Usually, to confirm the diagnosis of strabismus, eye position and strabismus angle determination, eye movement examination, compensatory head position, binocular visual function examination and refractive examination are required. Parents often wonder why eye position tests are needed when they can clearly see the strabismus at a glance. What is the purpose of these examinations? Simply put, eye position is the position of the two eyeballs. Our eyeballs are mainly controlled by 6 muscles that control their position. If one of the muscles is too strong or weak, the eye position may be skewed. Therefore, by examining the position of your child’s eye, you can determine if he or she has strabismus and what type of strabismus it is (internal or external strabismus); you can also generally determine the degree of deviation of the eye (also known as oblique angle of vision). So how do you do an eye position check and determination of strabismus? Clinically, the main methods are corneal reflection examination, masking de-masking test and trigeminal neutralization examination. These examinations are usually repeated several times at different times, and they are very important for the diagnosis of strabismus and the design of surgery. Parents often see the doctor shine a flashlight in their child’s eye, which is actually a corneal reflectance examination. The flashlight is placed directly in front of the child’s line of sight and the child is asked to look at the light of the flashlight. If there is no strabismus, the light will be reflected in the center of the pupil of both eyes. If the light in the two eyes is not symmetrical: the light in one eye (the eye that looks at the object, the child’s good eye) is in the center of the pupil, and the light in the other eye (called the deviated eye, the diseased eye) is not in the center, then the child has strabismus. In addition, the doctor can determine the degree of strabismus roughly based on the distance between the reflection point of the diseased eye and the center of the pupil. This test may seem simple, but it requires a high level of cooperation from the child. Because children with strabismus are often young, active, and afraid of the doctor, they may not cooperate with the test, so it may be necessary to repeat the test several times. Moreover, the corneal reflection can only determine the obvious eye position skew, i.e., dominant strabismus, and roughly estimate the strabismic angle; however, for the determination of some strabismus with fusion function, such as intermittent strabismus, other tests such as masking and de-masking test and trigeminal neutralization test should be done. Warm tip: Strabismus: It refers to the degree of skew of strabismus. Dominant strabismus: the deviation of the eye cannot be overcome by the fusion function of both eyes, which is the dominant strabismus, including the constant strabismus that cannot be orthotropic at any time and the intermittent strabismus that is sometimes orthotropic and sometimes deviated. Occult strabismus: Under normal circumstances, the eyes are in proper position when both eyes are looking at the same time, but when the fusion function of the brain is disturbed (e.g., one eye is covered), the eye position is deviated, and once the disturbed factor is removed, the eye position is restored to proper position, i.e., occult strabismus. Occult strabismus is very common and does not need to be treated when there are no symptoms.