ANATOMY OF THE MIDDLE EAR By Dr. Syed Salman Hussaini
THE MIDDLE EAR CLEFT The middle ear cleft consists of the − tympanic cavity, − Eustachian tube and − mastoid air cell system. The tympanic cavity is an irregular, air-filled space within the temporal bone between the tympanic membrane laterally and the osseous labyrinth medially. It contains the ossicles, muscles and structures, like the tympanic segment of the facial nerve,
THE TYMPANIC CAVITY Divided into three compartments the epitympanum (upper), the mesotympanum (middle) and hypotympanum (lower). The epitympanum or attic, lies above the level of the malleolar folds and is separated from the mesotympanum and hypotympanum by a series of mucosal membranes and folds. The mesotympanum lies opposite the tympanic membrane. The hypotympanum lies below the level of the inferior part of the tympanic sulcus and is continuous with the mesotympanum above.
THE LATERAL WALL The lateral wall of the tympanic cavity is formed by the bony lateral wall of the epitympanum superiorly, tympanic membrane centrally and bony lateral wall of the hypotympanum inferiorly. The lateral epitympanic wall is wedge-shaped in section and its sharp inferior portion is also called the outer attic wall or scutum (Latin: shield). It is thin and easily eroded by cholesteatoma, leaving a telltale sign on a high resolution coronal CT scan.
Three holes are present in the bone of the medial surface of the lateral wall of the tympanic cavity. The petrotympanic fissure is 2 mm long which opens anteriorly just above the attachment of the tympanic membrane. It receives the anterior malleolar ligament and transmits the anterior tympanic branch of the maxillary artery to the tympanic cavity. The chorda tympani nerve enters the medial surface of the fissure through a separate anterior canaliculus (canal of Huguier) which is sometimes confluent with the fissure. It then runs posteriorly between the fibrous and mucosal layers of the tympanic membrane, across the upper part of the handle of the malleus and then continues within the membrane, but below the level of the posterior malleolar fold.
The nerve reaches the posterior bony canal wall just medial to the tympanic sulcus, enters the posterior canaliculus. It then runs obliquely downwards and medially through the posterior wall of the tympanic cavity until it reaches the facial nerve. The point of entry of the chorda tympani into the facial nerve bundle is usually at the level of the inferior third of the facial canal on its anterior wall. During cortical mastoidectomy, the fibrous strands of the tympanomastoid suture line can often be confused with the chorda tympani although the angle of the white strands of the suture line is different from the angle of the chorda. The nerve carries taste sensation from the anterior two- thirds of the same side of the tongue and secretomotor fibres to the submandibular gland.
THE ROOF The roof of the epitympanum is the tegmen tympani It is a thin bony plate that separates the middle ear space from the middle cranial fossa. It is formed by both the petrous and squamous portions of the temporal bone . The petrosquamous suture line, which does not close until adult life, can provide a route of access for infection into the extradural space in children. Veins from the tympanic cavity running to the superior petrosal sinus pass through this suture line.
THE FLOOR The floor of the tympanic cavity separates the hypotympanum from the dome of the jugular bulb. Its thickness varies according to the height of the jugular fossa. Occasionally, the floor is deficient and the jugular bulb is then covered only by fibrous tissue and a mucous membrane. At the junction of the floor and the medial wall of the cavity there is a small opening that allows the entry of the tympanic branch of the glossopharyngeal nerve into the middle ear.
THE ANTERIOR WALL The anterior wall of the tympanic cavity is rather narrow as the medial and lateral walls converge. The lower-third consists of a thin plate of bone covering the carotid artery. This plate is perforated by the – superior and inferior caroticotympanic nerves (which carry sympathetic fibres to the tympanic plexus) and – tympanic branches of the internal carotid artery. The middle-third - tympanic orifice of the Eustachian tube. It is oval and 5 x 2 mm in size. Just above this is a canal containing the tensor tympani muscle that subsequently runs along the medial wall of the tympanic cavity enclosed in a thin bony sheath. The upper-third is usually pneumatized and may house the anterior epitympanic sinus, a small niche anterior to the ossicular heads, which can hide residual cholesteatoma in canal wall up surgery.
THE MEDIAL WALL The medial wall separates the tympanic cavity from the internal ear. The promontory is a rounded elevation occupying much of the central portion of the medial wall. It covers part of the basal coil of the cochlea and usually has small grooves on its surface containing the nerves which form the tympanic plexus. Sometimes the groove containing the tympanic branch of the glossopharyngeal nerve may be covered by bone, thereby forming a small canal. The promontory gently inclines forwards to merge with the anterior wall of the tympanic cavity, but is more steeply sloped posteriorly.
Behind and above the promontory is the oval window. It is a kidney-shaped opening that connects the tympanic cavity with the vestibule, which is closed by the footplate of the stapes and its surrounding annular ligament. Its size varies with the size of the footplate, but on average it is 3.25 mm long and 1.75 mm wide. The oval window niche can be of varying width depending on the position of the facial nerve superiorly, and the prominence of the promontory inferiorly.
The round window niche lies below and a little behind the oval window niche from which it is separated by a posterior extension of the promontory called the subiculum. Another ridge of bone, the ponticulus, leaves the promontory above the subiculum and runs to the pyramid on the posterior wall of the cavity. The round window niche is most commonly triangular in shape, with anterior, posterosuperior and posteroinferior walls. The latter two meet posteriorly and lead to the sinus tympani.
The round window membrane is usually out of sight, obscured by the overhanging edge of the promontory forming the niche and mucosal folds within it. The membrane is roughly oval in shape, about 2.3 x 1.9 mm in dimension and lies in a plane at right angles to the plane of the stapes footplate. It tends to curve towards the scala tympani of the basal coil of the cochlea, so that it is concave when viewed from the middle ear. It appears to be divided into an anterior and posterior portion by a transverse thickening.
The facial nerve canal (or Fallopian canal) runs above the promontory and oval window in an anteroposterior direction. It has a smooth rounded lateral surface that often has microdehiscences When the bone is thin or the nerve exposed by disease, there are two or three straight blood vessels clearly visible along this line of nerve. These are the only straight blood vessels in the middle ear and indicate that the facial nerve is very close by. The facial nerve canal is marked anteriorly by the processus cochleariformis, a curved projection of bone, concave anteriorly, which houses the tendon of the tensor tympani muscle as it turns laterally to the handle of the malleus. Behind the oval window, the facial canal starts to turn inferiorly as it begins its descent in the posterior wall of the tympanic cavity.
The region above the level of the facial nerve canal forms the medial wall of the epitympanum. The dome of the lateral semicircular canal is the major feature of the posterior portion of the epitympanum, lying posterior and extending a little lateral to the facial canal. In well - aerated mastoid bones, the labyrinthine bone over the superior semicircular canal may be prominent, running at right angles to the lateral canal and joining it anteriorly at a swelling which houses the ampullae of the two canals. In front and a little below this, above the processus cochleariformis, may be a slight swelling corresponding to the geniculate ganglion, with the bony canal of the greater superficial petrosal nerve running for a short distance anteriorly.
THE POSTERIOR WALL The posterior wall is wider above than below. Upper part a large irregular opening - the aditus ad antrum, that leads back from the posterior epitympanum into the mastoid antrum. Below the aditus is a small depression, the fossa incudis, which houses the short process of the incus and its suspensory ligament. Below the fossa incudis and medial to the opening of the chorda tympani nerve is the pyramid, a small hollow conical projection with its apex pointing anteriorly. This houses the stapedius muscle and tendon, which inserts into the posterior aspect of the head of stapes. The canal within the pyramid curves downwards and backwards to join the descending portion of the facial nerve canal.
The facial recess is a groove which lies between the pyramid with facial nerve, and the annulus of the tympanic membrane . This is shallower lower down where the facial nerve canal forms only a slight prominence on the posterior wall. The facial recess is, therefore, bounded – medially by the facial nerve and – laterally by the tympanic annulus, – with the chorda tympani nerve running obliquely through the wall between the two. The chorda always runs medial to the tympanic membrane. The angle between the facial nerve and the chorda allows a posterior tympanotomy, allowing access to the middle ear from the mastoid without disruptiong the tympanic membrane.
The sinus tympani is a posterior extension of the mesotympanum and lies deep to both the promontory and the facial nerve. This extension of air cells into the posterior wall can be extensive, and is probably the most inaccessible site in the middle ear and mastoid. The sinus can extend as far as 9 mm into the mastoid bone when measured from the tip of the pyramid. The medial wall of the sinus tympani becomes continuous with the posterior portion of the medial wall of the tympanic cavity where it is related to the oval and round window niches and the subiculum of the promontory. On rare occasions it can communicate with the mastoid air cells.
Cholesteatoma which has extended to the sinus tympani from the mesotympanum is extremely difficult to eradicate. The worst region for access is above the pyramid, posterior to an intact stapes and medial to the facial nerve. A retrofacial approach to this region via the mastoid is not possible because the posterior semicircular canal blocks access.
THE CONTENTS OF THE TYMPANIC CAVITY The tympanic cavity contains the – ossicles, – two muscles, – the chorda tympani and – the tympanic plexus. The ossicles are the malleus, incus and stapes that form a semi-rigid bony chain for conducting sound. The malleus is the most lateral and is attached to the tympanic membrane, whereas the stapes is attached to the oval window.
THE MALLEUS (the hammer) The malleus is the largest of the three ossicles, measuring up to 9 mm in length. It comprises a head, neck and handle or manubrium. The head lies in the epitympanum and is suspended by the superior ligament, which runs upward to the tegmen tympani. The head of the malleus has a saddle-shaped facet on its posteromedial surface to articulate with the body of the incus by a synovial joint. Below the neck of the malleus, the bone broadens and gives rise to the lateral process, the anterior process and the handle. The lateral process is a prominent landmark on the tympanic membrane and receives the anterior and posterior malleolar folds from the tympanic annulus.
The chorda tympani crosses the upper part of the malleus handle on its medial surface above the insertion of the tendon of tensor tympani, but below the neck of the malleus itself. The neck of the malleus connects the handle with the head and amputation of the head by cutting through the neck leaves both chorda tympani and tensor tympani intact. A slender anterior ligament arises from the anterior process to insert into the petrotympanic fissure. The handle runs downwards, medially and slightly backwards between the mucosal and fibrous layers of the tympanic membrane.
The handle is very closely attached to the membrane at its lower end, there is a fine web of mucosa separating the membrane from the handle in the upper portion before it becomes adherent again at the lateral process. This can be opened surgically to create a slit without perforating the membrane to allow a prosthesis to be crimped around the malleus handle in certain types of ossicular reconstruction. On the deep, medial surface of the handle, near its upper end, is a small projection into which the tendon of the tensor tympani muscle inserts.
THE INCUS (the anvil) The incus articulates with the malleus and has a body and two processes. The body lies in the epitympanum and has a cartilage- covered facet corresponding to that on the malleus. The body of the incus is suspended by the superior incudal ligament that is attached to the tegmen tympani. The short process projects backwards from the body to lie in the fossa incudis to which it is attached by a short suspensory ligament. The long process descends into the mesotympanum behind and medial to the handle of the malleus, and at its tip is a small medially directed lentiular process. It has been called the fourth ossicle because of its incomplete fusion with the tip of the long process, giving the appearance of a separate bone or at least a sesamoid bone. The lenticular process articulates with the head of the stapes.
THE STAPES (the stirrup) The stapes is shaped like a stirrup and consists of a head, neck, the anterior and posterior crura and a footplate. The head points laterally and has a small cartilage- covered depression for a synovial articulation with the lenticular process of the incus. The stapedius tendon inserts into the posterior part of the neck and upper portion of the posterior crus. The two crura arise from the broader lower part of the neck and the anterior crus is thinner and less curved than the posterior one. Both are hollowed out on their concave surfaces, which gives an optimum combination of strength and lightness. The two crura join the footplate, which usually has a convex superior margin, an almost straight inferior margin and curved anterior and posterior ends.
The average dimensions of the footplate are 3 mm long- and 1.4 mm wide, and it lies in the oval window where it is attached to the bony margins by the annular ligament. The long axis of the footplate is almost horizontal, with the posterior end being slightly lower than the anterior.
THE STAPEDIUS MUSCLE The stapedius arises from the walls of the conical cavity within the pyramid as well as from the downward curved continuation of this canal in front of the descending portion of the facial nerve. A slender tendon emerges from the apex of the pyramid and inserts into the stapes. The muscle is supplied by a small branch of the facial nerve.
THE TENSOR TYMPANI MUSCLE It arises from the walls of the bony canal lying above the Eustachian tube. Parts of it also arise from the cartilaginous portion of the Eustachian tube and the greater wing of the sphenoid. The muscle then passes backwards into the tympanic cavity where it lies on the medial wall, a little below the level of the facial nerve. The bony covering of the canal is often deficient in its tympanic segment where the muscle is replaced by a slender tendon. This enters the processus cochleariformis where it is held down by a transverse tendon as it turns through a right angle to pass laterally and insert into the medial aspect of the upper end of the malleus handle. It is supplied by mandibular nerve from its branch, the medial pterygoid nerve.
THE CHORDA TYMPANI NERVE It enters the tympanic cavity from the posterior canaliculus 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. Then passes medial to the upper portion of the handle of the malleus above the tendon of tensor tympani. Continues forwards and leaves by way of the anterior canaliculus, which subsequently joins the petrotympanic fissure.
THE TYMPANIC PLEXUS It is formed by the – tympanic branch of the glossopharyngeal nerve (Jacobsons nerve) and – caroticotympanic nerves, which arise from the sympathetic plexus around the internal carotid artery. The nerves form a plexus on the promontory and provide the branches to the mucous membrane lining the tympanic cavity, Eustachian tube and mastoid antrum and air cells. The plexus also provides branches to join the greater superficial petrosal nerve and the lesser superficial petrosal nerve that contains all the parasympathetic fibres of the glossopharyngeal nerve.
THE MUCOSA OF THE TYMPANIC CAVITY Mucus-secreting respiratory mucosa bearing cilia. Three distinct mucocilary pathways can be identified – epitympanic, – promontorial and – hypotympanic, (largest). Each of these pathways coalesces at the tympanic orifice of the Eustachian tube. The mucous membrane lines the bony walls of the tympanic cavity, and extends to cover the ossicles and their supporting ligaments.
It also covers the tendons of the two middle ear muscles and carry their blood supply. These folds separate the middle ear space into compartments. The only route for ventilation of the epitympanic space from the mesotympanum is via two small openings between the various mucosal folds - the anterior and posterior isthmus tympani. Prussaks space is found between the pars flaccida and the neck of the malleus, bounded by the lateral malleolar fold. This space can play an important role in the retention of keratin and subsequent development of cholesteatoma.
THE BLOOD SUPPLY OF THE TYMPANIC CAVITY Arise from both the internal and external carotid system. The overlap is extensive and great variability is present. Supply is from the anterior tympanic, stylomastoid, maxillary, posterior auricular, middle meningeal, ascending pharyngeal, artery of pterygoid canal and internal carotid arteries. The anterior tympanic and stylomastoid arteries are the biggest. Anterior tympanic artery br. of Maxillary Artery supplies Tympanic membrane; malleus and incus; anterior part of tympanic cavity. Stylomastoid artery br. of Posterior Auricular artery supplies Posterior part of tympanic cavity; stapedius muscle and Mastoid air cells.
THE EUSTACHIAN TUBE It is a dynamic channel that links the middle ear with the nasopharynx. Length = 36 mm (reached by the age of 7). It runs downwards from the middle ear at 45° and is turned forwards and medially. Consists of two unequal cones, connected at their apices. The lateral third is bony and arises from the anterior wall of the tympanic cavity. Medial two-thirds cartilaginous part. Its narrowest portion is called the isthmus, where the diameter is only 0.5 mm or less.
It is lined with respiratory mucosa containing goblet cells and mucous glands, having ciliated epithelium on its floor. At its nasopharyngeal end, the mucosa is truly respiratory; but in passing along the tube towards the middle ear, the number of goblet cells and glands decreases, and the ciliary carpet becomes less profuse. It runs through the squamous and petrous portions of the temporal bone, gradually tapering to the isthmus. A thin plate of bone forms the roof, separating the tube from the tensor tympani muscle above. The carotid canal lies medially and can impinge on the bony Eustachian tube.
The cartilaginous part of the tube is around 24 mm long and consists of a fibrocartilaginous skeleton to which attached the peritubal muscles. At its upper border, the cartilage is bent over to resemble an inverted J, forming a longer medial cartilaginous lamina and shorter lateral cartilaginous lamina. The cartilage is fixed to the base of the skull in a groove between the petrous part of the temporal bone and the greater wing of the sphenoid, which terminates near the root of the medial pterygoid plate. Thus, the back (posteromedial) wall is composed of cartilage and the front (anterolateral) wall comprises cartilage and fibrous tissue. The apex of the cartilage is attached to the isthmus of the bony portion, while the wider medial end protrudes into the nasopharynx, lying directly under the mucosa to form the torus tubarius.
In the nasopharynx, the tube opens 1-1.25 cm behind and below the posterior end of the interior turbinate. The opening is triangular in shape and is surrounded above and behind by the torus. The salpingopharyngeal fold stretches from the lower part of the torus downwards to the wall of the pharynx. The levator palati, as it enters the soft palate, results in a small swelling immediately below the opening of the tube. Behind the torus is the pharyngeal recess or fossa of Rosenmuller. Lymphoid tissue is present around the tubal orifice and in the fossa of Rosenmuller, and may be prominent in childhood.
MUSCLES ATTACHED TO THE EUSTACHIAN TUBE The tensor palati muscle arises from the bony wall and from along the whole length of the lateral cartilaginous lamina that forms the upper portion of the front wall of the cartilaginous tube. From these broad origins the muscle descends, converges to a short tendon that turns medially around the pterygoid hamulus and then spreads out within the soft palate to meet fibres from the other side in a midline raphe. The tensor palati separates the tube from the otic ganglion, the mandibular nerve and its branches, the chorda tympani nerve and the middle meningeal artery. It is supplied by the Mandibular Nerve.
Salpingopharyngeus is attached to the inferior part of the cartilage of the tube near its pharyngeal opening, and it descends to blend with the palatopharyngeus. Levator palati arises from the lower surface of the cartilaginous tube and from the lower surface of the petrous bone, and from fascia forming the upper part of the carotid sheath. It first lies inferior to the tube, then crosses to the medial side and spreads out into the soft palate. Salpingopharyngeus and the levator palati are supplied from the pharyngeal plexus. The ascending pharyngeal and middle meningeal arteries supply the Eustachian tube. The veins drain into the pharyngeal plexus and the lymphatics pass to the retropharyngeal nodes. The nerve supply arises from the pharyngeal branch of the sphenopalatine ganglion (Vb) for the ostium, the nervus spinosus (Vc) for the cartilaginous portion and from the tympanic plexus (IX) for the bony part.
The extent of pneumatization of the temporal bone varies according to heredity, environment, nutrition, infection, and eustachian tube function. There are five recognized regions of pneumatization: the middle ear, mastoid, perilabyrinthine, petrous apex, and accessory. The mastoid region is subdivided into the mastoid antrum, central mastoid, and peripheral mastoid. The bony labyrinth divides the perilabyrinthine region into supralabyrinthine and infralabyrinthine areas. The apical area and the peritubal area comprise the petrous apex region. The accessory region encompasses the zygomatic, squamous, occipital, and styloid areas.
There are five recognized air cell tracts. The posterosuperior tract runs at the juncture of the posterior and middle fossa aspects of the temporal bone. The posteromedial cell tract parallels and runs inferior to the posterosuperior tract. The subarcuate tract passes through the arch of the superior semicircular canal. The perilabyrinthine tracts run superior and inferior to the bony labyrinth, whereas the peritubal tract surrounds the eustachian tube. The anterior petrous apex is pneumatized in only 10 to 15% of specimens. Most often, it is diploic; in a small percentage of cases, it is sclerotic.
The mastoid antrum is an air-filled sinus in the petrous part of temporal bone. It communicates with the middle ear by the aditus. Antrum is well developed at birth. Volume = 2 ml (adult). The roof of the mastoid antrum and mastoid air cell space form the floor of the middle cranial fossa. The medial wall relates to the posterior semicircular canal. More deeply and inferiorly is the dura of the posterior cranial fossa and the endolymphatic sac.
Posterior to the endolymphatic system is the sigmoid sinus, which curves downwards only to turn sharply upwards to pass medial to the facial nerve and then becomes the dome of the jugular bulb in the middle ear space. The posterior belly of the digastric muscle forms a groove in the base of the mastoid bone. The digastric ridge inside the mastoid lies lateral to the sigmoid sinus and the facial nerve and is a useful landmark for finding the nerve. The periosteum of the digastric groove continues anteriorly and part of it becomes the endosteum of the stylomastoid foramen and subsequently of the facial nerve canal.
MacEwens triangle is a direct lateral relation to the mastoid antrum and is formed by – a posterior prolongation of the line of the zygomatic arch and – a tangent to this, that passes through the posterior border of the external auditory meatus. In most of the population, the mastoid air cell system is fairly extensive with air cells. Alternatively, the mastoid antrum may be the only airfilled space in the mastoid process when the name acellular or sclerotic is applied. This condition occurs in 20 percent of adult temporal bones and is seen in individuals with chronic ear disease. Normally lining of the mastoid is a flattened, nonciliated epithelium without goblet cells or mucus glands.