Induced tropia test eases diagnostic difficulties in children

Take-Home Message: The induced tropia test refines fixation testing and can better identify children with amblyopia or an organic visual deficit.

By Lynda Charters; Reviewed by Stephen P. Christiansen, MD

Boston-The induced tropia test is a valuable tool that medical students, residents, and fellows should be able to perform, suggested Stephen P. Christiansen, MD.

“This test refines fixation testing and can better identify children with amblyopia or an organic visual deficit,” said Dr. Christiansen, professor and chairman, Department of Ophthalmology, Boston Medical Center, Boston University School of Medicine, Boston.

Fixation testing is performed in all children; in the youngest children, visual acuity is evaluated by fixation behavior, typically monocularly, he said.

Performing this test requires a visually interesting accommodative target-such as a brightly colored finger puppet-that is presented separately to each eye. In children with normal vision, the vision is described as central, steady, and maintained.

In a child with strabismus, determining the preferred eye for fixation is straightforward, Dr. Christiansen noted.

“However, when the ocular alignment is normal or near normal, determining the preferred eye is much more difficult,” he said. “We typically described acuity in the non-preferred eye as ‘normal’ if the fixation is determined to be maintained either through a blink, a pursuit movement, or for longer than 3 seconds.”

In a child with straight eyes or a microtropia, however, assessing the visual acuity based on fixation is more difficult.

“The induced tropia test should be used with these children,” Dr. Christiansen said.

Fixation testing

Kenneth Wright, MD, and colleagues first reported the induced tropia test in 1981 (Arch Ophthalmol. 99:1242-1246). To perform the test, a 16-prism diopter base-down prism is presented in front of one eye. This creates a vertical retinal disparity that allows simultaneous observation of the quality and maintenance of fixation in both eyes, Dr. Christiansen explained.

He discussed various scenarios that may be encountered, such as that of a child with equal visual acuity in both eyes and alternating fixation. When the base-down prism is presented to the right, the eyes flip back and forth between the two retinal images. The clinician can presume that because the child alternates fixation so readily, the visual acuity is normal.

In some cases, the prism may create blurring. In this situation, the fixation may not be rapidly alternating. By moving the prism to the other eye, if the behavior is the same, the visual acuity is likely normal.

Dr. Christiansen also described cases in which children do not switch fixation and the clinician has to force them to do so to determine whether they can fixate with either eye.

“If the patient does not switch fixation with either eye, that is a hint,” he said.

In another case, a child may have a fixation preference but has equal vision in both eyes and maintains fixation through a smooth pursuit movement with the non-preferred eye.

“This suggests that there is normal visual acuity in that eye,” he said.

In an amblyopic left eye, for instance, the child can fixate with the right eye and not with the left eye. The child can be forced to fixate with the amblyopic left eye, but the fixation cannot be maintained.

The test may be less reliable in children who are tired, uncooperative, or developmentally delayed. Another consideration is that even a strong fixation preference may not indicate amblyopia in the non-preferred eye, Dr. Christiansen noted.

Stephen P. Christiansen, MD

E: Stephen.Christiansen@bmc.org

This article was adapted from Dr. Christiansen’s presentation during Pediatric Subspecialty Day at the 2014 meeting of the American Academy of Ophthalmology. Dr. Christiansen has no financial interest in any aspect of this report.