External and middle ear

The arterial supply of the extrinsic auricular muscles is derived mainly from the posterior auricular artery.

Auriculares anterior and superior are supplied by temporal branches of the facial nerve and auricularis posterior is supplied by the posterior auricular branch of the facial nerve.

In man these muscles have very little obvious effect. However, despite the paucity of auricular movement, auditory stimuli may evoke patterned responses from these small muscles and electromyography can detect the ‘crossed acoustic response’, used to determine auditory threshold levels and brainstem latencies, which is elicited by this means in investigative clinical neurology.

Intrinsic muscles

The intrinsic auricular muscles are helicis major and minor, tragicus, antitragicus, transversus auriculae and obliquus auriculae (Fig. 36.5). Helicis major is a narrow vertical band on the anterior margin of the helix, passing from its spine to its anterior border, where the helix is about to curve back. Helicis minor is an oblique fasciculus covering the crus of the helix. Tragicus is a short, flattened, vertical band on the lateral aspect of the tragus. Antitragicus passes from the outer part of the antitragus to the tail of the helix and the antihelix. Transversus auriculae, located on the cranial aspect of the auricle, consists of scattered fibres, partly tendinous, partly muscular, which extend between the eminentia conchae and the eminentia scaphae. Obliquus auriculae, also located on the cranial aspect of the auricle, consists of a few fibres which extend from the upper and posterior parts of the eminentia conchae to the eminentia scaphae.

Fig. 36.5 Auricular muscles of the left ear.

(From Sobotta 2006.)

The intrinsic auricular muscles are supplied by branches of the posterior auricular and superficial temporal arteries.

The intrinsic auricular muscles on the lateral aspect of the auricle are innervated by the temporal branches of the facial nerve, and those on the cranial aspect of the auricle are innervated by the posterior auricular branch of the facial nerve.

The intrinsic muscles modify auricular shape minimally, if at all, in most human ears: helicis major can draw the auricle forwards and upwards. Rare individuals can modify the shape and position of their external ears.

Vascular supply and lymphatic drainage

Arteries

The posterior auricular branch of the external carotid artery is the dominant blood supply (Imanishi et al 1997). It supplies three or four branches to the cranial surface of the auricle: twigs from these arteries reach the lateral surface, some through fissures in the cartilage, others round the margin of the helix. The posterior auricular artery ascends between the parotid gland and the styloid process to the groove between the auricular cartilage and mastoid process. The superior auricular artery has a constant course and connects the superior temporal artery and the posterior auricular arterial network: this branch can provide a reliable vascular pedicle for retro-auricular flaps (Moschella et al 2003) The auricle is also supplied by anterior auricular branches of the superficial temporal artery, which are distributed to its lateral surface, and by a branch from the occipital artery.

Veins

Auricular veins correspond to the arteries of the auricle. Arteriovenous anastomoses are numerous in the skin of the auricle and are thought to be important in the regulation of core temperature.

Lymphatic drainage

The posterior aspect of the pinna drains to nodes at the mastoid tip. The tragus and upper part of the pinna drain into pre-auricular nodes, while the remainder of the pinna drains to upper deep cervical lymph nodes.

Innervation

The sensory innervation of the auricle is complex and not fully determined. This is perhaps because the external ear represents an area where skin originally derived from a branchial region meets skin originally derived from a postbranchial region. The sensory nerves involved are the great auricular nerve, which supplies most of the cranial surface and the posterior part of the lateral surface (helix, antihelix, lobule); the lesser occipital nerve, which supplies the upper part of the cranial surface; the auricular branch of the vagus, which supplies the concavity of the concha and posterior part of the eminentia; the auriculotemporal nerve, which supplies the tragus, crus of the helix and the adjacent part of the helix; and the facial nerve, which together with the auricular branch of the vagus probably supplies small areas on both aspects of the auricle, in the depression of the concha, and over its eminence. The details of the cutaneous innervation derived from the facial nerve require further clarification. It is possible that as the auricular branch of the vagus traverses the temporal bone and crosses the facial canal, approximately 4 mm above the stylomastoid foramen, it contributes an ascending branch to the facial nerve, and that in this way fibres of the vagus are carried via the facial nerve to the pinna (see p. 494).

EXTERNAL ACOUSTIC MEATUS

The external acoustic meatus extends from the concha to the tympanic membrane: it is approximately 2.5 cm from the floor of the concha and approximately 4 cm from the tragus. It has two structurally different parts: its lateral third is cartilaginous and its medial two-thirds is osseous (Figs 36.6A, 36.7, 36.8). It forms an S-shaped curve, directed at first medially, anteriorly and slightly up (pars externa), then posteromedially and up (pars media), and lastly anteromedially and slightly down (pars interna). It is oval in section, its greatest diameter is obliquely inclined posteroinferiorly at the external orifice, but is nearly horizontal at its medial end. There are two constrictions, one near the medial end of the cartilaginous part, the other, the isthmus, in the osseous part about 2 cm from the bottom of the concha. The tympanic membrane, which closes its medial end, is obliquely set, which means that the floor and the anterior wall of the meatus are longer than its roof and posterior wall.

Fig. 36.6 The external, middle and inner regions of the left ear. A, Anterior aspect. B, A more detailed view of the relationships of structures in the middle and inner ear. (B, From Sobotta 2006.)

Fig. 36.7 The left auditory apparatus as if viewed through a semi-transparent temporal bone. Note the genu in the facial nerve at the site of the geniculate ganglion.

Fig. 36.8 Bone-windowed, coronal CT scan of external ear canal and middle ear showing the curvature of the floor and roof, and the obliquity of the tympanic membrane (arrow) at the medial end of the external auditory meatus.

(By courtesy of Dr Steven Connor.)

The lateral, cartilaginous part is approximately 8 mm long. It is continuous with the auricular cartilage and attached by fibrous tissue to the circumference of the osseous part. The meatal cartilage is deficient posterosuperiorly, and the gap is occupied by a sheet of collagen. Two or three deep fissures (of Santorini) exist in its anterior part: tumours of the external acoustic meatus escape the confines of the canal through these fissures and spread into the adjacent soft tissues.

The osseous part is approximately 16 mm long, and is narrower than the cartilaginous part. In sagittal section it is oval or elliptical and it is directed anteromedially and slightly downwards, with a slight posterosuperior convexity. Its medial end is smaller than the lateral end and it terminates obliquely. The anterior wall projects medially approximately 4 mm beyond the posterior and is marked, except above, by a narrow tympanic sulcus or anulus, to which the perimeter of the tympanic membrane is attached. Its lateral end is dilated and mostly rough for the attachment of the meatal cartilage. The anterior, inferior, and most of the posterior, parts of the osseous meatus are formed by the tympanic plate of the temporal bone, which in the fetus is only a tympanic ring. The posterosuperior region is formed by the squamous part of the temporal bone. The outer wall of the meatus is bounded above by the posterior zygomatic root, below which there may be a suprameatal spine.

Relations of the meatus

The condylar process of the mandible lies anterior to the meatus and is partially separated from the cartilaginous part by a small portion of the parotid gland. A blow on the chin may cause the condyle to break into the meatus. The middle cranial fossa lies above the osseous meatus and the mastoid air cells are posterior to it, separated from the meatus only by a thin layer of bone. Its deepest part is situated below the epitympanic recess, and is anteroinferior to the mastoid antrum: the lamina of bone which separates it from the antrum is only 1–2 mm thick and provides the ‘transmeatal approach’ of aural surgery.

Vasculature and lymphatic drainage

The arterial supply of the external acoustic meatus is derived from the posterior auricular artery, the deep auricular branch of the maxillary artery and the auricular branches of the superficial temporal artery. Associated veins drain into the external jugular and maxillary veins and the pterygoid plexus. The lymphatics drain into those associated with the pinna.

The sensory innervation of the external acoustic meatus is derived from the auriculotemporal branch of the mandibular nerve, which supplies the anterior and superior walls, and the auricular branch of the vagus, which supplies the posterior and inferior walls. The facial nerve may also contribute via its communication with the vagus nerve.

EXTERNAL SURGICAL APPROACHES TO THE MIDDLE EAR

Surgical access to the middle ear can be achieved by per-meatal, endaural and postauricular approaches.

Provided the external acoustic meatus is wide enough, the tympanic membrane can be elevated by incising the skin of the bony meatus circumferentially, leaving a vascular pedicle superiorly. The canal skin is elevated from the underlying bone until the fibrous anulus of the tympanic membrane is visualized, and this can then be elevated from the tympanic groove and the middle ear mucosa incised so that the tympanic membrane can be reflected forwards and upwards. This per-meatal approach is called a tympanotomy and is used for stapedectomy, ossuloplasty, myringoplasty and the removal of small middle ear tumours.

If the external acoustic meatus is too narrow to allow adequate visualization of the middle ear, or if access is required to the mastoid aditus and antrum, superficial soft tissues must be displaced. The two main external approaches to the middle ear are the endaural and postauricular approaches.

The endaural approach involves making an incision in the notch between the tragus and the helix. This is carried down to expose the lower margin of temporalis (which can be used to harvest a robust fascial graft for reconstruction) and the bone of the bony external acoustic meatus. The cartilaginous meatus is separated from the bony meatus and reflected laterally as a conchomeatal flap. The bony meatus can then be widened by drilling away bone (canaloplasty), which gives more space to manipulate the delicate structures of the middle ear as well as improving subsequent visualization of the tympanic membrane when the incision has healed.

The postauricular approach involves making an incision approximately 1 cm behind the postauricular skin crease and deepening the incision to the periosteum of the mastoid process, dividing the posterior auricular muscles on the way. Grafts can be harvested from the temporalis fascia. The periosteum is incised and elevated to expose the bony external acoustic meatus from behind. The skin over the junction of the bony and cartilaginous meatus is incised to allow the cartilage of the auricle and meatus to be swung forward on its blood supply and so expose the bony meatus and mastoid process. Access can then be gained by drilling and elevating a tympanomeatal skin flap as described for the endaural approach.

Temporalis fascia is the most popular tissue used as a free graft for repair of the tympanic membrane because it is easily obtainable. In recent years, tragal perichondrium has become a popular alternative: it has the additional advantage that the cartilage can also be harvested and used to reinforce the repair.

More extensive resections of the temporal bone are undertaken using extended pre- or postauricular incisions into the temporal region and neck. The blood supply of the pinna is sufficient to maintain viability despite significant elevation and undermining.

MIDDLE EAR

The middle ear is an irregular, laterally compressed space in the petrous part of the temporal bone. It is lined with mucous membrane and filled with air, which reaches it from the nasopharynx via the pharyngotympanic tube (Figs 36.6, 36.7, 36.9). The middle ear contains three small bones, the malleus, incus and stapes, collectively called the auditory ossicles, which form an articulated chain connecting the lateral and medial walls of the cavity, and which transmit the vibrations of the tympanic membrane across the cavity to the cochlea.

Fig. 36.9 A, Axial CT scan at level of the ossicular mass (white arrow) in the epitympanum, showing the mastoid antrum and its relationship to the lateral (horizontal) semicircular canal (black arrow). B, Axial CT scan at the level of the entrance of the pharyngotympanic tube, showing its relationship to the internal carotid artery (black arrow).

(By courtesy of Dr Steven Connor.)

The essential function of the middle ear is to transfer energy efficiently from relatively weak vibrations in the elastic, compressible air in the external acoustic meatus to the incompressible fluid around the delicate receptors in the cochlea. Mechanical coupling between the two systems must match their resistance to deformation or ‘flow’, i.e. their impedance, as closely as possible. Aerial waves of low amplitude and low force per unit area arrive at the tympanic membrane, which has 15–20 times the area of the stapedial footplate that contacts the perilymph in the inner ear: the force per unit area generated by the footplate is increased by a similar amount, while the amplitude of vibration is almost unchanged.

Protective mechanisms incorporated into the design of the middle ear include the presence of the pharyngotympanic tube (to equalize pressure on both sides of the delicate tympanic membrane); the shape of the articulations between the ossicles; and the reflex contractions of stapedius and tensor tympani in response to sounds of fairly high intensity (preventing damage caused by sudden or excessive excursions of the ossicles).

The space within the middle ear can be subdivided into three parts. These are the mesotympanum or tympanic cavity proper, which is opposite the tympanic membrane; the epitympanum or attic, which is above the level of the membrane, and contains the head of the malleus and the body and short process of the incus; and the hypotympanum, which is in the floor of the cavity between the jugular bulb and the lower margin of the tympanic membrane. The vertical and anteroposterior diameters of the mesotympanum and hypotympanum are each approximately 15 mm; the transverse diameter is 6 mm superiorly and 4 mm inferiorly, narrowing to 2 mm opposite the umbo. The cavity is bounded laterally by the tympanic membrane and medially by the lateral wall of the internal ear, the promontory. It communicates posteriorly with the mastoid antrum and the mastoid air cells, and anteriorly with the nasopharynx via the pharyngotympanic tube (Figs 36.6, 36.7).

The tympanic cavity and mastoid antrum, auditory ossicles and structures of the internal ear are all almost fully developed at birth and subsequently alter little. In the fetus the cavity contains a gelatinous tissue which has practically disappeared by birth, when it is filled by a fluid which is absorbed when air enters via the pharyngotympanic tube. The tympanic cavity is a common site of infection in childhood.

BOUNDARIES OF THE TYMPANIC CAVITY

The tympanic cavity has a roof and a floor, and lateral, medial, posterior and anterior walls.

Roof

A thin plate of compact bone, the tegmen tympani, separates the cranial and tympanic cavities, and forms much of the anterior surface of the petrous temporal bone. It is prolonged posteriorly as the roof of the mastoid antrum and anteriorly it covers the canal for tensor tympani. In youth, the unossified petrosquamosal suture may allow the spread of infection from the tympanic cavity to the meninges. In adults, veins from the tympanic cavity traverse this suture to reach the superior petrosal or petrosquamous sinus and thus may also transmit infection to these structures through a process of thrombophlebitis.

Longitudinal fractures of the middle cranial fossa almost always involve the tympanic roof, accompanied by dislocation of the ossicular chain, rupture of the tympanic membrane, or a fractured roof of the osseous external acoustic meatus, which can be seen as a notch. Such injuries usually cause bleeding from the ear, with escape of cerebrospinal fluid if the dura mater has been torn (CSF otorrhea).

Floor

The floor of the tympanic cavity is a narrow, thin, convex plate of bone which separates the cavity from the superior bulb of the internal jugular vein. The bone may be patchily deficient, in which case the tympanic cavity and the vein are separated only by mucous membrane and fibrous tissue. Alternatively, the floor is sometimes thick and may contain some accessory mastoid air cells. A small aperture for the tympanic branch of the glossopharyngeal nerve lies near the medial wall.

Lateral wall

The lateral wall consists mainly of the tympanic membrane, but also contains the ring of bone to which the membrane is attached (see Figs 36.6B, 36.14A),. The lateral epitympanic bony wall is wedge-shaped in section and its sharp inferior portion is known as the outer attic wall or scutum. This part is easily eroded or blunted by cholesteatoma, a feature easily detected on CT scans (see Fig. 36.8). There is a deficiency or notch in the upper part of this ring, close to which are the small openings of the anterior and posterior canaliculi for the chorda tympani and the petrotympanic fissure. The posterior canaliculus for the chorda tympani is situated in the angle between the posterior and lateral walls of the tympanic cavity just behind the tympanic membrane, at a variable position approximately level with the upper end of the handle of the malleus. It leads into a minute canal that descends in front of the facial canal and ends in it about 6 mm above the stylomastoid foramen. The canaliculus transmits the chorda tympani and a branch of the stylomastoid artery to the tympanic cavity. The chorda tympani leaves the tympanic cavity through the anterior canaliculus, which opens at the medial end of the petrotympanic fissure.

Fig. 36.14 The chorda tympani nerve. A, Oblique vertical section through the left temporal bone, to show roof and lateral wall of the middle ear, the chorda tympani and the mastoid antrum. B, Chorda tympani crossing the tympanic membrane, auroscopic view.

(B, By courtesy of Mr Simon A Hickey).

The petrotympanic fissure is a mere slit approximately 2 mm in length which opens just above and in front of the ring of bone to which the tympanic membrane is attached. It contains the anterior process and anterior ligament of the malleus and transmits the anterior tympanic branch of the maxillary artery to the tympanic cavity.

Tympanic membrane

The tympanic membrane separates the tympanic cavity from the external acoustic meatus (Fig. 36.10; see also Figs 38.6, 36.14). It is thin, semi-transparent, and almost oval, though somewhat broader above than below. It lies obliquely, at an angle of approximately 55° with the meatal floor. Its longest, anteroinferior diameter is 9 to 10 mm, and its shortest is 8 to 9 mm. Most of its circumference is a thickened fibrocartilaginous ring or anulus which is attached to the tympanic sulcus at the medial end of the meatus. The anulus contains radially orientated smooth muscle cells in several locations that possibly play a role in controlling blood flow or maintaining tension (Henson et al 2005). The sulcus is deficient superiorly, i.e. it is notched. Two bands, the anterior and posterior malleolar folds, pass from the ends of this notch to the lateral process of the malleus. The small triangular part of the membrane, the pars flaccida, lies above these folds and is lax and thin. The major part of the tympanic membrane, the pars tensa, is taut. The handle of the malleus is firmly attached to the internal surface of the tympanic membrane as far as its centre, which projects towards the tympanic cavity. The inner surface of the membrane is thus convex and the point of greatest convexity is termed the umbo. Although the membrane as a whole is convex on its inner surface, its radiating fibres are curved with their concavities directed inwards.

Fig. 36.10 Auroscopic view of left tympanic membrane. Note that a bright cone of light is seen in the anteroinferior quadrant of the membrane when it is illuminated.

(By courtesy of Mr Simon A Hickey.)

Microstructure

Histologically, the tympanic membrane is composed of an outer cuticular layer, an intermediate fibrous layer and an inner mucous layer.

The cuticular stratum is continuous with the thin skin of the meatus. It is keratinized, stratified squamous in type, devoid of dermal papillae and hairless. Its subepithelial tissue is vascularized and may develop a few peripheral papillae. Ultrastructurally, it is typically 10 cells thick and has two zones, a superficial layer of non-nucleated squames, and a deep zone which resembles the epidermal prickle cell layer (stratum spinosum). There are numerous desmosomes between cells, the deepest of which lie on a continuous basal lamina, but lack epithelial pegs and hemidesmosomes. The cells of this stratum have a propensity for lateral migration and differentiation not shared with any other stratified squamous epithelia in the body.

The fibrous stratum consists of an external layer of radiating fibres which diverge from the handle of the malleus, and a deep layer of circular fibres, which are plentiful peripherally, but sparse and scattered centrally. Ultrastructurally, the filaments are 10 nm in diameter, and are linked at 25 nm intervals. They have a distinctive amino acid composition, and may consist of a protein peculiar to the tympanic membrane. Small groups of collagen fibrils appear at 11 weeks in utero, interspersed with small bundles of elastin microfibrils. Older specimens contain more typically cross-banded collagen fibrils and an amorphous elastin component. The fibrous stratum is replaced by loose connective tissue in the pars flaccida.

The mucous stratum is a part of the mucosa of the tympanic cavity, and is thickest near the upper part of the membrane. It consists of a single layer of very flat cells, with overlapping interdigitating boundaries and desmosomes and tight junctions between adjacent cells. The cytoplasm contains only a few organelles: the luminal surfaces of these apparently metabolically inert cells have a few irregular microvilli and are covered by an amorphous electron-dense material. There are no ciliated columnar cells.

Innervation

The tympanic membrane is mainly innervated by the auriculotemporal nerve, and appears to perceive only pain. There is a minor, inconstant and overlapping sensory supply from cranial nerves VII, IX and X.

The auricular branch of the vagus arises from the superior vagal ganglion and is joined soon after by a ramus from the inferior ganglion of the glossopharyngeal nerve. It passes behind the internal jugular vein and enters the mastoid canaliculus on the lateral wall of the jugular fossa. It traverses the temporal bone and crosses the facial canal about 4 mm above the stylomastoid foramen. At this point it supplies an ascending branch to the facial nerve. Fibres of the nervus intermedius may pass to the auricular branch of the vagus here, which may explain the cutaneous vesiculation that sometimes accompanies geniculate herpes. The auricular branch then traverses the tympanomastoid fissure, and divides into two rami. One ramus joins the posterior auricular nerve and the other is distributed to the skin of part of the cranial surface of the auricle, the posterior wall and floor of the external acoustic meatus, and to the adjoining part of the outer surface of the tympanic membrane. The auricular branch therefore contains somatic afferent nerve fibres, which probably terminate in the spinal trigeminal nucleus. Stimulation of the vagus nerve, e.g. in syringing the ear, can have a reflex reaction on heart rate.

Myringoplasty

Persistent perforation of the tympanic membrane caused by infection or trauma causes hearing impairment and predisposes to continuing infection as a result of contamination with organisms from the external acoustic meatus. This condition is known as chronic suppurative otitis media of the tubo-tympanic type. Myringoplasty is a surgical procedure that uses a connective tissue scaffold or graft to support healing of the perforation. The commonest technique involves the elevation of the tympanic anulus and the placement of a piece of fibrous connective tissue, e.g. part of the fibrous deep fascia which invests the lateral surface of temporalis or the perichondrium of the tragal cartilage, onto the undersurface of the tympanic membrane to close the perforation. The healed edges of the perforation are stripped of epithelium to encourage healing and scar formation. The fibrous tissue supports the healing tympanic membrane and may in part be incorporated into the repair. Once the perforation is healed, the vibratory function of the tympanic membrane is usually restored to normal.

Medial wall

The medial wall of the tympanic cavity is also the lateral boundary of the internal ear. Its features are the promontory, fenestra vestibuli (fenestra ovalis, oval window), fenestra cochleae (fenestra rotunda, round window) and the facial prominence (Fig. 36.11).

Fig. 36.11 Medial wall of the left tympanic cavity, anterolateral aspect. A, Lateral wall and adjacent parts of anterior and superior walls removed; facial canal and carotid canal opened. B, Section along the axis of the petrous part of the temporal bone.

(From Sobotta 2006.)

The promontory is a rounded prominence furrowed by small grooves which lodge the nerves of the tympanic plexus. It lies over the lateral projection of the basal turn of the cochlea. A minute spicule of bone frequently connects the promontory to the pyramidal eminence of the posterior wall. The apex of the cochlea lies near the medial wall of the tympanic cavity, anterior to the promontory. A depression behind the promontory is known as the sinus tympani.

The fenestra vestibuli is a kidney-shaped opening situated above and behind the promontory, and leading from the tympanic cavity to the vestibule of the inner ear. Its long diameter is horizontal and its convex border is directed upwards. It is occupied by the base of the stapes, the footplate: the circumference of the footplate is attached to the margin of the fenestra by an anular ligament.

The fenestra cochleae is situated below and a little behind the fenestra vestibuli, from which it is separated by a posterior extension of the promontory, called the subiculum. Occasionally, 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 fenestra cochleae lies completely under the overhanging edge of the promontory in a deep hollow or niche, and is placed very obliquely. In dried specimens it opens anterosuperiorly from the tympanic cavity into the scala tympani of the cochlea, but in life it is closed by the secondary tympanic membrane. This is somewhat concave towards the tympanic cavity and convex towards the cochlea, and is bent so that its posterosuperior one-third forms an angle with its anteroinferior two-thirds. The membrane is composed of an external layer derived from the tympanic mucosa; an internal layer, derived from the cochlear lining membrane; and an intermediate, fibrous layer.

The prominence of the facial nerve canal indicates the position of the upper part of the bony facial canal (Fallopian canal) which contains the facial nerve. The canal crosses the medial tympanic wall from the cochleariform process anteriorly, runs just above the fenestra vestibuli, and then curves down into the posterior wall of the cavity. Its lateral wall may be partly deficient.

Posterior wall

The posterior wall of the tympanic cavity is wider above than below. Its main features are the aditus to the mastoid antrum, the pyramid, and the fossa incudis (Fig. 36.11).

The aditus to the mastoid antrum is a large irregular aperture which leads back from the epitympanic recess into the upper part of the mastoid antrum. A rounded eminence on the medial wall of the aditus, above and behind the prominence of the facial nerve canal, corresponds to the position of the lateral semicircular canal.

The pyramidal eminence is situated just behind the fenestra vestibuli and in front of the vertical part of the facial nerve canal. It is hollow and contains the stapedius muscle. Its summit projects towards the fenestra vestibuli and is pierced by a small aperture which transits the tendon of stapedius. The cavity in the pyramidal eminence is prolonged down and back in front of the facial nerve canal; it communicates with the canal by an aperture through which a small branch of the facial nerve passes to stapedius.

The fossa incudis is a small depression in the lower and posterior part of the epitympanic recess. It contains the short process of the incus, which is fixed to the fossa by ligamentous fibres.

Mastoid antrum

The mastoid antrum is an air sinus in the petrous part of the temporal bone. Its topographical relations are of considerable surgical importance. The aditus to the mastoid antrum, which leads back from the epitympanic recess, opens in the upper part of its anterior wall. The lateral semicircular canal lies medial to the aditus. The descending part of the facial nerve canal is anteroinferior. The medial wall is related to the posterior semicircular canal (see Ch. 37). The sigmoid sinus lies some distance posteriorly: the distance can be extremely variable and is dependent on the degree of pneumatization of the mastoid. The roof is formed by the tegmen tympani, and so the antrum lies below the middle cranial fossa and the temporal lobe of the brain. The floor has several openings which communicate with the mastoid air cells. The lateral wall, which offers the usual surgical approach to the cavity, is formed by the postmeatal process of the squamous part of the temporal bone. This is only 2 mm thick at birth but increases at an average rate of 1 mm a year, attaining a final thickness of 12–15 mm. In adults, the lateral wall of the antrum corresponds to the suprameatal triangle (Macewen’s triangle) on the outer surface of the skull. This is palpable through the cymba conchae: the superior side of the triangle, the supramastoid crest, is level with the floor of the middle cranial fossa; the anteroinferior side, which forms the posterosuperior margin of the external acoustic meatus, indicates approximately the position of the descending part of the facial nerve canal; the posterior side, formed by a posterior vertical tangent to the posterior margin of the external acoustic meatus, is anterior to the sigmoid sinus.

The adult capacity of the mastoid antrum is variable, but on average is 1 mL, with a general diameter of 10 mm. Unlike the other air sinuses in the skull, it is present at birth, and is indeed then almost adult in size, although it is at a higher level relative to the external acoustic meatus than it is in adults. In the very young, the thinness of the lateral antral wall and the absence or under-development of the mastoid process means that the stylomastoid foramen and emerging facial nerve are very superficially situated.

Mastoid air cells

Though the mastoid process antrum is well developed at birth, the mastoid air cells are merely minute antral diverticula at this stage. As the mastoid develops in the second year, the air cells gradually extend into it and by the fourth year they are well formed, although their greatest growth occurs at puberty. They vary considerably in number, form and size. Usually, they interconnect and are lined by a mucosa with squamous non-ciliated epithelium, continuous with that in the mastoid antrum and tympanic cavity. They may fill the mastoid process, even to its tip, and some may be separated from the sigmoid sinus and posterior cranial fossa only by extremely thin bone, which is occasionally deficient (Fig. 36.11). Some may lie superficial to, or even behind, the sigmoid sinus, and others may be present in the posterior wall of the descending part of the facial nerve canal. Those in the squamous part of the temporal bone may be separated from deeper cells in the petrous part by a plate of bone in the line of the squamomastoid suture (Körner’s septum). Sometimes they extend only minimally into the mastoid process, in which case the process consists largely of dense bone or trabecular bone containing bone marrow. Varieties of the mastoid process are recognized. The three types most commonly described are pneumatized (with many air cells); sclerotic or diploic (with few or no air cells); and mixed (contain both air cells and bone marrow).

The mastoid process may have no air cells at all in 20% of skulls. Alternatively, air cells may extend beyond the mastoid process into the squamous part of the temporal bone above the supramastoid crest; into the posterior root of the zygomatic process of the temporal bone; into the osseous roof of the external acoustic meatus just below the middle cranial fossa; or into the floor of the tympanic cavity very close to the superior jugular bulb. Rarely, a few may excavate the jugular process of the occipital bone. An important group may extend medially into the petrous part of the temporal bone, even to its apex, and are related to the pharyngotympanic tube, carotid canal, labyrinth and abducens nerve. Some investigators maintain that these are not continuous with the mastoid cells, but grow independently from the tympanic cavity. All of these extensions of the mastoid air cells are pathologically important since infection may spread to the structures around them.

The mastoid air cells are innervated by a meningeal branch of the mandibular division of the trigeminal nerve.

Mastoiditis

Mastoiditis is a potentially dangerous, life-threatening condition that develops as a result of the spread of bacterial infection from the tympanic cavity via the aditus to the mastoid antrum and associated mastoid air cells. In particular, the infection may spread through the tegmen tympani to the dura mater of the middle cranial fossae, to cause an extradural collection. This in turn may cause necrosis of the adjacent dura mater and infection may spread to form a subdural empyema in the subarachnoid space, or an abscess in the substance of the adjacent temporal lobe. Bacterial meningitis may also develop. Similar spread may be seen into the posterior cranial fossa. In both sites the infection may prove fatal.

Infection may spread laterally through the cortical bone of the lateral aspect of the mastoid process to form a subperiosteal postauricular abscess (Bezold’s abscess), or through the cortical bone of the tip of the mastoid process to the attachment of the posterior belly of digastric and sternocleidomastoid, which stimulates painful muscular contraction and torticollis.

Anterior wall

The inferior, larger area of the anterior wall of the tympanic cavity is narrowed by the approximation of the medial and lateral walls of the cavity (Fig. 36.6). It is a thin lamina and forms the posterior wall of the carotid canal. It is perforated by the superior and inferior caroticotympanic nerves and the tympanic branch or branches of the internal carotid. The canals for tensor tympani and the osseous part of the pharyngotympanic tube open above it, the canal for tensor tympani being superior to that for the pharyngotympanic tube. Both canals incline downwards and anteromedially, to open in the angle between the squamous and petrous parts of the temporal bone, and are separated by a thin, osseous septum. The canal for tensor tympani and the bony septum runs posterolaterally on the medial tympanic wall, and ends immediately above the fenestra vestibuli. Here, the posterior end of the septum is curved laterally to form a pulley, the processus trochleariformis (cochleariform process), which is a surgical landmark for the identification of the geniculate ganglion of the facial nerve. The tendon of tensor tympani turns laterally over the pulley before attaching to the upper part of the handle of the malleus.

Pharyngotympanic tube blockage in children

The pharyngotympanic tube serves to ventilate the middle ear, exchanging nasopharyngeal air with the air in the middle ear, which has been altered in its composition via transmucosal gas exchange with the haemoglobin in the blood vessels of the mucosa. The tube also carries mucus from the middle ear cleft to the nasopharynx as a result of ciliary transport.

In children, the pharyngotympanic tube is relatively narrow. It is prone to obstruction when the mucosa swells in response to infection or allergic challenge: obstruction results in a relative vacuum being created in the middle ear secondary to transmucosal gas exchange, and this in turn promotes mucosal secretion and the formation of a middle ear effusion. Because of the collapsibility of the pharyngotympanic tube, the vacuum thus created can overcome the distending effect of the muscles of the tube and ‘lock’ the tube shut. The resultant persistent middle ear effusion, otitis media with effusion (glue ear), can cause hearing loss by splinting the tympanic membrane and impeding its vibration. It can also provide an ideal environment for the proliferation of bacteria, with the result that an acute otitis media may develop. It is possible to relieve the vacuum and unlock the tube, and then remove the effusion by myringotomy, i.e. by surgically creating a hole in the tympanic membrane. This hole will generally heal rapidly and it is common practice to insert a flanged ventilation tube (a grommet or tympanostomy tube) to keep the hole open. Migration of the outer squamous layer of the tympanic membrane eventually displaces the tube and the myringotomy heals.

Otitis media

It is assumed that acute otitis media usually arises as a result of ascending infection from the nasopharynx via the pharyngotympanic tube to the middle ear cleft. From there it may extend to the mastoid aditus and antrum. Swelling secondary to the infection may result in the closure of both exits from the middle ear, i.e. the pharyngotympanic tube and the aditus, with subsequent accumulation of pus under pressure, which causes lateral bulging and inflammation of the tympanic membrane. The latter may burst, releasing mucopurulent discharge into the external acoustic meatus, which results in a release of the pressure in the middle ear and a diminution in the levels of pain. After a brief period the discharge dries up, and for the most part the resultant perforation of the tympanic membrane heals. Normal ventilation and drainage of mucus from the middle ear is restored once the swelling in the pharyngotympanic tube resolves. On occasion the process will fail to produce a perforation of the tympanic membrane and the inflammatory exudates will not drain. The immune defence system sterilizes the exudates of organisms, resulting in a sterile mucoid effusion, otitis media with effusion (see above).

AUDITORY OSSICLES

A chain of three mobile ossicles, the malleus, incus and stapes, transfers sound waves across the tympanic cavity from the tympanic membrane to the fenestra vestibuli. The malleus is attached to the tympanic membrane and the base (footplate) of the stapes is attached to the rim of the fenestra vestibuli. The incus is suspended between them, and articulates with both bones.

Malleus

The malleus is the largest of the ossicles, and is shaped somewhat like a mallet (Fig. 36.12). It is 8–9 mm long and has a head, neck, handle (manubrium) and anterior and lateral processes. The head is the large upper end of the bone and is situated in the epitympanic recess. It is ovoid in shape, articulates posteriorly with the incus, and is covered elsewhere by mucosa. The cartilaginous articular facet for the incus is narrowed near its middle and consists of a larger upper part and a smaller lower part, orientated almost at right angles to each other. Opposite the constriction, the lower margin of the facet projects in the form of a process, the spur of the malleus. The neck is the narrowed part below the head, and inferior to this is an enlargement from which the anterior and lateral processes project.

Fig. 36.12 The left auditory ossicles.

The handle of the malleus is connected by its lateral margin to the tympanic membrane (Figs 36.6, 36.10; see also Fig. 36.14A). It is directed downwards, medially and backwards. It decreases in size towards its free end, which is curved slightly forwards and is flattened transversely. Near the upper end of its medial surface there is a slight projection to which the tendon of tensor tympani is attached. The anterior process is a delicate bony spicule, directed forwards from the enlargement below the neck, and connected to the petrotympanic fissure by ligamentous fibres. In fetal life it is the longest process of the malleus and is continuous in front with Meckel’s cartilage. The lateral process is a conical projection from the root of the handle of the malleus. It is directed laterally and is attached to the upper part of the tympanic membrane and, via the anterior and posterior malleolar folds, to the sides of the notch in the upper part of the tympanic sulcus.

Ossification

The cartilaginous precursor of the malleus originates as part of the dorsal end of Meckel’s cartilage. With the exception of its anterior process, the malleus ossifies from a single endochondral centre which appears near the future neck of the bone in the fourth month in utero. The anterior process ossifies separately in dense connective tissue and joins the rest of the bone at about the sixth month of fetal life.

Incus

The incus is shaped less like an anvil, from which it is named, than a premolar tooth with its two diverging roots. It has a body and two processes (Fig. 36.12). The body is somewhat cubical but laterally compressed. On its anterior surface it has a saddle-shaped facet for articulation with the head of the malleus. The long process, rather more than half the length of the handle of the malleus, descends almost vertically, behind and parallel to the handle. Its lower end bends medially and ends in a rounded lenticular process, the medial surface of which is covered with cartilage and articulates with the head of the stapes. The short process, somewhat conical, projects backwards and is attached by ligamentous fibres to the fossa incudis in the lower and posterior part of the epitympanic recess.

Ossification

The incus has a cartilaginous precursor continuous with the dorsal extremity of Meckel’s cartilage. Ossification often spreads from a single centre in the upper part of its long process in the fourth fetal month; the lenticular process may have a separate centre.

Stapes

The stapes is also known as the stirrup. It has a head, neck, two limbs (processes or crura) and a base (footplate) (Fig. 36.12). The head (caput) is directed laterally and has a small cartilaginous facet for articulation with the lenticular process of the incus. The neck is the constricted part supporting the head, and the tendon of stapedius is attached to its posterior surface. The processes diverge from the neck and are connected at their ends by a flattened oval plate, the base, which forms the footplate of the stapes. The footplate is attached to the margin of the fenestra vestibuli by a ring of fibres (the anular ligament). The anterior process is shorter, thinner and less curved than the posterior.

Ossification

The stapes is preformed in the perforated dorsal moiety of the hyoid arch cartilage of the fetus. Ossification starts from a single endochondral centre, which appears in the base in the fourth fetal month and then gradually spreads through the limbs of the stapes to reach the head.

At birth the auditory ossicles have reached an advanced state of maturity.

Ossicular ligaments

The ossicles are connected to the tympanic walls by ligaments (Fig. 36.6B): three for the malleus and one each for the incus and stapes. Some are mere mucosal folds which carry blood vessels and nerves to and from the ossicles and their articulations, and others contain a central, strong band of collagen fibres.

The anterior ligament of the malleus stretches from the neck of the malleus, just above the anterior process, to the anterior wall of the tympanic cavity near the petrotympanic fissure. Some of its collagen fibres traverse this fissure to reach the spine of the sphenoid, and others continue into the sphenomandibular ligament. The latter, like the anterior malleolar ligament, is derived from the perichondrial sheath of Meckel’s cartilage. The anterior malleolar ligament may contain muscle fibres, called laxator tympani or musculus externus mallei. The lateral ligament of the malleus is a triangular band which stretches from the posterior part of the border of the tympanic incisure to the head of the malleus. The superior ligament of the malleus connects the head of the malleus to the roof of the epitympanic recess.

The posterior ligament of the incus connects the end of its short process to the fossa incudis. The superior ligament of the incus is little more than a mucosal fold passing from the body of the incus to the roof of the epitympanic recess.

The vestibular surface and rim of the stapedial base are covered with hyaline cartilage. The cartilage encircling the base is attached to the margin of the fenestra vestibuli by a ring of elastic fibres, the anular ligament of the base of the stapes. The posterior part of this ligament is much narrower than the anterior part: it acts as a kind of hinge on which the stapedial base moves when stapedius contracts and during acoustic oscillation.

Ossicular articulations

The ossicular articulations are typical synovial joints. The incudomalleolar joint is saddle-shaped and the incudostapedial joint is a ball and socket articulation. The articular surfaces are covered with articular cartilage, and each joint is enveloped by a capsule rich in elastic tissue and lined by synovial membrane.

Movements of the auditory ossicles

The handle of the malleus faithfully follows all movements of the tympanic membrane. The malleus and incus rotate together around an axis which runs from the short process and posterior ligament of the incus to the anterior ligament of the malleus. When the tympanic membrane and handle of the malleus move inwards (medially), the long process of the incus moves in the same direction and pushes the stapedial footplate towards the labyrinth and the perilymph contained within the labyrinth. The movement of the perilymph causes a compensatory outward bulging of the secondary tympanic membrane. These events are reversed when the tympanic membrane moves outwards. However, if the movement of the tympanic membrane is considerable, the incus does not follow the full outward excursion of the malleus, and merely glides on it at the incudomalleolar joint, so preventing a dislocation of the footplate of the stapes from the fenestra vestibuli. When the handle of the malleus is carried medially, the spur at the lower margin of the head of the malleus locks the incudomalleolar joint, and this necessitates an inward movement of the long process of the incus. The joint is unlocked again when the handle of the malleus is carried outwards. The three bones together act as a bent lever so that the stapedial footplate does not move in the fenestra vestibuli like a piston, but rocks on a fulcrum at its anteroinferior border, where the anular ligament is thick. The rocking movement around a vertical axis, which is like a swinging door, is said to happen only at moderate intensities of sound. With loud, low-pitched sounds, the axis becomes horizontal, and the upper and lower margins of the stapedial footplate oscillate in opposite directions around this central axis, thus preventing excessive displacement of the perilymph.

Otosclerosis, stapedectomy and stapedotomy

Otosclerosis is a hereditary localized disease of the bone derived from the embryonic otic capsule in which lamellar bone is replaced by woven bone of greater thickness and vascularity. The position of the focus of new bone formation determines its effect on the function of the ear. When new bone develops around the footplate of the stapes it may fix the footplate to the margin of the fenestra vestibuli and prevent it from moving. This impedes the passage of vibrations of the tympanic membrane passing through the ossicular chain to the inner ear, producing a conductive hearing loss. Complete deafness does not result, because vibrations can still pass directly to the cochlea via the bones of the skull, albeit in a markedly less efficient manner.

Stapedectomy is a surgical procedure designed to bypass the fixation of the stapes footplate caused by otosclerosis. The tympanic membrane is temporarily elevated for access to the middle ear and, under microscopic control, the incudostapedial joint is disarticulated using microinstruments. The limbs of the stapes and stapedius are then both divided, usually with microscissors or a laser, and the superstructure of the stapes removed. A small hole (stapedotomy) is then made in the fixed footplate of the stapes using a microdrill, reamer or laser to expose the fluids of the inner ear. In some, the whole footplate becomes detached (stapedectomy). A small graft of connective tissue derived from local fascia or perichondrium is used to seal the hole with a flexible membrane. A small piston, usually made of Teflon or titanium incorporating a wire made of stainless steel, platinum or titanium, is crimped onto the long process of the incus and placed in the perforation in the stapes footplate. The tympanic membrane is then returned. The connection between the tympanic membrane and the inner ear is thus reconstituted and hearing restored.

MUSCLES

There are two intratympanic muscles, tensor tympani and stapedius.

Tensor tympani

Tensor tympani is a long slender muscle which occupies the bony canal above the osseous part of the pharyngotympanic tube, from which it is separated by a thin bony septum (Fig. 36.6). It arises from the cartilaginous part of the pharyngotympanic tube and the adjoining region of the greater wing of the sphenoid, as well as from its own canal. It passes back within its canal, and ends in a slim tendon which bends laterally round the pulley-like processus cochleariformis and attaches to the handle of the malleus, near its root.

Tensor tympani receives its arterial blood supply from the superior tympanic branch of the middle meningeal artery.

Tensor tympani is innervated by a branch of the nerve to medial pterygoid (a ramus of the mandibular division of the trigeminal nerve) which traverses the otic ganglion without interruption to reach the muscle.

Tensor tympani draws the handle of the malleus medially, and so tenses the tympanic membrane and helps to damp sound vibrations: its action also pushes the base of the stapes more tightly into the fenestra vestibuli.

Stapedius

Stapedius arises from the wall of a conical cavity in the pyramidal eminence on the posterior wall of the tympanic cavity, and from its continuation anterior to the descending part of the facial nerve canal. Its minute tendon emerges from the orifice at the apex of the pyramid and passes forwards to attach to the posterior surface of the neck of the stapes. The muscle is of an asymmetric bipennate form. It contains numerous small motor units, each of only six to nine muscle fibres; a few neuromuscular spindles exist near the myotendinous junction.

Stapedius receives its arterial blood supply from branches of the posterior auricular, anterior tympanic and middle meningeal arteries.

Stapedius is supplied by a branch of the facial nerve which is given off in the facial canal.