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Anatomy of middle ear

By

Dr. T. Balasubramanian M.S. D.L.O.

 

Synonyms: Middle ear cleft, Tympanum

The middle ear cleft includes the tympanum (middle ear cavity proper), the eustachean tube, and the mastoid air cell system. The tympanic cavity is an air filled irregular space contained within the temporal bone. It also contains the three auditory ossicles (malleus, incus and stapes) along with their attached muscles. For the purpose of description the tympanic cavity may be considered as a box with four walls, a roof and a floor. The corners of this hypothetical box is not sharp.

Lateral wall: The lateral wall of the tympanum / middle ear is partly bony and partly membranous. The central portion of the lateral wall is formed by the tympanic membrane, while above and below the tympanic membrane there is bone, forming the outer lateral walls of the epitympanum (attic) and hypotympanum respectively. The lateral wall of the epitympanum (attic) also includes that part of the tympanic membrane lying above the anterior and posterior malleolar folds - this portion of the ear drum is also known as pars flaccida. This portion of the tympanic membrane lacks the middle fibrous layer, hence the name. The lateral attic wall (bony portion) is wedge shaped, its lower portion is also called the outer attic wall (scutum). Scutum actually means sheild in latin. This bony portion is thin and its lateral surface forms the superior portion of the deep portion of the external meatus.

Three openings are present in the bone of the medial surface of the lateral wall of the tympanic cavity. The first opening is the posterior canaliculus for the chorda tympani nerve. This opening is situated at the junction between the lateral and posterior walls of the tympanic cavity. This opening is usually present at the level of the upper end of the handle of the malleus. This opening leads to the bony canal which descends through the posterior wall of the tympanic cavity. Since chorda tympanic nerve traverses this canal it is also known as the canal for chorda tympani nerve. This canal also contains a branch from the stylomastoid artery which usually accompanies the chorda tympani nerve.

The second opening is the petrotympanic (Glaserian) fissure. This fissure opens anteriorly just above the attachment of the tympanic membrane. This opening is infact a small slit about 2 mm long. It receives the anterior malleolar ligament. It also transmits the anterior tympanic branch of the maxillary artery to the tympani cavity.

The third is the canal of Hugier. It lies medial to the Glaserian fissure. The chorda tympani nerve enters through this.

Roof: The roof of the middle ear cavity is formed by the tegmen tympani. It is this tegmen tympani which separates the middle ear cavity from the dura of the middle cranial fossa. This tegmen tympani is formed in part by the petrous portion of the temporal bone, and the squamous portion of the temporal bone. The suture line between these two components is known as the petrosquamous suture line. This suture line is unossified in the young, and does not close until adult life is reached. Through this suture veins from the middle ear may pass to the superior petrosal sinus.

Floor: The floor is much narrow. Infact it is narrower than the roof of the middle ear cavity. This portion of the middle ear cavity lies in close relationship with the jugular bulb. The middle ear cavity is separated from the jugular bulb by a thin piece of bone. Rarely, the floor may be deficient and the jugular bulb in these patients is separated from the middle ear cavity only by fibrous tissue and mucous membrane. At the junction of the floor and the medial wall of the middle ear there is a small opening which allows the entry of tympanic branch of glossopharyngeal nerve to pass into the middle ear. This nerve takes an important part in the formation of tympanic plexus.

Anterior wall: The anterior wall of the tympanic cavity is very narrow. This is because the medial and lateral walls converge anteriorly. The anterior wall can be divided into two portions; the upper and lower portions. The lower portion of the anterior wall is larger than the upper portion. It has a thin plate of bone which separates this portion from the internal carotid artery as it enters the skull. This plate has two openings for the carotico tympanic nerves. The upper opening transmits the superior carotico tympanic nerve and the inferior opening transmits the inferior carotico tympanic nerve. It is through these nerves that sympathetic nerves reach the tympanic plexus. The upper smaller part of the anterior wall has two tunnels placed one below the other. The upper tunnel transmits the tensor tympani muscle, and the lower tunnel transmits the bony portion of the eustachean tube.

Medial wall: The medial wall separates the middle ear from the inner ear. The most prominent portion of the medial wall of the middle ear cavity is the promontory. It is a rounded projection occupying most of the central portion of the medial wall of the middle ear. This projection is raised by the underlying basal turn of the cochlea. The promontory has numerous small grooves on its surface. These grooves contain the tympanic plexus of nerves. Behind and above the promontory is the oval window (fenestra vestibuli). This is a oval shaped opening connecting the tympanic cavity with the vestibule. In life this is closed by the foot plate of stapes and its surrounding annular ligament. The long axis of the fenestra vestibuli is horizontal. Its inferior border is concave. The size of the oval window varies, but on an average it is 3.25mm long and 1.75 mm wide. Above this fenestra vestibuli is the canal for facial nerver (horizontal portion) and below lies the promontory. Hence the fenestra vestibuli lies at the bottom of a depression also known as fossula that can be of varying depths depending on the position of the facial nerve and the prominence of the promontory.
The fenestra cochlea (round window) lies just below and behind the oval window. It is closed in life by a membrane known as the round window membrane (secondary tympanic membrane). The secondary tympanic membrane appears to be divided into an anterior and posterior portions by the presence of a transverse thickening. The diameter of the round window membrane is between 1.8 to 2.3 mm. It is made up of three layers; the outer mucosal, middle fibrous and an inner endothelial layer. The membrane of the fenestra cochleae does not lie at the end of the scala tympani but forms part of its floor. The ampulla of the posterior semicircular canal is the closest vestibular structure to this membrane. The nerve supplying the ampulla of the posterior semicirular canal (singular nerve) lies close to this secondary tympanic membrane. The secondary tympanic membrane forms a landmark for the position of the singular nerve. This is useful during surgical procedures like singular neurectomy for treatment of intractable vertigo. These two windows (oval & round) are separated by the posterior extension of the promontory. This is known as the subiculum. Rarely a spicule of bone arises from the promontory above the subiculum and runs to the pyramid on the posterior wall of the middle ear cavity. This spicule of bone is known as the ponticulus. The round window faces inferiorly and a little posteriorly, lying completely under the cover of the promontory and hence usually is difficult to visualise. The round window niche is usually trianglular in shape, having anterior, posterosuperior and posteroinferior walls. The posterosuperior and posteroinferior walls meet posteriorly leading on to the sinus tympani. This sinus tympani is a difficult area to visualise. Cholesteatoma may lurk in this area making it difficult to remove. This is one of the commonest causes of cholesteatoma recurrence after mastoidectomy. Small mirrors known as the zinne mirror can be used to visualise this area indirectly. Since sinus tympani lies under the pyramid, removal of the pyramid during surgery will bring the sinus tympani area into view. The facial nerve canal is another important anatomical structure present in this wall. This nerve runs above the promontory and fenestra vestibuli in an anteroposterior direction. The canal may occasionally be deficient leaving an exposed facial nerve. This is a dangerous anatomical variant because this nerve can easily be traumatised during any surgical procedures in the middle ear cavity. Even infections of the middle ear mucosa can cause facial nerve palsy in patients with an exposed facial nerve. The anterior end of the facial nerve canal is marked by the presence of a bony process known as processus cochleariformis. This curved projection of bone is concave anteriorly and it houses the tendon of the tensor tympani muscle as it turns laterally to the handle of the malleus. Behind the fenestra vestibuli, the facial nerve turns inferiorly to begin its descent in the posterior wall of the tympani cavity.

The region above the level of the facial nerve canal forms the medial wall of the epitympanum or attic. The dome of the lateral semicircular canal extends a little lateral to the facial canal and is the major feature of the posterior portion of the epitympanum. In well pneumatised bones this dome of the lateral canal can be very prominent.



Figure showing pyramid of middle ear and its relations

Posterior wall: The posterior wall of the middle ear is wider above than below. In its upper part it has an important opening known as the aditus. This aditus helps the middle ear communicate with the mastoid air cell system. Aditus is a large irregular opening connecting the mastoid antrum to the middle ear cavity. Below the aditus is a small depression known as the fossa incudis. Fossa incudis houses the short process of the incus. Below the fossa incudis lies the pyramid.

Pyramid is a small conical projection which is hollow and its apex pointing anteriorly. It contains the stapedius muscle, the tendon of which passes forwards to insert into the neck of the stapes. The canal within the promontory curves downwards and backwards to join the descending portion of the facial nerve canal. Between the promontory and the tympanic annulus is the facial recess. The facial recess is bounded medially by the facial nerve and laterally by the tympanic annulus. Running through the wall between the two with varying degress of obliquity is the chorda tympani nerve. This nerve always run medial to the tympanic membrane. Drilling over the facial recess area between the facial nerve and the annulus in the angle formed by the chorda tympani nerve can lead into the middle ear cavity. This surgical approach to the middle ear cavity through this area is known as the facial recess approach. This approcah is suitable for surgeries involving the round window niche like placement of electrodes during cochlear implant procedures. Hypotympanum can also be approached through this approach.

 

Figure showing sinus tympani of middle ear

 

Contents of the middle ear:

The most important content of the middle ear is air. The air flows into the middle ear through a patent eustachean tube. The other contents are:

Chain of three ossicles which help in sound transmission; the malleus, incus and stapes. Two muscles, chorda tympani nerve and the tympanic plexus of nerves.

Malleus: This bone is shaped like a hammer hence the name. This is the largest of the three ossicles of the middle ear cavity. It has a head, neck and three processes arising from below the neck. The overall length of the malleus ranges between 7.5 - 9 mm. Its head lies in the attic region of the middle ear effectively dividing the attic into an anterior portion and a posterior one. The anterior portion lie anterior to the handle of the malleus, while the posterior portion lie behind the handle of the malleus. During surgical procedures for attic cholesteatoma clipping of this head will improve the exposure in the attic region. The head of the malleus on its posteriomedial surface has an elongated saddle shaped cartilage covered facet for articulation with the incus. This articular surface is constricted near its middle dividing the articular facet into a larger superior and a smaller inferior portions. The inferior portion of the articular facet lies at right angles to that of the superior portion. This projecting lower portion is also known as the cog or spur of the malleus. Below the neck the bone broadens and gives rise to the following: the anterior process from which a slender anterior ligament arises to insert into the petrotympanic fissure; the lateral process which receives the anterior and posterior malleolar folds from the annulus tympanicum, and the handle which runs downwards, medially and slightly backwards between the mucous and fibrous layers of the tympanic membrane. On the deep medial surface of the handle there is a small projection into which the tendone of the tensor tympani muscle inserts. Additionally the malleus is supported by the superior ligament which runs from the head to the tegmen tympani.


 

Figure showing malleus and its articular facets

Incus: This bone is shaped like an anvil. It articulates with the malleus and has a body and two processes. The body lies in the attic and has a cartilage covered articular facet corresponding to that of the malleus. The short process projects backwards from the body to lie in the fossa incudis. It is infact attached to the fossa incudis by a short ligament. The long process of the incus descends into the mesotympanum behind and medial to the handle of the malleus. At its tip there is a small medially directed lenticular process which articulates with the stapes. The long process of the incus has precarious blood suppy. This portion of the incus is prone for undergoing necrosis in disease conditions.

Figure showing incus and its articular facets

The stapes: The stapes consists of a head, neck, two crura and a base (footplate). The head of the stapes points laterally and has a small cartilage covered depression for articulation with the lenticular process of the incus. The tendon of the stapedius muscle attaches to the posterior part of the neck and the upper part of the posterior crura. The neck of the stapes gives rise to two crura, the anterior crura is thinner and less curved than the posterior crura. The two crura join the foot plate which closes the oval window during life. The average dimensions of the foot plate is 3mm x 1.4mm. The long axis of the foot plate is almost horizontal, with the posterior end being slightly lower than the anterior.

 

Figure showing stapes bone

Muscles of the middle ear:

Stapedius muscle: arises from the walls of the concial cavity within the pyramid. A slender tendon emerges from the apex of the pyramid and inserts into the stapes. This muscle is supplied by a small branch from the facial nerve.The stapedial tendon is inserted into the neck of the stapes. On contraction this muscle rocks the stapes backwards holding it firm against the annular ligament preventing excessive transmission of sound into the inner ear. This muscle has a protective role to play. It protects the inner ear from insults caused by loud noise. Patients with facial nerve palsy have hyperacusis because of lack of action of this muscle.

Tensor tympani muscle: This long slender muscle arises from the walls of the bony canal which lie above the canal for the eustachean tube. Parts of the muscle also arise from the cartilagenous portion of the eustachean tube and the greater wing of sphenoid. From these origins the muscle passes backwards into the tympanic cavity lying on the medial wall of the middle ear just below the level of the facial nerve. The bony covering of the canal is often deficient in its tympanic segment where the muscle is replace by its tendon. This tendon enters the processus cochleariformis, turns at right angles inserting into the medial aspect of the upper end of the handle of the malleus. This muscle is supplied by the mandibular nerve by way of a branch from the medial pterygoid nerve, which passes through the otic ganglion without synapsing. This muscle tenses the tympanic membrane by holding the handle of the malleus thus helping the middle ear in better sound perception.

Chorda tympani nerve:

This is a branch of the facial nerve. It enters the middle ear cavity through the posterior canaliculus which is present at the junction of the lateral and posterior walls. It runs across the medial surface of the tympanic membrane between the mucosal and fibrous layers passes medial to the upper portion of the handle of the malleus. Here it lies above the tendon of the tensor tympani muscle, continues forwards and leaves by way of the anterior canaliculus placed within the petrotympanic fissure. It joins the lingual branch of the V nerve with which it is distributed to the anterior 1/3 of the tongue.

Tympanic plexus:

Is found over the promontory. It is formed by the tympanic branch of the glossopharyngeal nerve, carotico tympanic nerves which supplies the sympathetic component. The tympanic plexus provide the following branches:

1. Branches to the mucous membrane lining the tympanic cavity, eustachean tube, mastoid antrum and its air cells

2. A branch joining the greater superficial petrosal nerve.

3. The lesser superficial petrosal nerve, which contain all the parasympathetic fibers of the IX nerve. This nerve leaves the middle ear through a small canal below the tensor tympani muscle where it receives parasympathetic fibers from the VII nerve by way of a branch from the geniculate ganglion. The full nerve passes through the temporal bone to emerge lateral to the greater superficial petrosal nerve on the floor of the middle cranial fossa, outside the dura. It then passes through the foramen ovale with the mandibular nerve and accessory meningeal artery to the otic ganglion. Post ganglionic fibers from the otic ganglion supply secretomotor fibers to the parotid gland by way of the auriculotemporal nerve.

The mucosal lining of the middle ear cavity is varies according to the location. The attic or the epitympanum is lined by pavement epithelium, while the middle ear proper is lined by cuboidal epithelium and the hypotympanum is lined by ciliated columnar epithelium.



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