Congenital oculomotor “apraxia” (Cogan syndrome) is a congential disorder characterized by an inability to make normal voluntary horizontal saccades when the head is stationary. Unusual eye and head movements are obligately tied together during attempts to change the position of the eyes. If the head is free to move and the patient is asked to look at an object to either side, the head is thrust to one side and the eyes turn in the opposite direction; the head overshoots the target, and the eyes, as they return to the central position, fixate on the target. Both voluntary saccades and the quick phase of vestibular nystagmus are defective. The pathologic anatomy is not understood but the condition abates over time. This same phenomenon is also seen in ataxia-telangiectasia (Louis-Bar disease, Chap. 37) and with agenesis of the corpus callosum.
Convergence insufficiency may give rise to diplopia and blurred vision at all near points; most cases are a result of head injury, some to encephalitis or multiple sclerosis. The ill-defined entity of divergence insufficiency causes diplopia at a distance because of crossing of the visual axes; in such patients images fuse only at a near position. This disorder may relate to changes in orbital fate and the position of the globe within the orbit. A special type of divergence paralysis, however, is seen with strokes in the rostral midbrain; these display an asymmetrical incompleteness of ocular abduction on both sides (pseudosixth palsy or convergence spasm). Based on scant clinical data, a center for active divergence has been postulated to reside in the rostral midbrain tegmentum.
The third (oculomotor), fourth (trochlear), and sixth (abducens) cranial nerves innervate the extrinsic muscles of the eye. Because their actions are closely integrated and many diseases involve all of them at once, they are suitably considered together.
The nuclei of the third (oculomotor) nerve consist of several paired groups of motor nerve cells adjacent to the midline, and ventral to the aqueduct of Sylvius at the level of the superior colliculi. A centrally located group of cells that innervate the pupillary sphincters and ciliary bodies (muscles of accommodation) is situated dorsally in the Edinger-Westphal nucleus; this is the parasympathetic portion of the oculomotor nucleus that subserves pupillary reactions to light and the near vision response. Ventral to this nuclear group are cells that mediate the actions of the levator of the eyelid, superior and inferior recti, inferior oblique, and medial rectus, in this dorsal–ventral order. This functional arrangement has been determined in cats and monkeys by extirpating individual extrinsic ocular muscles and observing the retrograde cellular changes (Warwick). Subsequent studies using radioactive tracer techniques have shown that medial rectus neurons occupy three disparate locations within the oculomotor nucleus rather than being confined to its ventral tip (Büttner-Ennever and Akert). These experiments also indicated that the medial rectus, inferior rectus, and the inferior oblique are innervated strictly ipsilaterally from the oculomotor nuclei, whereas the superior rectus receives only crossed fibers, and the levator palpebrae superioris (lid elevators) has bilateral innervations. Vergence movements are under the control of medial rectus neurons and not, as was once supposed, by an unpaired medial group of cells (nucleus of Perlia).