Received: 30 January 2019
Accepted: 7 November 2019
First Online: 9 December 2019
Ethics approval and consent to participate
: The study protocol which included a written consent to participate has received approval by the Commission of Ethics in Scientific Research, Medical University - Varna, Bulgaria (Protocol № 57, Year 2016; Final report accepted with Protocol № 75/07.06.2018).
: Not applicable.
: The authors declare that they have no competing interests.