Middle Ear Flashcards
Middle Ear
The middle ear together with the eustachian tube, aditus, antrum and mastoid air cells is called middle ear cleft
It is lined by mucous membrane and filled with air.
The middle ear extends much beyond the limits of tym-panic membrane which forms its lateral boundary and is sometimes divided into: (i) mesotympanum (lying opposite the pars tensa), (ii) epitympanum or the attic (lying above the pars tensa but medial to Shrapnell’s membrane and the bony lateral attic wall) and (iii) hypotympanum (lying below the level of pars tensa)
Protympanum
The portion of middle ear around the tympanic orifice of the eustachian tube is sometimes called protympanum
Roof and floor
The roof is formed by a thin plate of bone called tegmen tympani. It also extends posteriorly to form the roof of the aditus and antrum. It separates tympanic cavity from the middle cranial fossa.
The fl oor is also a thin plate of bone, which separates tym-panic cavity from the jugular bulb. Sometimes, it is con-genitally def i cient and the jugular bulb may then project into the middle ear; separated from the cavity only by the mucosa.
Ant wall
The anterior wall has a thin plate of bone, which separates the cavity from internal carotid artery. It also has two open-ings; the lower one for the eustachian tube and the upper one for the canal of tensor tympani muscle.
Posterior wall
The posterior wall lies close to the mastoid air cells. It pres-ents a bony projection called pyramid through the summit of which appears the tendon of the stapedius muscle to get attachment to the neck of stapes. Aditus, an opening through which attic communicates with the antrum, lies above the 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 thchorda tympani and above, by the fossa incudis (Figure 1.10). Surgically, facial recess is important, as direct access can be made through this into the middle ear without dis-turbing posterior canal wall
Medial wall
The medial wall is formed by the labyrinth. It presents a bulge called promontory which is due to the basal coil of cochlea; oval window into which is fixed the footplate of stapes; round window or the fenestra cochleae which is covered by the secondary tympanic membrane. Above the oval window is the canal for facial nerve. Above the canal for facial nerve is the prominence of lat-eral 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 cochle-ariform process also marks the level of the genu of the facial nerve which is an important landmark for surgery of the facial nerve. Medial to the pyramid is a deep recess called sinus tympani, which is bounded by the subiculum below and the ponticulus above.
Clinical importance
Its bony covering may sometimes be congenitally dehiscent and the nerve may lie exposed making it very vulnerable to injuries or infection.
Lateral wall
The lateral wall is formed largely by the tympanic membrane and to a lesser extent by the bony outer attic wall called scu-tum. The tympanic membrane is semitransparent and forms a “window” into the middle ear. It is possible to see some structures of the middle ear through the normal tympanic membrane, e.g. the long process of incus, incudostapedial joint and the round wind
Aditus ad antrum
ADITUS AD ANTRUM Aditus is an opening through which the attic communicates with the antrum. The bony prominence of the horizontal canal lies on its medial side while the fossa incudis, to which is attached the short process of incus, lies laterally. Facial nerve courses just below the aditus.
Air cells
Mastoid cells are well pneumotized intervening septa are thin.
Diploetic. Mastoid consists of marrow spaces and a few air cells.
3. Sclerotic or acellular. There are no cells or 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.
Cells
- Zygomatic cells (in the root of zygoma).
- Tegmen cells (extending into the tegmen tympani).
- Perisinus cells (overlying the sinus plate).
- Retrofacial cells (round the facial nerve).
- Perilabyrinthine cells (located above, below and behind the labyrinth, some of them pass through the arch of superior semicircular canal. These cells may communi-cate with the petrous apex).
- Peritubal (around the eustachian tube. Along with hypotym-panic cells they also communicate with the petrous apex).
- Tip cells (which are quite large and lie medial and lateral to the digastric ridge in the tip of mastoid).
- Marginal cells (lying behind the sinus plate and may extend into the occipital bone)
Cells
- Zygomatic cells (in the root of zygoma).
- Tegmen cells (extending into the tegmen tympani).
- Perisinus cells (overlying the sinus plate).
- Retrofacial cells (round the facial nerve).
- Perilabyrinthine cells (located above, below and behind the labyrinth, some of them pass through the arch of superior semicircular canal. These cells may communi-cate with the petrous apex).
- Peritubal (around the eustachian tube. Along with hypotym-panic cells they also communicate with the petrous apex).
- Tip cells (which are quite large and lie medial and lateral to the digastric ridge in the tip of mastoid).
- Marginal cells (lying behind the sinus plate and may extend into the occipital bone)
DEVELOPMENT OF MASTOID .
Mastoid develops from the squamous and petrous bones.
The petrosquamosal suture may persist as a bony plate—the Korner’s septum, separating superf i cial squamosal cells from the deep petrosal cells. Korner’s septum is surgically impor-tant as it may cause diff i culty in locating the antrum and the deeper cells; and thus may lead to incomplete removal of dis-ease at mastoidectomy (Figure 1.14). Mastoid antrum cannot be reached unless the Korner’s septum has been removed
OSSICLES OF THE MIDDLE EAR (FIGURE 1.15)
There are three ossicles in the middle ear—the malleus, incus and stapes.
The malleus has head, neck, handle (manubrium), a lateral and an anterior process. Head and neck of malleus lie in the attic. Manubrium is embedded in the fi brous layer of the
malleus has head, neck, handle (manubrium), a lateral and an anterior process. Head and neck of malleus lie in the attic.
Malleus
Manubrium
Manubrium is embedded in the fi brous layer of the tympanic membrane. The lateral process forms a knob-like projection on the outer surface of the tympanic membrane and gives attachment to the anterior and posterior malleal (malleolar) folds.
The incus has a body and a short process, both of which lie in the attic, and a long process which hangs vertically and attaches to the head of stapes.
Stapes
The stapes has a head, neck, anterior and posterior crura, and a footplate. The footplate is held in the oval window by annular ligament.
The ossicles conduct sound energy from the tympanic membrane to the oval window and then to the inner ear fl uid.
INTRATYMPANIC MUSCLES
There are two muscles—tensor tympani and the stapedius;
the former attaches to the neck of malleus and tenses the tympanic membrane while the latter attaches to the neck of stapes and helps to dampen very loud sounds thus prevent-ing noise trauma to the inner ear.
Muscles
Stapedius is a second arch muscle and is supplied by a branch of CN VII while tensor tympani develops from the fi rst arch and is supplied by a branch of mandibular nerve
Blood supply
Ant tympanic branch of maxillary artery
Stylomastoid branch of post auricular artery
