Clinical Vignette
Sam, a 13-year-old M/C shepherd cross, is presented for an acute onset of stumbling, falling, and leaning to the left. He was normal at 7:30 in the morning and then at noon the owner found Sam having difficulty rising. Sam had been confined to the house during the morning. According to the owner, when Sam tried to walk he appeared to support himself well but veered to the left when walking. On presentation Sam was ambulatory with good strength but had generalized ataxia and a base wide stance. He had (1) a left head tilt, (2) would lean, fall, and veer to the left when gaiting, (3) had a horizontal nystagmus with the fast phase to the right, (4) normal mentation, (5) good postural test reactions on both sides, (6) normal myotatic reflexes, and (7) the remainder of the cranial nerve examination was normal. This history fits best with which categories of disease? The neurologic examination is most consistent with a lesion localized to which part of the nervous system and why? What are your differential diagnoses for Sam? What tests would you select and why or, would you do any tests at all?
Problem Definition and Recognition
Head tilt is a postural deficit of the head and neck and is most commonly caused by an abnormality of the vestibular system. The head is twisted to one side on its long axis so that one ear is lower than the other; this is called a “roll.” It should be distinguished from a turning of the head and neck, where the head remains parallel to the ground (a “yaw,”), which is usually caused by spasms of the cervical muscles or by brain stem disease. To help distinguish between these two head posture changes, it is useful to observe the patient’s head from straight in front or by viewing the head from above. Also, animals with head tilt due to vestibular disease frequently have asymmetric ataxia and nystagmus. Animals with acute external or middle ear disease may tilt their head toward the affected side. In these patients though, there are no signs of ataxia. Facial nerve dysfunction or Horner’s syndrome may be present if middle ear disease is causing the head tilt but there will be no vestibular signs or other neurologic deficits in this instance. So although vestibular disease is by far the most common cause of head tilt, the clinician should understand that it is not an exclusive vestibular sign.
Pathophysiology
Head tilt is caused by an imbalance of tone in the cervical spinal muscles, which are controlled in part by the vestibular system. The vestibular system (Fig. 43-1) includes the receptors in the inner ear, which detect changes in position of the head with respect to gravity and acceleration of the head, the vestibular nerve (cranial nerve VIII), the vestibular nuclei in the brain stem (of which there are four on each side), the flocculonodular lobes of the cerebellum, and ascending and descending sensory pathways for the recognition of trunk, limb, and head position (de Lahunta and Glass 2009). The vestibular system provides information that allows the animal to maintain a normal posture of the head, trunk, and limbs, and position of the eyes relative to the movement of the head at any time. Part of this is accomplished by the vestibulospinal tract, which provides tonic stimulation to the extensor motor neurons of the ipsilateral limbs. The vestibular system also assists in control of some types of eye movements by way of connections to the medial longitudinal fasciculus. The medial longitudinal fasciculus then connects to the nuclei of cranial nerves III, IV, and VI for control of eye position and the extraocular muscles. This is the neuro anatomic basis for physiologic nystagmus, the fast and slow movements of the eyes when the head is turned from side to side.
FIGURE 43-1. Schematic anatomy of the vestibular system. III, oculomotor nucleus; IV, trochlear nucleus; VI, abducent nucleus; VII, facial nucleus; C, cranial nerve VIII—cochlear portion; V, cranial nerve VIII—vestibular portion; F, flocculus; FN, fastigial nucleus; MLF, medial longitudinal fasciculus; N, nodulus; S, saccule; SN, sympathetic neurons; U, utricle; UMN, upper motor neuron; VN, vestibular nucleus. (With permission from de Lahunta and Glass 2009.)
Lesions of any part of the vestibular system produce one or more of the following signs: head tilt, usually to the side of the lesion; ataxia, which is usually asymmetric; falling, leaning, veering, or rolling to the side of the lesion; and different forms of nystagmus.
It is important to differentiate central vestibular disease (which involves the brain stem or cerebellum) from peripheral disease (which involves only the receptor organ or vestibular nerve) in the inner ear. Peripheral lesions (inner ear or vestibular nerve) disrupt normal posture through an imbalance of sensory input to the central nervous system. When this happens, the vestibular nuclei in the brain stem are not “fed” proper information from the inner ear sensory organ so that these nuclei can then provide the proper special proprioception input to the head, trunk, eyes, and limbs. Central lesions (brain stem or cerebellum) may produce the same effect, but, in addition, usually damage other sensory (spinocerebellar tracts, medial lemniscus) or motor (reticulospinal, vestibulospinal, corticospinal, rubrospinal) pathways, causing ipsilateral proprioceptive deficits or paresis (see Chapters 41 and 42).
Diagnostic Plan
Anatomic Diagnosis
The signs of vestibular disease—head tilt, asymmetric ataxia, falling to one side, and nystagmus—are easily recognized. Differentiation of central lesions from peripheral lesions is essential for management and prognosis. In fact, many times the clinician can provide the client a fairly accurate prognosis simply from the neurologic examination findings. As a general rule, diseases that result in central vestibular signs carry a far worse prognosis than those that cause peripheral vestibular signs.
The most useful distinction is the presence of postural test reaction deficits or motor weakness which are seen with central disease but not with peripheral vestibular disease. Animals with acute vestibular syndromes may be difficult to examine because of their severe disorientation (especially cats), and therefore caution should be advised when picking up these patients. These patients will have a tendency to grab for any support available and that support could be your arms or legs. With careful testing of the postural test reactions, an animal with peripheral vestibular disease should have a normal response, while an abnormal response will be seen in those with central disease. In addition, if the patient is able to walk and weakness is observed, this finding would also point to a central lesion. The character of the nystagmus may also be helpful to differentiate peripheral from central disease. Nystagmus in peripheral disease is consistently horizontal with the fast phase away from the side of the lesion and/or head tilt. At times the nystagmus may be rotary in nature, but if pure vertical nystagmus is seen then central disease is more likely. In addition, with peripheral disease the nystagmus does not change direction when the head is held in a different position, although the severity of the nystagmus may change. Nystagmus in central disease may be horizontal, rotary, or vertical and frequently changes direction when the head is held in a different position. A couple of other seldom-mentioned clues that point to central disease are (1) with central disease the nystagmus may only be seen when the animal is placed in different positions (induced nystagmus) or (2) the fast phase of the horizontal nystagmus is toward the side of the head tilt.
Other neurologic deficits may be associated with vestibular disease. The facial nerve (cranial nerve VII) may be involved with both central and peripheral lesions but for different reasons. In central disease, it is due to involvement of the facial nuclei in the brain stem, which are located in close proximity to the vestibular nuclei, but with peripheral disease, it is due to the simultaneous inner ear and middle ear (where the facial nerve courses) disease. Horner’s syndrome (miosis, ptosis, enophthalmus, and prolapse of the third eyelid) may coexist with peripheral vestibular disease (if middle and inner ear disease coexist), due to the fact that the sympathetic nerve passes through the middle ear; it is very rare to see sympathetic dysfunction in central vestibular disease. Involvement of other cranial nerves (such as cranial verves V, VI, IX, and X) usually signifies central disease unless there is clinical evidence of a polyneuropathy. Table 42-1 summarizes the signs of vestibular disease and compares those to cerebellar and proprioceptive ataxias.
Two less common vestibular presentations are bilateral vestibular disease and “paradoxical” vestibular disease. Bilateral disease is characterized by a more symmetric ataxia, with swaying/swinging movements of the head. The patient has great difficulty stopping and holding the head at the midline position. Therefore, a classic side-to-side swinging motion of the head is seen. There is no pathologic nystagmus nor physiologic nystagmus with bilateral disease. Paradoxical vestibular disease is a unique syndrome in which the head tilt and loss of balance are directed toward the side opposite of the central lesion. This usually results from involvement of the cerebellar peduncle (de Lahunta and Glass 2009). It is paradoxical, in that some of the deficits indicate left-sided disease, while others indicate right-sided disease. In clinical practice, the side of the unilateral lesion will be determined on the neurologic examination by demonstrating postural test reaction deficits or hemi-paresis and ataxia, on the side ipsilateral to the lesion. The caudal cerebellar peduncle lesion will be contralateral to the direction of the head tilt with paradoxical vestibular system disease.
Etiologic Diagnosis
Most peripheral vestibular syndromes are caused by inflammation/infection in the inner ear or are idiopathic in nature. Idiopathic vestibular disease is diagnosed by exclusion of other causes. An increased incidence of peripheral vestibular disease (with or without facial paralysis) associated with hypothyroidism has been reported, but signs do not always resolve with treatment for the hypothyroidism. Therefore, an exact cause-and-effect association has not been proven. Rarely, neoplasia, drug toxicity, trauma, or congenital anomaly is the cause (Table 43-1).
Central vestibular disease may be caused by a variety of disorders. The history provides information on the rate of onset, course, previous illness, age, and previous medication. A few general categories of disease can be defined from this information (Table 43-2). Confirmation of the diagnosis requires additional tests. Infection of the middle and inner ear, although rare, may extend into the brain stem via the vestibular nerve. Therefore, it is important to always exclude peripheral infection, even if there are signs of central disease. A diagnostic plan is outlined in Table 43-3.
TABLE 43-1. Etiology of peripheral vestibular disease
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