The sixth (abducens) nerve arises at the level of the lower pons from a paired group of cells in the floor of the fourth ventricle, adjacent to the midline. The intrapontine portion of the facial nerve loops around the sixth-nerve nucleus before it turns anterolaterally to make its exit; a lesion in this locality therefore causes a homolateral paralysis of the lateral rectus and facial muscles. It is important to note that the efferent fibers of the oculomotor and abducens nuclei have a considerable intramedullary, that is, fascicular, portion (Fig. 13-4A and B). After leaving the brainstem, the nerve sweeps upward along the clivus and then runs alongside the third and fourth cranial nerves; together they course anteriorly, pierce the dura just lateral to the posterior clinoid process, and run in the lateral wall of the cavernous sinus, where they are closely applied to the internal carotid artery and first and second divisions of the fifth nerve (Fig. 13-5 and see “Cavernous Sinus Thrombosis” in Chap. 33).
The cells of origin of the fourth (trochlear) nerve are just caudal to those of the oculomotor nerves in the lower midbrain. Unlike all other cranial nerves, the fourth nerve emerges from the dorsal surface of the lower midbrain and then decussates a short distance from its origin, just caudal to the inferior colliculi. The nerves proceed circumferentially and ventrally around the midbrain toward the entry of the nerve into the posterior cavernous sinus. Each nucleus therefore innervates the contralateral superior oblique muscle. The long extraaxial course and the position of the nerves adjacent to the brainstem is a putative explanation for the common complication of fourth-nerve palsy in head injury (see Chap. 34). The superior oblique muscle forms a tendon that passes through a pulley structure (the trochlea) and attaches to the upper aspect of the globe. When the eye is adducted, the muscle exerts an upward pull, but being attached to the globe behind the axis of rotation, it causes depression and intorsion of the eye; in abduction, it pulls the ocular meridian toward the nose, thereby causing intorsion (i.e., clockwise in the right eye and counterclockwise in the left from the examiner’s perspective).
Together with the first division of the fifth nerve, the third, fourth, and sixth nerves enter the orbit through the superior orbital fissure.
Under normal conditions, all the extraocular muscles participate in every movement of the eyes; for proper movement, the contraction of any muscle requires relaxation of its antagonist. Clinically, however, an eye movement can be thought of in terms of the one muscle that is predominantly responsible for an agonist movement in that direction, for example, outward movement of the eye requires the action of the lateral rectus; inward movement, action of the medial rectus. The action of the superior and inferior recti and the oblique muscles varies according to the position of the eye. When the eye is turned outward, the elevator is the superior rectus and the depressor is the inferior rectus. When the eye is turned inward, the elevator and depressor are the inferior and superior oblique muscles, respectively. The actions of the ocular muscles in different positions of gaze are illustrated in Fig. 13-6 and Table 13-2.
The term binocular diplopia refers to the symptom of double vision caused by a misalignment of the visual axes of the two eyes. It is only present when both eyes are open and can see. Put another way, covering one eye usually obliterates double vision. In contrast, monocular diplopia persists when one eye is closed and is often due to lenticular or retinal disease or is nonorganic.