As you embark on a literature search, begin with a scoping search to locate existing literature syntheses: "... a scoping review is an iterative process whereby existing literature is identified, examined and conceptually mapped, and where gaps are identified. Think of a scoping review as a first step in doing any systematic literature review or large study." ( http://hlwiki.slais.ubc.ca/index.php/Scoping_studies )
For example, you might need overview, "background" information on a PROBLEM, PATIENT OR POPULATION:
A disease: type 2 diabetes, Parkinson's, autistic disorder, UTI, osteoarthritis
A procedure: knee replacement
A patient population: "patients with indwelling urinary catheters," "post-op C-section," toddler with ear infection.
And, you might need overview, general reference content on an INTERVENTION: