Porth's Essentials of Pathophysiology, 4e

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Nervous System

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is continuous posteriorly with the tympanic cavity and anteriorly with that of the nasopharynx. Infections from the nasopharynx can travel from the nasopharynx along the mucous membrane of the eustachian tube to the middle ear, causing acute otitis media. Toward the nasopharynx, the eustachian tube becomes lined by columnar epithelium with mucus-secreting cells. Hypertrophy of these mucus-secreting cells is thought to contribute to the mucoid secretions that develop during certain types of otitis media. EustachianTube Disorders Abnormalities in eustachian tube function are important factors in the pathogenesis of middle ear infections. There are two important types of eustachian tube dysfunction: abnormal patency and obstruction (Fig. 38-18). The abnormally patent tube either does not close or does not close completely. In infants and children with an abnormally patent tube, air and secretions often are pumped into the eustachian tube during crying and nose blowing. Obstruction can be functional or mechanical. Functional obstruction results from the persistent collapse of the eustachian tube due to a lack of tubal stiffness or poor function of the tensor veli palatini muscle that controls the opening of the eustachian tube (see Fig. 38-18B). It is common in infants and young children because the amount and stiffness of the cartilage supporting the eustachian tube are less than in older children and adults. Changes in the structure

ear if showering, and limit insertion of hearing aids or ear phones until pain or discharge subside. Persistent external otitis in diabetic or immunocompromised persons may evolve into osteomyelitis of the skull base, often called malignant external otitis. 48 Usually caused by Pseudomonas aeruginosa, it begins in the floor of the ear canal, and may extend to the middle fossa floor, and even to the contralateral skull base. Persons with the disorder usually present with complaints of severe pain, foul- smelling ear discharge, fever, granulation tissue in the ear canal, possibly vertigo (dizziness), and in advanced cases, cranial nerve palsies. Diagnosis is confirmed by demonstration of osseous involvement on CT and radionuclide imaging. Disorders of the Middle Ear and EustachianTube The middle ear, or tympanic cavity, is a small, air-filled, mucosa-lined cavity in the petrous portion of the temporal bone 2,46 (see Fig. 38-17). It is bounded anteriorly by the tympanic membrane, and spanned by three tiny bones, the auditory ossicles , which are connected by two synovial joints and are covered with the epithelial lining of the cavity. There are two openings in the medial wall of the middle ear—the oval (vestibular) window and the round (cochlear) window—that communicate with the inner ear. The three auditory ossicles (the malleus, the incus, and the stapes) connect the tympanic membrane with the oval window. 2,46 The malleus (“hammer”) has its handle firmly fixed to the upper portion of the tympanic membrane. The head of the malleus articulates with the incus (“anvil”), which links the malleus to the stapes (“stirrup”), whose footplate fits into the oval window. Arrangement of the ear ossicles is such that their lever movements transmit vibrations from the tympanic membrane to the oval window and from there to the fluid in the inner ear. The middle ear is connected to the nasopharynx by the eustachian or auditory tube, which is located in a gap in the bone between the anterior and medial walls of the middle ear (see Fig. 38-17). The eustachian tube serves three basic functions: (1) ventilation of the middle ear, along with equalization of middle ear and ambient pressures; (2) protection of the middle ear from unwanted nasopharyngeal sound waves and secretions; and (3) drainage of middle ear secretions into the nasopharynx. 49 The nasopharyngeal entrance to the eustachian tube, which usually is closed, is opened by the action of the tensor veli palatini muscle, which is innervated by the trigeminal cranial nerve (CN V). Opening of the eustachian tube, which normally occurs with swallowing and yawning reflexes, provides the mechanism for equalizing the pressure of the middle ear with that of the atmosphere. This equalization ensures that the pressures on both sides of the tympanic membrane are the same, so that sound transmission is not reduced and rupture of the tympanic membrane does not result from sudden changes in atmospheric pressure, as occurs during airplane travel. The eustachian tube is lined with a mucous membrane that

A

Normal patency

TVP

B

Functional obstruction

Floppy tube

Poor TVP function

C

Mechanical obstruction

Intrinsic

Inflammation

Extrinsic

Tumor or adenoids

FIGURE 38-18. Disorders of the eustachian tube.TVP, tensor veli palatini: (A) normal patency, (B) functional obstruction, (C) mechanical obstruction. (Developed from Bluestone CD. Recent advances in the pathogenesis, diagnosis, and management of otitis media. Pediatr Clin North Am. 1981;28(4):727–755. With permission from Elsevier Science.)