The middle ear is an irregular air-filled, mucous membrane–lined space in the temporal bone. It is situated between the tympanic membrane laterally and the lateral wall of the internal ear medially. The middle ear together with the eustachian tube, attic, aditus, mastoid antrum and mastoid air cells is called middle ear cleft. It houses ear ossicles, stapedius muscle and tensor tympani muscle and chorda tympani nerve and tympanic plexus. Stapedius muscle and tensor tympani muscle attaches the ossicles to the middle ear muscles. The middle ear communicates with the mastoid antrum area posteriorly and the nasopharynx (via the eustachian tube) anteriorly. The basic function of middle ear is to transmit vibrations of the tympanic membrane to the internal ear.
Relations of middle ear
The middle ear is traditionally divided into three parts: the epitympanum (upper compartment), the mesotympanum (middle compartment) and hypotympanum (lower compartment).
- Epitympanum or the Attic – It lies above the level of pars tensa and malleolar folds but medial to pars flaccida and the bony lateral attic wall (also known as scutum). It is separated from the mesotympanum and hypotympanum by a series of mucosal membranes and folds.
- Mesotympanum or the tympanic cavity – It lies opposite to the pars tensa.
- Hypotympanum – It lies below the level of pars tensa and tympanic sulcus. The retrotympanum is the area lying in the posterior wall of the tympanic cavity, whereas the protympanum is area lying in the anterior wall of the tympanic cavity close to the Eustachian tube opening.
Diagram showing Retrotympanum and Protympanum area
Boundaries of middle ear.
Middle ear can be likened to a six-sided box with a roof, a floor, anterior, posterior, lateral, medial walls.
- The roof (tegmental wall) is formed by a thin layer of bone known as tegmen tympani (petrous part of the temporal bone) which separates the middle ear from the middle cranial fossa. It also extends posteriorly to form the roof of the aditus and antrum. A bony crest (cog) projects from the roof caudally to lie anterior to the head of the malleus. Cog divides the epitympanum into larger posterior epitympanic space from the smaller anterior epitympanic space, where residual cholesteatoma may be left if not formally explored in canal wall-up surgery.
- The floor (jugular wall) is also a thin layer of bone, which separates hypotympanum from the dome of the jugular bulb (internal jugular vein). Occasionally, it is congenitally deficient and the jugular bulb may then project into the middle ear separated from the cavity only by fibrous tissue and a mucous membrane. Jacobson’s nerve also enters the middle ear through the floor. This should be kept in mind while raising the tympanomeatal flap inferiorly.
- The anterior wall (carotid wall) is a wafer thin layer of bone (up to 3mm thick), which separates the cavity from internal carotid artery. It also has two openings; the lower large opening for the entrance of the eustachian tube into the middle ear and the upper smaller opening for the canal of tensor tympani muscle. The foramen (anterior canaliculus) for the exit of the chorda tympani nerve from the middle ear is also associated with this wall.
- The posterior wall (mastoid wall) is only partially complete. The lower part of this wall consists of a bony partition between the middle ear and mastoid air cells. Superiorly, the epitympanum compartment (attic) is continuous with the aditus, through which it communicates with the mastoid antrum. Posterior wall presents a bony projection, a small elevation called pyramid through which the tendon of the stapedius muscle appears and get attached to the neck of stapes. Aditus lies above the level of pyramid. Facial nerve runs in the posterior wall just behind the pyramid. Facial recess or the posterior sinus is a depression in the posterior wall lateral to the pyramid. It is bounded medially by the vertical part of VIIth nerve, laterally by the chorda tympani and above, by the fossa incudis. Surgically, facial recess is important, as direct access can be made through this into the middle ear without disturbing posterior canal wall.
- The lateral wall (Membranous wall). It is formed by the bony lateral wall of the epitympanum (scutum) superiorly, the tympanic membrane centrally and the bony lateral wall of the hypotympanum inferiorly. Scutum (Latin: ‘shield’) is a wedge shaped thin bone and it easily gets eroded by cholesteatoma, leaving a tell-tale sign on a high-resolution coronal CT scan. The tympanic membrane is semi- transparent and forms a “window” into the middle ear. Few structures of the middle ear (such as long process of incus, incudostapedial joint, round window and eustachian tube) can be seen through the normal semi-transparent tympanic membrane.
- The medial wall (Labyrinthine wall) separates the middle ear from internal ear. It is formed by the lateral wall of the labyrinth. It presents a rounded bulge (the promontory) produced by the basal coil of the cochlea and occupies largely the central portion of the medial wall. On promontory there is tympanic plexus, which consists of tympanic branch of the glossopharyngeal nerve [IX] and branches from the internal carotid plexus. Tympanic plexus supplies the mucous membrane of the middle ear, the mastoid area, and the eustachian tube. Other important structures present on the labyrinthine wall are the oval and round windows. The oval window is a kidney shaped structure. Footplate of stapes is attached to the oval window which transfer vibrations initiated by the tympanic membrane to the cochlea. Round window (fenestra cochleae) is covered by the secondary tympanic membrane. Above the oval window is the canal for facial nerve. Its bony covering may sometimes be congenitally dehiscent and the nerve may lie exposed making it very vulnerable to injuries or infection. Above the canal for facial nerve is the broader ridge of bony prominence of lateral semicircular canal. Just anterior to the oval window, the medial wall presents a hook-like projection called processus cochleariformis. The tendon of tensor tympani takes a turn here to get attachment to the neck of malleus. The cochleariform process also marks the level of the first genu of the facial nerve which is an important landmark for surgery of the facial nerve. The Sinus tympani is bounded medially by the pyramid, laterally by the posterior semicircular canal and above by the ponticulus (bony ridge present between pyramid and promontory) and below by the subiculum (bony ridge present between round window niche to the styloid eminence region).
MASTOID ANTRUM
It is a large, air-containing space (9 mm height, 14 mm width and 7 mm depth) in the upper part of mastoid.
It communicates with the middle ear through the aditus. The antrum, but not the air cells, is well developed at birth and by adult life has a volume of about 2ml. Its roof is formed by tegmen antri, which is a continuation of the tegmen tympani and separates it from the middle cranial fossa. The lateral wall of antrum is formed by a plate of bone which is on an average 1.5 cm thick in the adult. It is marked externally on the surface of mastoid by suprameatal (MacEwen’s) triangle. The mucous membrane lining the mastoid air cells is continuous with the mucous membrane throughout the middle ear. Therefore infections in the middle ear can easily spread into the mastoid area.
Boundaries of mastoid antrum
Boundaries of mastoid antrum:
- Roof: It is formed by the tegmen antri, which separates mastoid antrum from the middle cranial fossa.
- Lateral wall: It is formed by a 1.5 cm thick plate of squamous part of temporal bone which is marked on the lateral surface of mastoid by suprameatal (Macewen’s) triangle. It is covered by postaural skin.
- Medial wall: It is formed by the petrous bone and related to the Posterior semicircular canal, Endolymphatic sac and Dura of posterior cranial fossa
- Anterior: Anteriorly mastoid antrum communicates with the attic through the aditus ad antrum.
- Posterior wall: It is formed by mastoid bone and communicates with mastoid air cells. Sigmoid sinus curves downwards.
- Floor: It is formed by mastoid bone and communicates with mastoid air cells.
Boundaries of Macewen’s triangle
- a. Linea temporalis (temporal line): A ridge of bone extending posteriorly from the zygomatic process (marking the lower margin of temporalis muscle and approximating the floor of middle cranial fossa).
- b. EAC: Posterosuperior margin of EAC
- c. Tangent: A tangent to the posterior margin of EAC.
Boundaries of Macewen’s triangle
THE MASTOID AND ITS AIR CELL SYSTEM
The mastoid consists of “honeycomb” air cells, which lie underneath the bony cortex. Depending on development of air cell, three types of mastoid have been described.
- Well-pneumatized or cellular. Mastoid cells are well-developed and intervening septa are thin.
- Diploetic. Mastoid consists of marrow spaces and a few air cells.
- Sclerotic or acellular. There are neither cells nor marrow spaces..
With any type of mastoid pneumatization, antrum is always present. In sclerotic mastoids, antrum is usually small and the sigmoid sinus is anteposed. Abscesses may form in relation to the air cells and may sometimes be located far from the mastoid region.
Depending on the location, mastoid air cells are divided into:
- Zygomatic cells are present in the root of zygoma.
- Tegmen cells are present in the tegmen tympani.
- Perisinus cells are present in the the sinus plate.
- Retrofacial cells are present around the facial nerve.
- Perilabyrinthine cells are present above, below and behind the labyrinth.
- Peritubal are present around the eustachian tube.
- Tip cells are present medial and lateral to the digastric ridge in the tip of mastoid.
- Marginal cells are present behind the sinus plate and may extend into the occipital bone.
- Squamosal cells are present in the squamous part of temporal bones.
KORNER’S SEPTUM
Squamous and petrous parts of temporal bone together forms the mastoid. In few instances petrosquamosal suture remains as a bony plate known as Korner’s septum, which separates superficial squamosal cells from the deep petrosal cells. Korner’s septum causes difficulty in locating the antrum and the deeper cells. Removal of Korner’s septum is must in order to reach mastoid antrum during surgery. Otherwise it will lead to incomplete removal of disease at mastoidectomy.
EAR OSSICLES.
There are three ossicles in the middle ear – the malleus, incus and stapes. The ossicles forms a semi-rigid osseous chain across the middle ear from the tympanic membrane to the oval window of the internal ear. The malleus is the most lateral ossicle and it is attached to the tympanic membrane, whereas the stapes ossicle is attached to the oval window. The ossicles conduct sound energy from the tympanic membrane to the oval window. Muscles related to the auditory ossicles modulate movement during the transmission of vibrations from the tympanic membrane.
Malleus (hammer): The malleus is the largest ossicle. It measures 8-9 mm in length. Parts of malleus include the head of the malleus, neck of the malleus, anterior and lateral processes, and handle of the malleus(manubrium).
- Head: The head of the malleus is the rounded upper part of the malleus which lies in the attic region. It is suspended by the superior ligament, which runs upwards to the tegmen tympani. Its saddle-shaped posterior surface articulates with the body of the incus by a synovial joint.
- Neck: Inferior to the head of the malleus is the constricted neck of the malleus which also lies in the attic region. The chorda tympani and tendon of tensor tympani lie below the neck of malleus, hence amputation of the head of malleus by cutting through the neck leaves both chorda tympani and tensor tympani intact.
- Anterior process: Below the neck of malleus there are the anterior and lateral processes. The anterior process is attached to the anterior wall of the middle ear by a ligament.
- Lateral process: The lateral process forms a knob-like projection on the outer surface of the tympanic membrane is attached to the anterior and posterior malleolar folds of the tympanic membrane.
- Handle of malleus: Below the anterior and lateral processes, is the handle of the malleus, which is embedded in the fibrous layer of the tympanic membrane. Tendon of the tensor tympani muscle inserts on the medial surface of the handle. Chorda tympani crosses the handle of malleus just above the insertion of the tensor tympani muscle.
Incus (Anvil). It is the second bone in the series of ear ossicles. Parts of malleus include the body of the incus and long and short process.
- Body. It articulates with the head of the malleus and lie in the attic. It is suspended by the superior incudal ligament coming from tegmen tympani.
- Short process. The short process projects backwards to lie in the fossa incudes. It is attached by a short suspensory ligament to the upper posterior wall of the middle ear.
- Long process. It hangs vertically downward into the mesotympanum almost parallel to the handle of the malleus. Tip of incus also known as the lenticular process and is at times been called the fourth ossicle because of its incomplete fusion with the tip of the long process. The lenticular process articulates with the head of the stapes and forms incudostapedial joint.
Stapes (stirrup): The stapes is the third and smallest bone in the ossicular chain. It measures about 3.5 mm. It is attached to the oval window by annular ligament. Parts of malleus include the head, neck, anterior and posterior crura and footplate.
Muscles associated with the ossicles. There are two middle ear muscles: tensor tympani and the stapedius.
Muscles associated with the ossicles
- Stapedius muscle. The stapedius arises from the apex of pyramid, which is a small projection on the posterior (mastoid) wall of the middle ear and passes forward to attach to the posterior surface of the neck of the stapes. In response to loud sounds (80 dB and above), the stapedius muscle contracts, pulling the stapes posteriorly, hereby dampens the loud sounds and prevents noise trauma to the inner ear (acoustic reflex). Stapedius is a second arch muscle and is supplied by nerve to stapedius which is a branch of CN VII.
- Tensor tympani muscle. This is a long, slender muscle arising from the bony canal present on the anterior wall of middle ear. It is present above the opening of eustachian tube. Its tendon turns through a right angle round the processus cochleariformis and passes laterally and insert into the medial aspect of the upper end of the handle of malleus. It tenses the tympanic membrane. Contraction of the tensor tympani pulls the handle of the malleus medially. This tenses the tympanic membrane, reducing the force of vibrations in response to loud noises. The muscle develops from the first arch and is supplied by medial pterygoid branch of mandibular nerve (V3).
TYMPANIC PLEXUS
Tympanic plexus
The tympanic plexus innervates the the medial surface of the tympanic membrane, tympanic cavity, mastoid air cells and the bony eustachian tube.
The tympanic plexus lies on the promontory and is formed by the
- Tympanic branch of glossopharyngeal and
- Caroticotympanic nerves (sympathetic fibres) from the plexus round the internal carotid artery.
The tympanic plexus also gives off a major branch (the lesser petrosal nerve), which supplies preganglionic para-sympathetic fibers to the otic ganglion. Tympanic branch of glossopharyngeal nerve carries secretomotor fibres for the parotid gland. Section of tympanic branch of glossopharyngeal nerve can be carried out in the middle ear in cases of Frey’s syndrome.
Course of secretomotor fibres to the parotid:
Inferior salivary nucleus → CN IX → Tympanic branch → Tympanic plexus → Lesser petrosal nerve → Otic ganglion → Auriculotemporal nerve → Parotid gland.
CHORDA TYMPANI NERVE
It is a branch of the facial nerve. It enters the middle ear through posterior canaliculus at the junction of the lateral and posterior walls of middle ear. It runs on the medial surface of the tympanic membrane between its mucosal and fibrous layers. On malleus, it runs medially just below the neck of malleus and above the attachment of tendon of tensor tympani. It further continue forwards and leave the middle ear through anterior canaliculus. It carries taste sensation from anterior two-thirds of tongue and supplies secretomotor fibres to the submaxillary and sublingual salivary glands.
MIDDLE EAR CLEFT MUCOSA
The middle ear mucosa is essentially mucus-secreting respiratory mucosa bearing cilia on its surface. Mucosa of the nasopharynx is continuous with that of the middle ear cleft. The mucosa wraps ossicles, muscles, ligaments and nerves in a similar way as peritoneum wraps various viscera in the abdomen – raising several mucosal folds and dividing the middle ear into various compartments. So, all the middle ear structures lie outside the mucous membrane. Middle ear contains nothing but the air; all the structures lie outside the mucous membrane. Mucous membrane of the nasopharynx is continuous with that of the middle ear cleft.
Histologically, the eustachian tube is lined by ciliated epithelium, which is pseudostratified columnar in the cartilaginous part, columnar in the bony part with several mucous glands in the submucosa. Tympanic cavity is lined by ciliated columnar epithelium in its anterior and inferior part which changes to cuboidal type in the posterior part. Epitympanum and mastoid air cells are lined by flat, non-ciliated epithelium.
COMPARTMENTS AND FOLDS OF MIDDLE EAR. Ossicles and their mucosal folds separate mesotympanum from epitympanum (attic).
| 1. | Compartments of Epitympanum | a. Prussak’s space: Its boundaries, which limit spread of infection to other compartments, are following: – Lateral: Pars flaccida – Medial: Neck of malleus – Floor: Lateral process of malleus – Roof: Ligament arising from neck of malleus |
| b. Attic compartments: Cog divides attic into two compartments – smaller anterior epitympanum and larger posterior epitympanum. The space between the lateral malleolar fold and lateral incudal fold provides communication with Prussak’s space. | ||
| 2. | Compartments of Mesotympanum | Inferior incudal space: Its boundaries are following – Superior: Lateral incudal fold – Medial: Medial incudal fold – Lateral: Posterior malleolar fold – Anterior: Interossicular fold |
| Anterior pouch of von Troeltsch: It lies between the following boundaries: – Medial: Anterior malleolar fold – Lateral: Portion of the tympanic membrane anterior to handle of malleus |
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| Posterior pouch of von Troeltsch: It is situated between the following boundaries: – Medial: Posterior malleolar fold – Lateral: Portion of the tympanic membrane posterior to handle of malleus. |
BLOOD SUPPLY OF MIDDLE EAR
Middle ear is supplied by eight arteries, out of which two are the main, i.e.
- Anterior tympanic branch of maxillary artery which supplies tympanic membrane, malleus, incus and anterior part of tympanic cavity.
- Stylomastoid branch of posterior auricular artery which supplies stapedius muscle and posterior part of tympanic cavity.
Six minor vessels are:
- Mastoid branch of stylomastoid artery which supplies mastoid air cells.
- Petrosal branch of middle meningeal artery (runs along greater petrosal nerve) which supplies the roof of mastoid and epitympanum.
- Superior tympanic branch of middle meningeal artery which supplies the canal for tensor tympani muscle, malleus and incus.
- Inferior tympanic branch of ascending pharyngeal artery which supplies the mesotympanum.
- Branch of artery of pterygoid canal (runs along eustachian tube) which supplies the mesotympanum and hypotympanum
- Tympanic branch of internal carotid which supplies the mesotympanum and hypotympanum
| Branch | Parent artery | Region supplied |
| Anterior tympanic | Maxillary artery | Tympanic membrane Malleus and incus Anterior part of tympanic cavity |
| Stylomastoid | Posterior auricular | Posterior part of tympanic cavity Stapedius muscle |
| Mastoid | Stylomastoid | Mastoid air cells |
| Petrosal | Middle meningeal | Roof of mastoid Roof of epitympanum |
| Superior tympanic | Middle meningeal | Malleus and incus Tensor tympani |
| Inferior tympanic | Ascending pharyngeal | Mesotympanum |
| Branch from artery | Artery of pterygoid canal | Meso- and hypotympanum |
| Tympanic arches | Internal carotid | Meso- and hypotympanum |
Veins drain into pterygoid venous plexus and superior petrosal sinus.
LYMPHATIC DRAINAGE OF EAR
The lymphatics of middle ear drain into retropharyngeal and parotid nodes. Eustachian tube lymphatics drain into retropharyngeal group of lymph nodes. Internal ear does not have any lymphatics.
———- End of the chapter ———–
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Learning resources.
- Scott-Brown, Textbook of Otorhinolaryngology Head and Neck Surgery.
- Glasscock-Shambaugh, Textbook of Surgery of the Ear.
- Livio Presutti, Textbook of Endoscopic Ear Surgery Principles, Indications, and Techniques.
- P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
- Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
- Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck surgery.
Author:
Dr. Rahul Kumar Bagla
MS & Fellow Rhinoplasty & Facial Plastic Surgery.
Associate Professor
GIMS, Greater Noida, India
msrahulbagla@gmail.com