Tests for Bilateral Vision Loss
Optokinetic response: Eliciting an optokinetic response is helpful if the reported visual acuity (VA) loss is severe and binocular. Optokinetic nystagmus is difficult to suppress and requires a VA of at least 20/400 in one eye.
Navigating skills: When very poor VA is reported in both eyes, it is helpful to watch patients navigate the waiting and examination rooms. The ability to navigate a complex environment without auditory cues is difficult to reconcile with vision of reported light perception or worse.
Tests for Unilateral Vision Loss
Duochrome testing: This can be helpful in establishing that patients with asymmetric VAs can see better than reported. When the duochrome chart is viewed through the glasses used for Worth 4 dot testing (one eye viewing through a red lens and one through a green lens), the eye viewing through the red lens only perceives the red portion of the screen and vice versa for the eye viewing through the green lens. Many patients do not recognize that each eye should only be able to see the letters on one side of the screen. For example, if the reported VA with conventional testing is 20/100 OD and 20/20 OS, but the patient can read the entire 20/20 line through the Worth 4 dot glasses, the vision has to be at least 20/20 in each eye.
Fogging the good eye: Progressively (and subtly) “fogging” the “good” eye in cases of markedly asymmetric VA can also be helpful. If a phoropter is available, the patient can be given a line perceived by the “good” eye but too small to be perceived by the “bad” eye. Progressive plus lenses can be added to the “good” eye until the lens is adequately strong to render the eye unable to see the image. If the patient can continue reading lines of equivalent size, it is clear that the “bad” eye has vision better than reported. However, the patient’s refraction must be taken into account, and one must be certain that the lens used over the “good eye” is actually strong enough to prevent it from reading the line used.
Tests for Unilateral or Bilateral Vision Loss
Ishihara color plates: Correct identification of the Ishihara color plates requires a VA of at least 20/400.
Stereopsis: Evaluating stereopsis can be helpful if the reported VA loss is subtle and ocular alignment is normal. The correlation between VA in the worse eye and number of Titmus dots perceived is well established.
Comparing distance VA to near VA (given proper refraction): This can be a quick and helpful way of assessing the reliability of a reported level of vision. VA can also be checked by starting at the 20/10 line and increasing the size slowly (“bottom up acuity”), or by trying multiple attempts at different 20/20 lines as the examiner explains that the optotypes are “getting bigger.”