Strabismus

A mouthful to say, this is the best way to refer to misaligned eyes, whether turned in, out, one higher or lower than the other.
Using this term means that on formal testing the person is unable to have or keep their 2 eyes looking at the same object.
It indicates that they are unable to use the eyes together (binocular vision) or that this is impaired, rather than a temporary misalignment that disappears when the 2 eyes are open (binocular vision is normal). 

A "squint" is not only a problem of the eye that turns, but of the whole control system of both eyes. The nerves and muscles that make the eyes turn send an identical signal that makes both eyes move the same amount, and togther. This is why surgery on both eyes is often used to correct the turn - any change in the position of an eye afects the total direction of the 2 eyes as a unit.

"Like a steering wheel"

To try and illustrate this you can think of what happens when you turn the steering wheel in your car

When you turn the steering wheel it sends a control signal of equal amount to each wheel, to turn in the direction that you have turned the wheel. The wheels of the car don't move independently, they have to move together. Your eyes are controlled the same way - nerve signals that go to a muscle in one eye to turn to the side are matched by a signal to the muscle that turns the other eye the same direction.

Imagine if your wheel alignment is out, one wheel could be pointing straight while the other is off-line. If you turn the wheel to bring that one straight, it will make the wheel that was pointing directly ahead shift it's alignment by the same amount, so that it is now no longer 'straight'. A child can often shift which eye they looking at you with, and the turn will seem to switch between the eyes.

ET1.jpg
Esotropia


Simply means one eye at a time is turned inwards, the other looking straight at the object. A common problem, often benefits from surgery.


Exotropia


One eye is turned outwards. This often happens moreso when the person looks at a distant object. This requires surgery if present a majority of the time.


Vertical misalignment


Reasonably common, especially when the person had Strabismus as a young child. This may also be due to a "forth nerve palsy", which usually causes the person to tilt their head to the side (this allows the eyes to realign).


Dissociated Deviations


An interesting problem, seen often with early onset esotropia. The eyes have astrong tendency to want to work independently of each other, so that one will drift off, often upwards.


Paralytic Problems


Denervated muscles don't work properly, so that the eyes are out of alignment when looking in some directions and not others. This is different to the 'wheel alignement' problem as discussed above, and more like one wheel having a mechanical fault that stops it from being able to turn outwards, for instance.


Restrictive problems


A little like paralysis, but most often caused by thyroid eye disease or damaged muscles, unable to move because they are 'stuck' or being held back by something (ie scar-tissue).

Why do my child's eyes cross?

This is actually quite a complex issue. However, it is not due to a problem with the eye muscles themselves in most childhood (or adult) strabismus. Instead, the brain circuits controlling eye alignment are miscalibrated, and send the wrong commands to the muscles.

There are a number of things that set this process of miscalibration in motion, including a congenital tendency, family history, long-sightedness (hyperopia, having to exert excessive fousing power) and poor vision in one eye.

Once the brain loses the ability to keep the 2 eyes aligned (motor control), the eye that is turning produces double vision. This is a very uncomfortable thing for our brains, so to stop the confusing signals it 'adapts' by shutting out the 2nd image. Once the turn is constant this area of suppressed vision forms a blanked out patch (a scotoma). The eyes then become locked in this position. 

Intervention.

It would then seem obvious that doing something to stop this process from taking hold or becoming permanent would be a good idea - that is why a child whose eye clearly turns should be seen without delay. 

How does operating on the muscles fix this problem?

Shifting the position of the muscles on the eyeballs mechanically straightens the eyes. This allows more normal activity to be retored to the brain circuits, which can then repair themselves to get the eyes locked together.