Visual acuity testing can be done without using letters, for example in children or other individuals who might be literacy challenged. Another example would be working with foreign adults who are only familiar with their own alphabets (e.g. Japanese).
There is the “tumbling E” chart which requires the patient to point fingers in the direction of the 3 lines in the letter “E.” The “E” may be oriented either up, down, or to the left or right.
There is also the “animal test” where stylized objects easily recognizable as, say, a bird or horse in silhouette, are used as test objects. There is also the Landolt C test which uses rings much like the letter C.” The patient then points to the direction of the opening in the ring (where the C has the break in it.
But how can we measure the vision in a patient who might not be able to respond, either verbally or with pointing? While it would hardly be anywhere near the level of visual assessment measured in a typical eye exam, it is possible to assess vision in even comatose patients. The eyes respond reflexively to certain stimuli such as rotating bands of black and white stripes. The Oculo-Vestibular test is used by neurologists as well.