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Middle Ear-Anatomy and Development

Structures of Tympanic Cavity

The entire middle ear cleft is lined by columnar ciliated and pavement epithelium. It is an exten­ sion of the respiratory mucous membrane from nasopharynx. The middle ear cleft consists of:
1. Eustachian tube
2. Tympanic cavity
3. Mastoid antrum
4. Aditus ad antrum
5. Mastoid air cells.

EUSTACHIAN TUBE

Eustachian tube connects tympanic cavity to nasopharynx, it is approximately 3.75 em long in average adult. It is directed upward, backward from lower opening in the lateral wall of naso­ pharynx, towards the upper opening in anterior wall of tympanic cavity. Whereas it is directed downward, forward medially from the tympanic cavity. The nasopharyngeal opening lies behind posterior end of inferior turbinate. Tympanic opening is higher than the pharyngeal opening. Tube is more horizontal and relatively wider, shorter in infants and young children.

The upper or poster lateral one-third is bony whereas lower or anteriomedial two third is cartilaginous. It is widest in entrance to tympanic cavity and narrow at its lower end, where tube is flattened at a diameter of 2 mm. Tubal tonsils are seen near the pharyngeal end of the tube which may at times cause Eustachian tube obstruction because of hypertrophy.
There is also presence of fibro fatty tissue related to membranous part of cartilaginous tube specially in the region of nasopharynx which is known as Ostmann’s pad of fat. This keeps the ET tube closed, thereby protecting the tube from nasopharyngeal reflux.

The fossa of Rosenmuller which lies behind the nasopharyngeal orifice is normally packed with small but well organized lymph nodes. It is the most common site for nasopharyngeal malignancy.

Blood Supply of Eustachian tube

Arterial supply: Ascending pharyngeal and middle meningeal artery and also from artery of pterygoid canal.

Venous Drainage: Pterygoid plexus.

Functions of Eustachian Tube

1. Ventilation of middle ear cleft-It plays a major role in equalizing middle ear pressure with atmospheric pressure.
2. Prevents reflux of nasopharyngeal secretion.
3. Clearance of middle ear secretions.

TYMPANIC CAVITY (the middle ear)

The tympanic cavity lies between the external and inner ear and shaped like a biconcave disc. The vertical and anteroposterior diameters are 15 mm, while the transverse diameter is 6 mm at the upper part, 2 mm at the center and 4 mm at the lower part.It is a six sided cavity with a roof, floor, anterior, posterior, medial and lateral walls. The tympanic cavity is divided into three parts:
• Epitympanum
• Mesotympanum
• Hypotympanum
Epitympanum (attic)
It is situated above the malleolar folds of the tympanic membrane. It contains the head of the malleus, incudomalleolar joint and body and short process of the incus. It connects the mastoid antrum via the aditus poster superiorly.

Mesotympanum
It is situated medial to the pars tensa of the tympanic membrane.

Hypotympanumi

It is situated below the level of the tympanic membrane.
Anterior mesotympanum and hypotympanum are lined by columnar ciliated epithelium. The posterior mesotympanum, aditus and mastoid area are lined by pavement epithelium.
Lateral Wall

It is formed by the tympanic membrane and partly by a portion of squamous part of the temporal bone. This wall separates the middle ear from external ear.
Medial Wall

It separates the middle ear from the inner ear. It has several important structures:
a) Promontory i the most prominent and bulging part of the medial wall formed by the basal turn of the cochlea.
(b) Bony Lateral Semicircular Canal lays poster superior to the promontory above the oval window.
(c) Oval Window (Fenestra vestibuli) lies between the middle ear and the scala vestibuli of the cochlea. It is closed by the footplate of stapes and the annular ligament.
(d) Round Window (Fenestra cochlea) is situated below and behind the promontory. The niche of the round window is directed posteriorly. It is closed by the secondary tympanic membrane and separates the middle ear from the scala tympani of cochlea.
(e) Facial Nerve runs in the bony fallopian canal above the oval window.
Anterior Wall

Anterior wall separates middle ear cavity from internal carotid artery. There are various structures passing through the anterior wall to the tympanic cavity. They are as follows:
(a) Canal for chorda tympani nerve (b) Canal for tensor tympani muscle (c) Eustachian tube
(d) Anterior malleolar ligament (e) Anterior tympanic artery
Posterior Wall

The upper part of the posterior wall shows the opening of aditus, which leads backwards from the posterior epitympanum to the mastoid antrum. Below the aditus there is a triangular bony projection known as processus pyramidalis through the apex of which is transmitted the stapedius tendon. The vertical portion of facial nerve courses down the posterior wall to its exit in the stylomastoid foramen.

• Facial recess (Suprapyramidal recess) and sinus tympani (Infrapyramidal recess)
Two recesses extend posteriorly from the mesotympanum that are often impossible to visualize directly. These spaces, the facial recess and sinus tympani, are the most com­ mon location for cholesteatoma persistence after ear surgery. The sinus tympani lies between the facial nerve and the medial wall of the mesotympanum and is very difficult to access surgically. The facial recess (suprapyramidal) is lateral to the facial nerve, bounded by the fossa incudis superiorly and the chorda tympani nerve inferiorly, posterosuperior meatal wall laterally and pyramid medially. It may be directly accessed via a posterior tympano­ torny approach, through the mastoid (posterior tympanotomy or facial recess approach).

• Sinus tympani (Infrapyramidal recess): The niche of two labyrinthine windows communicates at the posterior extremity with the deep recess which is known as sinus tympani. Laterally it is separated from the facial recess by the pyramid.
Floor

It is formed by a thin plate of bone which separates the middle ear from the bulb of the intemaljugular vein lodged in the jugular fossa. In the presence of a bony dehiscence in this area the jugular bulb may come into the middle ear to become a content of it.
Roof

The roof of the middle ear is separated from the middle cranial fossa by a thin plate of bone known as tegmen tympani and tegmen antri.
Ventilatory Anatomy
Normally the middle ear cleft is well ventilated. The air comes through the eustachian tube from the nasopharynx to the anterior mesotympanum. From here the air column goes up to the anterior epitympanum via the isthmus tympanic anticus and then goes backward to the posterior epitympanum. Part of this air passes through the aditus to ventilate the mastoid air cells and part of it comes down via isthmus tympanic posticus to ventilate the posterior mesotympanum. In a well pneumatized mastoid, ventilation of the posterior mesotympanum takes place also through the posterior wall. From the posterior mesotympanum, air percolates to the hypotympanum. Disorder of this ventilatory anatomy has a great bearing in the etiopathogenesis of various inflammatory diseases of the middle ear.

Contents of the Middle Ear

(a) Ossicles
Three tiny bones which conduct the sound from the ear drum to the oval window.
• Malleus (hammer) is the largest and lateral most ossicle measuring 8 mm in length. It has a head, neck, handle and anterior and lateral processes. The head is situated in the epitympanum. A lateral (short) process projects laterally from the neck while the handle is firmly fixed to the pars tensa of the ear drum.
• Incus (anvil) has a body, short process and long process. The body articulates with the head of malleus in the attic and the short process projects into the attic. The long process project downwards behind the handle of malleus, running parallel to it and articulates with the head of the stapes via the lenticular process.
• Stapes (stirrup) is the smallest ossicle mea­ suring about 3.5 mm and consists of head, neck, footplate and anterior and posterior crura. The footplate of stapes is held to the oval window by the annular ligament.
(b) Muscles

• The tensor tympani and stapedius muscles decrease the movement of the ossicles.
• The tensor tympani is inserted to the neck of malleus. First arch muscle supplied by branch of mandibular nerve
• The stapedius is inserted to the neck of the stapes. Second arch muscle supplied by branch of facial nerve, i.e. nerve to stapedius.
(c) Mucosal folds and ligaments-keep the ossicles in place.

(d) Nerves

• Chorda tympani is a branch of the facial nerve which carries the sense of taste. It enters the middle ear cavity from the posterior wall, runs forwards and lateral to the incus and medial to the malleus, escaping out through the anterior wall.
• The tympanic plexus lies on the promontory.
It is formed by tympanic branch of glossopharyngeal nerve and sympathetic fibers from the plexus around the internal carotid artery. It also carries the secretomotor to the parotid gland. Tympanic plexus innervates the medial surface of tympanic membrane, tympanic cavity, mastoid air cells and bony eustachian tube. Tympanic branch of glo sopharyngeal nerve can be sectioned in middle ear for treating the Frey’s syndrome.
(e)Vessels: Plexus of vessels of stylomastoid artery and from caroticotympanic artery.

Mastoid

The mastoid consists of three parts
1. Aditus ad antrum is a short canal connecting the epitympanum with the mastoid antrum. The short process of incus lies on its floor. The facial nerve runs in its canal in the floor, while the lateral semicircular canal lies in the medial wall. The bone lateral to the aditus appears like a bridge during ear operations.

2. Mastoid antrum is the largest air cell in the mastoid bone. The antrum is an important landmark in the surgery of the mastoid bone, and is always present.
• Anteriorly, the antrum receives the aditus. The facial nerve also lies anterior to the antrum.
• Medially, it is related to the horizontall po terior semicircular canal.
• Roof is formed by the tegmen antri.
• Lateral wall is formed by the cortex of the mastoid bone which lies medial to the suprameatal triangle. Its thickness can be upto 15 mm or 1.5 cm.
Mac Ewen’s (Suprameatal) triangle-It forms bony surface marking of the antrum. It is bounded by temporal line of supramastoid crest and posterosuperior bony meatal wall and the line drawn connecting the supprameatal crest and the bony meatal wall.
Posteroinferiorly, the antrum communicates with numerous mastoid air cells. Sinodural angle (Citelli’s angle) Angle between tegmen antri and sigmoid sinus.

3. Mastoid air cells are variable in number, size and distribution. They communicate with the mastoid antrum.
There are three types of mastoid process: (a) Cellular, with large and numerous air cells.
(b) Diploic, with small and less numerous air cells.
(c) Sclerotic, with air cells practically absent.
• The cellular mastoid account in about 80% of subjects and is considered to be normal. The diploic and sclerotic types may be due to the blockage of the eustachian tube. The air cells are located mainly in petromastoid and squamous parts of the temporal bone.

Development of Mastoid

Mastoid develops from squamous and petrous bone. The persistent petrosquamosallamina (bony plate)-the Korner’s Septum, is surgically important as it may cause difficulty in locating the antrum. It divides mastoid air cells into medial and lateral group. Mastoid antrum lies medial to the septum which may be difficult to reach or may lead to incomplete removal of disease during mastoidectomy. So to reach antrum, Korner’s septum has to be removed.

Development of mastoid process depends entirely on development of sternocleidomastoid muscle. Hence, its development does not begin until the end of first year of life, when the infants begin to hold their head erect. It does not form a definite elevation until the end of the second year and achieves its definite size only at puberty. So there is no actual mastoid process at birth and mastoid portion of temporal bone remains flat and stylomastoid foramen remains in surface of mastoid process. The facial nerve will be lying very superficial and may be injured in conven­ tional postauricular mastoid incision. So to avoid injury to the nerve, postauricular incision has to be done more horizontally.

Relations of the Middle Ear

External ear lies lateral to the ear drum.
Temporal lobe of the brain and meninges are above the antrum, aditus and epitympanum. The tegmen plate separates the middle ear cleft from the structures in the middle cranial fossa.
Cerebellum is posteromedial to the mastoid air cells.
Inner ear is medial to the antrum, aditus and tympanum.

Horizontal semicircular canal is an important landmark which lies posterosuperior to the facial nerve.
Fifth and sixth cranial nerves lie close to the apex of the petrous pyramid.
Facial nerve-The horizontal part runs downward in the medial wall of the tympanum.

The vertical part runs downward behind the tympanum and in front of the mastoid cells and emerges out through the stylomastoid foramen. Lateral sinus is posterior to the mastoid cells. Jugular bulb is in close contact with the floor of the tympanum.
Internal carotid artery is anterior to the tympanum.

Blood Supply
The blood supply of middle ear is from the branches of:
• Middle meningeal artery
• Maxillary artery
• Ascending pharyngeal artery
• Stylomastoid branch of the posterior auricular artery.

 Nerve Supply
Sensory: Tympanic branch of the ninth cranial nerve (Jacobson’s nerve) supplies through the tympanic plexus.
Motor: Tensor tympani muscle is supplied by the mandibular division of the trigeminal nerve and the stapedius muscle is supplied by the facial nerve.

Lymphatic Drainage
The lymphatics drain to the preauricular and the retropharyngeal lymph nodes.

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