Congestion of mastoid mucosa and influence on middle ear pressure - Effect of retroauricular injection of adrenaline

Micro-CT scanning of temporal bones has revealed numerous retroauricular microchannels, which connect the outer bone surface directly to the underlying mastoid air cells. Their structure and dimensions have suggested a separate vascular supply to the mastoid mucosa, which may play a role in middle ear (ME) pressure regulation. This role may be accomplished by changes in the mucosa congestion resulting in volumetric changes, which ultimately affect the pressure of the enclosed ME gas pocket (Boyle's law). Further, such mucosa congestion may be susceptible to α-adrenergic stimulation similar to the mucosa of the nose. The purpose of our study was to investigate these hypotheses by recording the ME pressure in response to adrenergic stimulation administered by retroauricular injections at the surface of the microchannels. In a group of 20 healthy adults we measured the ME pressure by tympanometry initially in the sitting position, and then in the supine position over a 5 min period with 30 s intervals. In each subject, the study included 1) a control reference experiment with no intervention, 2) a control experiment with subcutaneously retroauricular injection of 1 ml isotonic NaCl solution, and 3) a test experiment with subcutaneously retroauricular injection of 1 ml NaCl-adrenaline solution. In both control experiments the ME pressure displayed an immediate increase in response to changing body position; this pressure increase remained stable for the entire period up to five minutes. In the test experiments the ME pressure also showed an initial pressure increase, but it was followed by a distinct significant pressure decrease with a maximum after 90 s. The test group was injected with both a 5 and 10% adrenaline solution, but the responses appeared similar for the two concentrations. Subcutaneous retroauricular injection of adrenaline caused a significant pressure decrease in ME pressure compared with control ears. This may be explained by the microchannels conveying the adrenaline to the underlying mastoid mucosa, where it may result in a vascular constriction and decongestion, ultimately resulting in a ME pressure decrease. These findings suggest that the microchannels contain vascular connections to the mastoid mucosa, and that the mastoid mucosa is susceptible to vasoactive mediators, which may play a role in ME pressure regulation. Further anatomical and physiological experiments should be carried out to confirm these suggestions including pharmacological interactions with the mastoid mucosa.