Ординатура / Офтальмология / Английские материалы / Neuro-Ophthalmology_Kidd, Newman, Biousse_2008

recognize the object by the visual information—this includes the picture that they just drew or copied. Interestingly, patients with associative visual agnosia are usually capable of identifying the objects by the other means, for instance, by touch, hearing, or verbal description63—traditionally known as “optic aphasia.” For example, associative visual agnosics would fail to recognize a key when it is presented visually but are able to recognize it if allowed to touch it. It is important to discriminate this condition from anomic aphasia (inability to name) by using both verbal and nonverbal tests. The anomic patient will be able to recognize and explain the use or function of the object although being unable to name it, whereas the visual agnosic will be unable to state what the object is or its function. Associative visual object agnosia occurs as a result of lesions in the ventral stream, particularly at the inferior temporoparietal junction and adjacent white matter, either unilateral or bilateral, resulting from PCA infarction,64 tumor, hemorrhage, and demyelination. This may be the result of a disconnection of the visual association and the temporolimbic memory areas or destruction of visual association cortices in the temporooccipital region.

In some patients, visual object agnosia may be category specific, and cases have been reported in which specific deficits in the identification of fruit and vegetables or animals have been observed. However, the most common dissociation reported in these patients is an impairment in the recognition of natural (living) objects relative to human-made (nonliving) objects. Posterior cortical atrophy, a term first proposed by Benson et al,65 usually gives rise to various cortical disorders, including visual object agnosia.

The visual association cortex may store the neuronal templates that are required to match a visual stimulus with visual memory, and functional imaging studies, including fMRI and PET, have shown activation in the ventral stream during visual object recognition tasks.66 It is not surprising, therefore, that these patients may have partial visual field defects as well as achromatopsia and sometimes prosopagnosia.

Prosopagnosia

Prosopagnosia derives from the Greek prosopon (face) and gnosis (knowledge). It is used to describe the inability to recognize the identity of familiar faces (retrograde defect) or, more rarely, to learn and recognize the identity of new faces (anterograde defect) and is arguably a restricted form of visual agnosia. These patients recognize the components of a face, the eyes, nose, mouth, and so forth, and know that together they represent a face, but they cannot tell whose face it is. However, these patients can usually identify their spouses by voice, perfume, or clothes although failing to recognize the face.

Face recognition is one of the most complex tasks undertaken by the human brain because faces are composed of multiple, complex curved surfaces and in addition show subtle dynamic changes, both short term with emotional state and long term with aging. Despite this, we are still able to easily recognize faces with differing expressions and age. Therefore, because of its complexity, authorities in the past regarded prosopagnosia as a combination of a generalized mental impairment combined with visual disturbances. However, a complex neural network involved in face recognition has now been defined along with cortical

areas containing neurons that are face sensitive.67 These neurons, which respond to face stimuli regardless of identity and not other simple or complex stimuli, have been identified in the inferotemporal (IT) cortex and the superior temporal sulcus (STS) of monkeys. A smaller number are also found in the amygdala, ventral striatum, and inferior prefrontal cortex. These neurons show preferences for facial orientation, for example, frontal versus profile. Neurons in IT are involved in facial identification and in STS in perception of gaze and head orientation. In humans, the homologue for IT is the fusiform facial area (FFA) in the medial occipitotemporal cortex, anterior to the V4 color area.68

Models of the distributed human neural system for face perception propose

that the core system consists of low-level visual processing of facial structure leading to a face percept.67,69,70 This takes place in the STS where recognition

of changeable aspects of faces such as perception of eye gaze, expression, and lip movement is undertaken, and the FFA where invariant aspects of face recognition (i.e., the perception of unique identity) are performed. This face percept then moves into an extended system where, for example, in the anterior temporal region it is matched to “face recognition units,” which store memories of previously encountered faces. Once a match has been made, person identity nodes are activated providing information such as name and biographical information. Other areas may have more specific roles such as the amygdala for processing of facial emotion and emotional responses and the auditory cortex for prelexical speech perception. It has been proposed that depending on precisely where a lesion in this system occurs different disorders of face perception will arise. For example, at one level the face percept may not be formed (an apperceptive prosopagnosia) or at another it may not be possible to match with “face recognition units” (an associative prosopagnosia).71

There is still debate about prosopagnosia as to how specific it is for face recognition. As shown by the case of Bruyer et al72 who was able to recognize individual cows, dogs, houses, streets, and cars but not faces, even those on playing cards, prosopagnosia can be very specific and leave other categorical specific identification intact. However, some patients with prosopagnosia do show difficulty in identification within other specific categories. For example, a farmer was no longer able to recognize his own cattle or a bird-watcher was unable to identify different species of birds. It is, therefore, a possibility that face perception may represent the ultimate expression of a system for detecting subtle differences in shapes and features of objects of a similar category and may not have a unique anatomic substrate or dedicated visual system.70

Lesions in the inferior temporo-occipital cortex, especially the lingual and fusiform gyri, which are part of the ventral stream, are associated with prosopag-

nosia. In most cases, bilateral lesions have been reported73,74; however, isolated right-sided lesions resulting in prosopagnosia have also been reported.75,76

The lesions resulting in prosopagnosia may give rise to visual field defects (usually a left or bilateral upper quadrantanopia or left homonymous hemianopia), achromatopsia, and topographical disorientation. Although the most common etiology is PCA infarction, it has been reported, in relation to primary brain tumor,77 hematoma,78,79 brain abscess, and surgical resections.80 Although most cases of prosopagnosia are acquired, congenital cases of prosopagnosia are also recognized.81 These patients may not recognize their face recognition deficits until they encounter social difficulties later on in life.

Disorder of Visuospatial Function

Balint’s syndrome, a triad of visual defects, was first described in 1890 by Rezso¨ Balint, an Austrian-Hungarian neurologist, in a patient with bilateral occipitoparietal lesions resulting from a stroke. The triad comprises simultanagnosia (an inability to perceive the components of a visual scene as a whole, with variable perception of isolated components), ocular apraxia (inability to voluntarily direct gaze toward a new object of interest), and optic ataxia (impaired target pointing under visual guidance despite normal limb function and joint positional sense). Gordon Holmes, a British neurologist, also reported a similar syndrome resulting from brain damage caused by gunshot injuries and considered the condition a “disturbance of visual orientation.” Most of the subsequent reported cases of Balint’s syndrome, either complete or incomplete, have been caused by hypotensive stroke associated with diffuse atherosclerosis or a cardiac bypass operation, multiple emboli or venous infarction,82 tumor (multiple metastases or a butterfly glioma), trauma, prion disease, human immunodeficiency virus infection,83–85 corticobasal ganglionic degeneration,86 adrenoleukodystrophy,87 and Alzheimer’s disease.88

As previously mentioned, patients described by Balint and Holmes had a rather widespread injury to the brain that was almost always bilateral. Recent evidence indicates that it is rather difficult to tie the triad to one single location in the brain. In addition, there have been cases of incomplete triads, for instance, optic ataxia alone without simultanagnosia or ocular apraxia, which certainly supports the idea of different locations responsible for the different components. In general, lesions at the parieto-occipital junction, often bilateral, are often found in patients with these visuospatial disorders. Patients with Balint’s syndrome are often severely disabled and appear almost blind, requiring assistance to avoid bumping into things. They are not able to direct their gaze toward the new stimuli and do not blink to threat.89

Although simultanagnosia contains the term agnosia, recent evidence suggests that it has nothing to do with agnosia. It is a disorder of visual perception in which the patient fails to appreciate all the visible items in a complex visual scene and has been termed “piecemeal vision.” For instance, when shown a picture of a spider on a hand, the patient might report only the spider and the hand is totally ignored. Such patients may be able to read, but only when they spell each letter out loud (single letter reading strategy). Patients with simultanagnosia appear to have defects in spatial integration and sustained attention but have relatively intact visual fields on formal testing. Patients with simultagnosia see only with macular vision, which restricts their overall perception of a visual scene, and they show unpredictable shifts of focus from region to region. Isolated simultanagnosia is caused by bilateral lesions of the superior occipital visual association cortex (Brodmann areas 18,19), with sparing of the parietal visuomotor control area. It may occur in Alzheimer’s disease and is a common finding in posterior cerebral atrophy.

Ocular apraxia, first described as psychic paralysis of gaze by Balint, is not well described. The most significant feature is the inability to generate voluntary saccades toward a particular object of interest, although reflexive saccades (saccades toward a novel target) are intact. Gaze is therefore relatively random and targets are found by chance. Other features include saccadic dysmetria, a disturbance

in maintaining fixation, and impaired convergence and pursuit eye movements. Ocular apraxia is considered to be a disorder of visuospatial integration and generation of volitional saccades rather than a “true” apraxia. Isolated oculomotor apraxia is rare but can be seen in bilateral parietal lobe disorders and Gaucher’s disease.

Optic ataxia (also called visuomotor ataxia or defective visual localization) describes a difficulty in reaching for an object by hand under visual guidance. However, patients with optic ataxia can reach to touch their own body parts relatively accurately with their eyes closed, which suggests that this disorder is not a primarily motoric dysfunction of the limbs or a feature of cerebellar dysfunction. Accuracies of pointing or grasping in patients with optic ataxia reduce as the object is moved further away from the central vision.90 Optic ataxia may result from lesions of the dorsal visual association areas (intraparietal sulcus and superior parietal lobe) or from a disconnection of the projections from the visuomotor centers in the parieto-occipital lobes to the frontal lobes, where reaching is programmed (before the movement is initiated). However, more recent studies (for review see reference 91) emphasized the importance of online processing (after the hand movement is initiated to reach a moved target), which is essential for reaching a peripheral target accurately. This could also explain why the pointing or grasping ability in patients with optic ataxia is less accurate in the peripheral field because foveal targets require less real-time processing than peripheral targets.

Positive Syndromes

As a result of either destructive or irritative lesions of the visual system, a variety of interesting positive visual phenomena can be observed. Visual illusions result from an alteration of the perceived visual image leading to its persistence (palinopsia), replication (polyopia), or distortion (dysmetropsia). Visual hallucinations are visual percepts generated internally, unrelated to the external visual world. They may be simple (spots of light, diffuse color) or complex (faces, objects, or visual scenes). They can be very distressing to the patient, particularly complex visual hallucinations, because they often lead to the patient privately questioning his or her sanity.

PALINOPSIA

Palinopsia, Greek palin (again) opsis (vision), refers to the perseveration of the visual image in time.92 The images are usually of a real object that has recently been visualized and are superimposed on certain parts of the current visual scene. For instance, Meadows and Munro93 reported a palinopsic patient who after looking at someone dressed up as Santa Claus saw an image of Santa Claus’s face superimposed on the face of other people at the party. Patients with palinopsia may also describe the visual persistence as they turn their gaze away from the object, giving rise to a smeared image. Two types of palinopsia have been reported: an immediate and a delayed type. In the immediate type, the image persists after the disappearance of the actual object or scene and usually

persists for several minutes. Some authorities have argued that this type of palinopsia is simply part of the spectrum of retinal afterimages,94 but others have disagreed.95 In the delayed type, the perseverated image appears a few seconds after the object has disappeared or the patient’s gaze has been redirected away from it. Some patients may have both types of palinopsia.93

The persistent image can occupy any location in the visual field and it usually moves as the eyes move, resembling a retinal afterimage. In some cases, the persistent image multiplies across the entire visual field.96 In addition, the location of the persistent image can be contextually specific, for instance, one face is copied and pasted on to other persons’ faces.93

The mechanism of palinopsia is still unclear. It may result from an epileptic discharge or occur as a release phenomenon. The most common cause of palinopsia is parieto-occipital damage involving either hemisphere but more commonly the right hemisphere, which can also cause an associated homonymous hemianopia. The common etiologies include cerebrovascular disease,97,98 tumor,99

tuberculoma,100 trauma, epilepsy,101 hallucinogenic drugs (mescaline or lysergic acid diethylamide [LSD]102,103), therapeutic medications (e.g., mitrazapine,104

maprotiline,105 nafazodone,106 and trazodone),107 or paroxetine withdrawal.108 There have also been case reports of palinopsia occurring in optic nerve disease without cerebral lesions.

CEREBRAL POLYOPIA

Cerebral polyopia is also a type of visual perseveration in space, when two or more copies of a visualized object are seen simultaneously.109,110 This condition

usually occurs under monocular or binocular viewing conditions. If the former, ocular pathology such as a cataract or retinal detachment must be excluded. Cerebral diplopia is used to describe the condition in which two copies of the image are perceived, whereas cerebral polyopia signifies more than two copies. Occipital lesions may result in cerebral polyopia and an associated visual field defect, whereas migraine usually causes transient and reversible cerebral polyopia.

VISUAL ILLUSIONS (DYSMETROPSIA)

A visual illusion describes a phenomenon when an object of interest is perceived differently from reality, usually in size, shape, and spatial orientation. Illusions of the spatial aspect can be divided into three categories: micropsia (objects are smaller than reality), macropsia (objects are perceived larger than reality), and metamorphopsia (objects are perceived as a distorted image). Among these, micropsia is probably the most common form of dysmetropsia. A survey of more than 3000 students111 revealed a surprisingly high incidence of dysmetropsia (9%)—the phenomenon being reported most often during high fever, falling asleep, and migraine.

Micropsia can occur as a physiologic phenomenon. One form of micropsia, convergence micropsia, is a condition in which the object is perceived as smaller when the person focuses at a point nearer that the object. The exact physiology remains uncertain, but it may be the result of a modification in the size of visual receptive fields that occurs during convergence. Pathologic micropsia has been reported

in cerebrovascular disease,112 psychogenic conditions, and most commonly migraine. Retinal conditions, especially with foveal involvement, such as macular edema,113 can also cause micropsia but are usually accompanied by metamorphopsia and are monocular. Hemimicropsia, a condition in which the perceived image is smaller in only one hemifield, has been reported in the hemifield contralateral to a cerebral lesion.114

Macropsia is much less common than micropsia. It can also occur as a feature of normal vision, similar to micropsia, when the object is viewed with the focusing point further away than the object. However, neither physiologic micropsia nor macropsia are likely to be a significant complaint. Pathologic macropsia has been reported in association with zolpidem,115 the scarring stage of macular edema. Hemimacropsia of cerebral in origin was reported in a patient with a left occipital lobe tumor.116

Metamorphopsia, when an object is perceived as being distorted in shape, usually occurs as a symptom of ocular disorders, especially retinal pathology such as macular edema, epiretinal membranes, and macular degeneration. As a consequence, it is usually monocular and is almost never symmetrical if the symptom exists binocularly. Cerebral metamorphopsia is extremely rare. It has mainly been reported in seizure disorders.117–119 Hemimetamorphopsia has been reported in left putaminal hemorrhage.120 Upside-down visual metamorphopsia, in which the object or complete visual scene is suddenly perceived as inverted by 180 degrees, is a rare form of metamorphopsia seen in brainstem and parieto-occipital pathology, usually resulting from a cerebrovascular accident or migraine.121 The degree of reversal can sometimes be partial, with tilting of varying numbers of degrees.

VISUAL HALLUCINATIONS

Visual hallucinations are visual percepts without real external stimuli. The object is perceived in the absence of an actual object(s), and, depending on the degree of alertness and pathology, the observer may or may not be able to appreciate that the seen object does not exist. The quality of the perceived image can range from a simple flash of light (phosphene) to a well-formed object, animal, or a person. Movement of the images is often perceived. Patients are often convinced that hallucinations are, in fact, genuine, and under these circumstances a history from relatives or carers refuting this belief is invaluable.

Visual hallucinations are not specific to dysfunction in a particular brain region but have been observed in many neurologic conditions as well as functional or psychiatric disorders. Visual hallucinations, in the absence of hallucinations in other sensory modalities, are suggestive of an organic pathology. The common disorders include confusional states secondary to metabolic derangement (hypoglycemia, electrolyte imbalance), adverse drug reaction, alcohol withdrawal, and neurodegenerative disorders (Alzheimer’s disease, Parkinson’s disease, Huntington’s disease).

Release hallucinations (Charles Bonnet syndrome) are isolated visual hallucinations often occurring in the area of a visual field defect but have also been observed when there is severe generalized visual loss. Charles Bonnet first described the condition in 1796; the patient was actually his grandfather who became blind from bilateral cataracts but who remained cognitively intact.

These hallucinations are considered to be the result of a release of the visual system, secondary to a reduction of visual information, which allows the emergence into consciousness of endogenous visual activity. Any type of visual loss, including ocular and cerebral pathology, can cause such release hallucinations. Cataracts, senile macular degeneration,122 and diabetic retinopathy are most frequent among the ocular causes, and cerebrovascular disease123 is the common-

est cause, secondary to a cerebral insult.124 They have also been observed to occur after brain resection,125,126 ocular enucleation,127 brimonidine eye drops,128 and

multiple sclerosis.129,130

The onset of visual hallucinations can occur immediately after the onset of visual loss but can also be delayed for as long as 10 years. The duration and frequency of each episode of hallucination can also be extremely variable, ranging from a few episodes lasting 2 to 3 seconds in duration to being continuous for several hours. Although occurring almost exclusively in adults, especially the elderly, they have been reported in young children.131

The incidence of release hallucination ranges from 12%132 to 57%124 in different series of patients with ocular pathology but may be underestimated because many patients may be too embarrassed to admit to experiencing hallucinations. Patients with release hallucinations are usually cognitively intact and many realize that the observed scenes are not real,132 and some even enjoy them. Loneliness, intraversion, and shyness are risk factors for developing release hallucinations in the elderly.133 Release hallucinations can be simple or complex in quality. Simple forms, the commoner among the two, consist of flashes of lights, lines, shapes, or phosphenes.124 Complex hallucinations composed of recognizable objects (both living and nonliving) and fantasized objects (angels, dragons). One of the commoner items visualized in the complex form are deceased friends or relatives.132 The distinction between simple and complex visual hallucinations does not have any localizing value.

Visual hallucinations can also be a feature of occipital lobe epilepsy. The distinction between epilepsy and the Charles Bonnet syndrome can be difficult, especially in the presence of brain lesions and visual field defects. As mentioned, the content of visual hallucinations does not help to differentiate between the two, but accompanying head or eye deviation and rapid blinking are suggestive of occipital lobe seizures. Other features in support of occipital lobe seizure include confusion, tonic-clonic limb movements, and automatisms. In addition to neuroimaging and electroencephalogram, single-photon emission computed tomography, and fluorodeoxyglucose positron emission tomography (FDG-PET) may help to confirm and localize occipital lobe epilepsy.134

Visual hallucinations are also a well-known feature of migraine with aura. The aura almost always precedes the headache by 20 to 30 minutes, but it can also occur in the absence of headache (acephalgic migraine). Enlarging scintillating scotoma, a scotoma surrounded by sparkling light, is almost pathognomonic of migrainous aura. The speed of enlargement increases as it expands because of the relative larger cortical representation of the central visual field than the peripheral. Other common features of the migrainous aura include spots, wavy or zigzag lines (fortification spectra), and shimmering effects in the visual scene. These can happen in the entire visual field, in a hemifield, or in only one quadrant of the visual fields and then progress to involve the whole visual field.

Bedside Testing of a Patient with Suspected Higher

Visual Disorder

Standard neuro-ophthalmologic examination includes visual acuity, visual fields, pupil light reactions, and funduscopy; together, these provide important clues in making a diagnosis of a higher visual disorder.

Visual field testing with a confrontation technique is an easy way to detect large visual field defects and it has the advantage of examining the most peripheral region of the visual field. Visual field defects detected by confrontation are usually a true deficit, although this technique lacks sensitivity. More detailed information, especially at large eccentricities, can be obtained by using the Bjerrum tangent screen visual field test. Static automated threshold perimetry provides many advantages, including more standardized testing procedures, and requires less technician skill. It also yields high reproducibility, which is very important for follow-up purposes, but the process is laborious. In general, kinetic perimetry requires less cooperation, can be done in cases of severe visual loss, and tests a wider area of the visual field, but it is very time consuming and dependent on technician skill and experience.

Full-field achromatopsia can be formally tested by using standard tests, for example, color plates or color arrangement tests. It is useful to have some objects of different colors (i.e., bottle caps) in the clinic, which can be used to test for gross hue discrimination deficits (i.e., ask the patient to point to the red cap). Standard pseudoisochromatic plates, for instance Ishihara color plates, can be used. In addition, Ishihara color plates or Hand Rittler Round plates also offer additional advantages such as the ability to differentiate between generalized loss of color perception (as seen in achromatopsia) and congenital color blindness (usually red-green defect). In addition, in hemiachromatopsia, the patient might miss the number on the color-deficient side when using the two-numbered plates. Asking the patient to trace a perceived outline with a finger on these pseudoisochromatic plates is sometimes required because this does not require language skills that might be defective (alexia or aphasia), as may particularly occur in left-sided lesions. The standard tests for color vision such as the Farnsworth Munsell 100 Hue test (tests hue discrimination at a fixed luminance), the Lanthony New Colour Test (tests hue discrimination as well as which colors are confused with grays), and the Lightness Discrimination Test (tests the range of gray from light to dark)135 are required to make a firm diagnosis. Patients with cerebral achromatopsia usually have abnormal hue discrimination with preserved perception of brightness.136 Disorders of motion perception cannot easily be tested by bedside examination.

The traditional testing for visual object agnosia is to ask the patient to name an object placed in front of him or her. Different objects, preferably real ones, may be used for the test, and they should be presented in the intact visual field where there is known reasonable acuity. The examiner should then check that the patient can actually see the object by asking the patient to give a verbal description (i.e., shape, color) or draw a copy. In associative visual agnosia, drawing and copying ability should be intact. The next step is to ask the patient to name the object. It must be noted at this stage that “naming” and “recognizing” are not the same. It is true that naming requires object recognition, but the reverse is not true. Intact recognition can be declared if the patient can express

an abstract aspect of the object (e.g., its use) even if he or she failed to name it (anomia). Prosopagnosia, when it occurs in isolation, can be very dramatic as the patient can recognize any object but cannot recognize pictures of famous faces or the face of family members. Pictures of famous faces (i.e., the presidents/prime ministers or famous celebrities) can be used to test face recognition; however, these pictures should not contain any significant cues (e.g., Elton John’s glasses or Abraham Lincoln’s beard) that might aid the patient recognition. It should be noted that these object recognition disorders, including prosopagnosia, rarely occur in isolation but are usually present with other cortical visual disorders, of which hemianopia and achromatopsia are the commonest.

Simultanagnosia can be tested clinically relatively easily. Any pictures that contain more than one object can be used for the test. Typically, patients with simultanagnosia tend to attend and describe only one object and fail to report the others. Ideally, the picture should be well balanced in its four quadrants, for instance the famous “Cookie Theft Picture” from the Boston Diagnostic Aphasia Examination.

Clinical Course, Prognosis, and Therapeutic Options

The overall prognosis for cortical visual disorders is poor. Complete resolution of defective functions rarely happens except in the case of reversible causes such as migraine or epilepsy. Some patients with acquired homonymous hemianopia may not recognize the deficit especially when a right hemispheric lesion results in visuospatial neglect. However, most patients do eventually become aware of their defective fields.

About 20% to 30% of all patients in neurologic rehabilitation centers have homonymous visual field disorders.137 Partial field recovery mainly occurs in the first 2 to 3 months in about 10% to 20% of patients, with the average visual field recovery of 5 degrees.138 After this, further recovery is rarely seen. Vision usually returns to the hemianopic field in stages, starting with the perception of light and followed by motion, form, color, and stereognosis in that order.139 Less than 10% of patients recover their full visual field. Poor recovery is particularly seen in elderly patients with a history of diabetes or hypertension, large lesions, and the presence of cognitive, language, or memory impairment. The quality of life is particularly affected in the case of hemianopia without macular sparing. Spontaneous recovery from visual agnosia resulting from carbon monoxide poisoning has been described in detail by Adler,140 but patients with visual agnosia generally do not show dramatic recovery because of the extensive damage to the visual cortices. Prosopagnosia also carries a poor prognosis.

In an attempt to treat homonymous hemianopia, two approaches have been studied: first, to gain the ability to cope and compensate for such a defect, and second, to enhance recovery of the area of defective visual field (see reviews in references 141 and 142). Numerous treatments have been tried in the hope that the area of the visual field defect will become smaller by using appropriate train-

ing or therapy. Some studies have shown a reduction in the area of visual field defect,47,143 but this has not always been replicated.144–146 In most patients who

improved with these techniques, the field enlargement does not exceed 5 degrees

of eccentricity, although there has been the occasional individual case with remarkable recovery. The more recent studies by Sabal and colleagues147–149

have shown some reduction in the area of visual field defect. However, fixation has not always been adequately controlled and a recent study in which this was achieved using a scanning laser ophthalmoscope (SLO) failed to replicate these results.150 Patients with a possible reversible area of field defect, as indicated by a poorly demarcated border on visual field testing, are more likely to recover than those with a sharply demarcated field defect.

To improve visual function in hemianopia, optical aids may occasionally be helpful. Hemianopic spectacles work on the basis that they permit the patient to look at an adjustable mirror and see the reflection of objects in the hemianopic field. It is conventionally placed alongside the left eye in left hemianopia and the reverse is true for right hemianopia. Hemianopic prisms can be carefully incorporated into the existing lenses to displace the image of objects from the hemianopic field into the intact one. Conventionally, only one lens is fitted because bilateral placement usually affects the visual acuity, which could be more troublesome. In addition, the central area must be trimmed to avoid diplopia in primary gaze.151 Use of prisms improved vision in several domains including recognition152 and activity of daily living.153 The fitting requires skilled opticians and the process is rather laborious.

Another strategy that may help to improve visual function in a hemianopic patient is to use visual search training to get the patient to increase exploration of their hemianopic field. It has been shown to successfully expand the visual search field (the perimetrically measured area that a patient can actively scan via eye movements without head movements154). Using a relatively simple visual search retraining program, which patients could use at home, it was found that reaction times to locate targets in the hemianopic field reduced and they improved in activities of daily living.155

Hemianopia also significantly impairs the ability to read. In a culture in which the reading is from left to right, patients with left hemianopia have great difficulty searching for the beginning of the next line. Right hemianopia, however, is generally accepted to cause more difficulty when reading as most people make a saccade toward the beginning or middle of the word, and as a result of the hemianopia the rest of the word is not seen. Hence, it is probably sensible for patients with a left hemianopia to try to read the word as a whole while focusing on the beginning of the word and for right hemianopes to make a saccade to the end of the word before reading it. Zihl and Kennard156 used this strategy to successfully train patients with hemianopia who by the end of the training were able to read faster with fewer errors by using larger saccadic jumps and shorter fixation periods. Wang157 reported a helpful trick used by his patient with right hemianopia; when reading an article, the patient rotated the article 90-degrees clockwise and read it up to down instead of left to right.

Rehabilitation for other types of cortical visual disorders has not attracted much interest from researchers, possibly because of the rarity and the lesser effect on quality of life in comparison with hemianopia. In addition, patients with these disorders often have extensive bilateral lesions, which cause additional deficits that complicate the rehabilitation planning. Furthermore, the response to such rehabilitation is not readily quantifiable as it is in hemianopia.