The oculomotor nerve, the third cranial nerve, is divided into a motor part bringing innervation to most extraocular muscles
(dorsal, ventral, and medial recti and ventral oblique) and a parasympathetic part responsible for pupillary constriction.
The motor part is evaluated by checking for physiological nystagmus (sometimes called doll's eyes or oculocephalic reflex) and strabismus. To test for physiological nystagmus, move the animal's head quickly from side to side while looking in the
animal's eyes. Physiological nystagmus is characterized by a quick eye movement phase in the direction of the head movement
and a slow phase in the opposite direction. When examining cats for physiological nystagmus, I prefer to hold them under their
forelimbs and then turn myself in both directions with the cat facing me. Lesions of the oculomotor nerve will cause a fixed,
ventrolateral strabismus (Figure 2) along with a lack of conjugate nystagmus. The third cranial nerve also innervates the levator palpebrae muscle, and a lesion
may cause ptosis. The parasympathetic part of the third cranial nerve is the efferent part of the PLR. To evaluate the PLR,
cover one eye, and use a powerful light such as a transilluminator or a good penlight to stimulate the tested eye (via the
optic nerve) and look for the pupil's contraction (via the oculomotor nerve). Once the pupil is constricted, quickly move
the light to the other eye to see if there is an indirect response as well. The contralateral pupil should already be constricted.
Repeat on the other side.
1. Testing the menace reaction. 2. Direction of strabismus after cranial nerve involvement.
The trochlear nerve (fourth cranial nerve) and the abducent nerve (sixth cranial nerve) are also assessed as part of the physiological
nystagmus and strabismus evaluation. The trochlear nerve innervates the dorsal oblique muscle of the eye. A lesion involving
this nerve causes a lateral, rotational strabismus (Figure 2). In dogs, a retinal examination is necessary to evaluate the rotational strabismus associated with trochlear nerve involvement
since the pupils are round and, thus, a rotational strabismus cannot be seen. The abducent nerve innervates the lateral rectus
muscle of the eye. A lesion involving this nerve will cause a medial strabismus (Figure 2). The abducent nerve is also responsible for the retraction of the eyeball in the orbit (innervation of the retractor bulbi
muscle), and this can be tested by gently touching the cornea with a finger or a moist cotton swab.
The trigeminal nerve, or fifth cranial nerve, is divided into a motor part and a sensory part. The motor part (mandibular
branch) innervates the masticatory muscles and is evaluated by palpating the temporal and masseter muscles on both sides and
by assessing the patient's ability to close its mouth. The sensory part is evaluated by assessing the palpebral reflex and
the nasal sensation reaction. For the palpebral reflex, gently touch the medial canthus (ophthalmic branch), lateral canthus
(maxillary branch), and base of the ear and the jaw (mandibular branch). The efferent part of the reflex is mediated by the
facial nerve and results in eyelid closure. The nasal sensation reaction is tested by touching or poking the upper lip, nasal
septum, or both to elicit a reaction from the patient (Figures 3 & 4). In addition to eyelid closure (reflex portion), you should see an aversion reaction (e.g. backward movement of the head, licking of the nose). This reaction requires not only trigeminal sensory function but also
good function of the contralateral somesthetic cortex of the brain.
3 & 4. Testing nasal sensation
The facial nerve, the seventh cranial nerve, innervates the muscles of the face. It is responsible for facial expression (absence
of innervation causes a droopy face, dropping or absence of ear movement) and eyelid closure. When evaluating eyelid closure,
make sure that the closure is complete by gently passing your finger along the eyelids' surfaces since a facial paresis might
be missed otherwise.