For each aspect of your PICOT question, you will need to gather multiple varying search terms to describe that topical area. Do not get "stuck" on one phrase or search term combination to describe your research question. Below are some examples of terms that could be used to search for research relevant to visual impairment.
Loss of Visual Acuity
Conducting a thorough literature review will require that you do many searches using varying combinations of terms across many databases. If you are having difficulty identifying terms, use Google to read a few articles or websites about your area of research to gather ideas for new terminology. Read abstracts and the full text of articles very carefully looking for terms and phrases researchers use to describe the condition or intervention. As you search each database, look for subject tags assigned to articles or additional suggested search terms.
Be a flexible searcher. Search term combinations will work differently in each database, so one combination that gives great results in one database may not work at all in another.
Also recognize that some databases are much smaller than others and may yield only one or two articles using very broad search terms. Other databases are extremely large and may yield hundreds or even thousands of hits using even very specific search term combinations.