Optical Disturbances

People with myopia have decreasing visual acuity at increasingly greater distances. Myopia first becomes troublesome during adolescence when it causes difficulty with seeing blackboards, watching movies, and driving. Because reading and other close-up activities, which require “near vision,” remain unimpaired, the lay public commonly labels people with myopia “nearsighted.”

The usual causes of myopia are optical rather than neurologic, such as a lens that is too “thick” or a globe that is too “long” (Fig. 12-3). Occasionally medicines cause myopia. For example, topiramate (Topamax), a widely prescribed antimigraine and antiepileptic drug (AED), may produce an acutely occurring but transient myopia. (Topiramate can also lead to angle closure glaucoma [see later].)

FIGURE 12-3 Image focusing in hyperopic (farsighted) and myopic (nearsighted) eyes. A, In normal eyes, the lens focuses the image on to the retina. B, In hyperopic eyes, the shorter globe or improperly focusing lens causes the image to fall behind the retina. C, In myopic eyes, the longer globe or improperly focusing lens causes the image to fall in front of the retina. Lenses can compensate for the refractive errors of hyperopia and myopia. Alternatively, laser or surgical “flattening” of the lens may correct myopia.

In myopia’s counterpart, people with hyperopia or hypermetropia have decreasing visual acuity at increasingly shorter distances. The lay public commonly labels them “farsighted” because they can only see distant objects, such as street signs, rather than closely held ones, such as newspapers and computer screens. In hyperopia or farsightedness, the lens is usually too “thin,” rendering its refractive strength insufficient. Occasionally the globe is too “short.”

In presbyopia, older individuals cannot focus on closely held objects because their relatively inelastic and dehydrated lenses are unable to change shape. With their impaired near vision, people with presbyopia, as well as those with hyperopia, tend to hold newspapers and sew with needles at arms’ length. Reading glasses usually can compensate for the refractory defect by bringing the focal point into the proper working distance. (Older individuals and those with diabetes also tend to have small pupils [miosis] that should not be mistaken for Argyll Robertson pupils [see later].)

Disruption of the accommodation reflex is another common cause of visual disturbance. Normally, when a person looks at a closely held object, efferent fibers from this reflex contract the ciliary body muscles to thicken the lens so that the image falls on the retina. These fibers also cause miosis and increase convergence muscle tone. For example, when a person begins to read the screen of an iPad, the eyes converge, the pupils constrict, and each lens thickens to provide greater refraction. In other words, the reflex accommodation focuses the image of closely held objects on the retina.

Because the parasympathetic nervous system mediates the accommodation reflex, many medications with anticholinergic side effects impair visual acuity for closely held objects (Fig. 12-4). These medicines – selective serotonin reuptake inhibitors (SSRIs), selective norepinephrine reuptake inhibitors (SNRIs), tricyclic antidepressants, and clozapine – produce blurred vision mostly because their substantial anticholinergic properties impair patients’ accommodation reflex. For example, duloxetine (Cymbalta) causes blurred vision in approximately 3% of patients; sertraline (Zoloft) and paroxetine (Paxil), 4%; and venlafaxine (Effexor) at 75 mg, 9%. This side effect may be unsuspected because these medicines can impair accommodation without producing other anticholinergic effects, such as dry mouth, constipation, and urinary hesitancy.

FIGURE 12-4 Accommodation and accommodation paresis. A, When looking at a distant object, parallel light rays are refracted little by a relatively flat lens on to the retina. B, Accommodation: when looking at a closely held object, ciliary muscle contraction increases the curvature of the lens, greatly refracting the light rays. C, Accommodation paresis: If the ciliary muscles are paretic, the lens cannot form a rounded shape. Its weakened refractive power focuses light rays from closely held objects behind the retina; however, parallel light rays from distant objects still focus on the retina. Therefore, accommodation paralysis blurs closely held objects but leaves distant ones distinct.

Abnormalities of the Lens, Retina, and Optic Nerve

Cataracts (loss of lens transparency) result from complications of old age (senile cataract), trauma, diabetes, myotonic dystrophy (see Chapter 6), and chronic use of certain medicines, such as steroids. In prolonged, high doses, phenothiazines and some second-generation neuroleptics may produce minute lens opacities, but ones rarely dense enough to impair vision.

Pigmentary changes in the retina can be a manifestation of injuries, degenerative diseases, diabetes, infection, or the use of massive doses of phenothiazines (Fig. 12-5). Among infants and children, nonaccidental head injury (child abuse), particularly violent head shaking or direct trauma, creates retinal hemorrhages. Other stigmata of repeated trauma – spiral fractures of the long bones, multiple skull fractures, and burns (see Chapter 22) – frequently accompany these retinal hemorrhages.

FIGURE 12-5 Massive doses of thioridazine (Mellaril) may induce retinal hyperpigmentation – described as “black bone spicules” or “salt and pepper.” Before these retinal pigmentary changes are visible on fundoscopic examination, patients may complain of blurred vision or impaired nighttime vision.

In 25% or more of Americans older than 65 years, the cells of the retina’s pigment epithelium, mostly in the macula, degenerate through a variety of mechanisms, including proliferation of the underlying blood vessels. When degeneration involves cells in the macula, a condition known as macular degeneration, it distorts patients’ critical central vision. Patients characteristically lose their reading ability. With their remaining peripheral vision, patients negotiate around their living areas. However, progressive deterioration deprives patients of all their eyesight. As with individuals who develop blindness from any cause, those beset by macular degeneration are at risk of losing their self-sufficiency, appearing to have cognitive dysfunction, and experiencing visual hallucinations (especially if they also have hearing or cognitive impairments [Box 12-1]).

Among acquired immunodeficiency syndrome (AIDS) patients, opportunistic organisms, such as cytomegalovirus, infect the retina. Current anti-AIDS regimens have greatly reduced visual complications. In a more benign situation, several medicines lend a visual discoloration. For example, digoxin, at toxic concentration, casts a transient yellow hue (xanthopsia [Greek, xanthos yellow + opsis sight]), and sildenafil (Viagra), a blue or yellow hue. On the other hand, a few, such as the AED vigabatrin (Sabril), sometimes cause permanent retinopathy.

Optic Nerve

Injuries of the optic nerves, which are projections of the central nervous system (CNS), result in visual loss that may be limited to a scotoma (an area of blindness [see Fig. 15-2]), but may encompass the entire visual field. In addition, because the optic nerves serve as the afferent limb of the light reflex, optic nerve injury also causes an afferent pupillary defect: When the examiner shines light into an eye with optic neuritis, both pupils fail to constrict; however, when the same light shines into the unaffected eye, both pupils normally constrict (see Fig. 4-2). With time, optic nerve injuries usually cause atrophy, revealed by the fundoscopic examination showing pallor of the optic head.

Optic nerve injuries may occur either as isolated conditions or ones accompanied by injuries of the cerebrum or other part of the CNS. One of the most common, inflammation of one or both optic nerves, optic or retrobulbar neuritis, causes sudden, painful visual loss (Fig. 12-6), as well as an afferent pupillary defect. In addition, they see “desaturated” colors with their remaining vision. For example, patients cannot appreciate the difference between fire engine red and brick red. In severe cases, they cannot distinguish red from green.

FIGURE 12-6 Neurologists consider the optic disk, which they can see on fundoscopic examination, to be the bulbar portion of the optic nerve. They consider the long segment of the optic nerve behind the eye its retrobulbar portion. Depending on where an inflammatory condition, such as multiple sclerosis, attacks the optic nerve, patients develop optic or retrobulbar neuritis. Visual loss, pain on eye movement, and color desaturation characterize optic and retrobulbar neuritis.

When optic neuritis affects the optic disk, which is the most anterior segment of the optic nerve, physicians will often see inflammation of the disk (papillitis) on fundoscopic examination. If the inflammation affects only the segment of the optic nerve posterior to the disk, as in retrobulbar neuritis, physicians will be unable to see any abnormality of the optic disk on fundoscopic examination.

Of the many conditions that cause optic neuritis, demyelinating illnesses, particularly multiple sclerosis (MS) and its close relative neuromyelitis optica (NMO), are the most common (see Chapter 15). Most importantly, optic neuritis frequently precedes the appearance of other manifestations of MS and complicates the course of most cases. If an otherwise asymptomatic patient develops optic neuritis and the magnetic resonance imaging (MRI) shows two or more hyperintense lesions in the brain, that patient has greater than a 70% risk of developing MS. On the other hand, an otherwise asymptomatic optic neuritis patient who has no MRI lesions has only a 25% risk of developing MS.

With recurrent optic or retrobulbar neuritis attacks, the optic nerve becomes atrophic, the disk white, the pupil unreactive, and the eye blind. A course of high-dose, intravenous steroids may shorten an attack, but probably does not alter the outcome. Physicians cautiously use steroids because they can produce mental aberrations, including euphoria, agitation, and, in the extreme, psychosis.

Toxins, including some medications, can also damage the optic nerves. In one scenario, alcoholics inadvertently drink methanol (CH₃OH), a solvent component of antifreeze and cooking fuels, such as Sterno, and an illicit adulterant of everyday ethanol (C₂H₅OH) drinks. Drinking methanol causes a combination of gastroenteritis, delirium, and visual problems, particularly blurry vision and scotoma. With severe methanol intoxication or merely its chronic intermittent consumption, optic nerves atrophy and victims become blind.

An inflammatory condition of the arteries that supply the optic nerve, temporal or giant cell arteritis, often leads to ischemia of the optic nerves. Moreover, the arteritis tends to spread to the cerebral arteries (see Chapter 9). Typically affecting only people older than 65 years, temporal arteritis often first causes a mild to moderate, prolonged (weeks to months) illness featuring headache accompanied by systemic symptoms, such as malaise, prolonged aches, and pains. The number and variety of these initial nonspecific symptoms understandably lend the appearance of depression or a somatoform disorder. However, physicians should avoid missing this diagnosis because, unless they promptly treat the patient with high-dose steroids, it can result in blindness and strokes. Finding giant cells and other signs of inflammation on a temporal artery biopsy will confirm the diagnosis.

Leber hereditary optic atrophy, an illness attributable to a mitochondrial DNA mutation, also involves the optic nerves, but no other part of the CNS or the musculature (see Chapter 6). Most commonly affecting young males, it causes visual loss culminating in blindness in one and then, within months, the other eye.

Several conditions simultaneously affect the cerebrum and the optic nerves. These conditions produce cognitive decline and personality changes as well as blindness. MS would be one example. Another is the storage lysosomal storage disease due to a deficiency in hexosaminidase-A, Tay–Sachs disease, which is almost always fatal by age 5 years.

Another classic example of simultaneous injury of both the optic nerve and cerebrum is an olfactory groove or sphenoid wing meningioma. This tumor compresses the near-by optic nerve (see Chapters 19 and 20) and burrows into the overlying frontal or temporal lobe. The cerebral damage can trigger complex partial seizures and cause cognitive decline and personality changes. At the same time, optic nerve damage causes optic atrophy and blindness in one eye.

Similarly, tumors of the pituitary region, such as adenomas or craniopharyngiomas, may also produce visual loss accompanied by psychologic changes. Unless detected and removed early, these tumors grow slowly upward to compress the optic chiasm and hypothalamus and downward to infiltrate the pituitary gland (see Fig. 19-4). Compression of the optic chiasm causes the pathognomonic bitemporal hemianopsia. Compression of the hypothalamus and pituitary causes headache and panhypopituitarism: decreased libido, diabetes insipidus, loss of secondary sexual characteristics, and sleep disturbances.

Open-Angle Glaucoma

Open-angle or wide-angle glaucoma occurs seven times more frequently than closed-angle glaucoma. People at greatest risk are those who are older than 65 years, diabetic, myopic, and relatives of glaucoma patients. Because symptoms are usually absent at the onset, this variety of glaucoma might be diagnosed only when an ophthalmologist detects elevated intraocular pressure, certain visual field losses, or changes in the optic nerve. Later, when central vision or acuity is impaired, the optic cup is abnormally deep and permanently damaged. Lack of symptoms in the initial phase of open-angle glaucoma is one of the most compelling reasons for annual ophthalmologic examinations.

Open-angle glaucoma usually responds to topical medications (eyedrops) or laser treatment. Psychotropic medications do not precipitate open-angle glaucoma. In general, patients with open-angle glaucoma may be given antidepressants and other psychotropic medications provided that their glaucoma treatment is continued.

Angle Closure Glaucoma

In angle closure glaucoma, which is also called closed-angle or narrow-angle glaucoma, intraocular pressure is usually elevated by impaired aqueous humor outflow at the filtration angle (see Fig. 12-7). In one variety, the fluid becomes trapped behind the iris. Patients with narrow-angle glaucoma usually are older than 40 years and often have a family history of the disorder, but they also frequently have a history of hyperopia and long-standing narrow angles. Few have had symptoms, such as seeing halos around lights, preceding an attack of angle closure glaucoma. In contrast to the relatively normal appearance of the eye in open-angle glaucoma, in acute angle closure glaucoma the eye is red, the pupil dilated and unreactive, and the cornea hazy. Moreover, the eye and forehead are painful, and vision is impaired.

Angle closure glaucoma is sometimes iatrogenic. For example, when pupils are dilated for ocular examinations, the “bunched-up” iris can block the angle (see Fig. 12-7, B). Likewise, medicines with anticholinergic properties, probably because they dilate the pupil, can precipitate angle closure glaucoma.

Despite the attention to the potential problem, the complication rate of glaucoma with tricyclic antidepressant use is low, and with SSRIs it is almost nonexistent. However, as neurologists and other physicians prescribe tricyclics for increasingly numerous conditions – chronic pain, urinary incontinence, headache, and diabetic neuropathy – many patients remain vulnerable. In addition, other medications for neurologic diseases, particularly topiramate, can cause angle closure glaucoma.

Whatever the cause of angle closure glaucoma, prompt treatment can preserve vision. Topical and systemic medications open the angle (by constricting the pupil) and reduce aqueous humor production. Laser iridectomy immediately and painlessly creates a passage directly through the iris that drains aqueous humor.

Because glaucoma poses such a threat, individuals older than 40 years should have intraocular pressure measured every 2 years and those older than 65 years, every year. Most patients who are under treatment for either form of glaucoma may safely receive psychotropic medications. Glaucoma medications, such as pilocarpine (a cholinergic medicine that constricts the pupils), and ophthalmic beta-blockers, such as timolol (Timoptic), may be absorbed into the systemic circulation and create psychologic and cardiovascular side effects, including orthostatic lightheadedness, bradycardia, and even heart block. Not surprisingly, elderly patients who use beta-blocker eyedrops sometimes experience brief periods of confusion.

Children are also susceptible to systemic absorption. For example, when given scopolamine or other atropine-like eyedrops for ocular examination, children often become agitated. On the other hand, marijuana, despite claims of its proponents, is no more effective than standard medications for treating glaucoma.

Anton Syndrome

The dramatic neuropsychologic phenomenon of Anton syndrome – blind patients explicitly or implicitly denying that they have lost all vision – characteristically complicates the sudden onset of blindness. Whether the blindness stems from a blast injury of both eyes, bilateral occipital lobe strokes, or other cause, the irrational response to blindness, rather than blindness itself, constitutes Anton syndrome. These patients typically respond, as those with anosognosia (see Chapter 8), by using denial as a defense. Sometimes they simply refuse to say that they have lost vision. Others blame external factors, like dim light, for their problem. Some may, if pressed, acknowledge visual loss but confabulate by “describing” their room, clothing, and various other objects. Anton syndrome allows blind patients to behave as though they still had normal vision and proceed to stumble about their room.

For example, a 76-year-old man sustained a right-sided posterior cerebral artery stroke that was superimposed on a prior left-sided posterior cerebral stroke. He first blamed his inability to see the examiner’s blouse on poor lighting and having misplaced his glasses. He then claimed to be uninterested in the exercise. When urged, still implicitly denying his blindness, he calmly described the blouse as “lovely” and “becoming,” at one time elaborating that it was “obviously finely sewn and made from fine material.”

Depending on the patient’s premorbid state and extent of the responsible cerebral lesion, signs of generalized cerebral cortex injury, especially delirium and dementia, may accompany Anton syndrome. In addition, if the cerebral lesion extends beyond the occipital lobes, amnesia (from bilateral temporal lobe injury) or anosognosia for other deficits (from right-sided parietal lobe injury) may accompany individual cases.

Palinopsia

Palinopsia (Greek, palin, again; opsis, vision), which may be likened to “visual perseveration,” consists of recurrent or persistent images following removal of the visual stimulus. Technically a form of hallucinations, the visions usually appear to the patient within an area of incomplete visual loss in the left lateral field. Patients with palinopsia often have recurrent images of objects, scenes, or family members. Frequently, several afterimages appear immediately in rapid succession as “visual echoes,” but sometimes only after hours or days. Because palinopsia consists of duplicate images of common inanimate objects, it usually does not elicit an emotional response.

Both the left lateral visual impairment and superimposed hallucinations stem from right-sided occipital and parietal lobe lesions. Palinopsia may represent a failure of the cortex to capture only a single image, but it rarely results from seizure activity. In palinopsia, compared to seizure activity, the visions are well formed, not so much hallucinations as reproductions of objects in the environment, and unaccompanied by changes in emotion or level of consciousness. Because occipital lobe discharges would be readily detectable, an EEG would readily identify seizures causing isolated repetitive visual hallucinations.

Agnosia

Another visual perceptual disturbance, visual agnosia, consists of the inability to appreciate the meaning of an object by its appearance, despite an intact visual system. Patients with visual agnosia simply cannot comprehend what they see. For example, a man would be able to describe a stop sign by saying that it is octagonal, red, and says “S-T-O-P,” but not be able to explain what action drivers must take.

Neurologists detect visual agnosia most often in patients with cognitive impairment from multiple small strokes or Alzheimer disease. They sometimes portray it as a disconnection between the visual and cognitive centers.

Visual agnosia is also a major component of the infamous, although uncommon, Klüver–Bucy syndrome. Neurosurgeons have produced this behavioral disorder in monkeys by resection of both anterior temporal lobes, which contain the amygdalae and components of the limbic system. The resulting limbic system damage produces visual agnosia so severe that the monkeys not only touch all objects, but they compulsively identify all objects by putting them into their mouth (“psychic blindness”). Their behavior can be repetitive, compulsive, and indiscriminate. When the Klüver–Bucy syndrome occurs in humans (see Chapter 16), they display a muted variation of psychic blindness, oral exploration, which consists of their placing inedible objects in their mouth, although only briefly, partly, and absent-mindedly.

Color agnosia is a particular inability to identify colors by sight. The affected individual’s problem is neither aphasia nor color blindness, which is a sex-linked inherited retinal abnormality. Patients with color agnosia cannot specify (by speech or writing) the name of colors. When shown painted cards, for example, they cannot say or write the name of the colors. Despite those deficits, patients can match pairs of cards of the same color, read Ishihara plates (pseudoisochromatic numbered cards), and recite the colors of well-known objects, such as the American flag. In striking contrast, they behave as though they appreciate colors.

In a related impairment, prosopagnosia, patients cannot recognize familiar faces (Greek, prosópon, face, person; agnosia, lack of knowledge). An inability to identify objects out of their usual (visual) context, such as a shirt pocket cut from a shirt, often accompanies prosopagnosia. Nevertheless, patients can continue to identify individuals by their voice, dress, and mannerisms. Although reports are not fully consistent, neurologists usually attribute prosopagnosia to either bilateral occipitotemporal or right-sided temporal lesions. Instead of relating the symptom to a structural lesion, they sometimes attribute it to neurodegenerative illnesses, such as Alzheimer disease or frontotemporal dementia.

In a variation of prosopagnosia, patients with right cerebral lesions cannot match pairs of pictures of unfamiliar faces. This condition represents a visual perceptual impairment possibly induced by a nondominant parietal lobe lesion.

Balint Syndrome

Balint syndrome, which neurologists attribute to bilateral parietal-occipital region damage from strokes or Alzheimer disease, consists of three related, admittedly overlapping, elements concerned with visual attention: ocular apraxia, optic ataxia, and simultanagnosia. Ocular apraxia, which neurologists sometimes call “psychic paralysis of fixation,” is the neuropsychologic inability of a patient to shift attention by looking away from an object to one located in the periphery of vision. Patients behave as though they were mesmerized by the original object or like a military radar system that has locked on to an approaching hostile aircraft. By briefly closing their eyes, which momentarily interrupts attention, patients can shift their gaze.

Optic ataxia, the second element of Balint syndrome, is the inability to look or search in a deliberate pattern. A common manifestation of this element consists of inability to read in methodical visual sweeps.

The third element, simultanagnosia, consists of being able to attend only to objects immediately in the center of vision. When confronted with objects in the center and the periphery of vision, patients will invariably ignore the one in the periphery even though it might be more important or attractive. Because of simultanagnosia, patients cannot comprehend complicated scenes or objects. For example, they would be unable to comprehend a baseball game, but instead able to see only an individual player or a base.

Psychogenic Blindness

The medical literature has stated that nonorganic visual loss, which neurologists continue to label psychogenic blindness, explains the symptom of visual loss in as many as 5% of children and adults. However, cases of psychogenic blindness that convincingly mimic true blindness are rare. Because people lack an intuitive knowledge of visual pathways, neurologists can readily detect nonanatomic patterns of psychogenic blindness. Even bedside testing can easily reveal its spurious nature.

Psychogenic blindness occurs in malingering and various psychiatric conditions, particularly in what the preliminary version of the Diagnostic and Statistical Manual of Mental Disorders (DSM), 5th edition, classifies as Conversion Disorder (Functional Neurological Symptom Disorder). One of the most common presentations of psychogenic blindness is monocular visual loss and ipsilateral hemiparesis. This combination defies the laws of neuroanatomy because the division of optic pathways at the optic chiasm provides that a cerebral lesion causing hemiparesis also causes hemianopsia – not monocular blindness. (Brainstem lesions may cause hemiparesis and diplopia, but not hemianopsia or monocular blindness.) In another presentation, individuals with psychogenic blindness often needlessly wear sunglasses. This ploy seems to serve several purposes: It signals that they are blind, reduces visual distractions, and prevents observers from seeing when their eyes establish eye contact.

Similarly, tubular or tunnel vision defies the laws of optics that dictate that the visual area should expand with increasing distance (Fig. 12-8). Important exceptions to this general rule, however, sometimes occur when patients with migraine with aura have constriction of their peripheral vision and in some patients taking vigabatrin.

FIGURE 12-8 The area seen by a person normally increases conically in proportion to the distance from the object. In tubular or tunnel vision, which defies the laws of optics, the “visible” area remains constant despite increasing distance.

To unmask psychogenic blindness, an uninhibited examiner simply might make childlike facial contortions or ask the patient to read some four-letter words. The patient’s reaction to these provocations would reveal the ability to see. When only one eye is affected by psychogenic blindness, fogged, colored, or polarized lenses in front of the unaffected eye will often confuse (or fatigue) a patient into revealing that vision is present.

Another technique that exposes intact vision is to draw a striped cloth or spin a vertically striped cylinder (drum) in front of a person. The moving striped surface will elicit optokinetic nystagmus unless true blindness is present. Likewise, having patients stare at a large, moving mirror irresistibly compels them to follow their own image.

In a different approach, neurologists offer patients eyeglasses with lenses having negligible optical strength. Wearing these glasses allows patients to extract themselves from psychogenic blindness without embarrassment.

If clinical tests are inconclusive, EEG and other electrophysiologic testing may help. Alpha rhythm overlying the occipital lobes of patients at rest with their eyes closed, and loss of that rhythm when they open their eyes, indicates an intact visual system. However, because patients’ anxiety or concentration suppresses alpha activity, its absence is not as meaningful as its presence. In visual-evoked response testing, another noninvasive electrophysiologic test, visual system injuries produce abnormal potentials (see Chapter 15).

Visual Hallucinations

Unlike auditory hallucinations, which typically result from psychiatric disorders, visual hallucinations in adults typically result from neurologic disorders. (In children, they may be a symptom of schizophrenia.) Specific qualities of visual hallucinations that should prompt a neurologic evaluation include stereotyped visions, alteration in the patient’s level of consciousness, and concurrent behavioral abnormalities. Patients usually require an imaging study, routine blood tests, urinary toxicology analysis, and an EEG.

Visual hallucinations can originate in dysfunction of the frontal, temporal, or occipital cortex. Problems as diverse as toxic-metabolic encephalopathy, dementia-producing diseases, medication side effects, and structural lesions can produce them (see Box 12-1).

Seizures

Elementary partial, complex partial, or frontal lobe seizures (see Chapter 10) can produce visual hallucinations. Seizure-induced visual hallucinations tend to be stereotyped and brief, can be “seen” in both eyes, and may even appear in a hemianopic area. They range from simple geometric forms in elementary partial seizures to detailed visions accompanied by sounds, smells, thoughts, emotions, and, characteristically, impairment of consciousness, in complex partial seizures.

Migraine Aura

The “aura” in migraine with aura (previous termed “classic migraine”) consists of sensory disturbances – olfactory, sensory, or visual. In almost all cases, auras include stereotyped visual hallucinations (see Fig. 9-2). The majority consist of distinctive crescent scotomata or scintillating, patterned zig-zag lines (fortification spectra) that move slowly across the visual field for 1–20 minutes before yielding to a hemicranial headache. In a potentially confusing situation, visual auras sometimes represent the sole manifestation of migraine. In rare individuals, migraine aura consists of elaborate visual distortions, such as metamorphopsia, in which individuals and objects appear, to the patient, to change size or shape, as in the celebrated Alice in Wonderland syndrome.

Narcolepsy

As an element of the narcoleptic triad (see Chapter 17), visual hallucinations intrude into a patient’s partial consciousness. Narcoleptic-induced visual hallucinations are essentially dreams composed of variable, unpredictable – not stereotyped – intricate visions accompanied by rich thoughts and strong emotions. They tend to occur while patients fall asleep (hypnagogic hallucinations) or awaken (hypnopompic hallucinations). As with normal dreams, these hallucinations are associated with flaccid, areflexic paresis and rapid eye movements (REMs).

Neurodegenerative Illnesses

Visual hallucinations are also a hallmark of neurodegenerative diseases that cause dementia, particularly Alzheimer, Lewy bodies, and Parkinson diseases or their treatment (see Chapters 7 and 18). When they are manifestations of these disorders, hallucinations tend to be visually complex, have a paranoid aspect, and occur predominantly at night. As a clue to dementia with Lewy bodies disease, visual hallucinations occur frequently and begin early in its course. In contrast, when visual hallucinations complicate Alzheimer disease, they occur in its late stages. Hallucinations in Parkinson disease are usually partly medication-induced.

Toxins

Many classes of medicines, in addition to anticholinergics or dopamine enhancers, may cause hallucinations. Similarly, illicit stimulants, by design, cause hallucinations. In general, medicine- or drug-induced hallucinations develop in the context of delirium. Even withdrawal from certain substances causes visual hallucinations. The best-known example is alcohol withdrawal causing delirium tremens (DTs). In this case, most patients have varied, sometimes frightening visions that stem from the environment. Agitation, confusion, fluctuating consciousness, sweating, and tachycardia accompany or overwhelm the hallucinations. In some DT patients, however, the hallucinations seem to petrify the patients who then become hypervigilant, reticent, and immobile.

Sensory Deprivation

Visual loss from either cortical blindness or bilateral eye injuries produces deprivation or “deafferentation” of the visual cortex. Without the physiologic input, unregulated visual cortex activity emerges as hallucinations. For example, soldiers sustaining extensive eye wounds have periods of “seeing” brightly colored forms and even entire scenes. Because of the potential adverse effects of blindness, cataract patients should avoid simultaneous bilateral ophthalmologic surgical procedures.

In contrast to visual hallucinations occurring in patients with dementia, delirium, or psychiatric disorder, blind individuals with no mental disorder experience visual hallucinations. When this phenomenon occurs, neurologists label it the Charles Bonnet syndrome. As if to emphasize that it originates in deafferentation, patients with hemianopsia may have hallucinations exclusively in their blind visual field.