Structure and Function of the Ear in Dogs
Dr. Bari Spielman
External ear. Because the pinna is covered in skin, generalized skin diseases can also involve the pinna. Such diseases include parasite infestations (mange), allergic skin disease and immune-mediated skin diseases. The pinnae are also exposed to the environment and can be afflicted with sunburn, frostbite and insect bites, or they can experience various forms of injury or trauma. Inflammation and infection of the externa ear canal is called external otitis. Causes of external otitis include parasites (e.g. ear mites), bacterial and fungal infections, allergies and other skin diseases, and tumors of the glands of the canal. Self-trauma to the pinna from scratching at the ear can result in bleeding between the cartilage and the skin of the pinna. Blood may collect in a pocket or swelling along the inside of the pinna and is a called an aural hematoma.
What Are Common Diseases of the Ear?
Middle ear. The most common disease of the middle ear is inflammation or infection, called otitis media. Otitis media may develop as an extension of otitis externa through a ruptured eardrum, or it may travel up the eustachian tube from the pharynx. Inflammatory polyps (benign growths of soft tissue) of the middle ear occasionally occur in the dog, but are less common than in the cat.
Inner ear. Otitis interna is infection and/or inflammation of the inner ear. It often arises from extension of infection from the middle ear. Bacteria and fungal agents are the most common causes of infections in the inner ear. Certain inflammations of the inner ear may arise without infection and can lead to loss of balance and varying degrees of deafness. Deafness is an uncommon problem in cats, but may be present as a congenital birth defect in white cats with blue eyes. Deafness may also develop secondary to certain drug toxicities, chronic infection, trauma, tumors or aging.
What Types of Diagnostic Tests Are Used to Evaluate the Ear?
For external ear disease a visual examination and special laboratory tests are important. The ear canals are examined using an otoscope. In most animals, an otoscopic examination can be performed in the exam room without sedation. If the ear is quite diseased and painful, however, the animal may require heavy sedation or even anesthesia to allow complete examination.
Samples of ear discharge are retrieved with a cotton swab and examined under a microscope (cytology) for yeast, ear mites, and bacteria. Skin scrapings of the external pinna may also be examined for parasites.
Bacterial and/or fungal culture and sensitivity tests are often indicated when infection is suspected.
Certain laboratory tests such as a blood count, biochemistry profile, thyroid tests, and allergy tests may also be indicated when external ear disease is found. These tests help to identify the presence of skin and systemic diseases that may affect the ear.
For middle and inner ear disorders, further diagnostic tests are indicated and are usually performed under anesthesia. A thorough otoscopic exam that includes visualization of the tympanic membrane (eardrum) is important. Radiographs (X-rays) of the skull are used to assess the tympanic bullae. Advanced imaging with computed tomography (CT) scan/magnetic resonance imaging (MRI) may also be helpful.
A myringotomy is a procedure whereby fluid is retrieved from the middle ear by inserting a small needle through the eardrum. This procedure is most often used when infection or inflammation of the middle ear is suspected based upon x-ray, CT or MRI findings. The fluid is examined under the microscope and submitted for culture.
Surgical exploration of the tympanic bulla is sometimes necessary if other diagnostic tests fail to reveal the underlying problem or if polyps or tumors are suspected. Any masses found are submitted for a biopsy.
When neurologic signs are also present, a complete neurologic examination is undertaken. If neurologic abnormalities of the cranial nerves (nerves of the head) are found, then a spinal tap (analysis of cerebrospinal fluid) may be helpful.
Sophisticated electrodiagnostic tests are available at specialized referral practices to assess functions of the middle and inner ear, as well as to detect deafness. Such tests include impedance audiometry and brainstem auditory-evoked responses. Your veterinarian may refer your dog to a veterinary neurologist for further specialized testing.