Role of the Mastoid in the Middle Ear Function

Secretory otitis media (SOM) is one of the causes of permanent hearing loss. The key factor in the pathogenesis of the SOM is a chronic under-atmospheric middle ear (ME) pressure, which is assigned mainly to an altered ME gas exchange in combination to an impaired Eustachian tube (ET) function. Recent research pointed to the importance of the mastoid in the ME pressure balance related to ambient, based on a much higher area-to-volume ratio compared to the ME cavity, which might be significant for pressure regulatory mechanisms like gas exchange and mucosal swallowing. Newer hypotheses require more studies. Ethical issues favour the use of animal models in studies for ME physiology and pathogenesis, often of rodents, which are readily available and easily maintained. They have a corresponding tympanic cavity, but not the mastoid. An extension of the tympanic cavity called bulla is considered to take over the role of the mastoid. The present work raises the hypothesis that there is a significant difference in the diffusion and perfusion of the bulla versus the tympanic cavity mucosa in the rabbit. The data could be relevant in the interpretation of the studies on ME gas physiology. The method consists of a manual measurement of the distances from the centres of the blood vessels to the surface of mucosa, of the thickness of mucosa layer, of the diameter and surface density of the vascular structures in 1 mm histological samples from tympanic cavity and bulla in 6 rabbits. Data from each region are used in comparative analysis. The results show statistically significant shorter diffusion distances (30.7 vs. 14.8, p < 0.001) and thinner mucosa in the bulla (46.3 vs. 29.3, p = 0.001), but significantly more blood vessels in the tympanic cavity (307 vs. 284, p = 0.002). The vascular diameter did not differ significantly in the tympanic cavity vs. bulla (31.2 vs. 29.7, p = 0.70). Histologically, the mucosa of the tympanic cavity presents a more cubical epithelium and a looser connective tissue, whereas the bulla was lined by a flat surface epithelium. Generally, the results might suggest a higher specialization of the rabbit bulla for the trans-mucosal gas exchange vs. the tympanic cavity, but more efficient swallowing-congestion modifications of mucosa lining in the tympanic cavity.