Meniere's disease: Anatomy of Inner Ear - NEET PG ENT

Meniere's disease is a problem with the inner ear that can lead to vertigo attacks and hearing loss. Meniere's illness typically only affects one ear. Anyone can develop Meniere's disease at any age. However, it typically begins between the ages of 40 and 60. It is believed to be a permanent ailment. However, some therapies can aid in symptom relief and decrease the long-term effects on your life.

Read this blog further to get a quick overview of this important topic for ENT  and ace your NEET PG exam preparation.

Anatomy of the Inner Ear

The inner ear constitutes a bony labyrinth(covering) and a membranous labyrinth(content). The membranous labyrinth exists inside the bony labyrinth. The bony labyrinth has a central chamber called the vestibule. The vestibule contains a spherical recess and an elliptical recess. Anterior to vestibule, there exists the cochlea, and posterior to it, there exist the semicircular canals. The semi-circular canal duct exists within the semicircular canal. The saccule is present inside the spherical recess whereas the Utricle is present inside the elliptical recess. The cochlear duct is present inside the bony cochlea. The Ductus reunions connect the saccule and the cochlear duct.

Utricular duct and circular duct join to form the endolymphatic duct, which goes to the sack.  Semicircular canal duct within the semicircular canal.

Fluids inside the Ear 

Endolymph is the fluid that is present inside the membranous labyrinth. It is produced from the cochlear duct and circulates to the entire membranous labyrinth. And reabsorbed in the cochlear duct (scala media) Perilymph is produced from the Cochlear aqueduct. Perilymph is an ultrafiltrate of CSF which fills the entire space present between the bony labyrinth and the membranous labyrinth. Endolymph is rich in potassium and perilymph is rich in sodium. This is necessary for generating a good amount of action potential at the cochlea receptors level, at the macula, which is the receptor in the Utricle and saccule, and at the semicircular canal. Thus, to generate a depolarization so that the impulses are conducted to the respective nerves, these electrolytes i.e., sodium and potassium are necessary.

Stria vascularis is a structure that produces and reabsorbs the endolymph. Consider a condition where there is excessive production of the endolymph and decreased reabsorption due to allergy/autoimmune cause/ genetic causes/ stress/ excessive salt and water consumption, etc. Stria vascularis is a structure that produces and reabsorbs the endolymph. As a result, the entire membranous labyrinth will swell up. But this membranous labyrinth cannot expand infinitely as it is enclosed within the bony labyrinth. Thus, bone is restricting the movement or the excessive swelling of the membranous labyrinth. Inside the membranous labyrinth, there is the presence of receptors. Organ of corti exists inside the cochlea. Organ of Corti is responsible for hearing. Thus, if this organ of Corti doesn’t get appropriate impulses because the entire Scala media is swollen up and there is excessive endolymph, there will be distortion in the conduction of impulses. This results in hearing loss(Sensorineural) in the patient. Both spherical recess and elliptical recess contain receptors called the macula and it is responsible for hearing and balance.

In the case of distorted information passing the macula, both of them will get affected and there will be hearing loss and imbalance. The semicircular canal ducts contain neurosensory epithelium/receptors within their amupllated ends called Crista. It is responsible only for balance. If distorted information passes the Crista, there will be an imbalance. Thus, the patient will have Vertigo because of improper conduction of impulses from the Crista and the macula. The patient will have tinnitus whenever the firing within the cochlea or the nerve gets affected. Overall there is gross enlargement of membranous labyrinth.

The Scala vestibuli is on the top, Scala media in the middle, and Scala tympani at the bottom. The scala media is separated from scala vestibular by Reissner’s membrane and from the Scala tympani by the Basilar membrane. Stria vascularis is responsible for the production of endolymph and reabsorption of the endolymph. Suppose there is increased production and decreased reabsorption of the endolymph in the cochlear duct by stria vascularis. The entire Scalla media would swell up.

Reissner’s membrane would stretch to produce micro tears. The fluid present in this scala vestibuli(perilymph) would mix with the fluid present in this scala media(endolymph). Perilymph is rich in sodium and endolymph is rich in potassium. Thus, sodium-potassium This imbalance is not just happening at the level of the cochlear since the fluids are connected all over the labyrinth. It would happen at the macula and the Crista, resulting in possible Vertigo, hearing loss, and tinnitus. The tears will automatically heal after some time. The sodium-potassium balance will again get restored after a certain time and the patient's symptoms will improve.

There won’t be constant vertigo/hearing loss/tinnitus but rather fluctuating symptoms. Whenever there is a pressure increment, they will have symptoms. Whenever there is healing/medications, the symptoms will decrease.

Types of Meniere's Disease

• Primary Meniere's disease - idiopathic (no cause identified)
• Secondary Meniere's disease - it can occur due to infection, truma, allergy, autoimmune diseases, or even stapedectomy.
• There is also a genetic predisposition as Chromosome 6 has been attributed to causing it although there is no direct causation. 

Predisposing Factors of Meniere’s Disease

• Allergy
• Sodium and water retention
• Hypoadrenalism and Hypopituitarism
• Hypothyroidism
• These endocrinological abnormalities cause retention of the majority of the fluids, not just endolymph.

Cardinal Features of Meniere's

• Vertigo
  • The Vertigo of Meniere's disease has a sudden onset. There may be an aura or tinnitus, after which the symptoms may appear.
  • It will last for a few minutes to a few hours. And resolves on its own.
  • It is accompanied by nausea, vomiting, and specifically vagal symptoms (Doesn’t happen in any other disease) such as sweating, pallor, bradycardia, abdominal cramping, etc.
• Hearing Loss
  • Since this is an inner ear disease, there is a sensorineural hearing loss because external and middle ear diseases would cause conductive hearing loss whereas any lesion in the cochlea and the retro cochlear pathway would result in sensorineural hearing loss.
  • It is typically fluctuating and not constantly present.
  • Only during an attack, the patient will have hearing loss. The hearing loss will subside once the attack has aborted/ once the symptom has disappeared.
  • They typically have involvement of low frequencies i.e, the 250-hertz or 500-hertz frequencies are affected.
  • The condition where one can hear double is called Diplacusis (specific to Meniere's)
• Tinnitus
  • Tinnitus present in Meniere’s disease is a roaring type (as though a lion is roaring).
• Tullio’s phenomenon
  • It occurs when they have an intolerance to loud sounds.
  • Whenever a patient with Meniere's disease listens to a loud sound, they get vertigo.

Low-Frequency Hearing Loss

 The Scala media at the apex of the cochlea is involved first, and then the base of the cochlea. Apex is responsible for recognizing low frequency sounds i.e125 hertz, 250 hertz, etc. and the base of the cochlea is responsible for recognizing high frequency sounds. There is Tonotopic organization of cochlea due to which the base of the cochlea is responsible for recognizing high frequency i.e., 20,000 hertz, 18,000 hertz, 16,000 hertz etc. , whereas at the apex recognizes low-frequency sounds(125 , 250 hz). The disease begins at the apex of the cochlea. Thus, the low frequencies are affected first. Although, as the disease progresses, all the frequencies will get affected. The apex of the cochlea is narrow when compared to the base. Thus, when the pressure increases, the apex will be the first one to be affected, resulting in low-frequency hearing loss.

Tullio’s Phenomenon

It is a condition where loud noise causes vertigo in a patient with Meniere's disease. This is because in a patient with Meniere's disease, the oval window lies in very close proximity to the saccule because it is dilated. The footplate covering the oval window lies a bit far away from the saccule in a normal patient. However, in Meniere's case, it lies in close proximity to the Saccule. Thus, when a sound enters through the malleus incus and hits the stapes and the oval window, These vibrations can be transmitted very quickly to the saccule. The saccule contains macula which is responsible for both hearing and balance, resulting in imbalance i.e. Vertigo. This can be provoked by loud sounds because the distended saccule lies against the oval window since the saccule is now dilated(Meniere's case). This phenomenon is called Tullio's phenomenon. This condition is also positive whenever the patient has a Perilymph Fistula or Congenital Syphilis or Superior Semicircular Canal (SSC) Dehiscence.

Examination

These tests are not 100% specific for Meniere's disease. 

• Otoscopy: Normal
• Nystagmus: Since it's a disease of the inner ear, typically the external auditory canal will be normal. But when encountering a patient with vertigo, the first thing to look for is misdiagnosis. If a patient is encountered during the phase of vertigo, after checking for misdiagnosis, It will be observed that the fast component of the Nystagmus is towards the healthy ear.
• Tuning Fork Test (TFT): It will show the presence of sensorineural type of hearing loss because it's a cochlear pathology.
• Rinne and Weber: Thus, Rinne will be positive whereas Weber will be towards the opposite ear. Absolute Bone Conduction (ABC) test will show decreased hearing sensitivity when compared to the examiner. 
• Pure Tone Audiometry: There is a low-frequency hearing loss observed. But if glycerol, a dehydrating agent is given, it will reabsorb the endolymph to a certain extent i.e., partially. As a result, endolymphatic pressure and pressure in the cochlea decrease. Thus, an improvement in the audiological scores is observed. This is called a glycerol test.
• Recruitment: It is a test done to identify a Cochlear pathology. Since the Menier' is a Cochlear disease, this test will be positive.
• Calorie test: Since the receptors are not able to send the impulses appropriately to the brain, there will be a Canal Paresis. This means that if the ear is irrigated with warm/ cold water, there would be no response/ decreased response, this is called Canal Paresis.

ENT Preparation Articles:

Electrocochleography

It is the diagnostic test for Meniere’s disease. Whenever the inner ear/Organ of Corti/Macula/Crista is being stimulated, there is a certain depolarization and action potential being generated at the receptor that will fire the impulses to the corresponding nerves. In a patient with Meniere's disease, The summating potential to action potential ratio i.e., SP/AP <  30% or 0.3 in normal individuals. But in Meniere's disease, SP/AP > 30% or 0.3.

The test is done by passing an electrode through the tympanic membrane and placing it on the promontory, which is the basal turn of the cochlea. When sound impulses are given from an external sound source, it activates the receptors in the cochlea and generates an action potential. The SP/AP ratio is also recorded by this probe and can conclude whether the patient is having Meniere's disease.

Audiogram for Meinere’s Disease:

The other test performed in patients with Meniere's disease is an audiogram where it is typically observed that the hearing loss is more at lower frequencies. Thus, at 250 hertz, the hearing threshold is at 70. X is for air conduction of your left ear and close bracket - is for bone conduction of your left ear. Both air conduction and bone conduction are affected because it's a sensorineural pathology. At lower frequencies, the hearing loss is higher whereas at higher frequencies, the hearing is better. The graph is of an upsloping type of audiogram.

When an audiogram is done, it is observed that there is a certain amount of hearing loss. However, after three hours of administration of glycerol, there is an improvement at every frequency i.e., improvement in the audiological score.

Role of MRI with Gadolinium Contrast:

Gadolinium is a contrast agent, which when injected, stains the CSF the cerebrospinal fluid. The ultrafiltrate of CSF i.e., perilymph fills scala vestibuli(SV) and scala tympani(ST) compartments of the cochlea.In a normal patient, if the contrast is given and performed MRI on, the dye must only be seen in SV and ST but not in SM. However, in a patient with Meniere’s disease, the dye is seen in all three compartments because of the tear in the reissner's membrane or vestibular membrane. 

Variants of Meniere’s Disease

It is not necessary that every patient with manias will manifest to you with all the three symptoms. Symptoms can be variable :

• Cochlear Meniere’s is a condition in which there is  Presence only of cochlear symptoms (only hearing loss) and no vestibular symptoms i.e., no vertigo.
• Vestibular Meniere’s: Only vertigo and no cochlear symptoms and no tinnitus.
• Lermoyez Syndrome: The reverse of Meniere's triad that is tinnitus, followed by sensorineural hearing loss and vertigo. Reverse of Meniere’s is Lermoyez Syndrome.
• Tumarkin's Otolithic Crisis: Sudden drop attacks without loss of consciousness. This occurs whenever there is a disruption of the autolytic membrane in the macula. The otolith is responsible for balance. Thus, the patient will fall suddenly but there is no loss of consciousness.

Secondary Meniere’s

These can occur secondary to:

• Congenital and acquired Syphilis(infections)
• Trauma
• Paget's disease
• Post Stapedectomy

Treatment: General Measures

• Low salt diet(<1.5-2g/day)
• Stop smoking as nicotine cause vasospasm
• Avoid excessive intake of water and caffeine.
• Correct hormonal imbalances like hypothyroidism, hypopituitarism
• Elimination of allergen(in 50% allergens are found)
• The treatment of the cause itself is going to be the treatment of the disease.

Pharmacological Treatment

In an acute phase, the treatment is to give vestibular sedatives i.e., suppress the labyrinth.Labyrinthine symptoms must not appear.in an acute attack, Diphenhydramine, dimenhydrinate, Promethazine, Prochlorperazine. However, in a continuation to a chronic state, Prochlorperazine can be given. Vasodilators like Carbogen and histamine drip can be given in patients with acute phase. In continuation into the chronic phase, nicotinic acid and beta histidine are used. Other drugs like diuretics can be helpful but aren't usually given on a regular treatment basis. These drugs are going to only abort the attack but do not treat the disease itself. So, for treatment of the disease, certain sorts of steroids must be given. Systemic steroid can cause glaucoma, osteoporosis, liver dysfunction, renal dysfunction, etc.

Thus a local acting steroid is preferred over system steroid is preferred. However, the local steroid is preferred more as there are less system adverse effects.The concentration of the drug that reaches the inner ear would be higher i.e., bio availability of the drug would be higher. Bioavailability will be higher in a local steroid as compared to systemic steroid.

Intratympanic steroid/ medication is administered through the tympanic membrane into the middle ear. 

The scala vestibuli is covered by the oval window and scala tympani by the round window. When administered, the drug goes into scala tympani through the round window membrane.The drug reaches the scala media through the basilar membrane. We only give 0.3-0.4 ml of the drug as the middle ear can hold only that volume. If this was to be given as an injection, the patient will have to take at least 4-6 doses of steroids. Intratympanic injections can be painful where repeated injections must be avoided. A micro catheter/wick can be placed from the external auditory canal and put on the round window under local anesthesia. Microwick and microcatheter are drug delivery devices that can be used for patients  with meniere’s disease.After intratympanic steroids, if the patient is not having improvement in symptoms, one can switch to gentamicin. Gentamicin, is an ototoxic medication i.e., it is going to destroy the labyrinth. Even though the vertigo will definitely disappear since it is getting destroyed and becomes a dead labyrinth. However, the patient will end up having a permanent SNHL.Another form of therapy is a Meniett device i.e., an intermittent pulse pressure device.It is kept in the external auditory canal. This generates intermittent low pressure and the pressure helps in resorption of the endolymph in the inner ear. 

Surgical Management

• Surgeries come into play when these pharmacological methods don’t work i.e., intractable vertigo despite being treated with intratympanic steroids or gentamicin.

Endolymphatic sac decompression-In the inner ear, the endolymphatic duct opens into the endolymphatic sac. The vestibule and the semicircular canal are supplied via the vestibular nerve, whereas the cochlea is supplied by the cochlear nerve. The vestibular nerve, in turn divides into superior and inferior vestibular nerves. There are attempts to open up the Endolymphatic sac so that the endolymph is drained. If continuous drainage is desired, a draining tube can be kept into it. That is called an endolymphatic shunt. If this is also not helpful for the patient, vestibular nerve sectioning is chosen where the worst vestibular nerve is cut or sectioned. This is done so that no afferent impulses will go from the inner ear to the brain . When there are no afferent impulses traveling, the patient will not have symptoms.

Other surgical options:

Some other forms of meniere’s disease are:

• Certain meniere’s disease: certain only if there is a histopathological confirmation of meniere's.
• Definite meniere’s disease: in this there are 2 or more episodes of vertigo lasting for 20 minutes or more. 1 audiometrical evidence of sensorineural hearing loss at the time of Vertigo.Tinnitus and aural fullness must be present. 
• Probable meniere’s disease: At least one episode of vertigo lasting for 20 minutes or longer, associated with one audiometric evidence of hearing loss, tinnitus and aural fullness.
• Possible meniere’s disease: Episodic attacks of vertigo (May or may not have), without documented hearing loss, tinnitus or aural fullness.

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