976
U N I T 1 0
Nervous System
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
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
Floppy tube
Functional obstruction
Normal patency
Mechanical obstruction
Tumor or
adenoids
Extrinsic
Intrinsic
Inflammation
Poor TVP
function
TVP
A
B
C
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.)
