Page 768 - Clinical Small Animal Internal Medicine

736 Section 8 Neurologic Disease

before you handle them at all. The less restraint you use, the patient’s environment is sometimes necessary to
VetBooks.ir the better your examination will be. As soon as you try to determine if the animal can see and avoid these objects
when walking around them.
restrain young small animals, the ensuing struggle will
Immediately after the menace test, the pupil size and
make the cranial nerve evaluation very difficult. Puppies
and kittens may be wrapped in a towel and placed response to light should be evaluated. Most patients have
between your thighs when you sit on the floor with your a dark‐colored iris, which makes seeing the border of the
back against the wall and your knees flexed. This same pupil difficult without some light assistance. Cats with a
floor position can be used for toy breeds. For all other yellow iris are an exception. You need a bright source of
small animals, we examine the cranial nerves while light such as that from a Welch Allyn halogen pen light
standing over the standing patient facing the same way as or an Energizer pen light.
the patient, similar to when we performed the postural Initially, hold the light on the midline over the nose
reactions. The cranial nerve examination can be per- just close enough so that you can see the margin of the
formed by the numbers I–XII or by regions. We prefer pupils and determine their size and if any anisocoria is
the latter method, starting with the eyes, where either present. Then bring the light source as close as possible
part or all of cranial nerves II–VIII are evaluated. to one eye without touching the patient’s face. If no
response occurs, move the light so that it shines on all
Vision – Menace and Pupils aspects of the ocular fundus. Quickly swing the light into
We always start with the menace response. Cover one the opposite eye and observe what happens to that pupil.
eye with your hand. In small patients, use the hand that Then quickly return the light source to the original eye.
is holding the head to cover one eye and menace the Keep repeating this swinging light action until you are
opposite eye. This reaction is a learned response that comfortable with your findings, and record them accu-
may not be developed until 10–12 weeks in puppies and rately. To prevent confusion, record what happens in
kittens. Before this age, you may be able to assess vision each eye when the light is directed into one eye, and per-
by determining their ability to follow objects moving form this maneuver for both eyes. See how this pupillary
in their environment. The menace response requires light reflex is recorded on the neurologic examination
normally functioning eyes, the optic nerves (cranial form in this chapter. In the normal patient, the pupil will
nerve II), and the visual pathway to the visual cortex in rapidly constrict when the light is directed into it (the
the occipital lobe via the lateral geniculate nucleus. The direct response), and the opposite pupil will also con-
efferent response involves association axons from the strict without moving the light (the indirect or consen-
occipital cortex to the motor cortex and then an UMN sual response). Because of the major axonal crossing that
pathway to the brainstem terminating in the contralat- occurs at the optic chiasm and at the level of the pretec-
eral facial nucleus in the medulla. This pathway may tal nuclei, light directed into one eye will cause a stimu-
include a connection through the cerebellum via the lus that will reach both oculomotor nuclei. When you
pontine nuclei. The majority of this central visual path- swing the light to the opposite eye, the pupil has already
way caudal to the optic chiasm is contralateral to the eye constricted from the light directed into the first eye, and
being menaced. Some normal patients need a mild stim- it stays constricted. As you repeatedly swing the light
ulus to get a response. I usually tap their orbital or frontal back and forth between the eyes, the pupils remain con-
bone region with my hand to arouse their attention and stricted in the normal animal.
to observe for an intact palpebral reflex to be sure the The process I have described here is the way in which
facial nerve is functioning. In the normal menace I observe the direct and indirect (consensual) pupillary
response, the menacing gesture elicits closure of the light responses, but avoid using these terms, which can
palpebral fissure. This closure is dependent on normal be confusing to students and clinicians alike. I will
facial nerve innervation of the orbicularis oculi. Be sometimes evaluate pupil size in a darkened room to
careful to avoid touching any part of the face, especially help determine the origin of anisocoria.
the hair of the eyelids or the long vibrissae (whiskers), Test yourself on the anatomic diagnosis of the follow-
and to avoid creating a sudden air movement that might ing examples of eye examinations. (OS is oculus sinister
stimulate the hair or skin of the face. This action creates and refers to the left eye. OD is oculus dextra and refers
a cranial nerve V–VII reflex. to the right eye. OU is oculi uterque and refers to both
If the menace response is absent, we immediately eyes.)
touch the eyelid to be sure the facial nerve innervation is
intact to cause palpebral fissure closure. If facial paralysis ● A patient has no menace response in OS with a normal
is present, then we look for eyeball retraction and third palpebral reflex. No anisocoria is observed. Light
eyelid elevation or head retraction movements to deter- directed into OS causes no response OU. Light directed
mine if vision is present. Setting up a maze of objects in into OD causes a normal response OU. As you swing