Cover Test & Strabismus Simulator

1. Establish fixation

   Ask the patient to fixate a clear target. Cover testing is commonly performed at near around 33 cm and at distance around 6 m.

2. Start with the cover-uncover test

   Cover one eye for a few seconds and watch the uncovered eye. Refixation movement of the uncovered eye indicates a manifest deviation (tropia). Repeat on the fellow eye.

3. Look for a latent deviation

   With a phoria, alignment is straight during binocular viewing, but the covered eye drifts when fusion is broken and refixates when uncovered.

4. Use alternate cover to dissociate fully

   Move the occluder from one eye to the other, allowing time for each uncovered eye to take up fixation. This more dissociative test reveals the total manifest plus latent deviation.

5. Measure with prism

   Add prism and increase its power during alternate cover testing until refixation movement is neutralized. The prism power at neutrality estimates the deviation in prism diopters.

For unilateral manifest horizontal deviations, also identify the deviating eye as right or left. A phoria is a binocular alignment relationship and is not usually assigned to one eye. Vertical deviations are commonly named for the higher eye; right hypertropia is equivalent to left hypotropia. Base-out is typically used to neutralize esodeviation and base-in for exodeviation. This simulator is for training only.