Clinical Vignette
A 7-year-old castrated male miniature schnauzer is presented for acute loss of equilibrium, head tilt, circling, and falling. Signs have been present for 12 hours.
Physical examination reveals normal vital signs. The dog is circling and falling to the right, and a right head tilt is present. When picked up, the dog tries to roll to the right. A spontaneous horizontal nystagmus is present with the quick or jerk phase to the left. When the head is elevated, the right eyeball has a ventrolateral position. There is no palpebral reflex on the right side and the right pupil is smaller than the left pupil in normal room light. Bilateral pupillary light reflexes are present and the dog has normal vision. Postural reactions and spinal reflexes are normal. The rest of the physical examination is normal.
Using the problem-oriented format described in Chapter 1, identify the problems in this dog and write an initial plan for each.
Problem Definition and Recognition
Nystagmus is rhythmic, involuntary movement of the eyes that may occur in a horizontal, vertical, or rotary direction. Strabismus is deviation of the eyeball from the normal straightahead position. It can be seen with the head in its normal plane and/or it may be induced when the head is extended. Nystagmus and strabismus are recognized by direct examination of the eyes.
Pathophysiology
Normal eyeball movement is controlled by cortical and brain stem centers that regulate the extraocular muscles via their innervation by cranial nerves III (oculomotor), IV (troche-lear), and VI (abducent). Table 48-1 lists the actions and innervations of the extraocular muscles.
Extraocular muscles are special skeletal muscles that contain two fiber types. One type is composed of well-organized myofibrils that are innervated by heavily myelinated nerves (fast conducting) and are responsible for rapid eye movements or saccades. The second fiber system is innervated by small myelinated nerves. These fibers are responsible for smooth, slow contractions that hold the eyes in position or are used in following movements. The extraocular muscles are richly innervated, which allow rapid and fine adjustment in eye position. The deviations in eyeball position that result from muscle or nerve dysfunction are listed in Table 48-2.
TABLE 48-1. Actions and innervations of the extraocular muscles
| Muscle | Action | Innervations |
| Dorsal rectus | Supraduction (pulls eyeball dorsally) | CN III |
| Medical rectus | Adduction (pulls eyeball medially) | CN III |
| Ventral rectus | Infraduction (pulls eyeball ventrally) | CN III |
| Ventral oblique | Extorsion (rotates eyeball laterally) | CN III |
| Dorsal oblique | Intorsion (rotates eyeball medially) | CN IV |
| Lateral rectus |
