Cranial Nerve II: Vision

| September 30, 2009 | 0 Comments

by Paulette Marie Gillig, MD, PhD, and Richard D. Sanders, MD
Dr. Gillig is Professor of Psychiatry and Faculty of the Graduate School, Department of Psychiatry, Wright State University, Dayton, Ohio; Dr. Sanders is Associate Professor, Departments of Psychiatry and Neurology, Boonshoft School of Medicine, Wright State University, and Ohio VA Medical Center, Dayton, Ohio.

Psychiatry (Edgemont) 2009;6(9):32–37

Series Editor: Paulette M. Gillig, MD, PhD, Professor of Psychiatry, Department of Psychiatry, Boonshoft School of Medicine, Wright State University, Dayton, Ohio

ABSTRACT

This article contains a brief review of the anatomy of the visual system, a survey of diseases of the retina, optic nerve and lesions of the optic chiasm, and other visual field defects of special interest to the psychiatrist. It also includes a presentation of the corticothalamic mechanisms, differential diagnosis, and various manifestations of visual illusions, and simple and complex visual hallucinations, as well as the differential diagnoses of these various visual phenomena.

KEY WORDS

psychiatry and neurology, vision, hallucinations, Bonnet syndrome, Anton’s syndrome, peduncular hallucinosis, hallucinations and epilepsy, visual agnosias, visual defects in schizophrenia, examination of the visual system, cranial nerve II

GENERAL THOUGHTS ON VISION TESTING

Vision is central to human life as an extremely efficient medium for carrying information and as a source of pleasure. Visual deficits are important in the diagnosis of conditions encountered by psychiatrists and have major implications for treatment planning. Fundoscopy will help rule in or out increased intracranial pressure or vascular disease as the cause of mental status changes or behavioral symptoms and signs. Tests of motion perception and the perception of the directions of others’ gaze can help to clarify whether or not a patient is developing schizophrenia.

Visual hallucinations, especially in the absence of auditory hallucinations, can be important symptoms of toxic metabolic conditions or some neurological disorders. For example, a patient with “peduncular hallucinosis” (see below) after a brainstem stroke experienced hallucinations while awake (but only with his eyes shut), and he saw a ball of light, which changed several days later to a man in a suit and a black butterfly, which eventually evolved into dwarves with egg-shaped heads, then figures dressed in 17th century Cavalier’s clothing, and finally into women in Victorian clothes.[1] In another instance, a 68-year-old man with an 11-year history of Parkinson’s disease, when treated with levodopa, reported the visual field to be skewed and then for 30 minutes saw numerous cats in the room who padded about silently. His wife discovered him appearing to be petting something at his feet. On another occasion the Pope chatted with him while he was traveling in the car to church.[1] Between hallucinations, the patient was aware that the experiences were not real, but during the hallucinations, he did not have this insight.

Another elderly woman of our acquaintance, legally blind secondary to macular degeneration, enjoyed the almost daily experience of “watching little children play on the sidewalk,” from her comfortable chair on her summer porch.

ANATOMY OF THE VISUAL SYSTEM

When light enters the eye, it reaches the receptor layer of rods (in the peripheral retina, insensitive to color) and cones (in the central retina, sensitive to color) in the retina. These receptors transmit impulses to the bipolar cells, which synapse on the ganglion cells, which become myelinated and form the optic nerve. Unlike the other cranial nerves, the optic nerve is truly part of the central nervous system, and has glial cells that are derived from oligodendrocytes, rather than Schwann cells, between its fibers. Therefore, the optic nerve can be subject to diseases that affect the glial cells of the central nervous system, such as childhood-onset metabolic storage diseases, migraine, and multiple sclerosis,[2] but not diseases that affect the Schwann cells (such as Guillain-Barre). The vascular supply of the optic nerve comes from the ophthalmic branch of the internal carotid artery.

The visual receptors in the retina project to form the optic nerve. Most, but not all, of the fibers of the optic nerve cross in the optic chiasm. The optic tract then projects to the lateral geniculate ganglion inside the thalamus, synapses, and the post-synaptic fibers project from there through the internal capsule to the calcarine cortex of the occipital lobe. Nearly 90 percent of the primary visual cortex (area 17) is activated by the maculae (central retina).

Each lateral geniculate ganglion receives some input from both eyes (because of the uncrossed fibers) but from only one half of the visual field. It is also notable that the lateral geniculate ganglion probably also projects some fibers to higher cortical regions. For example, the geniculate ganglion (and the calcarine cortex) also project to the parietal cortex, and these connections are concerned with visual alerting reactions and with other aspects of perception.[3,4] The geniculate ganglion receives input from other systems, such as the auditory system, and this input helps the visual system direct attention to the relevant part of the visual field.[3,4]

Within this arrangement, there are two parallel visual systems with different roles projecting to distinct layers of the calcarine cortex, and from there projecting to distinct cortical areas.[5–7] Magnocellular neurons are heavily myelinated; respond to large, low-contrast stimuli; and carry impulses at high speed at the cortical level activating the dorsal visual system (including inferior parietal and middle temporal areas). Parvocellular neurons are less myelinated, respond to small, high-contrast stimuli, and carry impulses more slowly, activating the ventral visual stream (including lateral occipital and inferior temporal areas). The magnocellular/dorsal system conveys critical but incomplete information to enable rapid responding, while the parvocellular/ventral system fills in the details. The magnocellular/dorsal system is impaired in schizophrenia.

DIAGNOSES OF VISUAL SIGNS AND SYMPTOMS

Diseases of the retina and optic nerve. Macular degeneration. At least 25 percent of Americans older than 65 years experience some degeneration of the retina’s pigmented epithelium, and sometimes this unfortunately involves the maculae.[2] When this occurs, central vision is lost. Peripheral vision initially is preserved, but it is also lost slowly over time. Like all blind people, persons with macular degeneration also can experience complex visual hallucinations.[8] Depression is also common in macular degeneration and is a major factor in overall disability.[9]

Scotoma and optic atrophy. Demyelinative disease (especially multiple sclerosis), toxins, nutritional deficiency, and vascular disease are the usual causes of scotomas. The optic disc eventually becomes pale (optic atrophy), and there will be a defective direct pupillary light reflex in the affected eye. Optic atrophy is characterized by a chalky white optic disc with discrete margins. Optic atrophy is also a late finding with increased intracranial pressure, or increased pressure within the eye itself, as in glaucoma.[8]

Retrobulbar neuropathy. Retrobulbar neuropathy can occur acutely, affecting the eyes either simultaneously or successively. There is a rapid diminution of vision, although the optic disc may appear normal. There is pain on movement of the eye and tenderness to pressure. The usual cause is a demyelinative process.[8]

Central retinal artery occlusion. In central retinal artery occlusion, the retina is diffusely pale and the central fovea is uncharacteristically prominent (cherry red). Central retinal artery occlusion is characterized by painless, sudden blindness in the affected eye. It is usually related to an embolism, but can be due to temporal arteritis.

Central retinal vein occlusion. In central retinal vein occlusion, the optic disc is massively swollen with diffuse hemorrhages and “cotton-wool” spots. Central vein occlusion also is relatively sudden in onset and worsens over a few days, and usually occurs without pain. Unlike occlusion of the artery, there may be modest improvement in vision over time. It usually is related to arteriosclerosis, possibly because the vein is compressed by the thickened arteriole within the adventitial envelope shared by both vessels, at the site of crossing.

Proliferative diabetic retinopathy. Diabetic retinopathy is characterized by multiple hemorrhages, exudates, and neovascularization throughout the retina.

Cytomegalovirus retinitis. Cytomegaloviris retinitis is characterized by retinal necrosis and hemorrhage.

Leber’s hereditary optic atrophy.
This hereditary condition is caused by a mutation in mitochondrial deoxyribonucleic acid (DNA). It commonly occurs in young men, who develop visual loss culminating in blindness in both eyes within months.

Lesions of the optic chiasm. Bitemporal hemianopia. Bitemporal hemianopia (losing the visual field in the lateral part on both sides) is caused by a lesion of the decussating fibers of the optic chiasm, usually caused by a pituitary adenoma, but sometimes by a saccular aneurism of the circle of Willis, a craniopharyngioma, or meningioma, among other causes.[10]

Other visual field defects. Homonymous hemianopsia. Homonymous hemianopsia, which is blindness in one-half of the visual field, is caused by a lesion behind the chiasm anywhere along the pathway to the occipital cortex or in the cortex. If the field defects in the two eyes are identical, the lesion is more likely to be in the calcarine cortex; whereas, if the field defects in the two eyes are different, the visual fibers in the optic tract or in the parietal or temporal lobe are more likely to have been affected.

Bilateral superior quadrantanopsia. Bilateral superior quadrantanopsia is most likely to occur from a lesion causing damage to the lower fibers of the optic radiation, such as a pituitary adenoma, that is growing upward, disrupting the inferior crossing fibers. In the rare case where only one temporal field is affected, the possibility of a contralateral temporal lobe lesion (including a seizure focus) should be considered and may be the only interictal finding in patients with partial complex seizures.[2]

Visual illusions and hallucinations. Visual hallucinations are often associated with “organic” conditions, but toxic and neurologic etiologies are really more likely only when the person has visual but not auditory hallucinations. The history must (a) distinguish between rudimentary and complex hallucinations, (b) clarify their association with sleep and intoxicating agents, and (c) assess vision.

Characteristics of visual illusions and hallucinations. Visual illusions and hallucinations can occur in relation to lesions in any part of the visual system, but are frequent with occipital lobe lesions. Lesions, such as strokes, disturb the equilibrium of visual system pathways and release hallucinations from the association cortex indirectly due to loss of normal corticocortical inputs[11] or loss of thalamocortical inputs to the visual association cortex. By contrast, the complex visual hallucinations that can occur in epilepsy usually are caused by pathology that directly stimulates the visual association cortex.[1,12–15]

Visual illusions (metamorphopsias). Visual illusions may present as distortions of form, size, movement, or color. Also, visual images may become disconnected from visual memories, resulting in a sense of strangeness or déjà vu, as in temporal lobe epilepsy. Visual illusions can include micropsia or macropsia, or a sense that the object is moving toward the patient. Objects also can appear to invert (especially where there is involvement of the vestibular system also), lose color, or change in orientation in space.[16] Illusions of these types occur with lesions of the occipital, occipitoparietal, or occipitotemporal regions, and the right hemisphere is more commonly involved. They are often associated with visual field defects.

Complex visual hallucinations: corticothalamic mechanisms. Visual hallucinations are likely caused by a release of inhibition of the visual association cortex or by direct stimulation of the cortex. Serotonin and acetylcholine are particularly important in the generation of visual hallucinations. The raphe lesion of peduncular hallucinosis (site of serotonin) results in excitation of the dorsal lateral geniculate nucleus, and its projections to the visual association cortex, causing the generation of visual hallucinations in the cortex.

Both the dorsal lateral geniculate nucleus and lateral pulvinar nuclei of the thalamus are under brainstem control and are junctions for brainstem modulation of inputs to visual cortex.[1,3,17–19]

Several neurological conditions can also be associated with complex visual hallucinations. These include narcolepsy-cataplexy, peduncular hallucinosis, Parkinson’s disease (treated), Bonnet’s syndrome, and epilepsy. In cases where other central nervous system functions are preserved (Bonnet’s syndrome), the patient is not distressed by the hallucinations and is able to gain insight into their nature. Insight is usually only lacking when there is generalized cognitive deficit (delirium or dementia) or a psychotic disorder.

The similarity of complex visual hallucinations across a variety of conditions and situations has led many observers to conclude that they may have a common mechanism: Complex visual hallucinations likely are caused by defective modulation of thalamocortical relationships leading to “release phenomena.”[2] This can occur in several ways, with the complex visual hallucination (ultimately involving the cortex) being the final common pathway for the following symptom expression: (a) direct irritation of cortical centers that integrate complex visual information, as in epilepsy; (b) lesions in the visual pathway that impair cortical input; and (c) brainstem lesions affecting ascending cholinergic and serotonergic pathways.

Hypnagogic hallucinations. Visual hallucinations of spots of light evolving into animals or of people wearing bright colors or engaged in active situations may occur occasionally in up to one-third of us when we are about to go to sleep, known as hypnagogic hallucinations.[20–22] These hallucinations may last a few seconds or minutes. The experience of these hallucinations is different than a dream, because the observer is “looking at” the scene, rather than being a participant in it. The emotional reaction to the experience is variable.

Peduncular hallucinosis. In peduncular hallucinosis, the hallucinations appear natural in form and move like a cartoon. Insight usually is preserved. Peduncular hallucinosis was originally described in association with a rostral brainstem lesion affecting the median raphe.

The hallucinations of peduncular hallucinosis usually start a few days after a brainstem stroke that affects the brainstem reticular formation or its projections to the thalamus. Usually they last minutes to several hours. There is often an associated disturbance of consciousness. Patients usually have insight into the nature of the hallucinations.

Visual hallucinations in Parkinson’s disease. The pathology of Parkinson’s disease extends beyond the nigrostriatal system,23 with widespread brainstem changes. Subtle deficits of visual pathways have been described and attributed to dopamine deficiency in the retina and perhaps also in central pathways, as well as significant cholinergic dysfunction related to a loss of cholinergic neurons from the basal nucleus of Meynert.[1] There is loss of the noradrenergic neurons of the locus coeruleus and serotonergic neurons of the raphe nuclei.

Nevertheless, hallucinations were not reported in Parkinson’s disease patients until specific treatments were tried that affected the cholinergic and dopaminergic systems. While the pathological changes of Parkinson’s disease may resemble those of peduncular hallucinosis, it is likely that current treatments, when combined with the disease process itself, may alter neurochemistry in these patients enough to produce complex visual hallucinations.

The visual hallucinations of the blind: Bonnet syndrome. Complex visual hallucinations in the context of vision loss occur most commonly in cases of macular degeneration.[24] Hallucinations occur in 10 percent of patients with severe visual loss. These hallucinations are not stereotyped, but tend to be vivid images of animals and figures, lasting briefly (when alert) or for hours, when drowsy.[25] They occur with eyes open, usually in the evening. Most patients (unless also suffering from dementia) have insight, and are not distressed by these hallucinations.[26]

Anton’s syndrome (visual anosognosia or cortical blindness). Cortical blindness occurs with bilateral posterior cerebral artery occlusions from anoxia or stroke, and sometimes occurs in multiple sclerosis. In cases of cortical blindness, the electroencephalogram (EEG) loses the usual posterior 8 to12Hz (alpha) rhythm. Pupils are normal and reactive. The lesions extend beyond the striate cortex to involve the visual association areas.[16]

Cortical blindness can be accompanied by Anton’s syndrome, where a cortically blind person acts as though or insists she or he can see or appears indifferent to a loss of sight and confabulates a description of a scene. When walking, the person may collide with objects and become injured. Where there is hemianopia rather than complete cortical blindness, the visual hallucinations may appear in the defective field or move from the intact field toward the hemianopic one. A much smaller lesion than that which causes hemianopia without any hallucinations can cause hallucinations alone, suggesting that an intact cortex with visual processing capacity is required and that the phenomenon is related to cortical release mechanisms.[27] The hallucinations are usually transient, lasting days or weeks, but can be permanent.

Case example. One 55-year old woman who developed a hemiparesis described acute left-sided weakness and left hemianopia, followed by a drowsy feeling and hallucinations affecting her left visual field. She saw enormous snakes in vivid colors, but was not afraid of them as she “realized they were not real.” Over subsequent weeks this visual symptom subsided, although the hemiparesis persisted.[1]

Visual hallucinations in focal epilepsy. Complex visual hallucinations associated with focal epilepsy are different from those already described, in that they are brief, stereotyped, and fragmentary. They may be associated with other seizure manifestations, such as altered awareness and automatisms. EEG will confirm that there is direct stimulation of the visual association cortical area that is responsible for the epileptic, complex, visual hallucinations.[28]

Simple, unformed visual hallucinations. Flashes of light, colored spots, or scintillating zigzag lines are related to lesions in the calcarine cortex and are observed in migraine. In addition, older persons can see zigzag flashes of light in the peripheral field of one eye due to vitreous deposits in the eye. The latter is a benign condition, referred to as “Moore’s lightning streaks.”

Visual agnosias. In visual agnosia, the patient cannot name or indicate the use of a seen object by spoken or written word or by gesture.[16] However, if the patient palpates the object, she or he can identify it at once, or by its smell, sound, and so forth. Patients with this symptom generally have a large lesion, often bilateral, in the occipital lobe(s).

Subtypes of visual agnosias exist. For example, the occasional patient has been unable to perceive simultaneously all the elements of a scene and to interpret the scene; some patients cannot identify a familiar face either by looking at the person or a picture (although the patient knows she or he is looking at a face and can point out the features), and may have trouble interpreting facial expressions. In these patients, visual field defects are almost always present, and usually the patients have bilateral lesions of the ventromesial occipitotemporal regions.[29]

Visual system defects in schizophrenia. Schizophrenia is associated neurological “soft signs,” including nystagmus. In addition, patients who have schizophrenia may have deficits in higher-order processing of visual information,[6] as well as an encoding deficit during face processing within the right fusiform face area of the brain (a brain area also affected in autism).[30]

HOW TO TEST CRANIAL NERVE II (OPTIC)

Fundoscopy. Intracranial pressure. Using an ophthalmoscope, observe the optic disc, physiological cup, retinal vessels, and fovea. Note the pulsations of the optic vessels and check for a blurring of the optic disc margin and a change in the optic disc’s color form its normal yellowish orange.

Papilledema. Papilledema is characterized by disc swelling, hemorrhages, and exudates. The initial change in the ophthalmoscopic examination in a patient with increased intracranial pressure is the loss of pulsations of the retinal vessels. This is followed by blurring of the optic disc margin and possibly retinal hemorrhages. Note the presence of a chalky white disc with discrete margins. Optic atrophy is a late finding with increased intracranial pressure. In papilledema, visual acuity is usually preserved.

Pupils. Pupils-Swinging light test for “Marcus-Gunn pupil:”[16] When moving a penlight back and forth from one eye to the other, normally both pupils change diameter when the light is moved from the normal eye (brain perceiving increased illumination) to the abnormal eye (brain perceiving less illumination). This is called an afferent pupillary defect (APD) or Marcus-Gunn pupil, commonly seen in optic neuritis.

Visual acuity. Visual acuity problems are too common in the general population to expect that they can assist in differential diagnosis. Studies comparing elderly patients with schizophrenia-like disorders to those with dementia and other organic mental disorders have not found differences in visual acuity,[31,32] although other differences in ocular health were noted. In the elderly, psychosis is associated with visual impairment, or at least uncorrected visual impairment.[33]

Visual fields. Visual fields are evaluated via “confrontation.” Face the patient one foot away at eye level. Tell the patient to cover the right eye with the right hand and look you in the eyes. Instruct the patient to remain looking you in the eyes and say “now” when your fingers enter from out of sight, into his or her peripheral vision.

Visual fields have not been found impaired in routine confrontation testing of patients with schizophrenia.[34,35]

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