CHAPTER 23
Neuro-ophthalmology
I. Anatomy and Examination
A. Photoreceptors
1. Rods: use rhodopsin pigment; mediate light perception
2. Cones: use iodopsin pigment; mediate color vision
B. Eyelids
1. Three muscles control lid position.
a. Levator palpebrae superioris
i. Main elevator of the upper lid
ii. Innervated by the superior division of cranial nerve (CN) III
iii. Right and left levators originate from the caudal central nucleus
b. Müller’s muscle: upper and lower eyelids; innervated by sympathetic fibers
c. Orbicularis oculi: closes eyelids; innervated mainly by ipsilateral CN VII
C. Ocular muscles
1. Horizontal eye movement
a. Lateral rectus—abducts the eye
b. Medial rectus—adducts the eye
2. Vertical eye movement
a. Superior rectus and inferior oblique—elevate the eye
b. Inferior rectus and superior oblique—depress the eye
3. Torsional eye movement: the superior muscles produce intorsion (superior oblique muscle) and the inferior eye muscles produce extorsion (inferior oblique muscle).
NB:
When eye is turned outward, eye depressor is inferior rectus, and when turned inward, is superior oblique.
D. Ocular motor nerves
1. Oculomotor nerve (CN III)
a. Innervates: superior rectus, inferior rectus, medial rectus, inferior oblique, levator palpebrae superioris, iris sphincter, ciliary muscle (required for focusing on near objects)
b. Controls all adduction, extorsion, and elevation of the eye; controls most depression; and contributes to intorsion through the secondary action of the superior rectus
c. Edinger-Westphal nucleus: rostral part of CN III, supplies the iris sphincter and ciliary muscle
2. Trochlear nerve (CN IV): innervates: superior oblique; intorsion, particularly during abduction
3. Abducens nerve (CN VI): innervates: lateral rectus muscle; eye abduction
E. Oculomotor systems
1. Vestibulo-ocular response system
a. Three semicircular canals
i. Resting firing rate increased by acceleration/rotation of the head.
ii. Canal function is initiated by head rotation toward it.
iii. Each canal works in tandem with one on the opposite side.
iv. Generate compensatory eye movements in the direction opposite the head motion to keep the eye stable during fixation
v. Project through the vestibular component of CN VIII
b. Otoliths
i. Also component of the vestibular system
ii. Saccule and utricle consist of maculae embedded in a gelatinous substance with calcium crystals.
iii. Detect linear accelerations of the head
2. Optokinetic response
a. Main function is to hold images steady on the retina during sustained head movement
b. Precise pathways are unknown but likely associated with pathways for smooth pursuit extending from visual association areas (Brodmann’s 18 and 19) to the horizontal pontine gaze center.
c. Smooth eye movement generated when large portions of the visual scene move across the retina, which usually happens when the head is moving
d. Generates a jerk nystagmus with the slow phase in the direction of stripe motion
e. Vestibulo-ocular response works together with optokinetic response: vestibulo-ocular response: rapid head rotation greater than 0.5 Hz; optokinetic response: slower rotations.
f. Pursuit
i. Main function is to hold an object of interest on the fovea
ii. Motion-sensitive regions of the extrastriate cortex, parieto-occipitotemporal junction, and the frontal eye fields supply input into superior colliculi, which control horizontal and vertical pursuit.
iii. Involved in horizontal ipsilateral pursuit
g. Saccades
i. Main function is to bring objects of interest onto the fovea
ii. Saccades are quick eye movements that shift gaze from one object to another
iii. Involve parietal and frontal eye fields
iv. Mediate saccades toward the opposite side
v. Superior colliculus involved in triggering saccades
vi. Parapontine reticular formation and medial longitudinal fasciculus are responsible for horizontal saccades,
F. Pupillary anatomy
1. Parasympathetic pathway
a. Muscles innervated
i. Iris sphincter: for pupillary constriction
ii. Ciliary muscle: for accommodation
b. Edinger-Westphal nucleus
c. CN III
i. Innervation of intraocular muscles is located in the inner aspect of CN III.
ii. Pupillomotor fibers are located on the outside (susceptible to compression).
iii. Courses within the cavernous sinus, where it bifurcates into an inferior (preganglionic pupillomotor fibers) and superior division
iv. Within the orbit, the parasympathetic fibers synapse in the ciliary ganglion and postganglionic parasympathetic fibers proceed anteriorly as short ciliary nerves to innervate the iris sphincter and ciliary muscles.
v. Acetylcholine released at both the preganglionic presynaptic terminal within the ciliary ganglion and the postganglionic neuromuscular junction
2. Sympathetic pathway
a. First-order neurons: originate in the posterolateral hypothalamus and synapse within the intermediolateral gray-matter column of the lower cervical and upper thoracic spinal cord
b. Second-order (preganglionic) neurons: arise from the ciliospinal center and exit the spinal cord through the ventral roots of C8 to T2 to synapse in the superior cervical ganglion
