External and middle ear

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CHAPTER 36 External and middle ear

By convention, the ear is subdivided into three parts, the external, middle and inner ear. It is largely, but not entirely, contained within the temporal bone. The ears not only receive, modulate, conduct, amplify and discriminately analyse the complex pressure waves that are sound, but also contain the end organs of balance.

TEMPORAL BONE

Each temporal bone consists of four components: the squamous, petromastoid and tympanic parts and the styloid process (Fig. 36.1). The squamous part has a shallow mandibular fossa associated with the temporomandibular joint (Ch. 31). The petromastoid part is relatively large: its petrous portion houses the auditory apparatus and is formed of compact bone. In contrast, the mastoid process is trabecular and variably pneumatized. The tympanic part has the form of a thin and incomplete ring whose ends are fused with the squamous part. The styloid process gives attachment to the styloid group of muscles. Two canals are associated with the temporal bone. The external acoustic meatus, visible on the lateral surface, conveys sound waves to the tympanic membrane. The internal acoustic meatus, evident on the medial surface, conveys the facial and vestibulocochlear nerves.

Squamous part

The squamous part lies anterosuperiorly and is thin and partly translucent. Its external temporal surface is smooth, slightly convex, and forms part of the temporal fossa to which temporalis is attached. Above the external acoustic meatus, it is grooved vertically by the middle temporal artery. The supramastoid crest curves backwards and upwards across its posterior part and gives attachment to the temporal fascia. The junction between the squamous and mastoid parts is approximately 1.5 cm below this crest, and traces of the squamomastoid suture may persist. The suprameatal triangle, a depression marking the position of the mastoid antrum (which is medial to the triangle at a depth of approximately 1.25 cm), lies between the anterior end of the supramastoid crest and the posterosuperior quadrant of the external acoustic meatus. The triangle usually contains a small suprameatal spine anteriorly.

The internal cerebral surface of the squamous part is concave and contains depressions which correspond to convolutions of the temporal lobe of the cerebral hemisphere. This surface is grooved by the middle meningeal vessels. Its lower border is fused to the anterior region of the petrous part, but traces of a petrosquamosal suture often appear in adult bones. The superior border is thin, bevelled internally and overlaps the inferior border of the parietal bone at the squamosal suture. Posteriorly it forms an angle with the mastoid element. The anteroinferior border, thin above and thick below, meets the greater wing of the sphenoid bone: it is bevelled internally above, and bevelled externally below.

The squamous part has a zygomatic process and a mandibular fossa.

Zygomatic process

The zygomatic process juts forwards from the lower region of the squamous part. Its triangular posterior part has a broad base that is directed laterally, presenting superior and inferior surfaces. The zygomatic process then twists anteromedially, so that its surfaces become medial and lateral.

The superior surface of the posterior part is concave. The inferior surface is bounded by anterior and posterior roots, converging into the anterior part of the process. The tubercle of the zygomatic root gives attachment to the lateral temporomandibular ligament at the junction of the roots. The posterior root is prolonged forwards above the external acoustic meatus, its upper border continuing into the supramastoid crest. Very rarely, the squamous part is perforated above the posterior root by a squamosal foramen, which transmits the petrosquamous sinus. The anterior root juts almost horizontally from the squamous part. Its inferior surface, with an anteroposterior convexity, forms a short semi-cylindrical articular tubercle and comes into contact with the articular disc of the temporomandibular joint. The tubercle forms the anterior limit of the mandibular fossa.

The anterior part of the zygomatic process is thin and flat and the temporal fascia is attached to its superior border. The inferior border is short and arched and gives origin to some fibres of masseter. The lateral surface is convex. The medial surface is concave and provides further attachment for part of masseter. The anterior end is deeply serrated and slopes obliquely posteroinferiorly to articulate with the temporal process of the zygomatic bone, forming the zygomatic arch. Anterior to the articular tubercle, a small triangular area forms part of the roof of the infratemporal fossa: it is continuous behind with the anterior root and in front with the infratemporal crest of the greater wing of the sphenoid.

Petromastoid part

The petromastoid part of the temporal bone, although morphologically one element, is more conveniently described as two parts, namely mastoid and petrous parts.

Mastoid part

The mastoid part is the posterior region of the temporal bone and has an outer surface roughened by the attachments of the occipital belly of occipitofrontalis and auricularis posterior. A mastoid foramen, of variable size and position, and traversed by a vein from the sigmoid sinus and a small dural branch of the occipital artery, frequently lies near its posterior border. The foramen may be in the occipital or occipitotemporal suture; it may be parasutural (40–50% of crania); or may be absent.

The mastoid part projects down as the conical mastoid process, and is larger in adult males. Sternocleidomastoid, splenius capitis and longissimus capitis are all attached to its lateral surface, and the posterior belly of digastric is attached to a deep mastoid notch on its medial aspect. The occipital artery runs in a shallow occipital groove which lies medial to the mastoid notch. The internal surface of the mastoid process bears a deep, curved sigmoid sulcus for the sigmoid venous sinus: the sulcus is separated from the underlying innermost mastoid air cells by a thin lamina of bone.

The superior border of the mastoid part is thick and serrated for articulation with the mastoid angle of the parietal bone. The posterior border is also serrated and articulates with the inferior border of the occipital bone between its lateral angle and jugular process. The mastoid element is fused with the descending process of the squamous part: below, it appears in the posterior wall of the tympanic cavity.

Petrous part

The petrous part is a mass of bone that is wedged between the sphenoid and occipital bones in the cranial base: it contains the acoustic labyrinth. It is inclined superiorly and anteromedially, and has a base, apex, three surfaces (anterior, posterior and inferior) and three borders (superior, posterior and anterior).

The base would correspond to the part that lies on the base of the skull and is separated from the squamous part by a suture. However, this suture disappears soon after birth. The subsequent development of the mastoid processes means that the precise boundaries of the base are no longer identifiable.

The apex, blunt and irregular, is angled between the posterior border of the greater wing of the sphenoid and the basilar part of the occipital bone. It contains the anterior opening of the carotid canal and limits the foramen lacerum posterolaterally.

The anterior surface contributes to the floor of the middle cranial fossa (Ch. 27) and is continuous with the cerebral surface of the squamous part (although the petrosquamosal suture often persists late in life). The whole surface is adapted to the inferior temporal gyri. Behind the apex is a trigeminal impression for the trigeminal ganglion. Bone anterolateral to this impression roofs the anterior part of the carotid canal, but is often deficient. A ridge separates the trigeminal impression from another hollow behind which partly roofs the internal acoustic meatus and cochlea. This, in turn, is limited behind by the arcuate eminence which is raised by the superior (anterior) semicircular canal. Laterally, the anterior surface roofs the vestibule and, partly, the facial canal. Between the squamous part laterally and the arcuate eminence and the hollows just described medially, the anterior surface is formed by the tegmen tympani, a thin plate of bone which forms the roof of the mastoid antrum, and extends forwards above the tympanic cavity and the canal for tensor tympani. The lateral margin of the tegmen tympani meets the squamous part at the petrosquamosal suture, turning down in front as the lateral wall of the canal for tensor tympani and the osseous part of the pharyngotympanic tube: its lower edge is in the squamotympanic fissure. Anteriorly the tegmen bears a narrow groove related to the greater petrosal nerve (which passes posterolaterally to enter the bone by a hiatus anterior to the arcuate eminence). The groove passes forwards to the foramen lacerum. A smaller and similar hiatus and groove may be found more laterally: they are related to the lesser petrosal nerve (which runs to the foramen ovale). The posterior slope of the arcuate eminence overlies the posterior and lateral semicircular canals. Lateral to the eminence, the posterior part of the tegmen tympani roofs the mastoid antrum.

The posterior surface contributes to the anterior part of the posterior cranial fossa and is continuous with the internal surface of the mastoid part. The opening of the internal acoustic meatus lies near its centre. A small slit leading to the vestibular aqueduct lies behind the opening of the meatus, almost hidden by a thin plate of bone. This contains the saccus and ductus endolymphaticus together with a small artery and vein. The terminal half of the saccus endolymphaticus protrudes through the slit between the periosteum and dura mater. The subarcuate fossa lies above these openings.

The irregular inferior surface is part of the exterior of the cranial base. Near the apex of the petrous part, a quadrilateral area is partly associated with the attachment of levator veli palatini and the cartilaginous pharyngotympanic tube, and partly connected to the basilar part of the occipital bone by dense fibrocartilage. Behind this region is the large, circular opening of the carotid canal, and behind the opening of the canal is the jugular fossa, which is of variable depth and size and contains the superior jugular bulb. The inferior ganglion of the glossopharyngeal nerve lies in a triangular depression anteromedial to the jugular fossa (below the internal acoustic meatus). At its apex is a small opening into the cochlear canaliculus, occupied by the perilymphatic duct (a tube of dura mater) and a vein draining from the cochlea to the internal jugular vein. A canaliculus for the tympanic nerve from the glossopharyngeal nerve lies on the ridge between the carotid canal and the jugular fossa. The mastoid canaliculus for the auricular branch of the vagus nerve is laterally positioned in the jugular fossa. Behind the jugular fossa, the rough quadrilateral jugular surface is covered by cartilage which joins it to the jugular process of the occipital bone.

The superior border, the longest, is grooved by the superior petrosal sinus. The attached margin of the tentorium cerebelli is fixed to the edges of the groove except at its medial end, where it is crossed by the roots of the trigeminal nerve. The posterior border, intermediate in length, bears a sulcus medially which forms, together with the occipital bone, a gutter for the inferior petrosal sinus. Behind this, the jugular fossa contributes (together with the occipital bone) to the jugular foramen and is notched by the glossopharyngeal nerve. Bone on either or both sides of the jugular notch may meet the occipital bone and divide the jugular foramen into two or three parts. The anterior border is joined laterally to the squamous part of the temporal bone at the petrosquamosal suture; medially it articulates with the greater wing of the sphenoid bone.

Two canals exist at the junction of the petrous and squamous parts, one above the other, separated by a thin osseous plate and both leading to the tympanic cavity: the upper canal contains tensor tympani, the lower canal is the pharyngotympanic tube.

Ossification

The four temporal components ossify independently (Fig. 36.2). The squamous part is ossified in a sheet of condensed mesenchyme from a single centre near the zygomatic roots, which appear in the seventh or eighth week in utero. The petromastoid part has several centres which appear in the cartilaginous otic capsule during the fifth month: as many as 14 have been described. These centres vary in order of appearance. Several are small and inconstant, soon fusing with others. The otic capsule is almost fully ossified by the end of the sixth month. The tympanic part is also ossified in mesenchyme from a centre identifiable about the third month; at birth it is an incomplete tympanic ring, deficient above, its concavity grooved by a tympanic sulcus for the tympanic membrane. The malleolar sulcus for the anterior malleolar process, chorda tympani and anterior tympanic artery inclines obliquely downwards and forwards across the medial aspect of the anterior part of the ring. The styloid process develops from two centres at the cranial end of cartilage in the second visceral or hyoid arch: a proximal centre for the tympanohyal appears before birth, and another, for the distal stylohyal, appears after birth. The tympanic ring unites with the squamous part shortly before birth, and the petromastoid fuses with it and the tympanohyal during the first year. The stylohyal does not unite with the rest of the process until after puberty and may never do so.

Once ossified, the tympanic cavity, mastoid antrum and the posterior end of the pharyngotympanic tube become surrounded by bone. The petrous part forms the roof, floor and medial wall of the cavity, while the squamous and tympanic parts, together with the tympanic membrane, form its lateral wall. At birth the middle and inner ears are adult size, and the tympanic cavity, mastoid antrum, tympanic membrane and auditory ossicles are all almost adult size. The anterior process does not join the malleus until 6 months later. The internal acoustic meatus is approximately 6 mm in horizontal diameter, 4 mm in vertical diameter and 7 mm in length at birth, and the adult diameters are 7.7 mm and 11 mm respectively.

After birth and apart from general growth, the tympanic ring extends posterolaterally to become cylindrical, growing into a fibrocartilaginous tympanic plate, which forms the adjacent part of the external acoustic meatus at this stage. This growth is not equal but is rapid in the anterior and posterior regions, which meet and blend. Thus, for a time, an opening (foramen of Huschke) exists in the floor: it usually closes at about the fifth year, but is sometimes permanent (in 5–46% of adult crania from ancient and modern populations). The external acoustic meatus is relatively as long in children as it is in adults, but the canal is fibrocartilaginous, whereas its medial two-thirds are osseous in adults. Surgical access to the tympanic cavity is via the mastoid antrum, and in children it is necessary to remove only a thin scale of bone in the suprameatal triangle to reach the antrum. The tympanic plate ensheathes the styloid process by posterior extension, and extends medially over the petrous bone to the carotid canal.

Initially, the mandibular fossa is shallow, facing more laterally, but it then deepens and ultimately faces downwards. Posteroinferiorly, the squamous part grows down behind the tympanic ring to form the lateral wall of the mastoid antrum. The mastoid part is at first flat, so that the stylomastoid foramen and rudimentary styloid process are immediately behind the tympanic ring. The mastoid part becomes invaded by air cells, especially at puberty. The lateral mastoid region grows downwards and forwards to form the mastoid process, which means that the styloid process and stylomastoid foramen become inferior. Descent of the foramen lengthens the facial canal. The mastoid process is not perceptible until late in the second year. The subarcuate fossa gradually fills and is almost obliterated.

In the neonate, the petrous and squamous parts of the temporal bone are usually partially separated by the petrosquamous fissure which opens directly into the mastoid antrum of the middle ear. The fissure closes in 4% of infants during the first year, but it remains unclosed in 20–40% up to the age of 19 years: it is a route for the spread of infection from the middle ear to the meninges. The neonatal internal acoustic meatus is about half the length of its adult counterpart. Its opening from the middle ear cavity is as large as it is in the adult, but the pharyngeal opening in the nasal part of the pharynx is relatively smaller. The course of the pharyngotympanic tube is horizontal in the newborn, whereas in the adult it passes from the middle ear downward, forward and medially.

EXTERNAL EAR

The external ear is not simply an ear-trumpet but the first of a series of stimulus modifiers in the auditory apparatus. It consists of the auricle, or pinna, and the external acoustic meatus. The auricle projects to a variable and individual degree from the side of the head and collects sound waves, which it conducts along the external auditory canal inwards to the eardrum, the tympanic membrane.

AURICLE (PINNA)

The lateral surface of the auricle is irregularly concave, faces slightly forwards, and displays numerous eminences and depressions (Fig. 36.3). It has a prominent curved rim, the helix. This usually bears a small tubercle posterosuperiorly, Darwin’s tubercle, which is quite pronounced around the sixth month of intrauterine life. The antihelix is a curved prominence, parallel and anterior to the posterior part of the helix: it divides above into two crura which flank a depressed triangular fossa. The curved depression between the helix and antihelix is the scaphoid fossa. The antihelix encircles the deep, capacious concha of the auricle, which is incompletely divided by the crus or anterior end of the helix. The conchal area above this, the cymba conchae, overlies the suprameatal triangle of the temporal bone, which can be felt through it, and which overlies the mastoid antrum. The tragus is a small curved flap below the crus of the helix and in front of the concha: it projects posteriorly, partly overlapping the meatal orifice. The antitragus is a small tubercle opposite the tragus and separated from it by the intertragic incisure or notch. Below it is the lobule, composed of fibrous and adipose tissues. It is soft, unlike the majority of the auricle which is supported by elastic cartilage and is firm. The cranial surface of the auricle presents elevations which correspond to the depressions on its lateral surface, and after which they are named (e.g. eminentia conchae, eminentia fossae triangularis).

image

Fig. 36.3 Lateral surface of the left auricle.

(By permission from Berkovitz BKB, Moxham BJ 2002 Head and Neck Anatomy. London: Martin Dunitz.)

A number of common abnormalities have been recognized and carry descriptive names or eponyms (Porter & Tan 2005).

Pre-auricular sinus

Six auricular hillocks, the embryological precursors of the auricle, form round the margins of the dorsal portion of the first pharyngeal cleft, three on the caudal edge of the first pharyngeal arch and three on the cranial edge of the second pharyngeal arch (see Ch. 40). They fuse to form the auricle and surround the dorsal end of the first branchial cleft from which the external acoustic meatus arises. Sinuses and cysts are often found just anterior to the root of the helix, near to the point of fusion of the hillocks derived from the first branchial arch and those derived from the second branchial arch. There is debate as to whether the abnormalities are epithelial inclusions between the hillocks or remnants of the first branchial cleft. The sinuses may be simple pits or complex branching sinuses that occasionally extend deeply towards the external acoustic meatus so that they lie close to the facial nerve. Clinically they may become chronically infected and require surgical excision: this may be technically demanding surgery given the close proximity to the facial nerve.

Skin

The skin of the auricle continues into the external auditory meatus to cover the outer surface of the tympanic membrane. It is thin, has no dermal papillae, and is closely adherent to the cartilaginous and osseous parts of the canal (inflammation of the canal skin is very painful because of this attachment to the underlying structures). The thick subcutaneous tissue of the cartilaginous part of the meatus contains numerous ceruminous glands that secrete wax, or cerumen. Their coiled tubular structure resembles that of sweat glands. The secretory cells are columnar when active, but cuboidal when quiescent; they are covered externally by myoepithelial cells. Ducts open either onto the epithelial surface or into the nearby sebaceous gland of a hair follicle. Cerumen prevents the maceration of meatal skin by trapped water. Antibacterial properties have been attributed to cerumen, but the evidence for this is lacking (Campos et al 2000, Pata et al 2003).

Two types of wax, wet and dry, are recognized. They are genetically determined. Dry wax is common in East Asians, while the wet type is more common in other ethnic groups (Yoshiura et al 2006). Overproduction, accumulation or impaction of wax may completely occlude the meatus, thereby hindering sound from reaching the tympanic membrane and also restricting the natural vibration of the drum. Although ceruminous glands and hair follicles are largely limited to the cartilaginous meatus, a few small glands and fine hairs are also present in the roof of the lateral part of the osseous part of the canal. The warm, humid environment of the relatively enclosed meatal air aids the mechanical responses of the tympanic membrane.

Cartilaginous framework

The auricle is a single thin plate of elastic fibrocartilage covered by skin, its surface moulded by eminences and depressions (Fig. 36.4). It is connected to the surrounding parts by ligaments and muscles, and is continuous with the cartilage of the external acoustic meatus. There is no cartilage in the lobule or between the tragus and the crus of the helix, where the gap is filled by dense fibrous tissue. Anteriorly, where the helix curves upwards, there is a small cartilaginous projection, the spine of the helix. Its other extremity is prolonged inferiorly as the tail of the helix and it is separated from the antihelix by the fissura antitragohelicina. The cranial aspect of the cartilage bears the eminentia conchae and eminentia scaphae, which correspond to the depressions on the lateral surface. The two eminences are separated by a transverse furrow, the sulcus antihelicis transversus, which corresponds to the inferior crus of the antihelix on the lateral surface. The eminentia conchae is crossed by an oblique ridge, the ponticulus, for the attachment of auricularis posterior. There are two fissures in the auricular cartilage, one behind the crus of the helix and another in the tragus.

Auricular muscles

Extrinsic auricular muscles connect the auricle to the skull and scalp and move the auricle as a whole. Intrinsic auricular muscles connect the different parts of the auricle.

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.