Do morphological differences in orientation of the Eustachian tube and dilator tubae muscle distinguish populations with high rates of middle ear disease?

Abstract

Middle ear disease or otitis media (OM) is among the most common pediatric conditions globally. It is nearly ubiquitous among every human population, usually occurring within the first three postnatal years. The etiology of OM is known to be rooted in the short, horizontal cartilaginous Eustachian tubes (CET) of infants which are mechanically inefficient relative to those of adults. Yet, despite the presence of OM among all human populations, Inuit and Aboriginal Australian communities have been documented as exhibiting some of the highest rates globally. Little information exists on the normal morphology of the bony nasopharyngeal boundaries of these groups. We tested whether they could be distinguished from other human populations in the orientation of the cartilaginous Eustachian tube and dilator tubae (DT) belly of the tensor veli palatini muscle (TVP), the sole dilatory apparatus of the CET. Any morphological feature distinguishing groups with higher rates of OM may impact susceptibility. We reconstructed soft tissue CET and DT morphology from adult dry crania. The sample represented eight populations: Alaska (n=156); Aleutian Islands (n=14); China (n=37); Greenland (n=28); Australia (n=39); Germany (n=22); India (n=15); and West Africa (n=19). Alaskans, Aleuts, Greenlanders, and Australians were considered populations with high rates of OM while the remainder were used as comparator populations with relatively lower documented rates of OM. We assessed morphology from coordinate data collected along the posterior edges of the medial pterygoid plates, the hard palate, the basicranial midline, and the osseous orifice of the Eustachian tube. Angular measures were assessed from the coordinate data and Bonferroni-Dunn-adjusted multiple t-tests were performed. Results showed that the Alaskans exhibited the most horizontal orientation of the DT relative to the axis of its common tendon with the TVP palatal sub-belly (an infant-like trait), being significantly more horizontal (P<0.05) than among all other groups. This trait could impact the function of the CET, especially among infants whose DT orientation would be even more horizontal. The only measure that distinguished Alaskans, Greenlanders, and Australians from all other populations was the sagittal orientation of the CET, an additional trait that may impact DT function. The presence of these morphological features may thus be related to increased rates of OM.