When you abbreviate, do you do it only once at the beginning of the literature review? i.e. Down syndrome (DS)?
The APA style does permit the use of abbreviations for freqently used words which are familiar to the audience who will be reading an article. In a literature review, such as found in an APA-style journal, the word should be explained the first time it is used. For this example, the author could go with:
Down Syndrome (DS)
when the term is first used. After that, it would be acceptable to use DS.
For more information, see section 4.25 of APA, 6th ed.