Visual Acuity

   This is possibly our most important test. Each eye is tested separately - a child may have profound vision loss due to amblyopia affecting one eye, yet see perfectly well with the opposite normal eye while both eyes are open.

   School age children can identify Snellen letters, with the traditional 20/xx notation. It may be helpful to recognize that this is a reference, not a fraction denoting the absolute level of visual function. Hence, 20/40 refers to the distance at which the test is done (20 feet), and the relative size of the letter, which is twice as large as a 20/20 letter. It does not mean that vision is one-half of normal.

   Younger verbal children can usually identify pictures, which are not as precisely accurate as Snellen letters but quite useful for detecting amblyopia and monitoring treatment. Pre-verbal children are observed using "central-steady-maintained" guidelines, which will provide reasonable diagnostic information for a trained observer.