Results General description of the literature Sources initially identified from each database were as follows: MEDLINE n=6, Embase n=6, Web of Science n=62, OpenGrey n=0 and GreyLit n=0. The screening process resulted in 30 documents which were reviewed based on their titles and abstracts. Following the screening 5 duplicate papers were removed and 25 papers were identified for full-text screening. Further studies were identified by searching the reference lists.