ENDOLYMPHATIC HYDROPS


Endolymphatic hydrops has long been held to be the pathologic basis for Meniere’s
disease. Endolymph, the potassium-enriched fluid in the inner ear, may be either
excessively synthesized or inadequately resorbed, resulting in expansion of the
endolymphatic space. Surgical ablation of the endolymphatic sac in experimental
animals has reproduced the histopathologic finding of endolymphatic hydrops seen
in temporal bone specimens of individuals with Meniere’s disease, although these
animals do not seem to experience the classic signs and symptoms associated with
Meniere’s disease in humans.
Endolymphatic hydrops typically involves the pars inferior of the labyrinth
(comprising the saccule and cochlea). Saccular hydrops may range from mild to
severe, based on the degree of membrane distension toward the stapes footplate.
Cochlear hydrops is typified by bowing of the Reissner membrane into the scala vestibuli;
severity of cochlear hydrops also varies according to the degree of convexity
toward the scalar wall of the modiolus. The pars superior (utricle and semicircular
canals) may also be involved in endolymphatic hydrops, although changes tend to
be less dramatic and occur less frequently.
Several mechanisms have been suggested to explain how endolymphatic hydrops
may produce the spontaneous attacks of vertigo characteristic of Meniere’s disease.

The most prominent theory holds that hydropic distension of the endolymphatic duct
causes rupture of the distended membranes, a phenomenon that has been observed
throughout the labyrinth. Membrane rupture allows the potassium-rich endolymph
to leak into the perilymphatic space and contact the basal surface of the hair cells
as well as the eighth cranial nerve. Initial excitation then subsequent inhibition of the
hair cells manifest as a direction-changing nystagmus and may underlie the clinical
phenotype of episodic vertigo.

Long-term declines in auditory and vestibular function may be the result of
repeated exposure of the vestibular hair cells to toxic levels of potassium-enriched
perilymph. The differential susceptibility of type I and type II hair cells in Meniere’s
disease supports the hypothesis that chronic perilymph toxicity may cause neurosensory
dysfunction. The vestibular neuroepithelium consists of type I and type II
hair cells as well as supporting cells. Both hair cell types have cuticular plates and
stereociliary bundles, reflecting their role in mechanosensory signal transduction.
However, the 2 hair cell types can be distinguished based on other morphologic
characteristics:

Type I hair cells are flask-shaped, have a round nucleus, and are
enveloped on their basal surface by an afferent nerve chalice.

In contrast, type II hair cells are cylindrical, and have oval nuclei and small
bouton-type nerve terminals from afferent and efferent nerve endings.
The sparse nerve endings on the basal surface of type II hair cells may
provide decreased protection against harmful ionic changes in the perilymph.

The physiologic and functional implications of the selective
depletion of type II hair cells in Meniere’s disease are still poorly understood.
Alternatively, it has been postulated that hydrops itself may occur in an episodic
manner, as a result of sudden increases in the secretory function of the stria vascularis
or of spontaneous obstruction of the endolymphatic sac. Hydropic distension may
then cause a mechanical deflection of the macula and crista of the otoliths and semicircular
canals, respectively, and thus vestibular hair cell depolarization, leading to the
sensation of vertigo. Long-term changes to the neurosensory function of the vestibular
apparatus may be the consequence of increased hydrodynamic pressure,
causing increased vascular resistance, compromised blood flow, and chronic
ischemic injury.

Several lines of evidence challenge the primacy of endolymphatic hydrops in the
pathophysiology of Meniere’s disease. As mentioned previously, experimentally
induced endolymphatic hydrops in animal models does not produce the clinical
phenotype of Meniere’s disease in these animals. Moreover, a double-blind study of
temporal bone specimens and associated clinical histories reported that all individuals
with Meniere’s syndrome diagnosed during life had evidence of endolymphatic
hydrops on postmortem examination of their temporal bones; however, not all individuals
with histopathologic evidence of endolymphatic hydrops had clinical histories
consistent with Meniere’s disease. If endolymphatic hydrops was central to the
development of Meniere’s disease, one would expect the correlation between the clinical
manifestations of Meniere’s disease and endolymphatic hydrops to be absolute.

Alternatively, studies increasingly suggest that endolymphatic hydrops may be
a marker of some other pathologic process that causes Meniere’s disease, such as
disordered cochlear homeostasis. Emerging evidence implicates the fibrocytes of
the spiral ligament, which play a crucial role in maintaining cochlear fluid homeostasis;
dysregulation of these cells seems to precede the development of hydrops. Triggers
for cytologic changes in the fibrocytes remain elusive, although this line of inquiry
shows promise for yielding the true pathologic basis for Meniere’s disease.

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