Strabism and how big is the field of vision

During my presentation on university we have discussed the strabism and how big is the field of vision for different types of strabism.

Fig. Strabism (Esotropia - the upper image, Exotropia - lower image)

Strabism is the disease when both eyes cannot simultaneously gaze at the same fixation point. Therefore the image from both eyes cannot be combined in a single image and this can cause so called "double vision" = diplopia (Light from the visual input passess through lens to the retina but to not corresponding places - from one eye it reaches the fovea but in the other eye not - therefore the subsequent fusion of image is not possible). There are two main different kinds of strabism: esotropia (axis of eyes intersect before fixation point) and exotropia (axes of eyes intersect behing fixation point). Less frequent are hyper- and hypotropia which correspond to the deviation in vertical line.

Fig. Esotropia and Exotropia

But what is the cause of strabism?

Strabism can be caused by different length of extraocular muscles or by their lack of coordination. Secondly, strabism can be also caused by problems with ocular nerves and subsequent problems with neural interpretation of visual input (this is called lazy eye or amblyopia and brain will favor one of the eye and suppress the other). What I was not sure whether the strabism is somehow connected to the problems with eye lens. My collegue told me that the problem is not primarily caused by problems with lens but without any cure the problems with eye lens can appear (caused by using only one eye etc.).

How can people with strabism see and work?

This is very interesting. When you use only one eye, you cannot percieve the depth. When using both eyes they can perceive "double vision" which can be quite dangerous in normal life: people with this problem can fall from stairs or have problems with driving. Therefore they frequently cover one of their eyes (especially the ill one which can cause further problems which I will discuss beneath). The brain itself will supress deviated eye in the central visual field where we have binocular vision.

Fig. Diplopia (double vision)

Do they have bigger/smaller field of vision compared to healthy human?

You maybe know that the horizontal field of vision for normal human is something as 200° (each eye about 160°, binocullar vision ~ 120°).

We could expect that people with exotropia should have bigger field of vision than people with normal vision and similarly people with esotropia should have this field of vision smaller. Anyway, this is not exactly true. People with strabism have "bigger field of vision" when using only one eye (the second eye is covered), but while brain cannot work with "double vision", it accomodates and the central part of the field (approximatelly 20° is excluded). Sometimes one eye is completly suppressed.

What is even more interesting is that when you completelly cover one of your eyes (to improve your vision) and stop using it, the muscles which are used for accomodation of lens will loose their strength and afterwards it can also cause total "deth" of one of your eyes (also neural connection are not working anymore).

And...how big is the field of vision for birds?

Just interested, whether birds with bigger eyes (and lens) have also bigger field of vision. And the answer is YES! Yes, they have. Something as 300° compared to 200° which is the normal humens' field of vision. Furthermore they can perceive even small changes in light intensity (owl), big distances, binocularly and some birds see also colors.

OK! But how to cure strabism?

There are several methods. One of them is to cover the healthy eye which will cause that the "ill" eye will have to work more. Other methods are mainly focused on strengthening and training extraocular muscles. One of my collegue is prepares set of games which can be used for diagnosing and curing strabism: here (unfortunatelly just in Czech).

One problem with curing strabism can be that after cure, the brain is not able to supress the deviated eye in the area of binocular vision and the patient can again perceive "double vision" (dyplopia).