Strabismus

A squint, or strabismus, is where the eyes are not lined up correctly on the target that a person is looking at. The movement of each eye is controlled by 6 muscles, and requires complex interactions between the visual environment and the brain to maintain fixation on a given object.

Strabismus is often described by the direction of the eye misalignment (eg esotropia, exotropia, hypotropia, hypertropia). It can also be described by its cause if there is involvement of one of the three cranial nerves (III, IV and VI) responsible for eye movement (eg third nerve palsy, fourth nerve palsy). Strabismus can be constant or intermittent. The misalignment may always affect the same eye (unilateral strabismus) or the two eyes may take turns being misaligned (alternating strabismus).

In children, whose visual systems are not fully matured, the brain ignores the visual input from the misaligned eye to prevent double vision, and this typically leads to the development of a ‘lazy eye’ with poorer vision (amblyopia), but seldom gives rise to persistent double vision. In adults, the visual system is mature, and a strabismus will usually give double vision unless the strabismus has been present from childhood. In both adults and children the strabismus will give cosmetic concerns as well, which can affect social confidence etc.

Strabismus in children is usually caused by the wrong signals being sent to the eye muscles, for reasons that are poorly understood. Strabismus due to a very strong focusing error of the eyes is also common in children, and may be completely, or at least partially, fixed by wearing the appropriate glasses. In adults strabismus is often due to childhood strabismus problems or previous surgery, and can also be due to ‘palsies’ of one or more of the 3 nerves that control the movements of the 6 muscles for each eye. It is not so common for a strabismus to be due to a problem of the eye muscles themselves, but thyroid eye disease or previous trauma to the eye socket may cause this.

Esotropia

Exotropia

See also Strabismus Surgery

The primary sign of strabismus is a visible misalignment of the eyes, with one eye turning in, out, up, down or at an oblique angle. Less noticeable cases of strabismus such as small angle strabismus, or intermittent or alternating cases may present with headaches or eyestrain, fatigue when reading and unstable or ‘jittery’ vision.

Not all strabismus needs to be fixed surgically. As mentioned some types of strabismus will be made much better by the use of glasses to fix a focusing problem. In adults, strabismus that is not causing double vision only needs fixing if the cosmetic appearance of the misaligned eyes is of sufficient concern. However, as the visual system of a child is not completely developed, fixing a strabismus earlier than later will often result in a more stable alignment long-term, even though the vision of each eye may not improve. For this reason, children with no double vision may still be offered surgery to straighten the eyes. The risks and benefits of surgery to straighten the eyes need to be weighed up for each individual.

Strabismus surgery involves operating on the eye muscles (even though they may not be the cause of the problem) to tighten and/or loosen them in such a way as to bring the eyes back in to proper alignment. This is almost always done under a general anaesthetic (with you asleep), and as a day stay procedure.

Should strabismus surgery not be desired, other alternatives include no treatment (with risks of ongoing diplopia, cosmetic concerns and/or visual development implications), prescribing special glasses with prisms incorporated into the lenses (if the deviation is small enough, and does not vary in different directions of gaze), or wearing a patch over the deviating eye (not an option for a child as would lead to severe amblyopia).