Normal Anatomy and Physiology
The middle ear consists of the tympanic membrane, three cavities (epitympanic, tympanic, and ventral), and the bony ossicles (malleus, incus, and stapes). The tympanic membrane has two parts: (1) the thin pars tensa that attaches to the manubrium of the malleus and (2), above the pars tensa, the thicker, pars flaccida. The main portion of the middle ear, the ventral tympanic bulla, has two compartments in the cat (ventromedial and dorsolateral). The air-filled bulla is lined with modified respiratory epithelium, which is either squamous or cuboidal and may be ciliated. The four openings in the middle ear are the (1) tympanic opening, (2) the vestibu-lar window, (3) the cochlear window, and (4) the ostium of the auditory tube. The auditory tube is the communication between the middle ear and caudal nasopharynx. The normal flora of the middle ear may be due to this pharyngeal communication, but the role of the auditory tube as a source of bacteria in otitis media is unknown. The tympanic opening is a common source of bacterial infection of the middle ear in dogs with otitis extema. The cochlear and vestibular windows are possible ports of entry for progression of otitis media or ototoxic substances into the inner ear.
Cranial nerve VII, or the facial nerve, the sympathetic innervation of the eye, and the parasympathetic innervation of the lacrimal gland are closely associated with the middle ear. The separation of the facial nerve from the middle ear is minimal along the rostral aspect of its course through the petrosal bone. The nerve supplies motor fibers to the superficial muscles of the head, the muscles of the external ear, the caudal belly of the digastricus, and the ossicular muscles. The nerve also supplies sensation of the vertical ear canal and concave surface of the pinna.
Postganglionic sympathetic nerve fibers course closely with those of the facial nerve to innervate the smooth muscles of the eye. Preganglionic parasympathetic fibers also pass through the middle ear to innervate the salivary and lacrimal glands.
The inner ear is located within the petrosal bone. The cochlea, vestibule (saccule and utricle), and semicircular canals form the membranous labyrinth, which is encased in bone, called the bony labyrinth. The vestibular system functions to maintain the position of the eyes, trunk, and limbs relative to the position of the head, responding to linear and rotational acceleration and tilting. The system consists of the saccule, utriculus, and semicircular canals and communicates with the middle ear via the vestibular window. Fluid within the semicircular canals tends to remain stationary during motion, bending the cilia of the cells in the utricle and saccule, causing depolarization. These stereocilia synapse with the dendrites of the vestibular portion of the eighth cranial nerve and the signal is conducted via cranial nerve VIII to vestibular nuclei in the myelencephalon, the spinal cord, centers in the cerebellum and cerebral cortex, and motor nuclei of cranial nerves III, IV, and VI. The result is coordination of the body, head, and eye movement. Projections to the vomiting centers are responsible for nausea and vomiting associated with vestibular disorders and motion. The cochlear system, involved with the translation of sound, consists of the spiral organ, or organ of Corti, cochlear duct, scala vestibule, and scala tympani. Transmission of sound through the tympanic membrane, ossicles, and cochlear window results in undulation of the basilar membrane of the spiral organ. Cilia bend and cause depolarization and transmission of a signal to cochlear nuclei, caudal colliculi, and cerebral cortex. The cochlear nuclei control reflex regulation of sound via projections to cranial nerves V and VII, which control the muscles of the ossicles. Other projections allow for conscious perception of sound.
Neoplasia of the Middle Ear
Neoplasia of the middle ear is rare; most cases represent extension of tumors originating in the external ear canal.
Inflammatory polyps are a non-neoplastic admixture of inflammatory and epithelial cells originating in the tympanic bulla in cats. Other sites of origin include the auditory tube and nasopharynx. Macrophages, neutrophils, lymphocytes, plasma cells, and epithelial cells are usually present on histopathologic examination. The cause is unknown, but ascending infection and congenital causes have been suggested. No age or sex predilection exists for the condition, but younger cats are more commonly affected (1 to 5 years of age). Signs can be unilateral or bilateral and depend on the location of the mass lesion. A single polyp can grow into the external ear canal, down the auditory tube into the nasopharynx, or both. Signs of concurrent otitis extema and media are common with polyps limited to the ear, but respiratory stridor, dyspnea, gagging, and dysphagia occur with growth into the pharynx.
Diagnosis is based on otoscopic and pharyngeal examinations. Radiographs of the bulla, nasal cavity, and pharynx may be considered, and CT or MRI can be used to diagnose the site and side of origin of inflammatory polyps/ Treatment consists of excision by traction or surgical excision via ventral bulla osteotomy. Regrowth is a problem in half of the cats treated by traction extraction alone, and Homer’s syndrome is common in cats after ventral bulla osteotomy.
Otitis interna is usually an extension of otitis media or neoplasia of the middle ear. A careful neurologic examination is imperative to the localization of vestibular signs. Clinical signs associated with otitis interna include head tilt, ataxia, horizontal or rotary nystagmus, circling or falling toward the side of the lesion, or ipsilateral nystagmus. The fast phase of nystagmus is usually away from the side of the lesion. Occasionally, animals will become nauseated or vomit. Homer’s syndrome or deficits in cranial nerve VII may accompany otitis media interna, but involvement of other cranial nerves, vertical or changing nystagmus, or the presence of conscious proprioceptive deficits or paresis indicate central rather than peripheral vestibular disease. Bilateral peripheral vestibular disease is rare, but the animal will not have a head tilt, nystagmus, or strabismus and may exhibit wide head excursions and a crouched stance or the inability to stand.
The diagnosis of otitis interna is based on history, clinical signs, and physical, neurological, and otoscopic examinations. Advanced imaging may be helpful in distinguishing the anatomic location of the disease process. Treatment with aggressive medical or surgical intervention appropriate to the localization is important in prevention of adjacent brain stem involvement.
Prognosis for Otitis Media and Interna
A fair prognosis can be given if aggressive surgical and medical therapy are possible. Cases with concurrent severe external ear canal changes require total ear canal ablation and lateral bulla osteotomy. Repeated infections after ventral bulla osteotomy or total ear canal ablation and lateral bulla osteotomy may be operated again with resolution of the condition. Resistant organisms, failure to respond to aggressive surgery, and significant osteomyelitis are associated with a poor prognosis. The neurologic signs associated with otitis media and interna may be permanent, but many animals learn to use visual cues and can compensate for vestibular deficits. Facial nerve deficits, Horner’s syndrome, and keratoconjunctivitis sicca are often permanent.
Ototoxic substances (Table Ototoxic Drugs) damage the cochlear or vestibular systems or both. Otic application of medication can also cause adverse effects through local inflammation of the tympanic membrane or the meatal window (or both), as well as resultant otitis media. Topical medications also cause adverse effects by systemic absorption. Ototoxic substances reach the inner ear after local application and absorption through the cochlear or vestibular windows or hematogenously. The most frequent cause of ototoxicity is the application of an ototoxic substance to the external ear canal in a pet with a ruptured tympanum, which results in distribution to the middle ear. Absorption by the inner ear is increased when inflammation of the cochlear window occurs with otitis media. Hematogenous distribution of otoioxins to the inner ear is inherent in some medications (e.g. aminoglycosides).
|Dihydrostreptomycin||Iodine & iodophores|
|Polymixin B & E||Nitrogen mustard|
The development of ototoxicity also depends on the vehicle of the preparation, chemical composition, drug concentration, concurrent medications, as well as the route, frequency, and duration of administration. Examples of increased risk of ototoxicity depending on the vehicle (e.g. combination of chlorhexidine and detergents) and concurrent medications (e.g. loop diuretics and aminoglycosides) have been described. Minimization of the risk of toxicity should be considered when any potentially toxic substance is administered either topically or systemically. The integrity of the tympanic membrane should be known prior to topical administration of any potentially ototoxic drug, and consequences of each drug should be considered in light of the animal’s health and concurrent therapies.
Idiopathic Vestibular and Facial Nerve Diseases
A complete neurologic examination is key to differentiating peripheral from central vestibular disorders. Head tilt, ataxia, horizontal or rotary nystagmus, and cranial nerve VII deficits may be seen with either condition. Central vestibular disease causes paraparesis, conscious proprioceptive deficits, other cranial nerve abnormalities, and vertical or changing nystagmus. Middle ear neoplasia, otitis media interna, idiopathic vestibular syndrome, and congenital vestibular disorders result in peripheral vestibular signs. Congenital vestibular disorders have been described in the German shepherd, Doberman pinscher, English cocker spaniel, Siamese, and Burmese breeds. Bilateral congenital vestibular syndrome has been described in beagles and Akitas. Clinical signs of head tilt and ataxia in these dogs and cats may be persistent or may improve; animals can be congenitally deaf.
Otitis media interna may be associated with facial paresis or paralysis if cranial nerve VII is affected by the inflammation. Otitis should be ruled out before diagnosing any animal with idiopathic facial nerve paralysis, because otitis requires aggressive management and the idiopathic condition can only be treated symptomatically or with acupuncture.
Acquired Late-Onset Conductive Deafness
Conductive deafness is due to lack of transmission of sound through the tympanic membrane and ossicles to the inner ear. Conditions that block sound transmission through the external ear canal, tympanic membrane, or middle ear and ossicles, such as otitis externa, otitis media, and otic neoplasia, cause conductive deafness. Less common causes of conductive deafness include trauma-induced fluid accumulation in the middle ear, atresia of the tympanum or ossicles, fused ossicles, or incomplete development of the external ear canal, which results in fluid accumulation in the middle ear. An increase in hearing threshold, absence of air-conducted hearing, and the presence of bone-conducted hearing on BAER suggest conductive deafness. The application of a bone-anchored hearing aid was described in one dog with conductive deafness after total ear canal ablation. It maintained bone-conducted hearing and tolerated the hearing aid anchored to the parietal bone Use of a bone-anchored device was required, because the dog did not have an external ear canal in which to place an earpiece. The hearing aid acted as an amplifier, and the dog seemed to respond to its use.
Acquired Late-Onset Sensorineural Deafness
Presbycusis, or decline in hearing associated with aging, may be due to one of the following: loss of hair cells and degeneration in the organ of Corti, degeneration of spiral ganglion cells or neural fibers of the cochlear nerve, atrophy of the stria vascularis, or changes in the basilar membrane. Because this condition occurs in older dogs and cats from 8 to 17 years of age, animals should be evaluated for concurrent causes of conductive deafness such as chronic otitis extema or media and otic neoplasia. BAER testing may demonstrate normal waveforms in response to high-intensity sound. If conduction is intact at an increased hearing threshold, use of an amplifying hearing aid may be beneficial. Pets may not tolerate occlusive types of ear pieces often used in hearing aids, and training to the ear piece should be done prior to application of the hearing aid.
Ototoxic substances, chronic exposure to loud noise, hypothyroidism, trauma, and bony neoplasia can also cause acquired late-onset deafness in dogs and cats. Ototoxicity can result in abolition of waveforms or an increase in hearing threshold on BAER. BAER testing can be used to re-evaluate patients for return of function after withdrawal of medication after exposure to ototoxic medication.
Congenital Sensorineural Deafness
Inherited sensorineural deafness usually results in complete loss of hearing in the affected ear by 5 weeks of age. Many breeds can be affected with the condition (Box Canine Breeds Associated with Inherited Deafness). The condition has been linked to coat color in many breeds of dogs and white cats. The condition is common in white cats, and mode of inheritance is thought to be autosomal dominant with incomplete penetrance.The condition is most common in white cats with blue irides. The correlation of white coat, blue eyes, and deafness is not perfect, but cats with two blue irides have a greater risk of deafness than cats with one blue iris, which have a greater risk of deafness than cats without blue irides. Total hearing loss occurs more often in longhaired white cats. The condition is common in certain breeds of dogs, such as dalmatians, which have a nearly 30% incidence of deafness (combining unilateral and bilateral deafness).
Canine Breeds Associated with Inherited Deafness
|American-Canadian shepherd||Italian greyhound|
|American cocker spaniel||Jack Russell terrier|
|American Staffordshire terrier||Labrador retriever|
|Australian cattle dog||Maltese|
|Australian shepherd||Miniature pincer|
|Beagle||Miniature poodle mongrel|
|Bichon frise||Norwegian dunkerhound|
|Border collie||Nova Scotia duck tolling retriever|
|Borzoi||Old English sheepdog|
|Boxer||Pit bull terrier|
|Bull terrier||Poodle (toy & miniature)|
|Catahoula leopard dog||Puli|
|Doberman pincer||Sealyham terrier|
|Dogo Argentino||Shetland sheepdog|
|English cocker spaniel||Shropshire terrier|
|English setter||Soft-coated Wheaton terrier|
|Fox terrier||Sussex spaniel|
|French bulldog||Tibetan spaniel|
|German shepherd||Tibetan terrier|
|Great Dane||Walker American foxhound|
|Great Pyrenees||West Highland white terrier|
The trait is associated with the dominant merle or dapple gene in collies, Shetland sheepdogs, Great Danes, and dachshunds. The incidence of deafness tends to increase with increasing amount of white in the coat, and dogs homozygous for the merle gene are usually deaf and may be solid white, blind, or sterile, The piebald or extreme piebald gene is associated with deafness in dalmatians, bull terriers, Great Pyrenees, Sealyham terriers, greyhounds, bulldogs, and beagles. Inheritance is thought to be autosomal recessive, but the trait may be polygenic.
Heterochromia irides and lack of retinal pigment are associated with white color in dogs and cats. Hearing loss may be associated with absence of pigment in the cochlear stria vascularis. Diminished blood supply and disorders of endolymph production, with changes in the chemical or mechanical properties of endolymph, lead to degeneration of the organ of Corti secondary to stria vascularis atrophy. Loss of hair cells and abnormalities of the cochlear duct, Reissner membrane, tectorial membrane, and internal spiral suicus are typical of cochleosaccular type of end-organ degeneration seen in these cases.,
Clinical signs of deafness may be recognized in puppies as young as 3 weeks of age by astute owners; definitive diagnosis of uni- or bilateral deafness is usually made by BAER testing at 5 to 6 weeks of age when the auditory system is completely developed and cochlear degeneration, if present, is complete.
Congenital Acquired Sensorineural Deafness Exposure to bacteria, ototoxic drugs, low oxygen tension, and trauma in utero or during the perinatal period rarely causes deafness in young animals.