Page 338 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Hearing Impairment
Sound waves are transmitted from the ear- through the air is impaired, bone conduction
drum (tympanum) via the ossicles to the fe- remains normal (→ A).
nestra vestibuli (vestibular window) (→ A). The hair cells can be damaged by sound
The transmitting apparatus in the middle ear stress (i.e., impingement of too loud a sound
acts as an impedance converter. Without it for too long) and ischemia. But thanks to its
Systems 98% of sound energy would be reflected away high glycogen content they can survive short
because of the markedly different resistances
periods of ischemia by anaerobic glycolysis.
to the sound waves in the air and in the fluid
Hair cells can also be damaged by certain
drugs—such as aminoglycoside antibiotics and
of the inner ear. Invagination of the fenestra
Neuromuscular and Sensory of the fenestra cochleae (cochlear window). via the stria vascularis, are accumulated in the
the chemotherapeutic agent cisplatin—that,
vestibuli results in simultaneous evagination
The eardrum normally protects the latter
endolymph. This results in inner ear hearing
against external sound waves and conducts
loss that affects air and bone conduction
the sound energy specifically toward the fe-
equally (→ B4). Both the hearing threshold
nestra vestibuli. Sound waves can also be
and the active component of basilar mem-
brane displacement are affected, so that dis-
transmitted to the bones of the skull and can
crimination
thus stimulate the inner ear. However, this re-
higher-frequency
different
of
depolarization of the inner hair cells can pro-
The oscillation of the fenestra vestibuli pro-
duces traveling waves in the inner ear, at first
duce an unusual and disturbing sound sensa-
10 quires a much greater energy of sound. tones is impaired (→ B5). Lastly, inadequate
tion (subjective tinnitus). This can also be
spreading along the scala vestibuli. The stereo-
cilia of the outer and inner hair cells are bent caused by inadequate excitation of neurons in
by evagination of the cochlear septum with the auditory pathway or the auditory cortex.
the basilar membrane and the organ of Corti Stiffening of the basilar membrane disturbs
at a frequency-dependent location (→ B1). the micromechanics and thus probably con-
+
This leads to the opening of K channels in the tributes to hearing loss in the elderly (→ B1).
cell membrane. The endolymph in which the Inner ear deafness can also be the result of
stereocilia of the hair cells are suspended abnormal endolymph secretion. Thus loop
+
(→ B2) has a very high K concentration (ca. diuretics at high dosage not only inhibit renal
–
+
+
+
150 mmol/L). K is secreted by the epithelial but also auditory Na -K -2 Cl cotransport. In
+
–
+
cells of the stria vascularis, by Na -K -2 Cl co- addition, there is a known (but rare) genetic
+
+
+
transport and by Na /K -ATPase in the anti- defect of the luminal K channel. The channel,
luminal membrane, as well as by a luminal K + which consists of two subunits (IsK/Kv LQT1 ), is
+
channel (→ B3). When the K channels in the also expressed in the heart (as well as other or-
+
membrane of the hair cells are opened, K en- gans), where it participates in repolarization. A
ters the cells and depolarizes them. This depo- defect of Kv LQT1 or IsK results not only in deaf-
larization then triggers the release of the ness but also in delayed myocardial repolari-
transmitter, especially in the inner hair cells. zation (long QT interval [Jervell, Lange–Nielsen
By contracting, the outer hair cells increase syndrome]). Abnormal absorption of endo-
the local traveling wave and thus the amount lymph can also cause deafness. The endo-
of stimulation of the hair cells. lymph space becomes evaginated, distorting
Causes of deafness. A tear in the eardrum, a the relationship between hair cells and tectori-
lesion in the ossicles, or immobilization of the al membrane (endolymph edema; → B6). Fi-
conduction apparatus, for example, caused by nally, increased permeability between the en-
a purulent middle ear infection, dampen dolymph and perilymph spaces may be re-
transmission to the fenestra vestibuli. Further- sponsible for Ménière’s disease, which is char-
more, if there is a hole in the drum, the fenes- acterized by attacks of deafness and vertigo
tra cochleae is no longer protected. This results (→ B7).
328 in middle ear hearing loss. While conduction
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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