Chapter 18 Otitis Interna and Vestibular Disease

Otitis interna, or infection of the inner ear, is a relatively common disorder in the dog. It is usually a result of extension of middle ear infection (see Chapter 14) and results in a characteristic set of clinical signs. These signs reflect the dysfunction of the inner ear organs—namely, the cochlea and vestibular apparatus. Cochlear dysfunction manifests as decreased hearing acuity. This may be difficult to detect in the dog without electrodiagnostic testing, especially if it is unilateral. However, often the astute animal caregiver is able to detect the subtle signs of hearing loss in the animal’s natural environment. This information can be elucidated by careful questioning of the caregiver. The vestibular system is responsible for the detection of acceleration and orientation in respect to the earth’s gravitational field, and it is absolutely essential for an animal to be able to maintain normal balance and posture. It comprises components located in the bony labyrinth of the inner ear and nuclei located in the medulla oblongata.

Vestibular dysfunction results in a variety of readily visible clinical signs, including head tilt, small-radius circling, nystagmus, and strabismus. The primary concern of the examining veterinarian when presented with an animal with vestibular disease is to determine whether the problem lies within the bony labyrinth (peripheral vestibular disease [PVD]) or in the brainstem vestibular system (central vestibular disease [CVD]). This determination helps when forming a diagnostic differential list and also allows for some sense of prognostication. For the most part, CVD carries a grim prognosis, whereas PVD is often treatable and carries a much better prognosis.

Components of the Vestibulocochlear System

The peripheral vestibular apparatus lies within the bony labyrinth of the petrous temporal bone. Important structures of the labyrinth include the utriculus and saculus of the maculae, the semicircular canals, and the cochlea. The maculae detect the pull of gravity and linear acceleration, whereas the semicircular canals detect angular and rotational acceleration. The cochlea houses the cochlear membrane, which is the receptor organ for the sense of audition. Information from the vestibular and cochlear systems leaves the peripheral apparatus in the vestibulocochlear nerve (cranial nerve VIII), which terminates in the brainstem. Neurons emanating from the cochlear apparatus course into nuclei within the brainstem and eventually project to the cerebral cortex, where the auditory information is processed, resulting in the sense of audition. The majority of the neurons from the peripheral vestibular system synapse in one of the four divisions of the vestibular nuclei that are located on both sides of the medulla oblongata, adjacent to the fourth ventricle. The vestibular nerve also projects directly to the cerebellum, which in turn sends projections back to the vestibular nuclei. Numerous projections from the vestibular nuclei are sent to different areas within the central nervous system.

Projections from the vestibular nuclei to the motor nuclei of the oculomotor (CN III), trochlear (CN IV), and abducent (CN VI) nerves control reflex eye movement (Figure 18-1).

Figure 18-1 Vestibular control of eye movements. When the head rotates, the semicircular canals detect the direction and speed of the rotation. The vestibular nuclei integrate the information and direct the oculomotor (III), trochlear (IV), and abducens (VI) nuclei to move the eye in the opposite direction at the same speed. This maintains a fixed image on the retina even as the head moves.

(From O’Brien DP: Comparative neurology, Columbia, MO, 2003, University of Missouri.)

Rotation of the head in one direction results in movement of the eyes in the opposite direction at the same speed. This reflex maintains a fixed image on the retina as the head moves. If the head is still moving when the eyes reach the furthest possible excursion in the opposite direction, the area of the pons that controls quick eye movements flicks the eyes quickly in the direction of the head movement, and the drift in the opposite direction begins again. Disease of the vestibular system creates the false perception of rotation, which produces a spontaneous drifting of the eyes in one direction with a quick reset in the opposite direction (spontaneous nystagmus). Projections to a separate area of the brainstem (the emetic center) are responsible for the nausea that may accompany vestibular disease.

Projections from the vestibular nuclei to the spinal cord maintain balance and support against gravity. They facilitate the ipsilateral large extensor muscle groups in all four limbs and the muscles of the neck that support the head (Figure 18-2).

Figure 18-2 Vestibular control of posture. The vestibular nuclei give rise to the ipsilateral vestibulospinal tract. The medial vestibulospinal tract projects to the neck muscles supporting the head against gravity. The lateral vestibulospinal tract facilitates the extensor muscles of the ipsilateral limb. It also inhibits the ipsilateral flexor muscles and the contralateral extensor muscles.

(From O’Brien DP: Comparative neurology, Columbia, MO, 2003, University of Missouri.)

Unilateral loss of this facilitation with vestibular disease produces the clinical signs of head tilt, leaning, and rolling toward the side of the lesion. The vestibular nuclei also send projections to the cerebral cortex, which mediates the conscious perception of movement and gravity.

Diagnosis: Otitis Interna (Otitis Media/Interna)

Otitis interna is a common cause of vestibular disease in dogs. It is almost always a result of infection of the middle ear (otitis media), but it must be noted that otitis media, in and of itself, does not cause vestibular disease. It is the extension of the infection into the petrosal bone, which houses the vestibular organs, that causes the signs associated with vestibular disease. Therefore, animals with signs consistent with PVD need to be thoroughly examined for evidence of middle ear disease. If present, it can usually be assumed that an otitis media/interna is causing the clinical signs, and treatment can begin.

Neurologic Examination

The first step in diagnosing animals with vestibular disease is the neurologic examination. Although patients with vestibular disease are often difficult to examine because of the severe clinical signs, the veterinarian can often get vital clues as to the location of the animal’s problem by concentrating on certain aspects of the neurologic examination. Remember, the ultimate goal is to decide whether the animal is suffering from PVD or CVD. Animals with CVD have, by definition, brainstem dysfunction. Therefore, treating these animals for a presumptive diagnosis of otitis media/interna is ultimately unrewarding.

All animals with vestibular disease, regardless of whether they have PVD or CVD, have certain common clinical signs. They typically have a nystagmus, a strabismus that is more apparent when the animal is placed in dorsal recumbency (positional strabismus); a head tilt; a tendency to circle in one direction; and a generalized ataxia. Nystagmus, by convention, is named according to the direction of the fast phase of the ocular movement. For example, an animal with a nystagmus characterized by horizontal movement of the pupil with the fast phase to the right has a “right horizontal nystagmus.” It is important to note that the fast phase is typically away from the side of the lesion; therefore the animal in the example above is likely to have a left-sided vestibular lesion. The head tilt, again by convention, is named according to the side of the head that is lower. In most cases the head tilt is directed toward the side of the lesion. The head tilt is often accompanied by a tendency to lean toward the side of the lesion. In severe cases the dog may roll toward the lesion. Circling, if present, should be described according to the direction of the circle (e.g., left circling) and by the radius of the circle (tight versus broad). Animals with vestibular dysfunction typically present with tight circles and circle toward the side of the lesion.

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