Regarding the middle ear ventilation routes in CSOM more detailed studies are required to establish the relevance and practical surgical implications. Objectives: 1) To describe the frequency of middle ear ventilation routes blockages in CSOM and normal ears. 2) To compare those findings in order to accept or reject the null hypothesis of similar distribution of variables in both groups. Design: Cross sectional, prevalence analytic study. Method: 46 CSOM ears taken to Tympanoplasty and 17 normal ears (cadavers). Results: Bivariate on middle ear ventilatory routes blockage between both groups were done, statistically significant difference was found for the distribution in both groups for blockage of the protympanum, isthmus, Eustachian tube diameter at the infundibulum in mm with p<0,05. Complete Tensor fold failed to demonstrate any statistically difference when compared in both groups. Eustachian tube diameter was reclassified in < ó ≥1,5 and it was introduced to a regression model with the other middle ear ventilation variables, the former demonstrated its capacity to explain the variability of the CSOM presence in 40%. Discussion: There was a statistically significant difference comparing both groups of CSOM and normal ears respect to the middle ear ventilatory routes blockage and the Eustachian tube diameter, this is relevant from a surgical and physio pathological point of view.