Author contribution: Mikhaleva LM — planned and supervised the study; provided equipment for the study; analyzed the obtained results; Voytkovskaya KS — analyzed the literature; collected, analyzed and interpreted the obtained data; processed microphotographs and wrote the manuscript; Fedorov ED — examined the patients; performed EGD and biopsy; analyzed and summarized the data; Shidiy-Zakrua AV — examined the patients, performed EGD and biopsy; Birukov AE — and Gracheva NA — performed pathological examination; Guschin MY — analyzed the literature.