Mechanical, histologic, and biochemical effects of canine rectal formalin instillation

Instillation of 4 percent formalin effectively treats radiation hemorrhagic proctitis; however, little is known regarding its side effects. The study contained herein was undertaken to determine rectal compliance and collagen content, mucosal and vascular histologic changes, and kinetics of formalin absorption following instillation. Fifteen mongrel dogs (50-60 pounds) were randomized into five experimental groups according to time elapsed from formalin treatment: control, acute, one week, two weeks, and four weeks. Formalin was instilled in 30-ml aliquots to a total volume of 400 ml. Rectal compliance (closed manometry system) was assessed pre-formalin and post-formalin at the designated time interval. Serum formalin metabolites were determined at time 0, 0.5, 1, and 3 hours. A segment of rectal wall was analyzed for collagen content, mucosal injury, and blood vessel density. Serum formalin levels peaked within 30 minutes, returning to normal by 3 hours. With the exception of one dog, toxic levels were not reached at any time during the study. No dogs experienced sepsis, fever, or altered gastrointestinal function. Acute and one-week dogs showed mild diffuse proctitis and mucosal slough, which healed within two weeks. Rectal compliance and collagen content were unchanged. Mucosal blood vessels decreased in number early (P = 0.03). Instillation of 4 percent formalin in sequential aliquots of a small volume that is kept in contact for a short period of time is safe. Serum formalin levels generally do not reach toxic levels, and the slight elevation in formalin concentration that was seen returns to normal within three hours. Formalin-induced proctitis heals within two weeks, and no long-term changes in rectal compliance or collagen content were seen.