Page 1083 - Small Animal Internal Medicine, 6th Edition

CHAPTER 58 Lesion Localization and the Neurologic Examination 1055

follows the movement. It may be necessary to set up a maze
BOX 58.8 of objects to assess vision in each eye.
VetBooks.ir Causes of Neck Pain (CN3) are responsible for pupil constriction. Pupil size
The parasympathetic axons of the oculomotor nerve
should be examined at rest in a well-lit room and then in a
Muscle
Polymyositis (immune, infectious) dimly lit room and the two eyes compared. The examiner
Muscle injury evaluates the ability of each pupil to constrict (parasympa-
thetic function) and dilate (sympathetic function) by shining
Bone a bright light in one eye, then swinging the light into the
Fracture/luxation other eye to observe the response, and then swinging it back
Atlantoaxial instability/subluxation again (Fig. 58.22, B). Loss of vision and pupillary abnormali-
Diskospondylitis/osteomyelitis ties are discussed in Chapter 61.
Wobbler syndrome
Neoplasia Examine for Strabismus, Nystagmus, and
Joint (Facetal Joints) Head Tilt
Polyarthritis (immune, infectious) To check for strabismus and nystagmus, the examiner must
Degenerative joint disease (osteoarthritis) determine whether the eyes are normally positioned in the
orbits and whether there is any spontaneous nystagmus
Nerve Root when the head is stationary. Spontaneous nystagmus at rest
Neoplasia is always abnormal and indicates a central vestibular (medul-
Compression (by disk, tumor, fibrous tissue, arachnoid lary) lesion, a lesion of the vestibular portion of CN8, or a
cysts) lesion of the cerebellum. A head tilt is common with a lesion

Meninges in any of these locations. Abnormal eye position (strabis-
Neoplasia mus) may occur when there is a vestibular disorder or
Infectious meningitis/meningomyelitis damage to the innervation of the extraocular muscles (inner-
Meningoencephalitis of unknown etiology (MUE) vated by CN3, 4, 6) (Figs. 58.23 and 58.24). Oculomotor
Granulomatous meningoencephalitis (GME) nerve (CN3) dysfunction can result in a ventrolateral stra-
Steroid responsive meningitis arteritis (aseptic meningitis) bismus and an inability to rotate the eye dorsally, ventrally,
Hemorrhage-induced inflammation or medially. Lesions of the abducent (CN6) nerve cause a
medial strabismus and an inability to look laterally, and
Brain lesions of the trochlear nerve (CN4) cause a dorsolateral
Mass lesion (neoplasia, inflammatory) rotation of the eye. Lesions of these nerves (CN3, 4, 6) often
Chiari malformation with syringomyelia occur together, producing complete external ophthalmople-
gia, usually due to a mass in the region of the paired cavern-
ous sinuses on the floor of the calvarium (cavernous sinus
examination of each individual cranial nerve can be under- syndrome).
taken (Table 58.6; also see Suggested Readings). Vestibular disorders may cause a ventral strabismus (eye
drop) on the side of the lesion that is only evident during
Evaluation of Menace Response, Vision, head and neck extension (positional strabismus). A quick
and Pupils assessment of the function of all these nerves can be accom-
The optic nerve (CN2) is an important component of the plished by moving the head from side to side and eliciting
afferent pathways for the menace response, vision, and the the vestibulo-ocular reflex (Fig. 58.25). As the head is turned
pupillary light reflex. To test the menace response, the exam- slowly to the right, the gaze of both eyes should slowly drift
iner covers one of the animal’s eyes and advances the hand left before jerking to the right to resume a central position.
in a menacing way toward the opposite eye, taking care to The examiner assesses these normal vestibular eye move-
avoid touching the eyelid or whiskers or generating an air ments (physiologic nystagmus, oculocephalic reflex) while
current that will stimulate the cornea, which is innervated moving the head in each direction.
by the sensory portion of the trigeminal nerve (CN5). (Fig. In addition to moving the head from side to side to deter-
58.22, A) It is a good idea to lightly stimulate the face before mine whether the eye movements are normal, the examiner
assessing the menace response to get the animal’s attention should hold the animal’s head still in each lateral position
and make certain there is an intact palpebral reflex (CN7) to determine whether an abnormal (positional) nystag-
and the animal is able to blink. The menace response is a mus develops. The head and neck should then be extended
cortically mediated learned response and will not be present and held in that position while the eyes are evaluated for a
until 10 to 12 weeks of age in puppies and kittens. In addition ventral strabismus and development of nystagmus. When
to the menace response, vision can be assessed by observing the head of a normal animal is held still in any position,
the animal’s response to its environment by making sudden there should be no nystagmus. In most animals with severe
movements and dropping cotton balls to see if the animal or acute central or peripheral vestibular lesions, a resting