Clinical Vignette
Misty, a 9-year-old female spayed sheltie, is presented for a 3-week history of progressive balance problems. The owner has noticed that Misty will stumble with all four limbs, and while standing, will sway forward and backward and side to side. Misty can support her weight well and is mentally alert. Your neurologic examination reveals good strength, normal mentation, normal cranial nerve examination, and normal tendon reflexes. Postural test reactions are normal except that Misty throws her feet too far laterally when hopped; Misty has generalized ataxia of the limbs and trunk while standing still, and a mild hypermetric gait. The history supports which category of disease? On the basis of the neurologic examination findings, where would you localize the lesion and then what tests would you recommend?
Problem Definition and Recognition
Ataxia and incoordination are synonyms that refer to poor coordination of muscle movements. There are three qualities of ataxia: (1) general proprioceptive (GP) ataxia (sometimes inappropriately referred to as sensory ataxia), (2) vestibular ataxia (see Chapter 43), and (3) cerebellar ataxia (sometimes inappropriately referred to as motor ataxia).
GP ataxia is a result of lack of kinesthesia, the lack of the sense of motion, or lack of awareness of the position of the neck, trunk, and limbs in space. A common mistake is to refer to a patient with lower motor neuron signs as being ataxic. They are not ataxic, just weak, therefore the patient still retains the ability to place their feet normally. The motor and GP pathways (long tracts) are located in close proximity in the spinal cord and brain stem. Lesions therefore can typically affect both systems so that ataxia and paresis are common concurrent signs. A patient with GP ataxia may stand with the paws placed more lateral than normal or on the dorsum of the paw. Upon moving, the limb may swing away from the trunk and circumduct or abduct more than normal. There may be a delay in the initiation of protraction of the limb while rising and the latter may also result in a longer stride than normal. With more involvement of the proprioceptive pathways, the animal may “knuckle over” with its paws while walking. Cervical spinal cord lesions may result in hypermetria due to damage of the spinocerebellar tracts, and this can be confused with cerebellar disease. These patients can be very hypermetric. The easiest way to separate these two patients is that the patient with cervical spinal cord disease will almost always have paresis and/or postural test reaction deficits while the cerebellar patient will not.
Vestibular ataxia is characterized by asymmetry and loss of balance. The patient tends to consistently lean, fall, or roll toward the side of the lesion. In addition, the patient many times has a head tilt; may circle propulsively in tight circles to one side, and has nystagmus (see Chapter 43).
Cerebellar ataxia is characterized by an inability to regulate the rate, range, or force of a movement and this is referred to as dysmetria. It is important to remember that there is no paresis with cerebellar disease. If paresis and cerebellar ataxia coexist, then there is most likely multifocal CNS disease. Truncal ataxia though is a very strong indicator for cerebellar disease. This is best observed while the animal is standing in place and not moving. The animal’s trunk will be observed to sway from side to side or forward and backward.
Pathophysiology
Ataxia is caused by disorders of the conscious or unconscious proprioceptive systems. The proprioceptive systems provide information about the location of the parts of the body. Without this information the movements are poorly coordinated. Receptors for proprioception are located in the skin, joints, tendons, and muscles. The nerve fibers are large and heavily myelinated. The pathways in the spinal cord are primarily in the dorsal and dorsolateral columns, and also consist of large fibers. These pathways terminate in the cerebellum and cerebral cortex. These large-fiber pathways are very susceptible to compression (especially those conditions that first compress the dorsal columns). Ataxia is the first clinical sign seen in compressive lesions affecting the spinal cord. Paresis commonly soon follows. Localization of lesions is essentially the same as described for paresis in Chapter 41. For example, ataxia of the pelvic limbs with normal thoracic limbs indicates a lesion caudal to T3. Ataxia of all four limbs indicates a lesion cranial to T2 (see Table 41-4).
The cerebellum coordinates motor activity by comparing the intended action with the performance (de Lahunta and Glass, 2009). It receives information from spinal pathways and from the cerebral cortex. The output of the cerebellum is to the brain stem and cortex. There are no direct cerebellar pathways to the spinal cord, and paresis is not seen with cerebellar lesions. Cerebellar ataxia has distinctive manifestations that are usually symmetric. Dysmetria is a disorder of the range of movements. They may be too long (hypermetria) or too short (hypometria). Clinically, hypermetria is easily recognized, since it causes a “goose-stepping” gait. Tremor, small oscillatory movements of the part, is common in cerebellar disease. Typically it is an intention tremor, one that becomes worse as the animal starts a movement and then it subsides at rest. Cerebellar disease usually causes dysmetria or tremor of the head, which can distinguish it from spinal tract disease that may also result in ataxia. Nystagmus in cerebellar disease is usually a fine tremor of the eyes, especially as the animal shifts its gaze. Jerk nystagmus, with a fast and slow component as seen in vestibular disease, may be seen with lesions involving the flocculonodular lobes, which are intimately related to the vestibular system. Generalized cerebellar disease may also result in a deficit in the menace reaction, although vision and the palpebral reflex will remain normal (Holliday 1980).
The vestibular system detects the position and movement of the head and trunk, providing the nervous system with information necessary to maintain normal posture. Ataxia associated with vestibular disease is usually asymmetric, with falling or rolling to one side. Head tilt and nystagmus are commonly seen in vestibular disease (see Chapter 43). The distinguishing characteristics of these three syndromes are listed in Table 42-1.
TABLE 42-1. Localization of lesions that result in ataxia
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