The Ear and Eye

CHAPTER 20


The Ear and Eye*




Ear


The ear is a specialized sense organ formed by a highly organized mixture of cutaneous, nasopharyngeal, osseous, and neurologic tissues. Within the ear are several air- and fluid-filled interfaces involved in the transduction of sound and pressure waves to action potentials that are conducted by the nervous system to the brain for interpretation and appropriate motor and cognitive responses.


In the context of interacting with animals, hearing is often considered of secondary or tertiary importance when compared to vision or olfaction. For many years, specific breeds predisposed to auditory dysfunction were maintained in colonies to facilitate investigation as animal models of human disease. More recently, throughout the world, deaf and hearing-impaired people are greatly benefiting from professionally trained hearing dogs. In addition, animal trainers, pet owners, and producers rely on a fully functioning auditory system to train, address, keep safe, or herd their animals. Therefore a much better understanding of conditions affecting this special sense is needed.


Many current textbooks focus on one species or one aspect of the ear. The focus of this chapter is to (1) clarify the anatomy of the ear by comparing and contrasting anatomic features of the domestic animal species, (2) address the responses to injury and the defense mechanisms protecting against injury, and (3) delineate various otic diseases that either affect many different species or are more unique to certain species.



Structure And Function



External Ear


The external ear comprises the auricle (also known as pinna) and external acoustic meatus terminating medially at the tympanic membrane. Developmentally, the external ear arises from tissue elevations called auricular hillocks, three from the first branchial or pharyngeal arch and three from the second branchial or pharyngeal arch. The first pharyngeal groove or cleft between the two arches forms the external acoustic meatus. As they come into apposition early in development, the auricle and external acoustic meatus form. The auricle is a highly mobile and flexible, cartilaginous structure that is covered by haired skin with adnexa more dense on the convex than concave surface. The structural characteristics of auricles are closely tied to breed specifications. Among the species and breeds covered in this chapter, auricles can be erect, semi-erect, lop-eared, pendulous, microtic, or folded.


The auricle functions to collect, focus, and direct sound down the funnel-shaped external acoustic meatus to the tympanic membrane (Fig. 20-1). Because the rostroauricular and caudoauricular muscles are innervated by motor branches of the facial nerve, the flexible (cartilaginous) auricle is able to move rostrally, laterally, and caudally about a central auricular axis. The position of the auricle can signal an animal’s behavior or emotion. Fearful cats often flatten their ears in a defensive posture, whereas angry horses often completely flatten their ears caudally as an initial warning before they strike. Alternatively, a dog may flatten its ears when content or when it is being verbally scolded.



The external acoustic meatus (ear canal) is a conical opening made up of elastic cartilage and bone (Fig. 20-2). The more lateral portions are composed of auricular cartilage that narrows and overlaps with annular cartilage. Dense fibrous connective tissue forms a bridge between the annular cartilage ring and the osseus portion of the external acoustic meatus. In cats and dogs, the osseous portion (1) is a very narrow rim of bone and (2) is a broad opening exposing the tympanic membrane that is readily visible during otic examination. In horses, ruminants, and pigs, the osseous portion of the external acoustic meatus is an elongate cylinder of bone with a narrow lumen (Fig. 20-3). In horses, the junction between the cartilaginous and osseous portions of the external acoustic meatus is grossly identified by an abrupt change from pigmented to nonpigmented epithelium. Visualization of the deeper portions of the external acoustic meatus in livestock species requires specialized equipment and heavy sedation.




Although there are wide species variations, the cartilaginous and osseous portions of the external acoustic meatus are lined by a thin epidermis formed by stratified squamous epithelium and a thin dermis that contains a relatively uniform allotment of sebaceous glands, fewer hair follicles, and greater ceruminous glands when comparing medial to lateral portions (Fig. 20-4). Sebaceous glands are composed of 6 to 10 club-shaped acini surrounded by thin fibrous tissue with ducts opening into associated hair follicles. Relative to ceruminous glands, they tend to be located in the more superficial dermis. Ceruminous glands are simple, coiled, tubular glands that resemble apocrine sweat glands. Their ducts open either into hair follicles or directly to the epidermal surface.



Although motor innervation of muscles of the external acoustic meatus is provided by the facial cranial nerve, sensory innervation is more complex. Branches of the trigeminal, facial, and vagal cranial nerves and branches of the second cervical spinal nerve innervate the skin. Sensory innervation to the mucosa of the external acoustic meatus is provided by the mandibular branch of trigeminal and auriculotemporal cranial nerves. The primary blood supply to the ear is through the caudal auricular artery, which branches into the lateral, intermediate, deep, and medial auricular arteries. The caudal auricular artery is a main branch of the external carotid artery.



Middle Ear



Tympanic Membrane (Tympanum): The tympanic membrane, also known as the tympanum, is an extremely thin, three-layered, semitransparent membrane peripherally suspended from the tympanic ring by a fibrocartilaginous ring. The tympanic membrane is formed when the endoderm of the first pharyngeal pouch comes into close contact with the ectoderm of the first pharyngeal cleft or groove. Most of the tympanic membrane is held under tension so it can be deformed by and responsive to sound waves (Fig. 20-5). It covers the medial extremity of external acoustic meatus, demarcating the junction between the external ear and middle ear. Both surfaces of the tympanic membrane have an air interface.



In most species, the tympanum is an oval-to-round structure, whereas in ruminants, it is shaped more like a broad triangle (Fig. 20-6). Embedded in the tympanum is the manubrium of the malleus. The placement of the manubrium in the tympanic membrane is highly variable between species. It is more centrally located in horses versus a more rostromedial location in ruminants and pigs.



The tympanum is divided into two sections, the pars tensa and the pars flaccida. The majority of the tympanic membrane is made up of the pars tensa, which is a very thin, translucent, and taut membrane that bulges convexly into the tympanic cavity (Fig. 20-6, E). The pars tensa is made up of three layers: (1) outer layer of keratinizing squamous epithelium derived from ectoderm of the first pharyngeal groove; (2) middle layer of thin, variably vascularized fibrous connective tissue originating from the pharyngeal wall; and (3) inner layer of very low cuboidal to nonkeratinizing squamous epithelium, which is of pharyngeal pouch origin.


The dorsal most portion of the tympanic membrane is the pars flaccida, which is roughly triangular, thicker, more vascular, and flaccid when compared to the pars tensa (see Fig. 20-6, E). Overlain by keratinized epithelium, the underlying stroma of the pars flaccida in dogs is made of loosely arranged collagen, rare mast cells, and few elastin fibers. This latter feature is in direct contrast to humans in which there are abundant elastin fibers. Visualized from the external acoustic meatus, this portion of the tympanic membrane may bulge into or away from the middle ear.


The tympanic membrane is placed roughly at a 45-degree angle relative to the central axis of horizontal portion of the external acoustic meatus (Fig. 20-7, A). However, commonly, the actual placement of the tympanic membrane is more variable with the external, concave surface of the tympanic membrane angled more rostrally (Fig. 20-7, B and C). Cats have a similar orientation to their tympanum (Fig. 20-8). Interestingly, the surface area of the tympanic membrane from an approximately 550-kg horse is larger than the tympanic membrane of a Maltese dog but is approximately 15% smaller than the surface area of the tympanic membrane of a German shepherd dog. Additionally, the tympanum of the fetal goat is approximately 20% larger than that of large dog breeds.



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Fig. 20-7 External acoustic meatus, tympanic membrane, tympanic cavity. Compare unlabeled contralateral side with labeled side for more structural detail.
A, Transverse section, rostral surface, dog. The tympanic membrane (arrows) extends medially towards the tympanic cavity (T) at an approximate 45-degree angle from dorsal to ventral, in relation to the central axis of the horizontal part of the external acoustic meatus (M). Portions of the rostral edge of the tympanic ring have been inadvertently removed during sample preparation. Brainstem (B); cerebellum (C). B, Transverse section, rostral (1) and caudal (2) surfaces, goat. The tympanic cavity (T) has been opened bilaterally. In the cranial view (1), the tympanic membrane is not visualized because it is hidden by the tympanic ring (arrows), which surround the membrane. The manubrium of the malleus (arrowheads) is minimally visible. However, from the caudal view (2), the tympanic membrane (arrows) is clearly visible and is positioned so that the concave or external surface is tilted rostrally. Brainstem (B); calvarium where brainstem would be positioned (C); tympanic ring (arrowheads). C, Transverse section, caudal surface, dog. The external or concave surface of the tympanic membrane (arrow) is angled almost fully rostral rather than lateral. Note that the septum bullae (asterisk) are short and incomplete in the dog when compared with a cat (see Fig. 20-10). Brainstem (B); Cerebellum (C); Tympanic cavity (T); Manubrium of the malleus (arrowheads); External acoustic meatus (M), Tympanic bullae (TB). D, Ventral-dorsal view, opened bullae. Bilaterally, the concave surface of the tympanic membranes (arrow) are tilted rostrally. Occipital condyles (O) appear at the bottom of the image. Extending rostrally and medially from the tympanic cavity are the auditory tubes (arrowhead), which provide direct communication between the tympanic cavity and nasopharynx. Manubrium of the malleus (arrow 1). (Courtesy Dr. B.L. Njaa, Center for Veterinary Health Sciences, Oklahoma State University.)




Tympanic Cavity: The tympanic cavity is an air-filled compartment surrounded by bone that is separated from the external ear by a thin tympanic membrane (tympanum) and is in direct communication with the pharynx via the auditory tube (also known as the eustachian or pharyngotympanic tube). Both the tympanic cavity and the auditory tube are derived from the endoderm of the first pharyngeal pouch. The epitympanic recess is the dorsal extremity of the tympanic cavity within which lies the head of the malleus and short curs of the incus. Ligaments stabilize and anchor the incudomallearis joint and the short crus of the incus within this recess (Fig. 20-9).



In many species, there is a bulbous, ventral portion of the tympanic cavity called the tympanic bulla (see Fig. 20-3, B and C). Within the bulla of the dog and cat is a bony septum referred to as the septum bulla. In the cat, the septum bulla abuts the petrous portion of the temporal bone and separates the tympanic cavity into two compartments: the dorsolateral epitympanic cavity and the ventromedial tympanic cavity (see Fig. 20-8). This separation is incomplete, which allows communication between the two compartments through a narrow opening between the septum bulla and petrous portion of the temporal bone and a larger opening at its caudal edge. In the dog, this septum is a much smaller, incomplete bony ridge that only makes contact with the petrous portion of the temporal bone rostrally and often has tiny, elongate bony spicules with bulbous ends (see Fig. 20-7, B). The mucosal surfaces of the tympanic bullae of cats and dogs are lined by an epithelium that varies, depending on location. Dorsally, close to the auditory tube opening, the mucosa is composed mostly of ciliated columnar cells mixed with goblet cells and basal cells similar to the cells in the nasopharyngeal mucosae (Fig. 20-10 and see Fig. 20-20). Ventrally, the number of ciliated cells and goblet cells decreases and the number of cuboidal, less differentiated cells increases. The surfaces of the petrous portion of the temporal bone, auditory ossicles, and tympanic membrane are lined by cuboidal to noncornifying squamous epithelium. In ruminants, camelids, and pigs, the ventral portion of the tympanic cavity or bulla is made of more numerous bony compartments lined by noncornifying squamous epithelium (Fig. 20-11). In cattle and pigs, these compartments are air-filled with direct communication with the tympanic cavity. In camelids and small ruminants, the tympanic cavity does not appear to directly communicate with the bullae. Horses do not have readily identifiable tympanic bullae.





Auditory Ossicles: A chain of three bones or auditory ossicles forms the mechanical transduction system of hearing: the malleus, incus, and stapes (Figs. 20-12 and 20-13). The malleus and the incus, as well as the tensor tympani, are derived from the mesenchyme of the first branchial or pharyngeal arch. The stapes and the stapedius muscle originate from the mesenchyme of the second branchial or pharyngeal arch.




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Fig. 20-13 Auditory ossicles and ossicular joints, cat.
A, Tympanic cavity, tympanic membrane, auditory ossicles, petrous portion of the temporal bone, auditory muscles, ventral view. The manubrium of the malleus is embedded in the tympanic membrane. The head of the malleus and incus are anchored in the epitympanic recess and form the incudomallearis joint. The long crus of the incus is shown articulating with the stapes to form the incudostapedius joint (arrowhead). Attached to the muscular process of the malleus (M) is the tensor tympani muscle (asterisk). Tympanic membrane (arrow). B, Histologic section of the incudomallearis joint (between the arrows), normal. The articulation of the malleus (M) and incus (I) is shown in the epitympanic recess. Similar to the petrous portion of the temporal bone, these ossicular bones lack a medulla. C, Histologic section of the incudostapedius joint. Positioned in the oval window is the stapes (S), held in place by a syndesmosis (arrow 1). The more ventral edge of the stapes has artifactually fractured. Articulating with the head of the stapes is the lenticular process of the long crus of the incus (I) to form the incudostapedius joint (arrow). The lenticular process of the incus is to the right of the incudostapedius joint. A portion of the short crus is positioned in the epitympanic recess and anchored by a ligamentous attachment (arrowhead). The facial nerve is present coursing through the facial groove (asterisk). Note the lack of complete bony encasement allowing communication with the tympanic cavity. Promontory (P), petrous portion of the temporal bone (T). H&E stain. (A and B courtesy Dr. B.L. Njaa, Center for Veterinary Health Sciences, Oklahoma State University.)



Malleus: The largest of the ossicles is the malleus. The manubrium of the malleus is embedded in the tympanic membrane (see Fig. 20-5). The most ventromedial convexity of the malleus is the “umbo” (see Figs. 20-6, F, and 20-12). The muscular process of the manubrium near the neck of the malleus is the attachment site of a thin tendinous portion of the tensor tympani muscle. Various ligaments stabilize the malleus in the epitympanic cavity by anchoring the long, thin rostral process, the neck, and the head of the malleus. The head of the malleus articulates with the articular surface of the body of the incus forming the incudomallearis joint (see Figs. 20-9, 20-12, and 20-13). In the horse and cow and in aged dogs and cats, the incudomallearis joint capsule is a narrow but thick ligament that makes disarticulation difficult and gives the external appearance of a falsely fused joint. In younger dogs and cats, the incudomallearis ligament is not nearly as tenacious and disarticulation is much less difficult.




Stapes: The stapes, named for its close resemblance to stirrups of a saddle, is often considered the smallest bone in the body.* However, its size and shape is somewhat variable, depending on species (Fig. 20-14). Its base or footplate is convex and firmly seated in the oval or vestibular window of the petrous portion of the temporal bone and anchored by the annular ligament of the stapes. This arrangement forms a syndesmosis between the stapes base and cartilage of the oval window (Fig. 20-15). The stapedius muscle, fittingly referred to as the smallest muscle in the body, is attached to the muscular process of the shorter caudal crus close to the head of the stapes. Vibrations of the tympanic membrane are linearly transduced to stapes vibrations that lead to fluid waves of the perilymph of the internal ear.





Middle Ear Muscles And Nerves


The middle ear has two muscles associated with the auditory ossicles that help modulate auditory transduction and a third muscle that controls patency of the auditory tube. The tensor tympani muscle originates rostrally and medially from the bony recess in the petrous portion of the temporal bone and makes its tendinous insertion onto the muscular process of the neck of the malleus (Fig. 20-16; see Fig. 20-13, A). It receives its innervation via a motor branch of the trigeminal nerve. Contraction of the tensor tympani muscle pulls the tympanic membrane medially and rostrally, placing greater tension on the auditory ossicle chain, which results in an increased resonant frequency of the auditory sound conduction system.



The stapedius muscle originates in the stapedius muscular fossa located dorsomedial to and obscured by the facial nerve as it courses through the facial groove of the temporal bone (Fig. 20-17). The stapedial branch of the facial nerve innervates this muscle as it converges into a thin tendon that inserts onto the muscular process of the short crus of the incus close to the head of the stapes. In the cat, the displacement variance of the stapes in its vestibular window is approximately 0.2 µm, whereas the maximal contraction of the stapedius muscle leads to dorsal and caudal stapedial bone displacement of between 40 and 60 µm. This displacement, which is perpendicular to the normal movement of the stapes, maximally attenuates sound transmission up to 30 decibels. Contraction of the stapedius muscle is an integral part of what is termed the acoustic reflex, defined as consensual, reflexive contraction of the muscle in response to stimuli (frequently sound) that leads to attenuated acoustic transmission.



The tensor veli palatine muscle arises from a groove in the petrous portion of the temporal bone medial and ventral to the tensor tympani muscle. Along with its nerve, the tensor veli palatine nerve, a branch of the trigeminal nerve, this long, slender muscle extends rostrally from the tympanic cavity parallel to the auditory tube. In concert with the levator veli palatine muscle, which is innervated by the facial nerve, coordinated contraction of these muscles open the pharyngeal orifice of the auditory tube.


Two cranial nerves provide motor branches to the muscles of the middle ear. Branches of the trigeminal nerve named for their respective muscles innervate the tensor tympani and tensor veli palatine muscles within the tympanic cavity. The facial nerve initially leaves the cranial cavity through the internal acoustic meatus (see Fig. 20-17), along with the vestibulocochlear cranial nerve, and then courses through the facial groove of the petrous portion of the temporal bone in close proximity to the oval window (Fig. 20-18). Several millimeters medial and lateral to the tendon of the stapedius muscle, the bony casing of the facial groove is incomplete, which allows direct communication between the epineurial connective tissue of the facial nerve and the tympanic cavity (see Fig. 20-18). This proximity and exposure of the facial nerve to the tympanic cavity explains why middle ear disease can manifest as facial nerve dysfunction. The facial nerve emerges from the middle ear through the stylomastoid foramen immediately caudal to the external acoustic meatus.




Auditory Tube (Eustachian Or Pharyngotympanic Tube)


In most mammalian species, the middle ear communicates with the pharynx through the auditory tube, which originates from the first pharyngeal pouch (Fig. 20-19). In the middle ear, the auditory tube opens into the most rostral and dorsal portion of the tympanic cavity called the epitympanic cavity. In the pharynx, the auditory tube originates from a narrow slit-like opening in the nasopharyngeal cavity and is lined by epithelium that is contiguous with the nasopharynx, namely ciliated, columnar pseudostratified epithelium mixed with goblet cells (Fig. 20-20). In many species, flanking the auditory tube are clusters of lymphocytes referred to as the tubal tonsil.




Infectious organisms can migrate via the auditory tube between the nasopharynx and the middle ear, thus serving as a portal of entry for each area. Additionally, the auditory tube is an important route for clearance of an infectious organism from the middle ear via the nasopharynx to the alimentary system.


Unique to the horse and other Equidae, guttural pouches (see Chapters 9 and 17) are enlarged diverticula of the auditory tubes that extend further rostrally, medially, and ventrally when compared to auditory tubes of other mammalian species. Although the precise function of guttural pouches remains controversial, their proximity to internal carotid arteries and their ability to inflate during vigorous exercise makes the idea of an extracalvarial brain cooling apparatus a provocative hypothesis.



Internal Ear


The internal ear is confined to a single bone, the petrous portion of the temporal bone. In most mammalian species, it is a triangular, wedge-shaped bone that forms the dorsomedial margin of the tympanic cavity. Often referred to as the hardest bone of the body, it is frequently slightly more yellow than surrounding bone and lacks the cancellous bony arrangement or medullary cavities present in other portions of the temporal bone. The internal ear is derived from a focal area of ectoderm referred to as the otic placode. This eventually forms an otic vesicle and through interaction with surrounding embryonic tissues, differentiates into this highly specialized tissue.


The internal ear is essentially made up of several membranous compartments, collectively known as the membranous labyrinth, derived from ectoderm, that contain endolymph. The membranous labyrinthine compartments include the cochlea, sacculus, utricle, and each semicircular canal with associated ampullae (Fig. 20-21). Surrounding the membranous labyrinth is a protective bony shell, derived from mesoderm, known as the osseous labyrinth (i.e., petrous portion of the temporal bone). The membranous labyrinth is classically a rostrally coiled tube or cochlea with an intermediate compartment or vestibule and a caudal semicircular canal region. However, the shape of the petrous portion of the temporal bone is very different. The beginning of the cochlea is denoted by the prominent bulge in the petrous portion of the temporal bone known as the promontory. Caudally, portions of this bone give the appearance of the tubular membranous labyrinth that likely represent the semicircular canals. Within the membranous labyrinth are biologic mechanosensory hair cells (see later) responsible for hearing (the auditory compartment) and for assessing head position, acceleration, and balance (the vestibular compartment).




Cochlea: The cochlea is the most complex portion of the membranous labyrinth comprising two closed-end tubular structures that are highly coiled (see Fig. 20-21). The central core of bony labyrinth around which the cochlea spirals nearly three times is the modiolus. Sound waves vibrate the tympanic membrane and are converted by coordinated movements of the malleus, incus, and stapes to fluid waves within the perilymph by vibrations of the vestibular or oval window. Fluid waves travel through the scala vestibuli toward the cupula, reaching the helicotrema, and then returning within the scala tympani toward the round (also known as cochlear) window. Positioned between the scala vestibuli and the scala tympani is the second closed compartment known as the cochlear duct or scala media. The cochlear duct is separated from the scala vestibuli by the vestibular membrane (also known as Reissner’s membrane), which transmits fluid waves of perilymph in the scala vestibuli into fluid waves of endolymph in the cochlear duct. Within the cochlear duct is a ribbonlike strip of extracellular matrix called the tectorial membrane. It is composed of several genetically distinct types of collagen (collagen types II, IX, and XI) and three distinct noncollagenous glycoproteins (α-tectorin, β-tectorin, and otogelin). The tectorial membrane rests on and is affixed to tips of hair cells that make up the mechanosensory portion of the organ of Corti that lies on the basilar membrane (Fig. 20-22). There are a series of three rows of outer hair cells and a single row of inner hair cells (see Fig. 20-21). Fluid waves in the cochlear duct result in movement of the tectorial membrane, which distorts inner and outer hair cells (neural-like cells), resulting in their depolarization and afferent transmission of action potentials via the cochlear branch of the vestibulocochlear cranial nerve to the vestibular nucleus.




Vestibular System: The vestibular system is made of several endolymph-filled compartments located in the caudal third to half of the petrous portion of the temporal bone. It represents a major sensory system that (1) maintains balance in concert with general proprioception and visual systems, (2) coordinates body posture, and (3) helps maintain ocular position in relation to the position or motion of the head. Included in the vestibular system are the semicircular canals, utricle, saccule, vestibular ganglia, vestibular portion of cranial nerve VIII (vestibulocochlear nerve), vestibular nuclei, and vestibular lobules of the cerebellum.


There are three semicircular canals oriented at right angles relative to each other occupying three planes. Each canal has a terminal dilation or ampulla that contains a specialized surface sensory organ called the crista. In aggregate, the sensory portion is referred to as crista ampullaris. Each crista is lined by specialized sensory hair cells that send continuous tonic neural signals to the vestibular nucleus. Deflection of these sensory hair cells during acceleration, deceleration, or rotation results in variation of the tonic signals sent to the vestibular nucleus. However, the hair cells are not activated during constant velocity.


Maculae are receptors located within the membranous utricle and saccule of the vestibule. The saccular macula is oriented in the vertical plane, whereas the macula of the utriculus is oriented in the horizontal plane. Surface lining neuroepithelial hair cells of the macula project into an otolithic membrane. Movement of the otolithic membrane causes deflection of the hair cells and triggers action potential. As in the case of the crista ampullaris, macular receptors provide a continuous tonic nervous input, with a net effect of maintaining static head positioning relative to gravity.


On stimulation of sensory nerve endings, action potentials are transmitted through bipolar cells whose cell bodies are located in the vestibular ganglia of the vestibular branch of the vestibulocochlear nerve. Signals travel to the vestibular nuclei in the medulla. From the vestibular nuclei, connections are made with the oculomotor, trochlear, and abducent nuclei of the rostral brainstem via the medial longitudinal fasciculus, the vestibulocerebellum by the caudal cerebellar peduncle, and the spinal cord via the vestibulospinal tract located in the ventral funiculus.




Portals Of Entry


Portals of entry into the ear are listed in Box 20-1.




External Ear



Extension from the External Environment: Extension from the external environment is a common portal of entry into the external ear. It is a specialized invagination of the skin that terminates medially at the tympanic membrane of the middle ear. As the external acoustic meatus gradually narrows, its funnel shape is conducive in directing foreign materials, fomites, parasites, and/or infectious microorganisms into the external ear and toward the tympanic membrane of the middle ear. Additionally, its moist environment favors colonization of skin and/or mucosa by pathogenic microorganisms (Table 20-1). Although dermatitides can affect any part of the dermis, including the external ear, occasionally, involvement of the ear is an important feature used to arrive at a definitive diagnosis.






Middle Ear



Extension through Perforation of the Tympanic Membrane: Extension from the external ear through a perforated tympanic membrane is a portal of entry into the middle ear (Fig. 20-23). In dogs with chronic otitis externa, secondary otitis media may occur in as many as 80% of affected dogs. At the time of clinical diagnosis, the tympanic membrane is most often intact, although some studies report perforations in over 40% of cases with otitis externa and concurrent otitis media. Based on the results of bacteriologic studies, it has been shown that a majority of dogs with concurrent otitis externa and otitis media have different bacteria isolated from each compartment. Thus it is unclear if the portal of entry in otitis media involves perforation of the tympanic membrane and spread of otitis externa into the middle ear with subsequent healing of the tympanic membrane or if otitis media results from bacteria ascending a poorly functioning auditory tube. Multiple studies implicate the latter mechanism (see later).







Migration along Vascular or Neural Pathways: Branches of the caudal auricular artery and the facial nerve traverse within the middle ear and have the potential to serve as pathways for spread of microorganisms and neoplasms from the brain to the middle ear. This migratory process likely occurs via the extracellular matrix of arteries and nerves (see Chapters 3, 10, and 14) and via anterograde axonal transport in nerves (see Chapter 14). As an example, cranial nerve sheath tumors (see Chapter 14) in the brain have been reported to migrate along branches of the facial nerve and vestibulocochlear nerves and enter the ear via the internal acoustic meatus.



Internal Ear



Extension from the Middle Ear: Extension from the middle ear is a portal of entry into the internal ear, thus otitis interna or labyrinthitis is most commonly thought of as occurring by direct extension from an infection of the middle ear. The most likely portal is the round or cochlear window. Based on studies in cats, the permeability of the round window is increased for elements (sodium) and macromolecules (tritiated albumin) during mild cases of experimentally induced otitis media. Penetration through the oval or vestibular window is less likely because of the annular syndesmosis that is formed between the petrous portion of the temporal bone and the stapes (Fig. 20-24). Although otitis media may be diagnosed in isolation, otitis interna is rarely diagnosed without concurrent otitis media.






Responses To Injury


The ear’s responses to injury are listed in Box 20-2.




External Ear


The external ear is an extension of the integument, and it responds to inflammatory stimuli similarly. All of the hallmarks of inflammation occur in otitis externa. Initially, there is reddening and warmth of the affected auricle associated with otitis externa caused by vascular dilatation and hyperemia. Transudation of fluid out of leaky vessels leads to edema affecting both the auricle (see Fig. 17-13) and external acoustic meatus. Edema within the tissues results in swelling of the tissues and discomfort when the tissues are touched. As the inflammatory response progresses, the transudate becomes an exudate, infiltrating the dermis of the external ear. Epithelial and adnexal changes, described later, result in further expansion of the external ear dermis. Eventually, the lumen of the external acoustic meatus may become so stenotic that hearing function becomes impaired.


As has already been described, the external acoustic meatus and auricle are lined by haired skin. Large, abundant, multiple, branching, and actively secreting sebaceous glands are most prominent in the deeper portions of the external acoustic meatus associated with hair follicles (see Fig. 20-5, A). Smaller, tubular, eccrine sweat glands, referred to as ceruminous glands, are located in the deeper layers of dermis. The epidermis, which is best studied in dogs, in response to inflammation, becomes hyperplastic and hyperkeratotic, although in some conditions it becomes ulcerated. Glandular changes include smaller, less abundant, less active sebaceous glands and more numerous, typically large, dilated ceruminous glands. Neutrophils, lymphocytes, and macrophages typically infiltrate the dermis, as well as the ectatic ceruminous glands (Fig. 20-25). Aggregates of lymphocytes form when the process is chronic. With increased chronicity, there is greater infiltration by fibroblasts and collagen, which can lead to more permanent stenotic changes. Finally, soft tissues, such as the dermis or auricular cartilage, may become ossified through metaplasia in the most chronically inflamed ears.



Auricles of lightly pigmented cats chronically exposed to ultraviolet (UV) light are prone to developing squamous cell carcinoma (described later in the chapter; also see Chapter 17). UVB light leads to cellular transformation of the epithelial cells leading to a clonal population of neoplastic squamous epithelial cells.



Middle Ear



Myringitis: Inflammation of the tympanic membrane is called myringitis and is most commonly caused by bacterial infection of the external or middle ear (see Fig. 20-39, B). Macroscopically, the tympanic membrane may be congested, hemorrhagic, or thickened, resulting in opacity. Microscopically, the tympanic membrane has all of the characteristics of acute inflammation and if the inciting cause is unresolved results in chronic inflammation (see Chapter 3). Prolonged and severe myringitis may lead to perforation of the tympanic membrane and prevent its healing.



Healing of the Tympanic Membrane: The tympanic membrane is a regionally well-vascularized membrane that has an air-interface along each surface. It can be perforated by traumatic injury, sudden exposures to high pressures, degradative enzymes and pressures from acute and chronic inflammation, chronic mite infestations, and neoplasms. Unique to the tympanic membrane, and likely related to its function, is its inherent ability to heal rapidly while maintaining its thin structure during healing through a process called epithelial migration. Unlike most other tissues, in which granulation tissue forms and bridges the defect followed by reepithelialization, the tympanic membrane closes the defect, first with migrating epithelial cells followed by a granulation tissue response that closes the mesenchymal portion of the tympanic membrane.


Within minutes to hours of the initial perforation, injured tissue at the edge of the perforation initiates an acute inflammatory response. Within hours, epithelial cells proliferate initially along the annulus and along the margin of the manubrium, corresponding to areas in which the blood supply is most prominent and epithelial stem cells of the tympanum are thought to reside. Proliferating epithelium advances toward the perforation using keratin as a scaffolding to initially bridge and reepithelialize the defect. As this epithelium is migrating across the defect, granulation tissue forms on the inner edge of the perforated membrane. Anchored to the epithelium that initially closes the perforation, the middle, mesenchymal layer of the tympanic membrane is repaired by this advancing granulation tissue. As granulation tissue remodels to its normal thin layer, the inner epithelial layer finally bridges the defect to complete the healing process. In an experimental animal model, 2.5-mm perforations were completely healed by 9 days after injury.



Goblet Cell Metaplasia and Impaired Mucociliary Clearance: An important defense mechanism of the middle ear is the mucociliary apparatus of the auditory tube and contiguous tympanic cavity. Chronic otitis media causes a decrease in the number of ciliated cells (ciliary atrophy) in the mucosa of the auditory tube. Studies in cats determined that neutrophil lysate (likely consisting of degradative enzymes) or lipopolysaccharide did not significantly reduce the number of ciliated cells in the auditory tube, but auditory tube obstruction and likely increased pressure in the middle ear resulted in a dramatic decrease in the number of ciliated epithelial cells. In the same study, there was a marked increase in the total number of goblet cells lining the tympanic cavity and bulla. It was hypothesized that obstruction of the auditory tube elevated partial pressure of carbon dioxide (pCO2) in the mucus layer of the mucosa, triggering mucosal stem cells to differentiate toward goblet cells rather than ciliated cells. The viscoelasticity of the mucus produced by these goblet cells was greater than normal and resulted in a marked decrease of mucociliary clearance of the middle ear, probably further contributing to auditory tube obstruction, increased pressure in the middle era, and metaplasia of ciliated cells to goblet cells.



Osteosclerosis of the Tympanic Bulla: In infections of the middle ear caused by microorganisms, mediators of acute and chronic inflammation, such as cytokines and degradative enzymes, may lead to excessive periosteal proliferation of new bone and osteosclerosis (thickening) of the wall of the tympanic bulla (Fig. 20-26; see Chapter 16). The bulla may become grossly distorted, and the lumen volume typically decreases. Inflammation also can contribute to bone lysis.





Horner’s Syndrome/Pourfour Du Petit Syndrome: Postganglionic sympathetic fibers that innervate the eye course through the middle ear with branches of the internal carotid artery via the tympano-occipital fissure. Smooth muscle that lifts the upper eye, retractor muscles of the third eyelid, smooth muscle that tonically pulls the eyes rostrally, and dilator muscles of the pupil are all innervated by these fibers. Thus, in otitis media, cells and mediators of inflammation can act on postganglionic sympathetic fibers that innervate the eye as they traverse through the middle ear, leading to primary demyelination and axonal injury.


Most commonly, injury resulting in partial paralysis of postganglionic sympathetic fibers causes Horner’s syndrome. This syndrome has the following clinical signs: miosis, enophthalmos, ptosis, a narrowed palpebral aperture, protrusion of the third eyelid, and peripheral vasodilation of the skin of the face. Resolution of signs typically follows clearance of the cause.


Injury to the postganglionic sympathetic fibers can much less commonly result in hyperexcitability or hyperirritability. This is most commonly reported in cats that have had their ears examined, sampled, and flushed while under anesthesia. Clinical signs include mydriasis, exophthalmos, widening of the palpebral aperture and cool skin over the face. In all cases reported, clinical signs resolved spontaneously. This hyperirritability syndrome is the opposite of Horner’s syndrome and is referred to by some as the Pourfour du Petit syndrome.



Internal Ear





Defense Mechanisms


Defense mechanisms of the ear are listed in Box 20-3.




External Ear





Adnexa and Cerumen: Cerumen is an oily emulsion that coats and protects the integument of the external acoustic meatus. Its naturally hydrophobic properties make it an important barrier to the entry of excessive moisture into the epidermal cells or underlying dermis. In normal ears of dogs, cerumen is made of sloughed superficial squamous cells mixed with ceruminous and sebaceous gland secretions. There is a high lipid content in cerumen made up of neutral lipids. In otitic ears, the cerumen changes because ceruminous glands are typically more numerous and active during periods of inflammation and thus contribute more to the content of cerumen (see Fig. 20-25). One consequence is a decrease in the lipid content, a decrease in hydrophobicity, and impairment of a natural barrier.


Cerumen in otitic ears becomes more acidic than normal, which is believed to impair bacterial growth. This outcome is due to greater contribution by ceruminous glands during periods of external ear inflammation. The pH of the external acoustic meatus is variable in dogs, ranging from 4.6 to 7.2. During periods of inflammation, the mean pH in acute otitis externa may decrease, whereas the mean pH in chronic otitis externa tends to increase. The mechanistic consequences of these changes are poorly understood.


Immunoglobulins IgA, IgG, and IgM have been identified in canine cerumen; however, the predominant immunoglobulin is IgG. This high level of IgG is believed to be related to transudation of serum in the inflamed external ear. Lysozyme and interleukins have also been identified in cerumen and provide an antimicrobial function.



Commensal Organisms: A mixture of bacteria and yeast normally populates the external acoustic meatus. Various species of “potentially pathogenic” microorganisms have been cultured from external ears of dogs with no evidence of disease. They include Bacillus spp., Corynebacterium spp., Escherichia coli, Micrococcus spp., Staphylococcus spp., Streptococcus spp., and yeast organisms, most commonly Malassezia spp. Rarely, Pseudomonas spp. and Proteus spp. have been isolated. Based on a single study of horses, Corynebacterium spp. and Staphylococcus intermedius were isolated from normal ears. Presumably, these organisms live symbiotically and defense mechanisms limit or prevent proliferation of a monoculture. However, the spectrum of microorganisms cultured from diseased external ears closely mirrors this list.




Middle Ear




Surfactant: Surfactant in the lung has long been recognized for its central role in reducing surface tension in alveoli, allowing them to expand and collapse normally during inspiration and expiration, respectively (see Chapter 9). However, recently, it has been shown that surfactant has other roles, especially in the function of the auditory tube. Surfactant is a complex mixture made up of 90% lipid and phospholipid and 10% surfactant proteins, designated surfactant protein-A (SP-A), SP-B, SP-C, and SP-D. Cuboidal epithelial cells lining the auditory tube are the source of surfactant and contain apical secretory granules analogous to what are seen in type II pneumocytes of the lung. One major difference between pulmonary surfactant and auditory tube surfactant is the ratio of phosphatidylcholine to sphingomyelin. Pulmonary surfactant has a ratio of 67 to 1, whereas auditory tube surfactant has a ratio of 2 to 1. This dramatic difference in auditory tube surfactant phospholipid content is reflected in a reduced ability to modify surface tension.


The most abundant surfactant protein in lung surfactant is SP-B, a strongly hydrophobic protein with high surface activity. However, auditory tube surfactant has a paucity of SP-B and relative abundance of SP-A and SP-D, two highly hydrophilic surfactant proteins. This difference may indicate a different function for auditory tube surfactant, namely, acting as a release agent and anti-adhesive rather than a surface tension modifying substance. Anti-adhesive properties would facilitate opening of the auditory tube.


Surfactant proteins are collectins that have two domains: (1) a lectin moiety that binds to the surface of foreign substances and organisms and (2) a collagenous domain that functions as a ligand for phagocytosis and complement activation (see Chapter 3). Finally, surfactant is believed to play a protective role against free radicals by early termination of free radical propagation, and protecting against oxidative injury. A lipid-binding pocket in surfactant proteins may prevent free radicals from propagating.

Sep 17, 2016 | Posted by in GENERAL | Comments Off on The Ear and Eye
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