Middle ear Anatomy

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“The structure injured by the primary blast wave, in order of prevalence, are the middle ear, the lung, and the bowel”

• The hollow organ containing gas or air are most readily damaged by blast waves. The homogeneous tissues like liver and muscles are least likely injured.
• In the underwater blast, injuries occur mostly in gastrointestinal tract and less commonly in lungs.
• The lung injuries can introduce air into the circulatory system. Death may occur from systemic air embolism.
• Tympanic membrane rupture is a sensitive marker for primary blast injuries.

Ref: Love & Bailey 24/e, p 298-300; Reddy 29e, p 216; Parikh, 6/e, p 4.183-4.184

The facial nerve passes in a canal situated in the posterior and medial walls of the middle ear. It is not associated with the anterior wall. 

Historically, congenital malformations of the ear have been divided into major and minor types with the latter limited to the middle ear alone.

Teunissen’s classification system is based on the site of involvement and aids in determining the appropriateness for surgery.

Maxillary antrum reaches adult size by 15 yrs of age. 

B i.e. Medial projection of middle ear cavity 

Tympanic nerve arises from the glossopharyngeal nerve, just below the jugular foramen. It passes through the floor of middle ear and reaches onto the mucous membrane covering the promontory, where it splits into branches and join branches of internal carotid plexus to form tympanic plexus.

Promontory is round bulging in the medial or labyrinthine wall of middle ear cavity produced by the first turn of cochlea.

B i.e. Presence of water in the middle ear

When seen in corona section, the cavity of the middle ear is biconcave, as the medial and lateral walls are closest to each other in the center.

• The nerve supply of middle ear is derived from tympanic plexus which lies over the promontory.
• The inferior ganglion of the glossopharyngeal nerve gives off the tympanic nerve which enters the middle ear through the tympanic canaliculus and takes part in formation of the tympanic plexus on the medial wall of middle ear.
• This distributes it fibres to the middle ear, and also to the auditory tube, aditus ad atrum mastoideum (aditus to mastoid antrum).
• Glossopharyngeal nerve —> Tympanic nerve/tympanic plexus —> Auditory tube

• Internal carotid A is related to anterior wall of middle ear.
• Round window is seen on the medial wall of middle ear.
• Sigmoid sinus is not related to middle ear (directly, it lies posterior to mastoid).

The tympanic end of the eustachian tube is bony and is situated in the anterior wall of middle ear.

The pharyngeal end of the tube is slit like and is situated in the lateral wall of the nasopharynx, 1-1.25 cm behind the posterior end of inferior tubinate.

• Roof – Thin plate called as tegmen tympani
• Floor – Jugular bulb
• Anterior wall – Internal carotid artery
• Posterior wall – Lies close to mastoid air cells
• Medial wall – labyrinth
• Lateral wall – tympanic membrane

i. Conduction of sound:

• It is done mainly by middle ear. Middle ear not just simply conducts the sound but converts sound of great amplitude and less force to that of less amplitude and greater force.
• This function of the middle ear is called as impedance matching mechanism or the transformer action.

ii. Transduction of mechanical energy to electrical impulse:

• Movements of the stapes footplate causes vibrations in scala vestibuli followed by scala tympani and is transmitted to the cochlear fluids which brings about movement of the basilar membrane.
• This sets up shearing force between the tectorial membrane and the hair cells.
• The distortion of hair cells gives rise to electrical nerve impulse.

A sound wave, depending on its frequency, reaches maximum amplitude on a particular place on the basilar membrane, and stimulates that segment (traveling wave theory of von Bekesy).

Higher frequencies are represented in the basal turn of cochlea and the progressively lower one toward the apex.

The symptoms of ear cancer depend on where the tumour is within the ear.

Types

The most common type of ear cancer is squamous cell cancer. Other types of cancer of the ear canal, middle or inner ear include:

• basal cell cancer
• melanoma
• adenoid cystic
• adenocarcinoma

Most common malignant tumor of middle ear and mastoid is squamous cell carcinoma.

Clinical Features

• It affects age group 40-60 years
• Slightly more common in females
• Most important predisposing cause is long standing CSOM
• Patient may present with chronic foul smelling blood stained discharge
• Pain is severe and comes at night.
• Facial palsy may be seen
• 0/E – Friable, hemorrhagic granulation or polyp are present.
• Diagnosis – made only on biopsy CT and angiography are done to see the extent of disease.

Metastasis occurs to cervical lymph nodes later.

Treatment of carcinoma of middle ear is combination of surgery followed by radiotherapy.

Surgery consists of radical mastoidectomy / subtotal or total petrosectomy depending on the extent of tumor.

A conical projection called the pyramid lies near the junction of the posterior and medial walls of the middle ear. It has an opening at its apex for the passage of the tendon of the stapedius muscle

The ossicles (auditory ossicles) are the three smallest bones in the body, the malleus, the incus, and the stapes.

They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea).

 Lateral wall

Ans. is ‘c’ i.e., Tympanomastoidectomy

Middle ear pappilomas

The middle ear papillomas are rare presentations and medical literature is mainly limited to case reports or case series.

These include aggressive pappilary tumors, schneiderian type of pappilomas and inverted pappilomas.

They are associated with hearing difficulty and vertigo and may be associated with Von Hippel Lindau syndrome.

They tend to be slowly growing, locally aggressive non metastasizing neoplasms

The approach for treatment of such pathology is usually radical and tympanomastoidectomy is considered the treatment of choice. This gives the best chance of cure.

Ans. is ‘a’ i.e., Semicircular canal

The middle – ear cleft in the temporal bone includes :‑

1. Eustachian tube
2. The middle ear (tympanic cavity)
3. Aditus which leads posteriorly to the mastoid antrum and air cells.