Strabismus and problems with extraocular muscles are frequently present whenever there are issues with uneven development of the eyes.
To direct our gaze wherever desired, and to be able to track the moving objects, human eyes are equipped with specialized muscles that move our two eyes jointly in our eye sockets. Humans have six extraocular muscles, that origin in the back of the orbit and insert on the eyeball. Four of them are rectus muscles – medial, lateral, superior and inferior rectus – and two of them are oblique muscles – inferior and superior oblique. They are innervated by three different cranial nerves that derive deep in the brainstem and are under voluntary as well as involuntary control.
Extraocular muscles are the effectors of one extremely complicated process that is eye movement. Eye movement as such is a complex concert of different eye movement patterns in different situations, some of them being smooth pursuit, saccades, movements pursuit, vestibulo-ocular reflex and rapid eye movement (REM) during sleep. We may be going into more detail on that in some of the future blogs, but today I just want to emphasize that eye movement is not as simple as one would perhaps imagine.
If we go back to the extraocular muscles, the healthy coordination of all 6 muscles in both our eyes is needed for us to be able to perceive a single image with good depth perception. If, however, the latter is not so strabismus can occur.
Strabismus, also named heterotropia or squint, is a disorder in which the two eyes misalign with each other. There are different types of strabismus that include esotropia (eyes are turned inward), exotropia (eyes deviate outward), and hypertropia (eyes are vertically misaligned). Furthermore, strabismus can be divided into concomitant (eyes deviation is independent of the position of gaze) and incomitant (eyes deviation changes with the position of gaze). In some cases, strabismus can also be present occasionally, whilst in most instances, it is present constantly (until properly addressed).
The onset of strabismus
Regarding the time of onset, strabismus can be classified as congenital, acquired, or secondary to other ocular pathology. Congenital strabismus is quite frequent as many infants are born with a slight deviation of their eyes. Thankfully, in the majority of cases, the two eyes align correctly with no treatment in months after birth. Strabismus often commences in early childhood and in many cases no specific cause can be identified – referred to as acquired strabismus. In some other cases, though, strabismus occurs as a complication of many ocular and neurological diseases or trauma to the eyes and adnexal parts of the eye.
If strabismus arises later in life, when the binocular function of the two eyes is already fully developed, a person will most likely experience very bothersome diplopia or double vision, as the two images one is getting from the eyes are of equal quality, but not aligned as they should be. If, on the other hand, strabismus commences early, when one’s binocular vision is not fully developed, a child might not notice any symptoms at all as the brain can easily shut down the image from the deviating eye that is of lesser quality than that from the leading eye – contributing to the development of amblyopia and impaired binocular vision.
Management of the problem
Whilst amblyopia development due to strabismus in children is very likely, strabismus is otherwise a disorder of binocular vision, so the primary goal of treatment is a single, clear, binocular vision in all directions of gaze. The latter can be achieved by a combination of specialized prism glasses, medication such as botulinum toxin, or surgery. In children, however, eye patching and vision therapy are often needed in order to treat amblyopia in the deviating eye.