Eye misalignment or strabismus can be measured by several different methods. The measurements help guide the surgical and medical management of strabismus. The age, vision, and level of cooperation of the patient determine which method is most accurate and feasible.
Light reflex testing (called Hirschberg testing) involves directing a patient to look at a point of light held about three feet from the patient’s face. If the light reflexes are located in the same spot in each pupil, the reflexes are symmetric and the eyes are straight. If the light reflexes fall asymmetrically in the pupils, strabismus may be present. Hirschberg testing estimates the size of the strabismus by determining how far the deviated light reflex is off-center. Krimsky light reflex testing involves holding a prism over one eye to center the deviated light reflex until the reflexes are symmetric. The amount of the prism needed to center the deviated light reflex estimates the size of the eye misalignment. Light reflex testing is the least accurate way to measure strabismus but may be the only means possible in young children and in those with vision too poor to fixate on a target well.
Cover testing is another method to evaluate strabismus. The patient is instructed to look at a target. While the patient is fixating on the target, one eye is covered. The uncovered eye is observed. If the uncovered eye is well aligned, it will not shift position when the other eye is covered. If the uncovered eye is misaligned, its position will change as the eye shifts to look at the target. For example, if the left eye is turned in, as the right eye is covered, the left eye will move outward to fixate on the target. If an eye is higher and the other eye is covered, the eye makes a downward movement when the other eye is covered.
Prisms are clear, triangular shaped objects that bend light. When held in front of an eye, the prism shifts the pathway of light coming into the eye. The amount the light is shifted is measured in a unit called prism diopters. Prism diopter measurements describe theamount of eye misalignment.
Prism and cover testing is used to determine the amount of strabismus present. A prism is held over the deviated eye and the eyes are alternately covered. When the proper strength of prism is place over the misaligned eye, there is no movement of the eyes when the cover is switched back and forth between the eyes. The eye alignment is measured in different gaze directions, on head tilts, and at near.
A person can look like they have strabismus because of an off-center light reflex and still have straight eyes. They will not have movement of the eyes on cover testing. This is called angle kappa.
Medical and surgical treatment of strabismus is based on the amount of eye misalignment present. Sometimes several office visits are needed to obtain accurate and complete strabismus measurements.
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