Surgical anatomy of the ear

The external ear consists of the pinna and the ear canal. The pinna is made of yellow elastic cartilage covered by tightly adherent skin. The external and middle ear develop from the first two branchial arches. The external ear canal is 3 cm in length, the outer two-thirds is cartilage and the inner third is bony. The skin on the lateral surface of the tympanic mem­brane and the inner two-thirds of the ear canal is highly specialised. It does not simply shed like the skin from the rest of the body. It migrates outwards from the tympanic membrane and along the ear canal. As a result of this migration most people’s ears are self-cleaning. Disorders of skin migration can result in ear disease (e.g. cholesteatoma). The external canal is richly innervated and the skin is tightly bound down to the perichondrium so that oedema in this region results in severe pain.

The lymphatics of the external ear drain to the retro auricular, parotid, retropharyngeal and deep upper cervical lymph nodes.

The middle ear contains the ossicles. Laterally it is bound­ed by the tympanic membrane, medially by the cochlea, anteriorly by the eustachian tube and posteriorly it communicates with the mastoid air cells (Fig. 40.1). Entwined in this tiny space is the facial nerve which pursues a tortuous course through the middle ear and exits the skull base at the stylomastoid foramen. Knowledge of the anatomy of the middle ear is important because infection can spread through it to the cranial cavity which lies millimetres away.

The tympanic membrane has three layers: an inner mucosal layer, a dense fibrous middle layer and the outer stratified squamous epithelium (skin). The upper portion that lies above the lateral process of the malleus is called the pars flaccida. The lower portion, making up the majority of the drum, is called the pars tensa (Fig. 40.2).

The tympanic membrane and ossicles act as a transformer system converting vibrations in the air to vibrations within the fluid-filled inner ear (perilymph). The evolution of the middle ear is interesting. Fish do not have one, whereas amphibians (e.g. salamanders) have a single strut for an ossicle. At an air—water interface there is a 30 decibels loss of sound energy. The mammalian middle ear overcomes virtually all of this potential loss of sound energy.

The inner ear comprises the cochlea and vestibular laby­rinth (saccule, utricle and semicircular canals). These struc­tures are embedded in dense bone called the otic capsule. The cochlea is a minute spiral of two and three-quarter turns. Within this spiral, perilymph and endolymph are parti­tioned by the thinnest of membranes (Reissner’s membrane). The endolymph has a high concentration of potassium similar to intracellular fluid, and the perilymph has a high sodium concentration similar to extracellular fluid. Maintenance of the ionic gradients is an active process and is essential for neuronal activity.

There are approximately 15 000 hair cells in the human cochlea. They are arranged in rows of inner and outer hair cells. The inner hair cells act as mechanicoelectric transducers, converting the acoustic signal into an electric impulse. The outer hair cells contain contractile proteins and have an efferent nerve supply from the brain. They serve to tune the basilar membrane on which they are positioned.

Each inner hair cell responds to a particular frequency and when stimulated it depolarises and passes an impulse to the cochlea nuclei in the brainstem.

The vestibular labyrinth consists of the semicircular canals, the utricle and saccule, and their central connections. The three semicircular canals are arranged in the three planes of space at right angles to each other. As in the auditory system, hair cells are present. In the lateral canals the hair cells are embedded in a gelatinous cupula, and shearing forces, caused by angular movements of the head, produce hair cell movements and generate action potentials. In the utricle and saccule the hair cells are embedded in an otoconial membrane which contains particles of calcium carbonate. These respond to changes in linear acceleration and the pull of gravity.

Impulses are carried centrally by the vestibular nerve, and connections are made to the spinal cord, cerebellum and external ocular muscles.

The sensory nerve supply of the ear is complex. The exter­nal ear is supplied by the auriculotemporal branch of the trigeminal nerve (V), and this supplies most of the anterior half of the pinna and the external auditory meatus. The greater auricular nerve (C2,3), together with branches of the lesser occipital nerve (C2), supply the posterior part of the pinna. The VIIth, IXth and Xth cranial nerves also supply small sensory branches to the external ear; this explains why the vesicles of herpes zoster affecting the VIIth nerve appear in the concha (see Fig. 40.29 later). The middle ear is supplied by the glossopharyngeal nerve (IX).

This complicated and rich sensory innervation means that referred otalgia is common and may originate from the nor­mal area of distribution of any of the above nerves. A classic example is the referred otalgia caused by a malignancy in the pyriform fossa of the pharynx or a cancer of the larynx.

Anatomy of the ear

  Referred otalgia has many causes (e.g. cancer of the larynx)

Middle ear is intimately related to the cranial cavity

The Vllth nerve has a tortuous course through the ear