eustachian tube Lying beyond the eardrum is the middle ear, a tiny air-filled cavity in the temporal bone of the skull, which is connected to the back of the throat by the eustachian tube. In adult humans, the eustachian tube, which was first described by Bartholomeo Eustachio in the sixteenth century, is just less than 4 cm long and lies at an angle of 45° relative to the horizontal plane. The bottom end of the eustachian tube, which opens into the nasopharynx, is composed of membrane and cartilage and is normally closed. However, it is essential for proper sound conduction through the ear that the eustachian tube opens periodically, so that the air pressure within each middle ear can be matched to that of the surrounding atmosphere. This occurs as a result of the contraction of the muscles that surround the eustachian tube during sneezing, forceful nose blowing, yawning, and swallowing (both when eating or drinking and throughout the day and night as build up of saliva and mucus stimulates the swallowing reflex). Opening of the eustachian tube also serves to drain any fluid that builds up in the middle ear into the nasopharynx.

The discomfort in the ears that sometimes follows marked changes in atmospheric pressure — caused, for example, by rapid descent in an aircraft or compression in a diving suit — is due to differences in pressure on either side of the eardrum. The unpleasant feeling can usually be overcome by yawning or some other means of opening the eustachian tubes. This is why scuba divers are taught to hold their noses and blow as they descend, and the same technique helps when an aircraft is landing.

The middle ear cavity, eustachian tube, and upper respiratory tract are lined by a continuous layer of mucous membrane. It is therefore not surprising that infections of the nasopharynx — including the common cold — readily reach the middle ear via the eustachian tube. Infants and young children are particularly susceptible to such acute infections, which usually cause ear pain and fever, possibly because the eustachian tube is wider, shorter, and more horizontal than it is in adults. This can lead to a build up of fluid in the middle ear in a condition known as otitis media with effusion or ‘glue ear’.

Accumulation of mucus in the eustachian tube, associated with inflammation of the middle ear, impedes the flow of air along the tube and results in negative pressure within the middle ear cavity. This causes the eardrum to be pushed inwards by the greater pressure of the atmosphere. Occlusion of the eustachian tube produces a sense of fullness in the affected ear and a mild conductive hearing loss, which may be increased if there is fluid in the middle ear. The sensation of popping in the ears that results when the nose is blown vigorously during a cold is due to air being forced up a blocked eustachian tube. Because of the increased difficulty in equalizing middle ear pressure, people with upper respiratory tract infections are more likely to suffer from ear discomfort when flying.

Andrew J. King

Bibliography

Bluestone, C. D. and and Klein, J. O. (1995). Otitis media in infants and children, (2nd edn). W. B. Saunders, Philadelphia.

Eustachian tube The tube that connects the middle ear to the back of the throat (pharynx) in vertebrates. It is normally closed, but during swallowing it opens to allow air into the middle ear, which equalizes the pressure on each side of the tympanum (eardrum). It was named after the Italian anatomist Bartolomeo Eustachio(?1520–74).

Eu·sta·chian tube / yoōˈstāsh(ē)ən; -kēən/ • n. Anat. a narrow passage leading from the pharynx to the cavity of the middle ear, permitting the equalization of pressure on each side of the eardrum.

Eustachian tube Small channel that connects the middle ear to the back of the throat. It opens when swallowing, to allow the pressure in the middle ear to remain the same as the pressure of air outside the body.

Eustachian tube (pharyngotympanic tube) (yoo-stay-shŏn) n. the tube that connects the middle ear to the pharynx. It allows the pressure on the inner side of the eardrum to remain equal to the external pressure. [ B. Eustachio (1520–74), Italian anatomist]