Pathology of the Head and Neck

Ear and Temporal Bone Chapter 8 237ry organ of sound reception, the organ of Corti. Wavesof vibration are conveyed from the perilymph to thewalls of the scala media, from which, through the endolymph,they affect the sensory cells of the organ ofCorti.The cochlear duct communicates with the vestibularendolymph-containing sacs through two fine canalsso that the endolymphatic system of the cochleaand vestibule is continuous, like the perilymphaticone. Gravitational acceleration of the head is detectedin a sensory organ arranged within endolymph-containingsacs in the vestibule (the utricle and saccule),and angular acceleration is detected within tubes emanatingin three dimensions from the utricle (lateral,posterior and superior semicircular canals). The sensorycells are located as a thickened portion of epithelium,the macula, in the saccule and utricle and araised prominence of epithelium, the crista, in expansionsof each semicircular canal, the ampullae. Thevestibular aqueduct contains the endolymphatic ductand sac, which constitute a blind offshoot of the endolymphaticsystem, probably functioning to absorbendolymph. The cochlear aqueduct is a communicationbetween the cerebrospinal fluid in the subarachnoidspace to the perilymph of the scala tympani nearthe round window. Cochlea, vestibule and semicircularcanals are surrounded by very dense bone, the oticcapsule.The cochlear and vestibular sensory structuresare supplied by a double nerve, the audiovestibularnerve or eighth cranial nerve, which enters the temporalbone through the internal auditory meatus. Thefacial nerve or seventh cranial nerve, enters the temporalbone through the same canal, and after a rightangledbend in the genu, where the geniculate ganglionis located, reaches the posterior wall of the middleear, from which it passes down through the mastoid toemerge in the region of the parotid salivary gland, afterwhich it provides motor nerve supply for the musclesof the face.8.1.3 HistologyThe ear and temporal bone are composed of many differenttissues, the normal histology of which is best consideredwhen describing the pathologic appearances ofthese parts.8.2 External Earand Auditory Canal8.2.1 Inflammatoryand Metabolic Lesions8.2.1.1 Diffuse External OtitisThis is a common condition, most frequently associatedwith Pseudomonas aeruginosa infection, but localtrauma is also an important causative factor. Histologicalfeatures include acute inflammation of the dermis,together with acanthosis and hyperkeratosis of the epidermis.8.2.1.2 PerichondritisPerichondritis most commonly affects the pinna, whereit may follow surgical trauma. As in the diffuse acute inflammationof the ear canal, Pseudomonas aeruginosa isthe most common infecting organism. Pus accumulatesbetween the perichondrium and cartilage of the pinna.This may interfere with the blood supply of the cartilageand so lead to its necrosis.8.2.1.3 Malignant Otitis ExternaMalignant otitis externa was first described as a severeinfection of the external auditory canal [16]. It usually(but not always [101]) affects elderly diabetics, resultingin unremitting pain, purulent discharge and invasion ofcartilage, nerve, bone and adjacent soft tissue. The causativeagent is usually Pseudomonas aeruginosa, but otherorganisms including fungi have been incriminated. Thecondition frequently goes on to ninth, tenth, eleventhand twelfth cranial nerve palsies and meningitis anddeath may result.Histopathological changes in the temporal bones oftwo patients who had been diagnosed clinically as havingmalignant otitis externa and were thought to havedied of this condition [123] were those of severe otitismedia and osteomyelitis of the jugular foramen secondaryto it. It seems likely that the manifestations of malignantotitis media are due to the spread of inflammationfrom the tympanic cavity and mastoid air spaces tothe petrous apex through bone marrow spaces by a processof osteomyelitis [85]. In recent years several patientswith AIDS have been reported to have malignant otitisexterna, and in one of them the presence of acute osteomyelitisof the skull base in addition supported the conceptof osteomyelitis as the pathologic basis for malignantotitis externa put forward above [121].