Agnosias

Chapter 11 Agnosias



Howard S. Kirshner



Chapter Outline



Visual Agnosias


Cortical Visual Disturbances

Cortical Visual Distortions

Balint Syndrome and Simultanagnosia

Visual Object Agnosia

Optic Aphasia

Prosopagnosia

Klüver-Bucy Syndrome

Auditory Agnosias


Cortical Deafness

Pure Word Deafness

Auditory Nonverbal Agnosia

Phonagnosia

Amusia

Tactile Agnosias


Tactile Aphasia

Summary


Agnosias are disorders of recognition. The general public is familiar with agnosia from Oliver Sacks’s patient, who not only failed to recognize his wife’s face but also mistook it for a hat. Sigmund Freud originally introduced the term agnosia in 1891 to denote disturbances in the ability to recognize and name objects, usually in one sensory modality in the presence of intact primary sensation. Another definition, that of Milner and Teuber in 1968, referred to agnosia as a “normal percept stripped of its meaning.” The agnosic patient can perceive and describe sensory features of an object yet cannot recognize or identify the object.


Criteria for the diagnosis of agnosia include: (1) failure to recognize an object; (2) normal perception of the object, excluding an elementary sensory disorder; (3) ability to name the object once it is recognized, excluding anomia as the principal deficit; and (4) absence of a generalized dementia. In addition, agnosias usually affect only one sensory modality, and the patient can identify the same object when presented in a different sensory modality. For example, a patient with visual agnosia may fail to identify a bell by sight but readily identifies it by touch or by the sound of its ring.


Agnosias are defined in terms of the specific sensory modality affected—usually visual, auditory, or tactile—or they may be selective for one class of items within a sensory modality, such as color agnosia or prosopagnosia (agnosia for faces). To diagnose agnosia, the examiner must establish that the deficit is not a primary sensory disorder, as documented by tests of visual acuity, visual fields, auditory function, and somatosensory functions, and not part of a more general cognitive disorder such as aphasia or dementia, as established by the bedside mental status examination. Naming deficits in aphasia or dementia are, with rare exceptions, not restricted to a single sensory modality.


Clinically, agnosias seem complex and arcane, yet they are important in understanding the behavior of neurological patients, and they provide fascinating insights into brain mechanisms related to perception and recognition. Part of their complexity derives from the underlying neuropathology; agnosias frequently result from bilateral or diffuse lesions such as hypoxic encephalopathy, multiple strokes, major head injuries, and neurodegenerative disorders and dementias.


Agnosias have aroused controversies since their earliest descriptions. Some authorities have attributed agnosic deficits to primary perceptual loss in the setting of general cognitive dysfunction or dementia. Abundant case studies, however, argue in favor of true agnosic deficits. In each sensory modality, a spectrum of disorders can be traced from primary sensory dysfunction to agnosia. We approach agnosias by sensory modality, with progression from primary sensory deficits to disorders of recognition.



Visual Agnosias



Cortical Visual Disturbances


Patients with bilateral occipital lobe damage may have complete “cortical” blindness. Some patients with cortical blindness are unaware that they cannot see, and some even confabulate visual descriptions or blame their poor vision on dim lighting or not having their glasses (Anton syndrome, originally described in 1899). Patients with Anton syndrome may describe objects they “see” in the room around them but walk immediately into the wall. The phenomena of this syndrome suggest that the thinking and speaking areas of the brain are not consciously aware of the lack of input from visual centers. Anton syndrome can still be thought of as a perceptual deficit rather than a visual agnosia, but one in which there is unawareness or neglect of the sensory deficit. Such visual unawareness is also frequently seen with hemianopic visual field defects (e.g., in patients with R hemisphere strokes), and it even has a correlate in normal people; we are not conscious of a visual field defect behind our heads, yet we know to turn when we hear a noise from behind. In contrast to Anton syndrome, some cortically blind patients actually have preserved ability to react to visual stimuli, despite the lack of any conscious visual perception, a phenomenon termed blindsight or inverse Anton syndrome (Ro and Rafal, 2006). Blindsight may be considered an agnosic deficit, because the patient fails to recognize what he or she sees. Residual vision is usually absent in blindness caused by disorders of the eyes, optic nerves, or optic tracts. Patients with cortical vision loss may react to more elementary visual stimuli such as brightness, size, and movement, whereas they cannot perceive finer attributes such as shape, color, and depth. Subjects sometimes look toward objects they cannot consciously see. One study reported a woman with postanoxic cortical blindness who could catch a ball without awareness of seeing it. Blindsight may be mediated by subcortical connections such as those from the optic tracts to the midbrain.


Lesions causing cortical blindness may also be accompanied by visual hallucinations. Irritative lesions of the visual cortex produce unformed hallucinations of lines or spots, whereas those of the temporal lobes produce formed visual images. Visual hallucinations in blindness are referred to as Bonnet syndrome (Teunisse et al., 1996). Although Bonnet originally described this phenomenon in his grandfather, who had ocular blindness, complex visual hallucinations occur more typically with cortical visual loss (Manford and Andermann, 1998). Visual hallucinations can occur during recovery from cortical blindness; positron emission tomography (PET) has shown metabolic activation in the parieto-occipital cortex associated with hallucinations, suggesting hyperexcitability of the recovering visual cortex (Wunderlich et al., 2000).


In practice, we diagnose cortical blindness by the absence of ocular pathology, the preservation of the pupillary light reflexes, and the presence of associated neurological symptoms and signs. In addition to blindness, patients with bilateral posterior hemisphere lesions are often confused, agitated, and have short-term memory loss. Amnesia is especially common in patients with bilateral strokes within the posterior cerebral artery territory, which involves not only the occipital lobe but also the hippocampi and related structures of the medial temporal region. Cortical blindness occurs as a transient phenomenon after traumatic brain injury, in migraine, in epileptic seizures, and as a complication of iodinated contrast procedures such as arteriography. Cortical blindness can develop in the setting of hypoxic-ischemic encephalopathy (Wunderlich et al., 2000), meningitis, systemic lupus erythematosus, dementing conditions such as the Heidenhain variant of Creutzfeldt-Jakob disease, or the posterior cortical atrophy syndrome described in Alzheimer disease and other dementias (Kirshner and Lavin, 2006).



Cortical Visual Distortions


Positive visual phenomena frequently develop in patients with visual field defects and even in migraine: distortions of shape called metamorphopsia, scintillating scotomas, irregular shapes (teichopsia, or fortification spectra), macropsia and micropsia, peculiar changes of shape and size known as the Alice in Wonderland syndrome (described by Golden in 1979), achromatopsia (loss of color vision), akinetopsia (loss of perception of motion), palinopsia (perseveration of visual images), visual allesthesia (spread of a visual image from a normal to a hemianopic field), and even polyopia (duplication of objects). All these phenomena are disturbances of higher visual perception rather than agnosias.


Two types of color vision deficit are associated with occipital lesions. First, a complete loss of color vision, or achromatopsia, may occur either bilaterally or in one visual hemifield with lesions that involve portions of the visual association cortex (Brodmann areas 18 and 19). Second, patients with pure alexia and lesions of the left occipital lobe fail to name colors, although their color matching and other aspects of color perception are normal. Patients often confabulate an incorrect color name when asked what color an object is. This deficit can be called color agnosia, in the sense that a normally perceived color cannot be properly recognized. Although this deficit has been termed color anomia, these patients can usually name the colors of familiar objects such as a school bus or the inside of a watermelon.



Balint Syndrome and Simultanagnosia


In 1909, Balint described a syndrome in which patients act blind, yet can describe small details of objects in central vision (Rizzo and Vecera, 2002). The disorder is usually associated with bilateral hemisphere lesions, often involving the parietal and frontal lobes. Balint syndrome involves a triad of deficits: (1) psychic paralysis of gaze, also called ocular motor apraxia, or difficulty directing the eyes away from central fixation; (2) optic ataxia, or incoordination of extremity movement under visual control (with normal coordination under proprioceptive control; and (3) impaired visual attention. These deficits result in the perception of only small details of a visual scene, with loss of the ability to scan and perceive the “big picture.” Patients with Balint syndrome literally cannot see the forest for the trees. Some but not all patients have bilateral visual field deficits. In bedside neurological examination, helpful tests include asking the patient to interpret a complex drawing or photograph, such as the “Cookie Theft” picture from the Boston Diagnostic Aphasia Examination and the National Institutes of Health Stroke Scale.


Partial deficits related to Balint syndrome have also been described, including isolated optic ataxia, or impaired visually guided reaching toward an object. Optic ataxia likely results from disruption of the transmission of visual information for visual direction of motor acts from the occipital cortex to the premotor areas. This function involves portions of the dorsal occipital and parietal areas as part of the “dorsal visual stream” (Himmelbach et al., 2009). A second partial Balint syndrome deficit is simultanagnosia, or loss of ability to perceive more than one item at a time, first described by Wolpert in 1924. The patient sees details of pictures, but not the whole. Many such patients have left occipital lesions and associated pure alexia without agraphia; these patients can often read “letter-by-letter,” or one letter at a time, but they cannot recognize a word at a glance (see Chapter 12). Robertson and colleagues (1997) emphasized deficient spatial organization as a contributing factor to the perceptual difficulties of a patient with Balint syndrome secondary to bilateral parieto-occipital strokes. Balint syndrome has also been reported in patients with posterior cortical atrophy and related neurodegenerative conditions involving the posterior parts of both hemispheres (Kirshner and Lavin, 2006; McMonagle et al., 2006).



Visual Object Agnosia


Visual object agnosia is the quintessential visual agnosia: the patient fails to recognize objects by sight, with preserved ability to recognize them through touch or hearing in the absence of impaired primary visual perception or dementia (Biran and Coslett, 2003). In 1890, Lissauer distinguished two subtypes of visual object agnosia: apperceptive visual object agnosia, referring to the synthesis of elementary perceptual elements into a unified image, and associative visual object agnosia, in which the meaning of a perceived stimulus is appreciated by recall of previous visual experiences.



Apperceptive Visual Agnosia


The first type, apperceptive visual agnosia, is difficult to separate from impaired perception or partial cortical blindness. Patients with apperceptive visual agnosia can pick out features of an object correctly (e.g., lines, angles, colors, movement), but they fail to appreciate the whole object (Grossman et al., 1997).Warrington and Rudge (1995) pointed to the right parietal cortex for its importance in visual processing of objects, and they found this area critical to apperceptive visual agnosia. A patient described by Luria misnamed eyeglasses as a bicycle, pointing to the two circles and a crossbar. Another study considered apperceptive visual agnosia related to bilateral occipital lesions a “pseudoagnosic syndrome” associated with visual processing defects, as compared to true visual agnosias, in which the right parietal cortex is deficient in identifying and recognizing visual objects. Recent evidence of the functions of specific cortical areas has included the specialization of the medial occipital cortex for appreciation of color and texture, whereas the lateral occipital cortex is more involved with shape perception. Deficits in these specific visual functions can be seen in patients with visual object agnosia (Cavina-Pratesi et al., 2010). On the other hand, a patient reported by Karnath et al. (2009) had visual form agnosia with bilateral medial occipitotemporal lesions.


Another way of analyzing apperceptive visual agnosia is by the focusing of visual attention. Theiss and DeBleser in 1992 distinguished two features of visual attention: a wide-angle attentional lens that sees the figure generally but perceives only gross features (the forest), and a narrow-angle spotlight that focuses on the fine visual details (the trees). They described a patient with a faulty wide-angle attentional beam; she could identify small objects within a drawing but missed what the drawing represented. Fink and colleagues (1996), in PET studies of visual perception in normal subjects, found that right hemisphere sites, particularly the lingual gyrus, activated during global processing of figures, whereas left hemisphere sites, particularly the left inferior occipital cortex, activated during more local processing. The ability of patients with apperceptive visual agnosia to perceive fine details but not the whole picture (missing the forest for the trees) is closely related to Balint syndrome and simultanagnosia.


As with most cortical visual syndromes, apperceptive visual agnosia usually occurs in patients with bilateral occipital lesions. It may represent a stage in recovery from complete cortical blindness. Deficits in recognition of visual objects may be especially apparent with recognition of degraded images, such as drawings rather than actual objects. Apperceptive visual agnosia can also be part of dementing syndromes (Kirshner and Lavin, 2006; McMonagle et al., 2006) (Fig. 11.1).


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Fig. 11.1 T2-weighted magnetic resonance images from a patient with progressive loss of vision, misidentification of objects, and inability to describe the whole of a picture, mentioning only small details. The clinical diagnosis was posterior cortical atrophy, a neurodegenerative condition.

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Jun 19, 2016 | Posted by in NEUROLOGY | Comments Off on Agnosias

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