Overview of Otitis Media and Interna
Otitis media, inflammation of the middle ear structures, is seen in small and large domestic animals, including dogs, cats, rabbits, ruminants, horses, pigs, and camelids. It can be unilateral or bilateral and can affect animals of all ages. Although typically sporadic, outbreaks are possible in herds. Otitis media usually results from extension of infection from the external ear canal through the tympanic membrane or from migration of pharyngeal microorganisms through the auditory tube. Occasionally, infection extends from the inner ear to the middle ear, or reaches the middle ear by the hematogenous route. Primary otitis media has been reported in certain breeds of dogs, particularly Cavalier King Charles Spaniels. Untreated otitis media can lead to otitis interna (inflammation of the inner ear structures) or to rupture of an intact tympanic membrane with subsequent otorrhea or otitis externa.
Signs of otitis media include head shaking, rubbing or scratching the affected ear, and tilting or rotating the head toward the affected side; self-trauma can lead to aural hematoma. The most common cause of recurrent otitis externa is undiagnosed otitis media. When otitis externa (see Otitis Externa) accompanies otitis media, the external ear canal may look inflamed and contain an abnormal discharge. The pinna or ear canal may be painful and malodorous, and the hair surrounding the base of the ear may be wet or matted. Because the facial (cranial nerve VII) and sympathetic nerves course through the middle ear, animals with otitis media may exhibit signs of facial nerve paralysis (eg, ear droop, lip droop, ptosis, collapse of the nostril) and/or Horner syndrome (eg, miosis, ptosis, enophthalmos, protrusion of the nictitating membrane) on the same side as the affected ear. Exposure keratitis and corneal ulceration may develop. With facial paralysis, the nasal philtrum or lip may deviate away from the affected side. These signs help to distinguish otitis media from simple otitis externa.
With otitis interna, inflammation impairs function of the vestibulocochlear nerve (cranial nerve VIII), resulting in hearing loss and signs of peripheral vestibular disease such as head tilt, circling, leaning or falling toward the affected side, general incoordination, or spontaneous horizontal nystagmus with the fast phase away from the affected side. Extension of infection from the inner ear to the brain leads to meningitis, meningoencephalitis, or abscesses, with signs referable to those conditions. In horses, severe otitis media/interna can result in fusion and fracture of the tympanohyoid joint; extension of the fracture line to the calvarium can lead to intracranial spread of infection or cause hematoma and death.
Whereas animals with otitis media/interna are usually alert, nonfebrile, and have a good appetite, those with meningitis or meningoencephalitis are usually depressed, febrile, and inappetent. A major differential diagnosis for otitis media/interna in ruminants is listeriosis. However, in listeriosis, cranial nerves other than VII and VIII may be affected, causing signs such as dysphagia or loss of facial sensation, and affected animals are usually depressed.
In large animals, otitis media and interna are presumptively diagnosed based on history and clinical signs. A history of bottle feeding or feeding of contaminated milk to neonates, concurrent or previous respiratory disease, chronic ear infection, or aural foreign body, in conjunction with typical signs of otitis media/interna, should prompt examination of the ear canal. Otitis media is confirmed by visualizing a bulging, discolored, or ruptured tympanic membrane. Although the tympanic membrane may be visualized using a simple otoscope in many cases, the anatomy of the ear canal hinders visualization in some species, such as horses and llamas; endoscopy, or video otoscopy, is an alternative approach. Imaging methods assist in diagnosis and assessment of lesion severity. Radiography can detect osseous changes in the tympanic bulla and fluid in the tympanic cavity if appropriate positioning and techniques are used. However, CT and MRI are more sensitive and are the preferred methods when feasible. In some cases, diagnosis is made only at necropsy, using special techniques to expose the tympanic region. Diagnosis of clinical otitis media/interna in one ear should always prompt examination of the other ear to determine whether subclinical otitis is present.
Otitis media has been reported to be present in 16% of dogs with otitis externa and in >50% of dogs with chronic otitis externa. The tympanic membrane has also been reported to be intact in >70% of cases, and the disease is usually bilateral in dogs. Primary and secondary causes and factors of otitis externa can lead to otitis media. Diagnosis can be challenging, because the tympanic membrane can be intact. Palpation of the tympanic membrane with a blunt instrument is not an accurate method of determining the patency of the tympanic membrane. Advanced imaging techniques (CT or MRI) are more sensitive than routine radiographs but are not 100% sensitive and specific. In some cases, exploratory (and therapeutic) bulla osteotomy may be necessary.
Treatment of otitis media/interna is most successful when started early in the course of the disease. Chronic cases are often refractory to treatment or recur after apparent remission. When otitis externa accompanies otitis media/interna, the ear should be examined closely for mites and foreign bodies, such as plant awns, and the discharge cultured for bacteria. Many aerobic and anaerobic bacteria have been cultured from the ears of animals with otitis media/interna, and mixed infections are common. Pathogens that warrant mentioning because of their frequency of isolation include Malassezia spp and Pseudomonas spp in small animals; Streptococcus suis in pigs; Streptococcus spp in horses; Mycoplasma spp in goats; and Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis in cattle. M bovis is particularly problematic in dairy calves fed unpasteurized waste milk from cows with intramammary infection. However, other pathogens, such as coliform bacteria, Staphylococcus spp, Neisseria spp, corynebacteria, and Trueperella pyogenes are frequently isolated from the ears of affected animals. Isolation of a bacterial pathogen(s) or mites from the ear helps direct initial treatment but does not necessarily imply causation of otitis media/interna, because the same organisms can be isolated from the external ear canals of apparently healthy animals.
Ear mites, when present, should be treated with an appropriate systemic antiparasitic agent (see Antiparasitics for Integumentary Disease). Topical acaricides can be instilled into the external ear canal once it is cleaned. Bacterial infection should be treated with appropriate systemic antimicrobial agents (see Antibacterials for Integumentary Disease), based on culture and susceptibility test results. If the tympanic membrane is intact, a culture can be obtained via a myringotomy incision with a tom cat catheter. It may be possible to aspirate fluid. If not, 0.5 mL of sterile water can be instilled into the bulla and the fluid aspirated for culture. No antimicrobial agents are labeled for treatment of otitis media/interna in food-producing animals in the USA, so extra-label drug use guidelines must be followed and prohibited drugs avoided.
In addition to antimicrobial and/or anthelmintic therapy, the external ear canal should be cleaned and flushed if otorrhea or otitis externa is present; physiologic saline or dilute antiseptic solutions, such as iodine, chlorhexidine, or hydrogen peroxide, are commonly used for flushing. Steroids or NSAIDs can help reduce inflammation and pain associated with otitis media/interna. Corneal ulceration, aural hematomas, and concurrent infections should be treated appropriately, if present, and the animal protected from further self-injury.
If the tympanic membrane is intact and otitis media/interna does not respond sufficiently to systemic antimicrobial and anti-inflammatory therapy, myringotomy (perforation of the tympanic membrane) can be performed to relieve pressure and enable culture and drainage of fluid from the tympanic cavity. In chronic, nonresponsive or recurrent cases of otitis media/interna, it may be necessary to perform bulla osteotomy or total ear canal ablation to establish sufficient drainage and enable effective lavage. Tympanostomy tubes can be implanted into the tympanic membrane after myringotomy to allow continuous drainage in Cavalier King Charles Spaniels with primary secretory otitis, but such tubes are not useful to drain more purulent exudate.
Early diagnosis and treatment of otitis media/interna can result in complete resolution of infection and clinical signs. However, with severe, chronic, or nonresponsive cases, owners should be advised that neurologic deficits and hearing loss may persist even if infection is resolved. In small animals, otitis media may be resolved only by surgery (total ear canal abalation), particularly if multidrug-resistant bacteria are present.