Nancy B. Cottrill,     Woburn, Massachusetts

Role of the Cranial Nerves in the Neuro-ophthalmic Examination

Most of the cranial nerves are involved in vision, PLRs, eye position, and movement. Cranial nerve II (CN II), or the optic nerve, is the afferent arm of the PLR and vision and transmits sensory information to the brain. Parasympathetic fibers of CN III, the oculomotor nerve, comprise the efferent arm of the PLR. CN III also innervates the levator palpebrae superioris muscle (to elevate the upper eyelid) and most of the extraocular muscles. CN IV, the trochlear nerve, innervates the dorsal oblique muscle that passes over a trochlea to rotate the globe medially. CN V, the trigeminal nerve, provides sensory input via the ophthalmic (medial) and maxillary (lateral) branches to the surfaces of the eye (cornea, conjunctiva, periocular skin) and provides afferent input to the lacrimal gland. CN VI, the abducens nerve, serves the lateral rectus muscle (to abduct the globe) and the retractor bulbi muscles. A helpful acronym for remembering the innervation of the extraocular muscles is LR₆, SO₄, similar to a chemical equation. This denotes that the lateral rectus and the retractor bulbi are innervated by cranial nerve 6 and the superior oblique by cranial nerve 4 (whereas all other extraocular muscles are controlled by CN III). CN VII, the facial nerve, innervates the facial muscles controlling eyelid closure, important in the menace and palpebral reflex. CN VIII, the vestibular nerve, is responsible for coordination of globe position and gaze.

Functional Anatomy of the Pupillary Light Reflex

Pupil size (and shape in the cat) is determined by the balance between parasympathetic and sympathetic innervation of the iris. Although emotional and other non–neuro-ophthalmic factors can affect this aperture, pupil size depends in large part on the amount of light illuminating the retina and the afferent and efferent pathways of the PLR. An understanding of the anatomy of the PLR is essential (Web Figure 80-2, B and C), along with recognition that the PLR reflex is subcortical (without cerebral cortical involvement) and that the reflex is not an indicator of vision. The response of only a few photoreceptors is required for the PLR to occur.

The afferent arm of the PLR for each eye consists of the retina, optic nerve, optic chiasm, and optic tract. Nerve fibers originating in the medial retina decussate to the opposite optic tract, whereas fibers from the lateral retina remain in the ipsilateral tract. In the dog approximately 75% of the optic nerve fibers decussate to the contralateral optic tract. In the cat approximately 65% of the optic nerve fibers decussate. The optic tracts carry impulses beyond the decussation and include both sensory visual fibers and pupillomotor fibers that differ according to their destinations. This fact is useful for localizing lesions by noting the presence or absence of vision and PLR in each eye. The pupillomotor fibers progress to the pretectal nucleus, and from there the majority of these fibers decussate to the parasympathetic nucleus of the oculomotor nerve (previously known as the Edinger-Westphal nucleus) on the contralateral side. This second decussation marks a return to the side of the originating (light) impulse. In contrast, the visual fibers proceed from the optic tract sequentially to the ipsilateral lateral geniculate nucleus, the optic radiation, and the occipital (visual) cortex. Therefore lesions that cause anisocoria and affect both PLRs and vision can be localized to the retina, optic nerve, optic chiasm, or optic tract (see Web Figure 80-2). By contrast, lesions affecting vision, but not PLRs, are located in the lateral geniculate nucleus, optic radiation, or occipital cortex.

The efferent arm of the PLR consists of parasympathetic fibers in the oculomotor nerve (CN III) that travel to the ciliary ganglion, postganglionic ciliary nerves, and finally the iris sphincter muscle. In the dog there are five to eight short ciliary nerves that contain both sympathetic and parasympathetic efferent fibers and are considered “mixed.” Therefore a lesion of the ciliary nerve in the dog results in a circular, midrange pupil. By contrast, the cat has only two short ciliary nerves, the nasal (medial) and malar (lateral) nerves; these are solely parasympathetic. Thus lesions involving the feline nerves cause more dramatic pupil dilation than in the dog because sympathetic function is still present and is now unopposed. If only the medial or lateral branch is damaged, dyscoria (altered pupil shape) results in a D-shaped or reverse D–shaped pupil in addition to anisocoria.

The sympathetic fibers innervate the iris dilator muscle. The three-neuron pathway that the sympathetic axons traverse is more complex. The central fibers originate in the hypothalamus and exit the brain with the tectotegmentospinal tract. The preganglionic cell bodies lie in spinal cord segments T1 to T3 and issue fibers that travel with the ventral spinal nerve roots. These pass cranially via the thoracic and cervical sympathetic trunk to terminate in the cranial cervical ganglion, caudal and medial to the tympanic bulla. Postganglionic fibers then exit the ganglion and pass through the cavernous sinus into the periorbita, nasociliary nerve, long ciliary nerve, ciliary body, and finally the iris dilator muscle.

The Neuro-ophthalmic Examination

The neuro-ophthalmic examination has a number of components including general observations, neurologic evaluation, assessment of the pupils and PLR, and general ophthalmic examination.