Understanding, detecting, and managing strabismus

Whereas many animals have eyes located on either side of their head (such as horses, for example), the eyes of humans look forwards – in the same direction. When normal, the eyes move in a coordinated manner, so that the object being looked at is centred in each eye. Because the eyes are set a small distance apart, the image in each eye is slightly different. The brain fuses the images coming from both eyes to produce a three-dimensional image that has depth. This three-dimensional vision, also known as stereoscopic or binocular vision, gives us depth perception. This allows us to judge distances more accurately, especially with objects close to us. Try to thread a needle with only one eye open and you will see the advantage of binocular vision! In order to achieve normal binocular vision, the eyes must see well, be aligned (looking in the same direction), and be focused properly on the same object. To maintain alignment, the eyes must also move in a coordinated manner, a process involving twelve different muscles (six in each eye). The four rectus muscles move the eyes up, down, to the right, and to the left, and the two oblique muscles have more complex actions, helping the eyes to look down and in (towards the tip of the nose) or up and in (towards the bridge of the nose). Three different cranial nerves are involved in the contraction and relaxation of these muscles and the main coordinating centre is in the brain. Misalignment of the eyes is called strabismus (or squint). Misalignment means that the eyes are not lined up to look at the same thing. In every case of strabismus or misalignment, one eye is fixed on what the person intends to look at (the fixing eye) and the other eye is looking at something else (the deviated eye). Types of strabismus 1 One eye can be deviated inwards (sometimes referred to as being ‘crosseyed’). This is called esotropia. 2 One eye can be deviated outward (sometimes referred to as a ‘wall eye’). This is called exotropia. 3 One eye can be deviated vertically, either upwards (hypertropia) or downwards (hypotropia).