Most people’s eyes begin to move in concert with each other from infancy (about three months to six months). Both eyes can adjust their position and focus at the same time, regardless of the direction they are facing, allowing the brain to assemble the exact image we see. About 2 percent of children are unable to look at the same object with both eyes at the same time for a variety of reasons, a condition called “strabismus. Strabismus occurs at about the same rate in both sexes. Although there is a family tendency, most relatives of strabismus patients do not have this eye condition. Types of strabismus: Strabismus can be a persistent misalignment of the eyes (called true strabismus); there are generally: internal strabismus, external strabismus, upward strabismus and downward strabismus. 1. Internal strabismus: Internal strabismus, commonly known as “crossed eyes”, is the most common type of strabismus. The symptom is that the eye is tilted to the nasal side. Some children are born with this condition, but in most cases, it begins to appear around the age of two to three. Some cases can be corrected with appropriate glasses, which can reduce the internal strabismus caused by excessive focal adjustment; however, in some cases, after glasses and masking treatment, eye muscle surgery is still necessary. 2. Exotropia: Exotropia, commonly known as “out of the window”, is second only to internal strabismus in terms of prevalence. The symptom is that the two eyes or one of the two eyes is deflected to the outside. Exotropia often begins to occur at the age of two or three. When exotropia first begins, the eyes may shift to the outside for a few seconds when the child is tired or ill, and then return to their original position. A common early symptom of exotropia is that a child will close one eye when playing outside in the sun because of the double image caused by the skewed eye. Upward strabismus” and “downward strabismus”: “Upward strabismus” means that one eye is higher than the other. Children with upward strabismus often tilt their heads to avoid double vision caused by upward strabismus. The “downward strabismus” is a rare condition in which one eye is lower than the normal eye position. It is rarely congenital, but is mostly caused by acquired factors such as trauma and fibrosis of the inferior rectus muscle due to abnormal thyroid function. What is “occult strabismus”: Sometimes the eye position is temporarily skewed when you are not paying attention or tired, but when you look at the object attentively, the eye position can return to the correct position. What is “pseudostrabismus”? Most infants under the age of one appear to have an “internal strabismus”. Because of the wide bridge of the nose and the common Oriental pattern of internal strabismus, the whites of the eyes appear to be small. In appearance, the black eyeballs (cornea) of both eyes appear to be “fighting” with each other across the skin of the nasal bridge, but in fact, the eye position is straight. This phenomenon of “pseudo-internal strabismus” is more obvious when the infant gazes to the left and right. We can check this by shining a flashlight on the eyes. A normal eye can see light reflecting off the cornea. If the eye position is positive, or “pseudostrabismus”, the light reflection point is in the center of the pupil; if it is “true strabismus”, the light reflection point is off the center of the pupil. Fortunately, this phenomenon disappears automatically when the baby grows up (because the bridge of the nose is higher). However, this has led parents or grandparents to believe that all cases of “internal strabismus” will disappear on their own. Because of this misconception, many parents delay taking their children to the ophthalmologist, thus missing the prime time for strabismus treatment. “True internal strabismus does not disappear automatically with age. How is strabismus detected? What is the importance of early detection? Some strabismus is only noticed when there is a difference in the appearance of the eye or a head tilt. If a strabismus in one eye is not detected until the age of 4-5, the vision of the deviated eye will be suppressed and cause amblyopia. Therefore, children should be examined for eye position, visual acuity, and stereoacuity at the age of 4-5, so that strabismic amblyopia can be detected and corrected early, as the prime time for amblyopia treatment is before the age of 6.