement may be deficient from birth. On the other hand, an injury at birth may prevent a particular eye-muscle from functioning. Again, injury and accident can put a nerve out of action, temporarily or permanently. Providing the sensory part of the eye and of the brain remain unaffected, the result of such a malfunction will be seeing double (diplopia), either part of the time and in particular areas of the visual field, or in the whole field and at all times. Whereas the child easily suppresses one of the images, the adult cannot do this as the neural pathways between the eyes and the visual cortex are too firmly established. The only remedy for double vision of this sort is to keep one eye covered up.

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