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A Discussion of Meniere's Disease
(Endolymphatic Hydrops)
Inner Ear Anatomy and Function
The inner ear is a delicate membraneous sense organ, which
is encased in a bony shell. It is suspended within a latice-like
bony framework, called the mastoid bone, which is located
behind the outer ear. The delicate inner ear membranes are
surrounded by a fluid called perilymph, which is rich in sodium,
and lies between the outer bony layer and the inner membraneous
layer. Within the membraneous layer circulates a second fluid
rich in potassium, called endolymph. The electrical potential
created by the separation of these two fluids is the driving
force of the inner ear.
The organ of hearing is called the cochlea, which resembles
a snail with its 2 ? turns; it is divided into three chambers
or scala. The scala media is separated from the scala tympani
by the basilar membrane and from the scala vestibuli by Reissner's
membrane. Only the scala media contains endolymph, whereas
the other two contain perilymph. The organ of Corti is housed
within the scala media, and rests on the basilar membrane.
Special cells with hair-like projections are attached to the
tectorial membrane, and project to the hearing nerve (cochlear
nerve). The nerve travels through the temporal bone (the bone
surrounding the outer, middle and inner ear) to the brainstem.
During it's course through the temporal bone, it is joined
by the two balance nerves (vestibular nerves) and the facial
nerve. All four travel together to the brainstem. Upon entering
the brainstem, the cochlear nerve relay's its message to nerves
that will travel to that particular region of the brain responsible
for decoding the message (temporal lobe).
The Mechanics of Hearing
Hearing occurs as sound enters the outer ear canal and causes
vibration of the tympanic membrane (eardrum) and subsequently
the movement of the middle ear bones. The piston-like action
of the stapes bone (stirrup) initiates a fluid wave within
the perilymph of the scala vestibuli. This "traveling
wave" in turn activates the hair cells of the organ of
Corti, causing the nerve to discharge. The hair cells are
responsible for converting the mechanical energy of the fluid
wave into an electrical signal, which will be processed by
the brain. There are actually two different types of hair
cells in the cochlea: inner and outer hair cells. Inner hair
cells are the special sensory receptors that receive the traveling
wave information and relay it to the brainstem. The outer
hair cells are responsible for amplifying the traveling wave
signal, and "fine tuning" the signal to a frequency
specific region of the cochlea.
Hearing loss may arise from the outer, middle and inner ear,
as well as from the cochlear nerve, brainstem or temporal
lobe. A loss that arises as a result of a blockage of sound
energy from reaching the inner ear is referred to as conductive;
whereas, a loss that results from injury to the inner ear
or central structures is called sensori-neural. Although a
clinical exam will often identify the type of hearing loss,
an audiogram (hearing evaluation) is usually necessary to
pinpoint the location, as well as the severity. Specialized
hearing evaluations, and (uncommonly) imaging studies
are sometimes performed if the cause of the loss is still
unknown.
The Mechanics of Balance
The balance portion of the inner ear is made up of three semicircular,
fluid filled canals which join a larger, globular structure
called the vestibule. Similar to the cochlea, the semicircular
canals are tubular structures filled with endolymph and surrounded
by perilymph. The hardest bone in the body, the labyrinthine
bone, surrounds the entire structure. Each semicircular canal
is oriented at right angles to the others, comprising vertical,
horizontal and posterior (behind) canals. The right and left
semicircular canals are mirror images of each other, so that
each and every direction of angular head motion is represented
by both ears. At the junction of each semicircular canal and
vestibule is a special receptor for angular rotational movements
of the head, referred to as the crista. The crista contain
hair cells embedded in a gelatinous matrix, with accompanying
nerve fibers. As the head turns in a particular direction,
the fluid within that semicircular canal turns in the opposite
direction bending the hair cells and inducing a neural discharge.
That signal is sent through the vestibular (balance) nerve
to the brain where it is interpreted, and adjustments are
made in eye movements and postural control. This ensures that
the eyes remain on a given target, and that the arms and legs
remain in a good position for maintaining stable posture.
Within the vestibule are hair cells that respond to changes
in head and body movements in the horizontal and vertical
planes. These "otolithic" hair cells are embedded
in a layer of calcium carbonate, making them top heavy and
therefore motion sensitive for both linear acceleration and
gravitational forces.
Nerve fibers from the crista and the otolithic organs form
two large balance nerves, the superior and inferior vestibular
nerves. They travel from the inner ear to the brainstem along
with the cochlear and facial nerves. Within the brainstem
they form an extensive neural network involving nerves from
the eyes, ears, the cerebellum, and positional receptors "proprioceptors"
located in the arms, legs and neck. The brain interprets this
information, and makes modifications in eye, head and body
position to maintain a fixed eye position, and erect posture.
Unfortunately, there are also connections to the thalamic
region of the brain, which is responsible for the nausea and
vomiting which accompanies most disturbances within the vestibular
system. A sensation of dysequilibrium may accompany any imbalance
or dysfunction within this neural network. Therefore, similar
symptoms of imbalance or "dizziness" may be experienced
by injury to the eye, ear, brain, and proprioceptors from
the extremities. Therefore it is often difficult to determine
the site of injury based on symptoms alone, and diagnostic
testing is necessary.
Meniere's Disease
Prosper Meniere was a French physician who described a new
disease process in the mid-1800's. The disease he identified
consisted of four symptoms: attacks of vertigo, ear fullness
or pressure, low-pitched tinnitus (ringing), and fluctuations
in hearing. At the time, the cause of the ailment was unknown,
but Meniere believed that it originated within the ear. Since
that time much has been learned about the disease and its
treatment; interestingly however, we still do not know the
cause of the disease Meniere described. There are now known
to be multiple variants of this disease process, with similar
pathologic consequences (abnormal tissue changes). Some forms
affect only hearing, others only balance and still others
have a few, but not all of the classic four symptoms.
Histopathology
It has been well characterized since the 1930's, that all
forms of Meniere's disease involve dilation of the scala media,
with subsequent bulging of Reissner's membrane into the scala
vestibuli. Eventually the membrane ruptures with mixing of
endolymph and perilymph, causing injury to the hair cells,
producing the characteristic symptoms. This process is called
"endolymphatic hydrops", meaning too much endolymph.
Only the disease process described by Meniere himself is should
be referred to as Meniere's disease, whereas all the others
are more properly termed hydrops, or endolymphatic hydrops.
The precise cause of the excessive fluid is unknown, whether
due to an overproduction of endolymph or from an under-resorption
is hotly debated. Either way, the net result is too much endolymph
with distressing symptoms. Although the etiology (cause) of
"Meniere's disease" is unknown, there are many known
causes of endolymphatic hydrops. These include allergy, immune
mediated, metabolic disorders, infections (syphilis), congenital
malformations of the ear and trauma. The known causes occur
infrequently compared to the idiopathic form (unknown cause),
but a search is often undertaken.
Diagnosis
The discussion you have with your physician is the most important
aspect of making the diagnosis. The vertigo spells are always
episodic, never occurring more than once a day, and often
separated by days, weeks or even years. Vertigo (sensation
of motion) in hydropic attacks can occasionally be heralded
by ear fullness, an increase in ringing (tinnitus) or a sudden
drop in hearing. Rarely, an improvement in hearing precedes
the attack (Lermoyez syndrome). The attack typically lasts
anywhere from 20 minutes to an entire day, depending on how
long it takes for Reissner's membrane to repair itself. The
vertigo is typically described as a spinning sensation (either
the environment or the individual spins), which is quite disabling,
and often associated with severe nausea and vomiting, sweating
and occasionally loss of bowel and/or bladder control. They
are not associated with severe headaches. The Tumarkin variant
of hydrops has sudden loss of postural control rather than
vertigo, thought to be due to involvement of the otolithic
component of the inner ear. After an attack of vertigo, one
may experience a period of unsteadiness lasting from hours
to weeks (occasionally years) as the brain compensates for
the loss of balance function.
Hearing loss in hydrops is quite variable, ranging from a
sudden, severe loss without return of function, to a gradual
decline over months to years. The most typical pattern is
that of hearing fluctuations, with periods of good and poor
hearing, associated with a gradual decline in word recognition.
The classic form of Meniere's disease consists of a low frequency
loss during an attack that may (or may not) return to normal
afterwards. Essentially, any pattern of hearing loss may be
associated with hydrops; therefore, the audiometric pattern
can not be relied upon to make the diagnosis. A demonstrable
fluctuation in hearing in one ear or the other helps to make
the diagnosis of hydrops.
Classically, low-pitched roaring sound is heard in the affected
ear prior to, or during an attack of vertigo. This too is
highly variable, and some individuals may experience constant
ringing in the ear (tinnitus). The precise etiology of tinnitus
is poorly understood, but thought to arise from a loss of
the usual spontaneous activity of the cochlear hair cells.
A sensation of fullness or pressure in the ear may accompany
the vertigo attack as well. This is thought to arise from
over-distension of the endolymphatic compartment with injury
to the hair cells.
Not unusually, patients with hydrops are quite sensitive to
changes in barometric pressure, and may experience more attacks
in the spring and fall. Hormonal changes that occur during
menses and pregnancy may trigger an attack. Tension, stress,
anxiety, allergy and exhaustion are also known triggers of
an attack.
Testing
A long list of disease processes must be excluded prior to
making the diagnosis of hydrops or Meniere's disease. Tests
of hearing, balance, blood count and chemistry and occasionally
imaging, are often performed. Pure tone and speech audiometry,
as well as electrocochleography (ECoG), are routinely performed.
The former is a standardized hearing evaluation, while the latter
is a more specific test of inner ear fluid balance. Balance
testing is performed if the diagnosis is in doubt, or persistent
dysequilibrium is present. When the diagnosis of hydrops is
entertained, an MRI or CT scan (imaging studies) is performed
to ensure that the symptoms are not being caused by a tumor.
It has been shown that nearly 1 patient in 10 with a tumor
on the balance nerve will demonstrate "classic Meniere's
symptoms". This does not mean that 1 out of 10 people
with hydrops has a tumor, as these tumors are exceptionally
rare, yet it remains an integral part of the diagnostic workup.
Treatment
Each and every therapeutic endeavor employed is aimed at eliminated
vertigo. Hearing loss, fullness, pressure and tinnitus that
occur with hydrops often defy known treatment, although a
few options are available. With any given treatment, some
patients will have improvement in these symptoms, while others
will have no change, and still others may have aggravation
of these symptoms. This likely represents the natural history
of the disease process. The mainstay of treatment for all
forms of hydrops is a low sodium diet and a diuretic. We encourage
our patients to maintain a very low sodium diet, less than
1800 mg of sodium each day. A pamphlet will be provided to
assist you with this. Additionally, a diuretic is often used
to eliminate extra fluid. Both of these maneuvers are employed
to help maintain fluid balance within the inner ear. As many
as 85% of patients will benefit from this treatment, either
with complete cessation of vertigo, or (at least) substantially
reduced attack severity. Some individuals are extremely salt
sensitive, having a vertigo attack within hours of sodium
indiscretion (hamburger and fries for example). Occasionally,
a sudden drop in hearing will respond to a steroid burst over
7-10 days. With progression of the disease, this often becomes
ineffective. Hundreds of other forms of medical therapy have
been tried and shown to be no better than placebo (sugar pill).
A vertiginous attack may be controlled through a variety of
vestibular suppressants, depending on the age and health status
of the individual.
For those patients who do not respond to medical therapy alone
(approximately 15%), surgical treatment is necessary to control
the vertigo. Four surgical options are available to everyone,
although those with good hearing are advised to consider only
those options which attempt to preserve normal hearing.
Endolymphatic Shunt Surgery
This minimally invasive procedure has been hotly debated for
many years. It involves an incision behind the ear and a mastoidectomy
(removing the bone behind the ear). The endolymphatic sac
is then identified within the mastoid bone, which is connected
to the inner ear endolymphatic compartment. The sac is opened
and a piece of surgical grade plastic is inserted inside.
The theoretical purpose of this operation is to "shunt"
excessive endolymph away from the inner ear. In reality, no
one knows why the operation is so successful. Perhaps it has
to do with altering the vascularity or immune function of
the sac, but it is doubtful that it has anything to do with
"shunting" fluid. It has been shown countless times
that this operation is successful in controlling vertigo in
75% of those with hydrops. The surgery has relatively few
risks, and can be performed as an outpatient.
Vestibular nerve section
Attacks of vertigo can be eliminated by cutting the balance
nerves on the affected side. The operation can be performed
either from above the ear, or behind the ear. It requires
a craniotomy (opening through the skull) to access the balance
nerves as they exit the inner ear. The vestibular nerves are
separated from the cochlear and facial nerves, and then divided.
Because of the intracranial nature of this operation, the
patient is observed in the intensive care unit overnight,
and remains in the hospital from 3 to 5 days. The risks of
the operation are higher than for an endolymphatic shunt,
however there is a much higher rate of vertigo control, up
to 97%.
Labyrinthectomy
For an individual with very poor hearing, this is the procedure
of choice. The operation begins with a mastoidectomy, but
then continues deeper to remove the inner ear semicircular
canals and vestibule. Some surgeons also cut the balance nerves
at the same time. Vertigo is controlled up to 99% of the time
with minimal risks, unfortunately hearing on the side of the
surgery is eliminated.
Intratympanic gentamycin
Gentamycin is an antibiotic which has as a side effect, injury
to the balance organs. This can be utilized to eliminate vertigo
in many individuals with hydrops. The eardrum on the side
of the disease is anesthetized (local anesthesia), and a tiny
needle is passed into the middle ear space. A small amount
of this antibiotic in injected into the middle ear, and the
patient remains in a recumbent position for 20-30 minutes.
Anywhere from 2-5 injections over the course of a month are
often necessary to affect vertigo control. Between injections
hearing and balance function are monitored. At the first sign
of either hearing loss or balance dysfunction, the treatments
are discontinued. Studies have shown that control of vertigo
can be accomplished in up to 80% of patients, however hearing
loss occurs in as many as 20% as well. For many elderly and
medically infirm patients, this is an excellent option.
Prognosis
Hydrops is a chronic disease, like diabetes or hypertension.
It needs to be treated medically (low sodium diet and diuretic)
like other disease processes for many years. Like other disease
states, it can be well controlled with therapy. Certain individuals
with hydrops will develop the disease in the opposite ear.
This occurs in 4-10% of patients with hydrops. This fact needs
to be taken into account when surgical options are entertained.
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