Dogs

The middle ear is phylogenetically derived from the pharynx and separates the external acoustic meatus from the cochlear and vestibular structures of the inner ear. Its major components include the tympanic cavity, tympanic membranes, ossicles (malleus, incus, and stapes), muscles (tensor tympani, stapedius), auditory (eustachian or pharyngotympanic) tube, nerves (facial, chorda tympani, tympanic branch of glossopharyngeal, and lesser petrosal), and tympanic plexus. The tympanic plexus receives parasympathetic innervation from the glossopharyngeal nerve and sympathetic innervation from the cranial cervical ganglion via the carotid plexus.

The tympanic cavity, formed by the tympanic component of the temporal bone, is a bony shell situated caudal and medial to the zygoma and temporomandibular articulation. The cavity has dorsal, middle, and ventral compartments (Figure 123-1) and is lined with respiratory mucosa contiguous, via the auditory tube, with that of the nasopharynx. The combined volume of the compartments is related to body weight.²¹ The dorsal component—the epitympanum or epitympanic recess—is the smallest of the three chambers and is largely occupied by the incus and part of the malleus. It is lined with cuboidal or squamous epithelium with few cilia. The middle component—the mesotympanum or true tympanic chamber—is a four-sided chamber lined with columnar or cuboidal epithelium and variable numbers of ciliated cells. The middle chamber is bounded laterally by the larger of two tympanic membranes, which separates it from the external ear; the smaller secondary membrane (the cochlear membrane) is located posteriorly within this chamber. On the medial aspect of the middle chamber, level with the tympanic membrane, is found the bony promontory that accommodates the cochlear structure. The ventral component of the tympanic cavity, termed the hypotympanum, is the largest of the chambers and is situated within the tympanic bulla, the spherical bone prominent ventrally. It communicates with the tympanic cavity proper through an anterolateral-facing elliptical opening on its dorsal aspect and is lined with cuboidal or squamous cells. Despite variations in the thickness of the bulla wall, which is thickest on its rostral and lateral aspects, the chamber’s interior profile remains regular.

Two membranes are found within the mesotympanic chamber: the tympanic membrane and the secondary cochlear (or round) membrane. The tympanic and cochlear membranes separate the chamber from the external acoustic meatus laterally and the inner ear medially, respectively. The tympanic membrane is a thin, translucent, triple-layered structure with contributions from the external and middle ear. The membrane is divided into the smaller pars flaccida and larger ventral, pars tensa (Figure 123-2). The pars flaccida occupies the dorsal portion of the membrane and covers a small area of the middle ear chamber that, in dogs, is separated from the remainder of the epi- and mesotympanum. The pars tensa comprises the majority of the membrane; the manubrium of the malleus is embedded in its dorsal aspect. The membrane is constructed of an inner epithelium derived from the pharyngeal pouch, a central fibrous layer also of pharyngeal origin, and an outer stratified squamous epithelium derived from the ectoderm of the outer ear. Its longitudinal axis slopes so that its ventral aspect is more medial, and its outer surface is concave because of its attachment to the malleus. The outer epithelial cells originate at the attachment of the malleus and migrate peripherally from this point to the external auditory meatus; this continual centrifugal movement of cells clears debris from the surface of the membrane and is also responsible for its repair in the event of rupture.

The opening to the auditory tube, also known as the eustachian or pharyngotympanic tube, is found in the rostral mesotympanic chamber. The tube is a short structure connecting the tympanic chamber with the nasopharynx. Its length varies from 1 to 2 cm, and its oval lumen between 1 and 2 mm in diameter. The wall of the tube is supported medially in the nasopharyngeal region by cartilage, in the tympanic region by the petrous temporal bone, and laterally by the tensor veli palatini muscle. The tube is lined with ciliated, pseudostratified, or columnar mucosa. Its middle segment has mainly ciliated cells that progress to a combination of goblet cells, nonciliated columnar cells, and ciliated cells at the tympanic opening. Mucus and goblet cells predominate at the pharyngeal end. Cannulation of the auditory tube via the pharyngeal opening for drainage purposes is feasible.⁴¹

The bones or ossicles of the middle ear include the malleus, incus, and stapes. The malleus is the most lateral ossicle and is comma shaped when viewed from its medial aspect. Its “tail” or manubrium is embedded in the fibrous middle layer of the tympanic membrane, whereas its medial head articulates with the body of the middle ossicle, the incus. The incus lies almost entirely within the epitympanic recess and articulates with the stirrup-shaped stapes medially. The stapes is oriented horizontally. Its base on the medial aspect is attached to the fibrocartilaginous ring around the rim of the vestibular, or oval, window and separates the tympanic cavity from the perilymphatic space of the inner ear. The ossicles are fixed within the mesotympanic cavity and epitympanic recess by a series of ligaments. These include the lateral ligament of the malleus that attaches it to the tympanic notch, the dorsal ligament of the malleus and incus that attaches them to the epitympanic recess, and the annular ligament of the stapes that attaches it to the vestibular window.

Two muscles are responsible for movement of the ossicles. The tensor tympani muscle is attached in the dorsal mesotympanic cavity and inserts on the small hook on the medial aspect of the malleus. Contraction of this muscle moves the malleus medially and increases the tension and convexity of the tympanic membrane to dampen sound vibrations. The stapedius muscle also tenses the stapes to limit its movement and similarly dampen vibrations.

A number of nerves supply or pass through the tympanic chambers. The facial nerve has mixed sensory and motor functions and reaches the internal auditory meatus in close association with the vestibulocochlear nerve. After separating from the vestibulocochlear nerve, it enters the facial canal within the petrous temporal bone and emerges caudal and medial to the tympanic bulla at the stylomastoid foramen. The facial canal of the petrous temporal bone is incomplete, and the nerve lies exposed in the dorsal aspect of the tympanic cavity near the vestibular window. Within the facial canal, the facial nerve gives branches into the stapedius nerve and the chorda tympani (tympanic nerve). The contribution from the facial nerve to the chorda tympani mixes with preganglionic parasympathetic vagal branches that pass across the medial aspect of the malleus and tympanic membrane before entering a small canal in the dorsal wall of the tympanic chamber and exiting the petrotympanic fissure to join the lingual nerve. The tympanic plexus is formed from the tympanic branches of cranial nerve (CN) IX and the caroticotympanic nerve. It spreads across the bony promontory before entering the lesser petrosal nerve. The tympanic plexus gives rise to preganglionic parasympathetic fibers to the parotid and zygomatic salivary glands, postganglionic fibers to the parotid gland and sensory fibers to the middle ear cavity. Postganglionic sympathetic fibers arise from the cranial cervical ganglion located deep to the bulla and pass to the eye to supply the smooth muscle of the dilator pupillae and nictitating membrane. These fibers follow the course of the internal carotid artery before joining the tympanic branch of CN IV.

The vascular supply to the middle ear—the tympanic artery—is derived from the maxillary artery and enters via a small foramen caudal to the temporomandibular joint. Other contributions arise from the meningeal and pharyngeal vessels.

Cats

The feline middle ear has a similar overall anatomic arrangement to that of dogs; however, its chambers show much more distinct separation into a larger ventral cavity (Figure 123-3), the hypotympanum, and a smaller rostrolateral component corresponding to the epitympanum and mesotympanum. The two components are almost completely separated by a bony septum and communicate on their medial aspect through a small slitlike opening that widens into a distinct foramen caudally. This separation gives rise to the characteristic “double shell” profile of the tympanic structures visible on anteroposterior radiographic views. The lining of the chambers is reported to contain more abundant ciliated and secretory cells than in dogs. The distribution of neural structures within the middle ear is similar to that in dogs. On entering the middle ear, the tympanic plexus distributes widely across the bony promontory but is reported to be more exposed, or possibly more sensitive, to iatrogenic trauma.

Septic Otitis Media

Septic otitis media is undoubtedly the most important disease of the canine tympanic chamber. In principle, bacterial ingress to the middle ear can arise via one of the following three routes: the external auditory meatus via the tympanic membrane, the nasopharynx via the auditory tube, or hematogenous distribution. Extension from the external meatus via the tympanic membrane is considered to be by far the most common route for bacterial invasion in canine middle ear disease, which is usually the sequela to longstanding otitis externa accompanied by chronic inflammation and accumulation of bacteria in the external acoustic meatus.

The precise mechanism by which bacteria are able to gain access to the tympanic chamber is still unresolved. Although it has always been assumed that the tympanic membrane must be damaged to permit ingress of microorganisms, various authors have shown that the membrane is intact or even thickened in many dogs with septic otitis media.49,52 This finding may support the possibility of transmembrane migration by the bacteria; it is equally conceivable, however, that temporary perforation and subsequent repair of the membrane permits ingress.

Alternatively, bacterial invasion of the integument that lines the osseous external auditory prominence and surrounds the membrane may explain their ingress to the middle ear. This is supported by the fact that the predominance of canine breeds affected by septic otitis media reflects that of dogs prone to otitis externa. In patients with chronic otitis externa, however, microbial flora of the external meatus and their antimicrobial susceptibility differ considerably from those organisms responsible for septic otitis media.9,40 The most commonly encountered microorganisms in canine middle ear infections are reportedly Staphylococcus intermedius, Pseudomonas spp., and Malassezia yeast.⁹ Cryptococcal infection of the tympanic chamber has been reported in cats.⁴

Other conditions that permit bacterial access to the middle ear include foreign body perforation and traumatic disruption of the external acoustic meatus. Not only may foreign bodies damage the tympanic membrane, but persistence of the foreign body in the external meatus and tympanic chamber can prevent its normal repair. Traumatic disruption at the level of the annular cartilage may be encountered as the result of bite wounds in dogs and cats. The subsequent inflammatory process may seal off the horizontal component of the external meatus adjacent to the tympanic membrane from the remainder of the meatus, effectively trapping bacteria within a secretory environment. The resultant abscessation disrupts the membrane, which allows the infection to spread to the middle ear and gives rise to para-aural discharge.⁵⁹ Para-aural abscessation can also occasionally occur as a sequela to iatrogenic stenosis of the external meatus after lateral wall resection or from residual integumentary remnants left within the bulla after total canal ablation.⁶⁷ Rarely, congenital atresia of the external acoustic meatus may lead to a similar disease process within the bulla.

Compared with dogs, bacterial middle ear infection in cats as a consequence of otitis externa appears to be rare; presumably, this reflects the differing causes of aural disease in the two species. Conversely, ascending infection via the auditory tube associated with episodes of viral nasopharyngeal infection is frequently implicated in development of middle ear polyps in cats even though this is not thought to be an important route of access to the middle ear in dogs. Hematogenous infections involving the middle ear are considered to be very infrequent in all species.

Pathologic changes recognized in the middle ear as a consequence of established bacterial infection include thickening of the tympanic membrane, and epithelial hyperplasia with thinning of the lamina propria and the development of associated granulation tissue.⁴⁹ A lymphoid and polymorphonuclear leukocyte infiltrate is usually evident. The wider availability of sectional imaging has allowed an increasing recognition of the incidence of extension of septic otitis media to involve the inner ear and adjacent intracranial structures.^60,72

Inflammatory Diseases of the Middle Ear

Congenital Anomalies

Congenital anomalies of the middle ear are uncommon; agenesis of the whole or part of the tympanic membrane is occasionally recognized. Congenital anomalies of the inner ear resulting in hearing or vestibular dysfunctions have been reported in a number of breeds,⁷⁰ including collapse of the scala media in the Dalmatian,⁷ neuroepithelial degeneration in Rottweilers,¹⁶ and changes in the organ of Corti in Pointers.¹⁵

Disorders of Middle Ear Drainage

Neoplastic Disease

The majority of tumors involving the middle ear are usually identified as extensions of neoplasms derived from structures of the external ear canal; tumors derived from structures of the tympanic cavity and auditory tube are rare. Tumors of true tympanic origin include ceruminous gland adenocarcinomas,48,53 squamous cell carcinoma,^31,79 carcinoma of unknown origin, and lymphoma.²² Neurologic signs (i.e., Horner syndrome, vestibular signs) indicating extension to involvement of the middle ear are occasionally recognized in dogs with malignancy of the external ear but are more commonly encountered in cats. Tumors of the inner ear are extremely rare, and many patients presented with vestibular signs are ultimately found to have central lesions.

Septic Otitis Media

Clinical signs of inflammatory or septic otitis media include otorrhea, otalgia, head shaking, and attention to the affected ear(s). The appearance of the discharge associated with otitis media varies; however, because most cases result from septic extension of otitis externa, purulent material is present in the external meatus. Signs of otalgia include discomfort on aural manipulation and behavioral changes. Additionally, animals may have intermittent alteration of head carriage with the head carried lower on the affected side if the disease is unilateral. Persistent head tilt is not encountered unless there is extension to the inner ear. Patients with severe inflammatory change that extends rostrally to involve the adjacent temporomandibular joint may show signs of pain on jaw movement and have a restricted range of movement. All of these signs are similar to, and often very difficult to differentiate from, those encountered in patients with severe otitis externa. Because most cases of otitis media in dogs are the result of preexisting otitis externa, this is of particular significance because involvement of the middle ear may be easily overlooked.