Page 770 - Clinical Small Animal Internal Medicine

738 Section 8 Neurologic Disease

nerve dysfunction. The last of these abnormalities causes When you have the head and neck in extension, look
VetBooks.ir the dorsal aspect of the pupil to rotate laterally in cats. at the position of the eyeballs, without retracting the
eyelids, to observe in small animals that both eyeballs
In dogs, in which the pupil is round, a fundic examina-
tion will show a lateral deviation of the retinal vein that
fissure and no sclera is exposed dorsal to the limbus.
emerges from the superior edge of the optic disk and have also elevated and are still centered in the palpebral
normally courses in a superior direction. A vestibular Sclera that is exposed dorsally indicates inadequate
strabismus is usually ventrolateral and is present only in elevation of the eyeball and either an oculomotor nerve
some positions of the head. dysfunction or a vestibular system dysfunction; the latter
disorder is a vestibular strabismus. With vestibular stra-
Nystagmus bismus, this eyeball deviation is present only in some
Nystagmus is an involuntary oscillation of the eyeballs. head positions, and the eyeball adducts normally when
In normal animals, a jerk nystagmus occurs when the physiologic nystagmus is examined.
head is moved rapidly. This normal physiologic nystagmus On rare occasions, a rapid resting pendular nystagmus
was observed when the eyes were examined for the func- is present at all times and in all positions of the head. The
tion of cranial nerves III and VI. Abnormal nystagmus eyeball excursions are rapid and to the same degree in
occurs with dysfunction of the vestibular system. In both directions. This ocular tremor is congenital and
severe cases, it is continual and is observed when the associated with anomalies of the retina or the decussation
head is resting in its normal posture, as well as in any of the optic axons at the optic chiasm. Vision is normal.
change of the head position. This disorder is a resting or This is most common in Siamese cats.
spontaneous nystagmus. In less severe and chronic
disorders, the abnormal nystagmus may be seen only Facial and Trigeminal Neurons
when the head is held in different positions. This disorder Up to this point in your cranial nerve examination, all of
is called a positional nystagmus. your evaluations have been directed at the eyes and
While you are holding the head in its normal resting orbital structures. Some of these evaluations have
position, look at the eyes for any resting nystagmus. To involved part of the innervation that is dependent on
facilitate this, use one hand to pull the facial muscles normal facial and trigeminal nerve function. At this
caudally enough so that the retraction of the eyelids will point, I reevaluate all of the facial and trigeminal nerve
expose the limbus of the eye to better observe it for any functions that are practical.
abnormal nystagmus. Move the head laterally to one For the facial nerve, repeat the palpebral reflex (cranial
side, hold it there, and observe for any development of a nerves V and VII) by touching both corners of the eyelids
positional nystagmus. Repeat this maneuver on holding for each eye. The sensory nerves are branches of both the
the head directed to the opposite side. Then hold the ophthalmic and maxillary nerves from the trigeminal
head and neck in extension and observe for any posi- nerve and the closure of the fissure is a function of the
tional nystagmus. If you are suspicious of subtle vestibu- palpebral branch of the facial nerve. Observe the sym-
lar system dysfunction and you have not observed any metry of the face and the position of the ears. The erect
abnormal nystagmus with the head held in a laterally ears of all cats, some breeds of dogs, and those dogs that
flexed or dorsally extended position, place the patient on have had ear crop surgery will not droop with facial
its back in dorsal recumbency. Extend the patient’s head paralysis because of the rigidity of the auricular cartilage.
and neck, and look for any positional nystagmus. Be sure You will feel atonia when you palpate the ear, and the ear
to record the direction of the abnormal nystagmus, as will not move when the skin of the ear canal is stimulated
well as the plane of rotation. The direction is defined as with your forceps or by blowing in the ear. You may be
the direction of the quick phase of the jerk nystagmus. able to palpate the loss of tone in the lips, and, in many
The plane of rotation may be horizontal, rotatory, or patients, you may observe a droop of the lips. Elevate the
vertical. As a rule, a consistently vertical nystagmus head and observe the lips at the angle of the mouth. More
with no rotatory component usually indicates involve- lip mucosa may be exposed on the denervated side (VII),
ment of the central components of the vestibular system. and the animal may tend to drool on that side. Facial
Look for possible change in this direction with different paralysis does not cause a deviation of the philtrum in
positions of the head. With peripheral vestibular sys- dogs and cats. Be aware that a deviated philtrum in a dog
tem disorders, the jerk phase of the nystagmus is always is most likely the result of tetany of the facial muscles on
directed away from the side of the lesion and is either in the side to which the philtrum is directed. This hemifa-
a horizontal plane or is rotatory. If the direction of the cial tetany can be confirmed by its disappearance with
nystagmus changes with different head positions, then local or general anesthesia. In older literature, this
a disorder of the central components of the vestibular observation is known as hemifacial spasm, which is a
system is present. misnomer.