Middle Ear Barotrauma

Excerpt

Otic Barotrauma (OBT) or ear barotrauma is a tissue injury to the ear secondary to inadequate pressure equalization between gas-filled body spaces and the external environment. Eustachian tube dysfunction (ETD) and middle ear barotrauma (MEBT) remain the most common complication of diving and clinical hyperbaric oxygen treatment (HBO).

The outside portion of the ear, the pinna, is composed primarily of ridged cartilage covered by skin. The external auditory meatus, or the opening to the external auditory canal (EAC), extends toward and ends at the tympanic membrane (TM). Posterior to the TM is the air-filled middle ear space containing three bony ossicles: the malleus, incus, and stapes. The umbo is the distal portion of the malleus and connects the bony ossicles to the TM. It is an expected anatomical finding on otoscopy and easily visualized. The other ossicles are visible when favorable anatomical conditions are present, such as a transparent TM. The third ossicle, the stapes, abuts the oval window, leading into the inner ear space. The inner ear space, containing the auditory (cochlea) and vestibular (semi-circular canals, utricle, and saccule) systems, is separated from the middle ear by the oval and round (labyrinthine) windows. The cochlea is responsible for sound transmission and is composed of three fluid-filled compartments: the scala vestibuli and scala tympani containing perilymph; and the scale media containing endolymph. The vestibular system, which is continuous with the cochlea, is responsible for spatial orientation and balance.

The middle ear space is covered by mucosa and is connected to the throat via the eustachian tube (ET), also referred to as the auditory tube. The ET opens just beyond the nasal openings in the posterior nasopharynx, allowing the drainage of fluid produced in the middle ear space. It is also responsible for the exchange of air between the nasopharynx and the middle ear space, maintaining equal pressure between the middle ear and the EAC. This is known as the equalization of the middle ear pressure. It is important to note that while equalization of the middle ear via the ET on ascent is passive, it requires an active maneuver on descent. Another less described air exchange takes place via the middle ear mucosa and mixed venous circulation. However, this transmucosal gas exchange is less important during rapid and large changes in ambient pressure occurring during diving, flying, or when being treated in a hyperbaric chamber.

Sound waves travel via the EAC producing TM vibration, which is then carried through, and amplified by, the bony ossicular chain. The stapes moves in and out against the thin membrane of the oval window, causing vibration and turbulence of the perilymph fluid within the cochlea, producing a wave of motion. The turbulence of the perilymph continues to the scala tympani and on to the round window where sound pressure is dissipated. This dissipation of pressure at the round window becomes an important factor in inner ear barotrauma (IEBT). Perilymph turbulence within the cochlea results in a shift of the basilar membrane, upon which sits the Organ of Corti. Movement of the mechanosensory hair cells in the Organ of Corti results in sound transmission in the form of nerve impulses, which are received by the central nervous system via the eighth cranial nerve.

While most minor damage secondary to MEBT heals rapidly and uneventfully, major trauma, such as a perforation of the TM or IEBT may take weeks or months to heal. Significant barotrauma may be associated with permanent complications such as hearing and balance deficits. This is why it is important that divers, passengers of pressurized aircraft, and clinical hyperbaric patients be taught and understand how to equalize middle ear pressure often and early during the course of pressurization. Prevention and recognition of ETD and MEBT remain important when evaluating and treating a pressure-related injury.