Strabismus is a disorder in which the eye muscles fail to allow the eyes to move in tandem. Instead, the eyes focus on two different objects -- which means the brain receives two different pictures, often causing double vision or difficulty with depth perception, among other issues. Without treatment, this can lead to more serious complications, such as amblyopia (or lazy eye).
While early treatment can prevent this, sometimes the underlying cause of the strabismus must be treated as well. Here’s a look at some of those causes.
Anatomical vs. Neurological
Each eye has six intraocular muscles that allow the eyeball to move about. These can differ in strength or length in either eye, making it difficult for the eyes to move the same. Additionally, the nerves of the eye may be abnormal. However, just as blindness can originate in two distinct manners, so can strabismus -- the problem may be with the anatomy of the eye or in the visual center of the brain.
There are many different types of strabismus. The main varieties of strabismus include esotropia (one or both eyes veer toward the nose), exotropia (toward the temples), hypotropia (horizontally lower than the normal eye), or hypertropia (horizontally higher than the normal eye).
Infantile Strabismus
Strabismus occurs fairly commonly in children. Additionally, about half of all child strabismus cases are a congenital form of strabismus, which means it is present at or right after birth.
While genetics plays a large role in childhood strabismus, there are other factors that may contribute to the congenital form as well. Drug or alcohol use by the mother, being born prematurely or having a below average birth weight may all increase the risk of congenital strabismus. The presence of certain genetic or congenital disorders (particularly those that affect the brain such as Down’s syndrome or cerebral palsy) or defects of the eyes may also make strabismus more likely.
Childhood Strabismus
Additionally, there is an acquired form of strabismus. It may appear gradually or quite suddenly. Different causes may be more likely to cause different types of strabismus. According to Merck Manual, common causes of acquired strabismus include: refractive error, tumors, head trauma, neurologic conditions (such as cerebral palsy, spina bifida, and palsy of the 3rd cranial nerve, 4th cranial nerve, or 6th cranial nerve), viral infections (such as encephalitis or meningitis), and acquired eye defects.
Hyperopia, or farsightedness, that has never been treated with corrective lenses can actually cause strabismus because the eyes have to work so hard to see. Noonan syndrome, congenital rubella, retinoblastoma, retinopathy of prematurity, and Apert syndrome are also closely linked with the presence of acquired or congenital strabismus in children.
Adult Onset Strabismus
Strabismus may also develop in adulthood. A stroke -- when a blood clot reaches the veins of the brain and cuts off oxygen causing cell death -- is one of the most common adult causes. While the cell death can interfere with the intraocular muscles, the visual cortex may also be damaged, leading to strabismus.
Grave’s disease, a thyroid disorder affecting the immune system, has a serious effect on the eyes and may cause strabismus. Other injuries diseases and disorders that can cause adult onset strabismus include botulism, shellfish poisoning, diabetes, eye or brain injuries , and several other diseases that result in vision loss.
