Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:It seems that systemic sympathetic denervation and lack of sympathetic compensation contribute to both pronounced morbidity and the development of PPH and OH (Jansen and Lipsitz 1995; Umehara et al. 2014; Mathias 1991; Teramoto et al. 1997).
Category: