Temporal bone

Facial canal

The tympanic part of the primitive facial canal, the facial sulcus, develops anteroposteriorly from the geniculate fossa to enclose the facial nerve. The mesenchyme that forms the facial sulcus undergoes endochondral ossification, while the bone that caps or closes the sulcus develops in membrane. Permanent elliptical congenital dehiscences, about 1 mm in length and caused by incomplete closure of the canal, may be found in the tympanic segment, most commonly at the second genu. These dehiscences leave the nerve vulnerable to iatrogenic damage, particularly during stapedotomy. The facial nerve, unprotected by bone, may overhang the stapes superstructure and foot plate, or sustain heat damage if lasers are used to perforate the footplate ( Fig. 14.1 ). The unwary mastoid surgeon can also traumatize a dehiscent facial nerve when peeling cholesteatoma matrix from the surrounding structures. Oval window atresia has been described in a few patients who present with non-progressive unilateral or bilateral mixed-type hearing loss. In this anomaly, the facial nerve is found to occupy the entire oval window area and the stapedial crura are absent. Surgical interventions to correct the conductive element of hearing loss have been described in which a fenestra into the vestibule is drilled immediately below the facial nerve and the incus extended with cements on which a piston can be placed. The provision of hearing aids is as effective and without risk of morbidity. More significant variants of the course of the facial nerve through the temporal bone have been described in detail and are often associated with other morphological abnormalities.

External auditory canal

The external auditory canal (external acoustic meatus) develops from the dorsal end of the first pharyngeal cleft. The groove extends inwards as a funnel-shaped primary meatus, and from this the cartilaginous element and a small part of the roof develop. The dorsal end of the first pharyngeal pouch, the tubotympanic recess, becomes filled with a solid plug of epidermal tissue and extends along the floor of the recess. The central cells of this plug degenerate to produce the inner part of the canal and the tympanic membrane. The osseous part of the external auditory canal is derived from the tympanic ring of the temporal bone.

Congenital aural atresia is often associated with dysmorphic features of the auricle (pinna), middle ear, cochlea, labyrinth and facial nerve, e.g. in Treacher Collins’, Crouzon's and Moebius’ syndromes. These anomalies, which are often bilateral, vary in severity from partial to complete atresia and rudimentary middle ear ossicles. There is often a so-called ‘atretic plate’ of bone between the mastoid component and the cartilaginous, fibrous and epidermal elements of the dysplastic external canal.

Inner ear

Development of the inner ear is extremely complex and a detailed account is outside the remit of this chapter. The process is dependent on genetic patterning and a cascade of transcription signals expressed by its intrinsic and adjacent tissues. Otic placodes, areas of neurectoderm remaining in the ectodermal epithelium after neurulation, appear lateral to the hindbrain at stage 9 (25–27 days post fertilization). Both invaginate as pits adjacent to the fifth and sixth rhombomeres of the hindbrain dorsal to the second pharyngeal cleft. At stage 12 (30–32 days post fertilization), each pit is nipped off from the surrounding ectoderm to form the otic vesicle. Regions of the vesicles differentiate to form the labyrinth and, slightly later, the cochlea, undergoing an intricate process of elongation and infolding. The surrounding mesenchyme chondrifies to become the otic capsule. Dehiscences of the otic capsule overlying the superior and posterior semicircular canals are not uncommon and can give rise to Minor's syndrome, which is characterized by autophony, a conductive hearing loss and vertigo in the presence of loud sounds, the Tullio phenomenon.

Congenital structural anomalies of the ear are seen in 2–6/1000 births and are associated with conductive, mixed or sensorineural hearing losses. About two-thirds are non-syndromic and caused by genetic factors that are increasingly being identified. Most are associated with sensorineural hearing loss and some children can be helped by cochlear implantation. CT and MRI have played a significant part in the classification and management of these anomalies, particularly the cochlear and labyrinthine dysplasias. Inner ear anomalies, usually unilateral, are also associated with an increased risk of cerebrospinal fluid leaks and bacterial meningitis, from which there is a significant morbidity and mortality.

Vascular anomalies

An anomalous course of the internal carotid artery may result from abnormal development of the stapediohyoid and cervical carotid arteries of the neck. It may enter the temporal bone in the hypotympanum and turn forwards beneath the oval window (fenestra vestibuli). The stapedial artery normally atrophies during the third month of fetal life; the hyoid remnant becomes the caroticotympanic artery that anastomoses with the inferior tympanic artery over the promontory. If the stapedial artery persists, it passes between the crura of the stapes, turns anteriorly and either replaces the middle meningeal artery or joins the arteries that accompany the three divisions of the trigeminal nerve.

The height of the jugular bulb, which may not be covered by bone, is variable and occasionally is extremely high. It can fill the middle ear cleft and give the otoscopic appearance of a blue drum. In some, it is in contact with the tympanic anulus and puts these patients at risk of serious haemorrhage when the anulus is lifted during a tympanoplasty. Undertaking a myringotomy if the appearance of the drum is mis­diagnosed as a cholesterol granuloma can be disastrous. The position of the sigmoid sinus or height of the jugular bulb must be considered carefully before undertaking a transtemporal approach to the internal auditory canal. The surgeon should be aware that the sigmoid sinuses may vary in size, one being dominant or even solitary ( Fig. 14.2 ).