Middle ear equalization, also known as ear clearing, is the process of balancing the pressure of gas in the middle ear with ambient pressure. Pressure regulation is principally achieved by passage of gas through the Eustachian tube, which connects the middle ear to the nasopharynx. The tube normally opens intermittently, including during swallowing and yawning, but deliberate equalization may be required when ambient pressure changes more rapidly than normal Eustachian tube function can compensate.^[1][2]

Equalization is particularly important during underwater diving, rapid changes in altitude and aviation, hyperbaric treatment, and work in compressed-air environments such as saturation diving. During an increase in ambient pressure, gas must enter the middle ear to prevent the middle-ear pressure from falling below that of the environment; during a decrease in ambient pressure, expanding middle-ear gas normally vents through the Eustachian tube.^[3]

Middle-ear pressure can be equalized naturally by swallowing or yawning, or deliberately using techniques such as the Valsalva, Frenzel and Toynbee manoeuvres and voluntary tubal opening. Failure to equalize can result in middle ear barotrauma. Inner-ear barotrauma may also occur during failed equalization or following forceful equalization manoeuvres, while asymmetric equalization between the ears can cause alternobaric vertigo.^[3][4]

The middle ear is a gas-filled cavity separated from the external auditory canal by the tympanic membrane (or 'ear drum'). It communicates with the nasopharynx through the Eustachian tube. One of the principal functions of the Eustachian tube is regulation of middle-ear pressure relative to ambient pressure; its other functions include clearance of middle-ear secretions and protection of the middle ear from nasopharyngeal secretions and pressure fluctuations.^[5]

The Eustachian tube is normally closed and opens intermittently during activities such as swallowing and yawning. Opening is produced principally by contraction of the tensor veli palatini muscle, with the levator veli palatini also contributing to the opening mechanism. When the tube opens, gas can pass between the nasopharynx and middle ear, allowing the middle-ear pressure to approach ambient pressure.^[5] Gas exchange also occurs continuously between the middle-ear cavity and surrounding tissues; consequently, maintenance of middle-ear pressure is a dynamic process involving both passive gas exchange and intermittent ventilation through the Eustachian tube.^[6]

When ambient pressure changes more rapidly than pressure can equilibrate through the Eustachian tube, a pressure difference develops across the tympanic membrane. During an increase in ambient pressure, such as during descent in water or descent from altitude, middle-ear pressure becomes lower than the pressure in the external auditory canal unless gas enters the middle ear. Conversely, when ambient pressure decreases, the gas in the middle ear expands relative to the surrounding pressure and must be vented through the Eustachian tube. In diving, equalization of the middle-ear space is therefore generally active during descent and normally passive during ascent.^[3]

The ability of the Eustachian tube to regulate pressure may be adequate under ordinary atmospheric conditions but insufficient during rapid changes in ambient pressure. This is recognised clinically as baro-challenge-induced Eustachian tube dysfunction, in which symptoms occur specifically during pressure changes such as diving or changes in altitude.^[7] Deliberate middle-ear equalization manoeuvres facilitate pressure regulation by opening the Eustachian tube or by creating a pressure gradient that moves gas through it.

Middle-ear equalization is required when changes in ambient pressure occur sufficiently rapidly that pressure in the middle ear cannot equilibrate through normal Eustachian tube function. Important situations include underwater diving, changes in altitude, and exposure to increased pressure in hyperbaric chambers or compressed-air workplaces such as saturation diving.^[8]

During underwater diving, ambient pressure increases with depth. On descent, compression of the gas in the middle ear produces a pressure difference across the tympanic membrane unless gas enters through the Eustachian tube to equalize the middle-ear pressure with the surrounding pressure. Equalization during descent therefore generally requires active opening of the Eustachian tube. During ascent, the reduction in ambient pressure causes middle-ear gas to expand and it normally vents passively through the Eustachian tube.^[3]

The proportional change in the volume of a gas for a given change in depth is greatest near the surface, so the requirement for pressure equalization is particularly marked during the initial part of a descent. Failure to equalize the middle ear can result in a progressively increasing pressure difference and middle ear barotrauma.^[3]

As the pressure difference increases, opening the Eustachian tube becomes progressively more difficult; at a differential of approximately 90 mmHg (12 kPa), the tube may become functionally locked closed, preventing voluntary opening and making a Valsalva maneuver unsuccessful.^[9]

In freediving, the diver descends while holding a single breath, so the volume of gas available in the lungs decreases progressively with increasing ambient pressure. This makes efficient middle-ear equalization particularly important and can limit the depth attainable during a dive.^[10] The Frenzel manoeuvre is commonly used because it generates nasopharyngeal pressure using the tongue and pharyngeal musculature rather than increased intrathoracic pressure; in one prospective study, experienced freedivers preferentially used the Frenzel manoeuvre.^[11]

At greater depths, freedivers may use mouth-fill equalization, in which gas is transferred from the lungs to the oral and nasopharyngeal spaces before further lung compression prevents its effective transfer, and is subsequently used for equalization during continued descent. Mouth-fill equalization is included in advanced freediving training programmes.^[12]

Changes in barometric pressure with altitude also require equilibration of middle-ear pressure. In aircraft, cabin pressure normally decreases during ascent and increases again during descent. Expanding middle-ear gas usually vents through the Eustachian tube during ascent, whereas during descent the tube may need to be opened actively to allow gas to enter the middle ear. Swallowing, yawning and deliberate equalization manoeuvres may be used for this purpose.^[13] Similar pressure changes may occur with other sufficiently rapid changes in altitude.^[8] Fighter pilots conducting dive bombing maneuvers may use Frenzel equalization.

Middle-ear equalization is also required during exposure to pressures greater than normal atmospheric pressure. During hyperbaric oxygen therapy, patients are pressurized in a hyperbaric chamber and must equalize the middle-ear spaces during compression. Difficulty with pressure equalization is a recognized problem during hyperbaric treatment and can prevent completion of a treatment session.^[14]

Similar exposures occur in occupational compressed-air work, including work in pressurized tunnels and other compressed-air environments. Occupational health guidance identifies the ears and paranasal sinuses as principal gas-containing spaces susceptible to effects of changing pressure in compressed-air workers.^[15]

Middle-ear pressure can be equalized by several methods. Some occur naturally, such as swallowing and yawning, while deliberate manoeuvres either increase pressure in the nasopharynx or actively open the Eustachian tube. Pressure-generating manoeuvres differ in the source of this pressure. The Valsalva manoeuvre increases intrapulmonary and intrathoracic pressure and transmits it to the nasopharynx through an open glottis, whereas the Frenzel manoeuvre closes the glottis and generates pressure locally in the oral cavity and nasopharynx using movements of the tongue and pharyngeal musculature.^[16]

Several maneuvers are used to facilitate opening of the Eustachian tube or to generate a pressure gradient across it:^[3]

• Swallowing and yawning can produce active dilation of the Eustachian tube through contraction of the paratubal muscles.^[17]
• The Valsalva maneuver increases nasopharyngeal pressure by attempted expiration with the mouth and nose occluded, which can produce active opening of the Eustachian tube.^[17] Forceful performance of the maneuver, particularly when the Eustachian tube is obstructed, can produce injurious pressure changes in the inner ear and is therefore discouraged.^[3][18]
• The Frenzel maneuver raises nasopharyngeal pressure by closing the glottis and compressing air in the nasopharynx using movements of the tongue and pharyngeal musculature, rather than by expiratory effort of the diaphram.^[19]
• The Toynbee maneuver consists of swallowing while the nostrils are occluded. Unlike the Valsalva and Frenzel maneuvers, it primarily uses the muscular opening associated with swallowing rather than sustained positive nasopharyngeal pressure.^[17][19]
• Voluntary tubal opening (French: béance tubaire volontaire, BTV) involves voluntary activation of the muscles that open the Eustachian tube. The technique can thereby maintain Eustachian-tube patency without generating positive nasopharyngeal pressure.^[3]
• Other named techniques used in diving include the Lowry and Edmonds techniques, which combine elements of the preceding methods.^[3]
• Mouthfill equalization – a technique used in deep freediving in which gas is transferred from the lungs to the mouth and upper airway during descent and retained above the closed glottis for subsequent equalization as lung volume decreases.^[20]

Comparative evidence for the effectiveness of individual techniques is limited. In a study of 35 healthy volunteers, no statistically significant overall difference in successful middle-ear pressure equalization was found between Valsalva, Toynbee and politzerization; in some participants for whom one technique was unsuccessful, another was effective.^[21] A pressure-chamber study of experienced freedivers found that the Frenzel was at least as effective as Valsalva in producing Eustachian-tube opening, although the study involved a small number of participants.^[19]

Middle-ear inflation can also be performed using an external pressure source. Politzerization introduces pressurized air through the nose to inflate the middle ear during swallowing and has been used medically to treat Eustachian-tube dysfunction.^[22]

Failure to equalize middle-ear pressure may result from impaired Eustachian tube function or from a change in ambient pressure that exceeds the capacity of the tube to ventilate the middle ear. Inadequate equalization can cause middle ear barotrauma. Inner ear barotrauma may also occur during failed middle-ear equalization or as a consequence of forceful equalization manoeuvres such as Valsalva.^[23] Unequal pressure between the two middle ears can also cause alternobaric vertigo.^[24]

Eustachian tube dysfunction (ETD) is characterized by impaired regulation of middle-ear pressure. Baro-challenge-induced Eustachian tube dysfunction is a recognized subtype in which symptoms develop specifically during changes in ambient pressure, such as during diving or changes in altitude. Otoscopy and tympanometry may be normal at normal ambient pressure, although a sufficiently large pressure challenge may result in middle-ear effusion or haemotympanum.^[25]

When ambient pressure increases without sufficient gas entering the middle ear, the resulting negative middle-ear pressure displaces the tympanic membrane inward. A persistent pressure difference can produce mucosal and tympanic-membrane oedema, transudation and haemorrhage, and at greater pressure differences may result in tympanic membrane perforation.^[23] As the pressure difference increases, opening the Eustachian tube becomes progressively more difficult; at a differential of approximately 90 mmHg (12 kPa), the tube may become functionally locked closed, preventing voluntary opening until the pressure difference is reduced.^[26]

During a decrease in ambient pressure, gas in the middle ear normally vents passively through the Eustachian tube. If this outflow is obstructed, expanding gas produces a positive middle-ear pressure relative to the environment and can cause barotrauma during decompression, sometimes termed a reverse block or reverse squeeze in diving.^[23]

Inner ear barotrauma results from a pressure gradient between the middle and inner ear, with pressure transmitted through the oval window and round window. It may occur in association with failed middle-ear equalization or with forceful equalization manoeuvres. In particular, a forceful Valsalva manoeuvre against an obstructed Eustachian tube can increase intracranial and inner-ear pressure while negative pressure persists in the middle ear, potentially injuring the round or oval window and producing a perilymphatic fistula.^[23] Inner-ear barotrauma may produce sensorineural hearing loss, tinnitus, vertigo or disequilibrium.^[27]

In divers, inner-ear barotrauma may be difficult to distinguish clinically from inner ear decompression sickness, which can produce similar cochlear and vestibular symptoms but results from inert-gas bubble formation rather than failure of middle-ear pressure equalization.^[28]

Alternobaric vertigo results from unequal pressure between the two middle ears following asymmetric pressure equalization. The resulting pressure difference is transmitted to the vestibular system through the oval and round windows and can produce transient vertigo, sometimes accompanied by nausea and disorientation. It occurs during exposure to changing ambient pressure, including diving and aviation, and is particularly associated with decompression or ascent.^[23][29]

The anatomical basis for middle-ear pressure equalization became established during the development of early otology. Bartolomeo Eustachi described the tube connecting the middle ear and nasopharynx in 1563, although its role in pressure regulation was not initially understood. In 1683, Guichard Joseph Duverney recognized that the tube allowed replacement and adjustment of air in the tympanic cavity. Antonio Maria Valsalva described the manoeuvre that bears his name in his 1704 treatise De Aure Humana Tractatus. Valsalva's original description was intended as a means of forcing material from the middle ear rather than specifically for equalization during changes in ambient pressure.^[30][31]

During the nineteenth century, Joseph Toynbee established that the Eustachian tube is normally closed and opens during swallowing, allowing ventilation of the middle ear. The manoeuvre subsequently named after him consists of swallowing while the nostrils are occluded.^[30] Adam Politzer subsequently investigated Eustachian tube function experimentally and demonstrated that the Toynbee manoeuvre could leave negative pressure in the middle ear. Between 1861 and 1863 he developed a method of actively inflating the middle ear by introducing air into the nose during swallowing, avoiding the need for direct catheterization of the Eustachian tube. The technique, later known as Politzerization, was published in 1863.^[30][32]

The Frenzel manoeuvre was described in 1938 by the German otolaryngologist Hermann Frenzel. It was subsequently taught to military personnel during the Second World War. Unlike the Valsalva manoeuvre, which generates pressure by increasing intrapulmonary pressure, the Frenzel manoeuvre uses movements of the tongue and pharyngeal musculature with the glottis closed to generate pressure in the nasopharynx. It later became widely used as a middle-ear equalization technique in diving, particularly freediving.^[33]

Later techniques were developed principally in diving medicine. Voluntary tubal opening (VTO; French: béance tubaire volontaire, BTV) became associated with the French physician Georges Delonca during the 1970s. Around the same time, the Lowry and Edmonds techniques were first described as combinations or modifications of established equalization manoeuvres and became incorporated into diving-medicine practice.^[34][35]

• Middle ear barotrauma
• Aerosinusitis - barotrauma of the sinuses often associated with air flight but also common in diving.