Introduction
The endolymphatic sac (ES) is a structure situated in the inner ear, together with
the cochlea and the semicircular canals, or vestibular system.[1] The ES may be responsible for homeostatic regulation of the inner ear, endolymphatic
fluid volume, immune response, elimination of inner ear cellular debris and floating
otoconia, membranous labyrinth pressure, acid/basic transport, and the secretion of
substances.[1]
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Despite being only 3 mm long in diameter, the ES does not have a very variable location
inside the inner ear.[8] Almost every alteration in this structure can cause a massive problem to the hearing,
including its loss. The Ménière disease (MD) is one of the most common pathologies
of the ES,[12] and it was first described by Prosper Ménière in 1861, with its clinical triad:
dizziness, tinnitus, and hearing deafness, with or without aural fullness. Over the
years, a lot has changed regarding the definition of this disease but some aspects,
like the etiology, are still in debate.[13]
The MD can be treated surgically. Two of the possible procedures are ES decompression
and vestibular neurectomy. Pharmacological and nonpharmacological treatments can also
help.[12]
[14]
In this review, our aim is to elucidate all the aspects that neurosurgeons should
know about MD.
Methodology
This article is divided into three parts. In the third part we review the MD's basic
aspects, clinical diagnosis and treatment. We focused on evidence of guidelines, books,
and PubMed (from 1933 to 2021) basic and translational research, using human and animal
ES tissue or cells, previous reviews about the subject, case reports, and papers about
surgical experience. The terms used individually and combined were: Endolymphatic sac; Ménière disease; Neurosurgery. Literature inclusion criteria were: English and Portuguese language only; individual
case studies and long-term follow-up studies were included, and duplicate studies
were excluded.
First, we briefly approached the basic aspects of MD and its relation to the ES, followed
by a review of the clinical diagnosis and treatment. This study may provide a basis
to guide neurosurgeons in the evaluation and treatment of this disease.
Results
Basic Aspects and Relation to the ES
The MD was first described by Prosper Ménière in 1861 with its clinical triad: dizziness,
tinnitus, and hearing deafness, with or without aural fullness.[13] However, although some of these original aspects are still present in guidelines,
a lot more has been discovered about this disease.
The MD commonly affects adults from 20 to 50 years[13] and is more likely to occur in women.[15] Both ears are affected with the same frequency. Despite the large number of recent
studies about the MD, the epidemiology is still unclear, bearing in mind how difficult
it is to make an early diagnosis.[13] Although challenging to measure, Alexander and Harris[15] estimate that the prevalence of MS in the United States is 190 per 100,000 habitants.
Part of the trouble in making an early diagnosis of MD lies in its unclear etiology.[13] Studies have raised many theories on its possible causes, but none of them were
confirmed. Some of the theories are related to the ES, such as the possibility of
a narrowing and shortening in the endolymphatic duct that could hamper the reabsorption
of endolymph in the ES, since a smaller duct would allow less endolymph to pass through
the duct. Thus, the retained endolymph would elevate the pressure in the endolymphatic
space.[13] Another possibility is a reduction of the ES itself, again hampering the reabsorption
properties of the ES since the contact surface is smaller.[13] In agreement with that, an experimental obliteration of the ES caused MD symptoms.[16]
It's also possible that due to the immunological properties of the ES,[2]
[5]
[6] the occurrence of severe inflammation on the ES contributes to tissue fibrosis,
further damaging the structure and its function, and generating MD.[13] Another theory, described by Cahali et al.,[13] is related to the stria vascularis, and claims that patients with MD have a significant
decrease in the number of vessels, with its transversal section also being smaller.
Thus, the vascular deficiency of the stria vascularis represents a possible etiology,
mostly because of its importance for the endocochlear potential and endolymph secretion.[13]
There are other explanations for the etiology of MD that are not related to the ES.
Glycemic and insulin disorders have been described as possible causes since 90% of
the patients with MD have alterations in the glucose and/or insulin levels. Besides
that, diet and exercise can diminish the symptomatology of the patients.[13] Hypothyroidism and estrogen insufficiency are among other possible causes.[13]
With fewer observational studies, some authors defend the hypothesis of food allergies
(meat, corn, wheat etc.) and stress causing the MD.[13]
Clinical Diagnosis
The diagnosis of the MD is mainly clinical. Cochlear and vestibular symptoms with
aural fullness, in the absence of neurological symptoms, characterize the disease.
The MD attacks typically last from minutes to hours, and 96% of the cases also present
neurovegetative symptoms. Bilateral tinnitus and hyperacusis are also very common,
and can persist during and after the crises, in some cases becoming chronic. Other
symptoms are: unilateral loud noise intolerance and sound frequencies distortion.[13]
Despite diagnosis criteria being mainly clinical, some exams can be done to confirm
the suspicion.[13] Magnetic resonance imaging (MRI),[16] pure tone audiometry, glycerol test,[17] otoacoustic transport, standard vestibular test, retrocochlear tests, and glycemic
and insulin curves[13] can be very helpful. Nevertheless, none of those exams are pathognomonic.[13]
The histopathological exam is the only one that comes close to confirming the diagnosis.
Paparella determined that the most important histopathology finding in the MD is the
endolymphatic hydrops in the cochlea and saccule. Also, saccule membrane bulging is
a common finding.[18]
Using all this information, in 2015, the Barany Society made a guideline for the diagnosis
of MD ([Fig. 1]) and created simple and easily applicable diagnosis criteria.[19]
Fig. 1 Ménière disease classification. Adapted from Lopez-Escamez JA et al.[19]
Treatment
The MD has 3 different types of treatment: pharmacological, nonpharmacological, and
surgical.[13] The pharmacological aim is to do a symptomatologic aid, since none of the drug trials
were proven to contribute to the healing process. Some of the choices include: hydrochlorothiazide,
which increases the potassium concentration in the endolymph, labyrinth depressors,
which have anticholinergics, antiemetics, and sedatives. Corticosteroids and vasodilators
can also be used, based on the etiology hypothesis of, respectively, fibrosis due
to severe inflammation of the ES or stria vascularis ischemia.[13]
Concerning the nonpharmacological treatment, it's possible to interfere in the patient's
quality of life with dietary recommendations and routine exercise. A very similar
diet for diabetic patients can be used to treat MD, since the high glucose levels
can be an etiological factor. Accordingly, a diet rich in potassium and with lower
sodium levels may also help. Alcohol consumption and smoking must stop, as well as
caffeine, of which can be consumed a maximum of 250 mg per day.[13]
Regarding surgical treatment, two procedures stand out: ES decompression and vestibular
neurectomy.[14] The ES procedure for MD was described for the first time by Portmann[20] and it is still used to treat patients' impairment with refractory MD. Hearing impairment
and vertigo are the symptoms that have the largest improvement: 19% and 81%, respectively.[14] The operation starts with a bone incision at the retroauricular sulcus level, followed
by mastoid opening to expose the lateral sinus. The bony wall of the lateral sinus
is separated from the adjacent dura mater (DM), which is then elevated to until it
reaches the adherence with the bone. After the vestibular aqueduct and the wall of
the fossa are on the same level, it is possible to open the ES. First, the endolymphatic
fossa is exposed and pierced in 2 to 3 mm on the ES wall. Afterwards, the opening
of the ES must be done in a very delicate way in the connecting point between the
dura mater and the rear wall of the petrosal bone.[20]
In 1933, Dandy[21] made the vestibular neurectomy common, but because of the collateral effects and
postoperative complications, the procedure was not widely done. In 1991, 58 years
later, Silverstein and Rosenberg[22] modified the previous technique and proposed what they called the “combined retrosigmoid/retrolabyrinthine
vestibular nerve section”. This technique demonstrated an 85% cure rate, with a 7%
chance of enhancing the vertigo. Regarding the hearing loss, Silverstein and Rosenberg
identified that 20% of the patients had a change on their audiometry compared to before
the surgery and 4% showed substantial hearing loss.[12]
The “combined retrosigmoid/retrolabyrinthine vestibular nerve section” makes a U-shape
incision in the postauricular area. Then, a mastoidectomy is required to expose the
posterior fossa and the lateral venous sinus. Following, a dural incision is made
to reach the cerebellum. The next step is to open the cerebellopontine angle (CPA)
arachnoid over the ninth cranial nerve, with the intention of releasing CPA liquid,
and releasing the pressure. With the decompression, the cerebellum disconnects itself
from the temporal bone allowing for CPA exposure without damaging the cerebellar retraction.
At this point, the facial nerve must be identified to prevent damaging it, and the
eighth cranial nerve must be examined. After all precautions are taken, the vestibular
nerve separation is performed in a cleavage plane between the cochlear and vestibular
fibers. If the identification of the cleavage plane fails, the dura mater is reflected
off the temporal bone, and the opening of the internal auditory canal is performed
using a diamond burr, to enable the division of the superior vestibular and posterior
ampullary nerves. To close, the mastoid air cells are sealed with bone wax and the
dura with watertight fashion suture. The bony defect is corrected with adipose tissue
to present fistulas.[22]
Conclusion
The MD is one of the most common pathologies of the inner ear and has been largely
studied over the years. Its diagnosis is mainly clinical, which allows a bigger autonomy
for medical professionals, but also demands a lot of experience. The latest guidelines
must help in the classification of the disease and offer a better basis for diagnosis
and treatment options.
The treatment, despite not being curative yet, has improved a lot recently. The pharmacological
treatment based on the possible etiologies, allied with proper diet and exercise,
is showing promising results. Surgical treatments, especially ES decompression, are
procedures that must be taken into consideration for patients who don't respond well
to noninvasive therapeutics, and for those who fill the necessary criteria. Knowing
this, we highlight the importance of a well-trained doctor to identify and offer the
best treatment possible. This study may provide a basis to acquire these skills.
