First, assess whether the pet is ataxic. Evaluate whether the pet's feet are randomly placed on the floor or whether the gait
is regular. Look for possible overreaching with the forelimbs, delayed protraction, hypermetria, and loss of balance. If the
animal is ataxic, characterize the ataxia. Vestibular ataxia is characterized by a head tilt, a wide-base stance, and a loss
of balance (e.g. falling to one side, leaning). The head is usually (but not always) tilted toward the side of the lesion. Sometimes, the
body can be completely curved with the concavity on the side of the lesion. Cerebellar ataxia is characterized by dysmetria
and a bouncing gait because of an inability to regulate the rate and force of movement.2 Hypermetria, or greater movement of the limb than normal, is most often recognized.2 It can be accompanied by intention tremors, which are fine tremors present on initiation of voluntary movements. They are
particularly visible in the head, especially when the animal is trying to smell something or is reaching for food. These tremors
are absent during sleep or complete rest. Strength is preserved with a pure cerebellar problem. Proprioceptive ataxia is almost
always accompanied by some degree of paresis (weakness or partial paralysis) and is characterized by the body's tendency to
deviate from the midline. Crossing of the limbs, dragging of the toes, knuckling (walking on the dorsal surface of the paw),
overreaching, and circumduction can also be seen.
Once the presence and type of ataxia are determined, evaluate whether weakness or lameness is present. If you notice a gait
abnormality, evaluate whether it affects only one limb, both hindlimbs, the ipsilateral limbs, or all four limbs. A marked
asymmetry may occasionally be present, and, although only one limb may seem to be affected, carefully assess whether a subtle
deficit is present in the contralateral limb. Keep in mind that a dog can get up and walk fairly well with only three limbs
(e.g. a dog with an amputated limb). If you see a problem in one limb in an animal that can hardly walk without falling and needs
help to get up, then most likely another limb (contralateral or ipsilateral) is affected also.
When describing ambulation, record the type of ataxia, if present, and the gait (normal, ambulatory paresis, nonambulatory
paresis, paralysis) and limbs affected. Again, it is important to evaluate whether voluntary movements are present when a
patient is nonambulatory by supporting the patient. If paresis is present, qualify it as upper motor neuron (normal to increased
spinal reflexes and muscle tone) or lower motor neuron (absent or reduced reflexes and tone).
When evaluating the cranial nerves, it is important to differentiate between a reflex and a reaction. A reaction implies conscious
manifestation and participation of the cerebral cortex.
The first cranial nerve, the olfactory nerve is an afferent nerve responsible for the perception of smell. It is rarely evaluated
because of the difficulty in stimulating only one side at a time and because of the concomitant stimulation of the sensory
fibers of the trigeminal nerve in the nasal mucosa when using a strong or irritating substance. Specific information from
the owner (e.g. the pet does not smell its food) might be more helpful.
The second cranial nerve, the optic nerve, is the afferent nerve for vision and the pupillary light reflex (PLR). It is evaluated
by the menace reaction and the PLR (see oculomotor nerve below). The menace reaction not only evaluates the second cranial nerve but also the ocular components, the optic chiasm and other
structures of the visual pathway, the contralateral occipital cortex, and the ipsilateral cerebellum and facial nerve. Test
the menace reaction by holding the pet's head, covering one eye with one hand, and making a forward gesture with the other
hand (Figure 1). The expected response is closure of the eyelids, possibly accompanied by a backward movement of the head and retraction
of the eye. When making the forward gesture, be careful not to get too close to the eye, as the air movement produced by the
hand movement might stimulate the sensory fibers of the cornea and produce a false positive reaction. Remember that the menace
reaction is a learned reaction and is absent in kittens and puppies up to about 12 weeks old.2