2004:227.)

Balint syndrome

A rare phenomenon resulting from bilateral occipitoparietal lesions is Balint syndrome. The syndrome consists of the triad of simultanagnosia, optic ataxia (disconnection between visual input and the motor system), and acquired ocular motor apraxia (loss of voluntary movement of the eyes while fixating on a target). Clinically, this triad of findings rarely occurs together.

Visual allesthesia

Patients with visual allesthesia see their environment rotated, flipped, or inverted. Such symptoms localize the damage to either the lateral medullary region (Wallenberg syndrome) or the occipitoparietal area.

Disorders of Awareness of Vision or Visual Deficit

Anton syndrome

A patient with cortical blindness may deny that there is any visual problem; this condition is termed Anton syndrome. Patients with Anton syndrome have no demonstrable visual behavior, but they hallucinate and confabulate visual images, claiming the ability to see. Anton syndrome is most common with bilateral occipital infarctions and has been described in patients with blindness from bilateral optic nerve lesions.

Stasheff SF, Barton JJ. Deficits in cortical visual function. Ophthalmol Clin North Am. 2001;14(1):217–242.

Riddoch phenomenon

Preservation of the perception of motion in a blind hemifield is termed staticokinetic dissociation, or the Riddoch phenomenon. When present in an otherwise complete homonymous hemianopia, it is thought to portend a better visual prognosis.

Blindsight

Cortically blind patients may have an unconscious rudimentary visual perception (blindsight). This condition may be caused by damage to the visual pathways through the superior colliculus or the connections between the lateral geniculate body and the extrastriate visual cortex.

Hemispatial neglect

Patients with hemispatial neglect (hemineglect) will not acknowledge seeing objects in an area of vision known to be intact. Confrontation testing using double simultaneous stimulation may be used to verify this condition (see Chapter 3 for a description of confrontation visual field testing). Hemispatial neglect is usually due to damage in the right hemisphere (eg, in the posterior parietal cortex, frontal eye fields, cingulate gyrus) that mediates attention in both hemifields.

Girken CA. Disorders of higher visual function. In: Kline LB, Bajandas FJ. Neuro-Ophthalmology Review Manual. 6th ed. Thorofare, NJ: Slack; 2007:233–244.)

CHAPTER 7

The Patient With Supranuclear Disorders of

Ocular Motility

The efferent visual system controls ocular movements. (The anatomy of this system is introduced in Chapter 1.) Like all efferent systems, the efferent ocular motor system consists of supranuclear and infranuclear pathways. This pathway distinction is clinically important because supranuclear disorders almost always affect both eyes similarly, whereas infranuclear disorders affect the eyes differently. The patterns of symmetric dysfunction that occur with supranuclear disorders typically do not produce diplopia (exceptions include skew deviation). Conversely, infranuclear lesions usually do produce diplopia. The supranuclear disorders are discussed in this chapter and the infranuclear disorders in Chapter 8.

Supranuclear pathways include

premotor and motor regions of the frontal and parietal cortices cerebellum

basal ganglia superior colliculi

thalamus (dorsal lateral geniculate nucleus and pulvinar)

brainstem centers (paramedian pontine reticular formation, 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)

neuromuscular junction extraocular muscles

Fundamental Principles of Ocular Motor Control

The afferent visual system of primates 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 devoted essentially 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 perform highly precise visual motor tasks.

Attention to peripherally placed objects is usually driven by the perception of a changing stimulus (eg, one that moves or that becomes brighter or larger). It is a basic principle of all sensory systems that any persistent, unchanging stimulus gradually produces an attenuated neural response. This phenomenon explains, for instance, why one does not attend to the constant tactile stimulus of wearing a wristwatch or clothing. Such a physiologic design improves the efficiency of neural communication.

Movement is an especially strong stimulus—often the primary one—that generates consciousness of an object in the environment. Fixating on a moving object presents a significant challenge, however, as the object must remain centered on the fovea while it and the viewer move in an unrelated, simultaneous fashion. Several ocular motor systems have evolved to meet this challenge, and they provide seamless object tracking over a wide range of relative velocities (Table 7-1).

Table 7-1

Relatively slow-moving (ie, less than 30° per second) targets are tracked by the pursuit system. (Consider that a target moving at this speed would cover one-third the distance from the primary position to the far extent of the temporal visual field in 1 second.) A slow-moving object can be tracked even if head movements are occurring simultaneously because of the influence of the vestibular ocular reflex (VOR), which produces eye movements in a direction opposite to those of head acceleration. The VOR response, however, attenuates fairly quickly (ie, within seconds) during a persistent period of stable head velocity. Any attenuation of the VOR response would reduce the capability of the subject to follow a moving target, which would cause blurring of vision. Thus, the optokinetic nystagmus (OKN) system supplements the VOR. It uses smooth pursuit to track a moving object but then introduces a saccade in the opposite direction when the maximum amplitude of the pursuit movement is reached or when the speed of the moving object exceeds the maximum velocity of the pursuit system. Both OKN and VOR can be suppressed by visual fixation on a target (eg, the discomfort of motion sickness can be lessened by visual fixation on a nonmoving target).

Faster-moving targets cannot be tracked by the pursuit system but can be followed by the relatively fast, back-to-back eye movements generated by the saccadic system. Saccades are “ballistic” movements—ones that generally cannot be altered once initiated. Relative movement of objects toward or away from the eyes activates one of the vergence systems. Convergence, which rotates both eyes inward, is activated by relative movement that brings an object closer. Divergence is activated by movement that produces increasing separation of the object from the viewer.