Below is information about the structure and function of the canine ear. We will tell you about the general structure of the ear, how the ear works in dogs, common diseases that affect the ear and common diagnostic tests performed in dogs to evaluate the ear.
What Is the Ear?
The ears are the paired receptor organs designed for the special senses of hearing and maintaining balance. Each ear is divided anatomically and functionally into regions called the external (outside) ear, the middle ear and the inner ear.
Where Is the Ear Located on a Dog?
The ears are located on both sides of the head. The external ear is identified by the visible portion of the ear, called the pinna. The pinnae of dogs vary greatly in their size and shape. The pinnae of some breeds are short and erect, while other breeds have long, floppy pinnae.
What Is the General Structure of the Canine Ear?
The ear is divided into three portions:
The external ear consists of the prominent earflap or pinna (also called the auricle) and the external ear canal (also called the auditory canal or meatus). The pinna is a funnel-shaped structure that collects sound and directs it into the external ear canal. The pinna is covered by skin, and the outer or posterior aspect is covered by fur. Numerous muscles are attached to the curved cartilage located between the inner and outer layers of skin around the ear, and these muscles allow the pinna to move and twitch. The external ear canal extends from the base of the pinna downward and inward towards the eardrum (also called the tympanic membrane). The external ear canal is L-shaped, with the L lying on its side. The canal forms an almost 90-degree angle between its two sections: the short, vertical outer section and the longer, horizontal inner section.
The middle ear includes the eardrum and the bony tympanic cavity (osseous bulla), which lies just past the ear drum. Within this tympanic cavity are found the auditory ossicles – three tiny bones that vibrate when stimulated by sound waves. These ossicles are named the malleus, stapes and incus (commonly known as the hammer, the stirrup and the anvil because of their resemblance to these objects). These three bones form a chain across the middle ear from the tympanum to the oval window of the inner ear. The middle ear is connected to the back of the throat (pharynx) by the auditory or eustachian tube. This tube allows air from the pharynx to pass in and out of the middle ear, which helps keep middle ear pressure normal. The middle ear is connected to the inner ear through the oval window, which lies against the stapes bone.
The inner ear is located within the petrous temporal bone of the skull and consists of two parts. The osseous or bony labyrinth houses a series of thin, fluid-filled membranes called the labyrinth. The inner ear contains three distinct structures: the cochlea (spiral tube), vestibule, and three semicircular canals. The cochlea contains the nerves that transmit the electrical impulses and is directly responsible for hearing. The vestibule and semicircular canals are responsible for maintaining balance or equilibrium. These tissues are supplied by the two branches of the 8th cranial nerve (the vestiblocochlear nerve), which transmits electrical impulses related to sound and balance back to the brain.
What Are the Functions of the Ear in Dogs?
The two main functions of the ear are to detect sound and allow for hearing, and to maintain balance.
Hearing. Sound first enters the external ear canal as sound waves. As these waves strike the eardrum, it begins to vibrate. These vibrations are then transmitted to the three small bones of the middle ear (the malleus, incus and stapes), which amplify the sound vibration. The end of the stapes is connected to the oval window of the inner ear. As the stapes vibrates, it transmits the sound vibrations to the cochlea, the snail shaped portion of the inner ear, which transforms the vibrations into nerve signals that are transmitted to the brain where they are interpreted as sound.
Balance. The other function of the ear is to help maintain balance. The three semicircular canals of the inner ear are oriented at right angles to each other. When the head turns, the resulting movement of fluid in these canals allows the brain to detect which way and how much the head is turning. Another part of the inner ear responds to gravity and sends information to the brain when the head is held still in a stationary position.
What Are Common Diseases of the Ear in Dogs?
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.
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.