Pharyngeal wall fold influences on the collapsibility of the pharynx

Obstructive sleep apnoea (OSA) is a disease that is characterised by recurrent pharyngeal obstruction during sleep. The pharynx is a hollow muscular tube lined with epithelium that performs the competing functions of breathing, where it is required to be open and swallowing where it is required to close. The mechanical process by which these large changes in luminal dimensions occur have not been considered, however in other biological tubes such as the oesophagus and the bronchial airways narrowing and closure occurs via folding of the mucosal surface. The transmural pressure (P) required to collapse a tube is related to the number of folds (n) formed during collapse by the equation P=n(2)-1, so that the more folds formed during narrowing and closure, the greater the transmural pressure required to collapse the tube. In biomechanical models, the bronchial airway is modelled as a 2-layer tube with an inner epithelial lining and an outer layer of muscle. These models predict that fold numbers will be reduced with thickening and stiffening of the outer layer, accompanied by an increase in collapsibility. We hypothesise that, similar to other biological tubes the pharynx narrows and closes via folding of the surface of the tube, and that the pharynx can also be modelled as a 2-layer tube. We further hypothesise that when compared to healthy subjects, subjects with OSA will have less pharyngeal wall folds during narrowing and closure, and that this reduction in fold numbers will contribute to an increase in pharyngeal collapsibility. In the absence of muscle activity, subjects with OSA have increased pharyngeal collapsibility when compared with healthy subjects, supporting an anatomical contribution to pharyngeal collapse. Histopathological studies of the pharyngeal epithelium in subjects with OSA demonstrate that, compared with age matched subjects, there is thickening of the epithelial surface with oedema of the submucosal layer, with a loss of tethering of the epithelium to the submucosal layer. These changes would tend to decrease fold numbers. There are no measurements of pharyngeal folds, however previous imaging data supports that narrowing of the pharynx occurs via folding. This hypothesis defines a novel anatomical risk for OSA. It also suggests a new therapeutic paradigm for the treatment of OSA, aimed at increasing the folds formed during narrowing and closure.