5 Neuro-Ophthalmology
Chapter 8: The Patient With Supranuclear Disorders of Ocular Motility
The efferent visual system, the anatomy of which is introduced in Chapter 1, controls ocular movements. Like all efferent nervous systems, the ocular motor system consists of supranuclear and infranuclear pathways. The supranuclear pathways originate from multiple locations and innervate the cranial nerve (CN) nuclei. Prenuclear is a more accurate term than supranuclear because some of these pathways (eg, the vestibular pathways) originate below the CN nuclei. However, the term supranuclear is used throughout the literature and will be used in this chapter as well.
The infranuclear pathways originate in the CN nuclei and innervate the extraocular muscles. The distinction between the supranuclear and infranuclear pathways is clinically important because supranuclear disorders almost always affect both eyes similarly, whereas infranuclear disorders affect each eye differently. The patterns of symmetric dysfunction that occur with supranuclear disorders typically do not produce diplopia, although exceptions such as skew deviation exist. Conversely, infranuclear lesions usually do produce diplopia.
Supranuclear pathways include
premotor and motor regions of the frontal and parietal cortices
basal ganglia
superior colliculi
thalamus (dorsal lateral geniculate nucleus and pulvinar)
brainstem centers (paramedian pontine reticular formation [PPRF], neural integrators, and vestibular nuclei)
Infranuclear pathways include
ocular motor nuclei
intramedullary segments of the ocular motor nerves
peripheral segments of the ocular motor nerves (coursing through the subarachnoid space, cavernous sinus, superior orbital fissure, and orbit)
Fundamental Principles of Ocular Motor Control
In primates, the afferent visual system is broadly designed to achieve 2 fundamental goals: (1) to detect objects and motion in the environment and (2) to provide a high level of spatial resolution for objects that command attention. The entire retina outside the fovea is essentially devoted to the detection of objects. Only the fovea, which occupies a tiny fraction of the total retinal area, provides the fine-quality resolution that allows us to read or to perform highly precise visual motor tasks. Therefore, if one is to discern the fine details of an object, the image of that object must be precisely located on the fovea.
The ocular motor system directs and holds the fovea in alignment with objects of interest. Six supranuclear ocular motor systems have evolved to meet this challenge (Table 8-1):
Ocular fixation system. Holds the image of a stationary object on the fovea when the head is not moving
Vestibular-ocular system. Holds the image steady on the fovea during brief head movements
Optokinetic system. Holds the image on the fovea during sustained head movements
Saccadic system. Rapidly brings the image of an object of interest onto the fovea
Smooth-pursuit system. Holds the image of a moving object on the fovea
Vergence system. Moves the eyes in opposite directions so that an image of an object of interest is simultaneously held on each fovea
These supranuclear systems are controlled by different anatomical pathways that converge at the level of the brainstem to innervate the ocular motor CN nuclei. Their separate origins allow selective disruption of these ocular motor systems by disease processes. Therefore, targeted clinical examination of these systems allows the clinician to identify the affected system and determine the responsible disease process.
Table 8-1 Ocular Motor Tracking Systems
Leigh RJ, Zee DS. The Neurology of Eye Movements. 5th ed. Contemporary Neurology Series. New York: Oxford University Press; 2015.
Wong AMF. Eye Movement Disorders. New York: Oxford University Press; 2008.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.