Introduction

Mental functions are the essence of mankind. The cognitive systems of the human brain allow us to build internal representations of the world, share them with others, understand others’ feelings and intentions, store information, reason, decide, and plan actions into the future anticipating their results.

Mental representations are based on a large-scale neuronal network called the conceptual representation system or semantic system. It comprises modality-specific and multimodal convergent regions [1] and is connected with more circumscribed and lateralized operational systems that allow us to translate thoughts into words (spoken or written), images, numbers, or other symbols, to store and retrieve information when necessary, and to make decisions or act upon them. Most of these operational abilities are subserved by distributed networks with areas of regional specialization, organized according to their specific processing capacities.

Cerebral lesions can produce different types of cognitive and behavioral dysfunction, according to the networks involved, but the pattern of impairment observed after ischemic stroke is relatively stereotyped, since it follows the distribution of vascular territories. Yet, there are exceptions to this trend. In the very acute stage of stroke cognitive symptoms are likely to be amplified by additional regions of ischemic penumbra, mass effects, and diaschisis (impairment of intact regions that are functionally connected with the damaged area) while in the chronic stage, functional reorganization and brain plasticity mechanisms make neuroanatomical correlations loose and less predictable. Likewise, in certain pathological processes such as hemorrhagic stroke, vasculitis, and cerebral venous thrombosis, the pattern of cognitive defects is less stereotyped due to the variability of lesion topography or to particular pathogenic mechanisms that may cause diffuse impairment.

In this chapter we will present the most common cognitive and neurobehavioral deficits secondary to stroke, according to symptom presentation.

Language Disorders

Aphasia is the main cognitive disturbance of left hemisphere stroke and has a marked impact on the individual’s quality of life, autonomy, and ability to return to work or previous activities. It occurs after perisylvian lesions involving the branches of the middle cerebral artery. Since these lesions are circumscribed, the conceptual representation system is not extensively affected, and these patients must not be confused with dementia. This is an important distinction that should be explained to the families and caregivers, for it can be misunderstood. In fact, deficits in oral comprehension increase the likelihood of discharge to a setting other than home [2].

A brief bedside evaluation of language comprises four cardinal tests (naming, verbal comprehension, repetition, and speech fluency) that are useful for classifying aphasic syndromes and for inferring lesions’ localization, since they have neuroanatomical correlates [3, 4]. Although these tests are also included in some brief tools for cognitive assessment, such as the “Mini Mental State Examination” (MMSE) or the “Montreal Cognitive Assessment,” language evaluation should be performed beforehand, because aphasia may preclude the assessment of orientation, memory, or executive functions.

Confrontation naming, which relies on a large network around the left Sylvian fissure, is one of the most sensitive tasks for the diagnosis of aphasia and the degree of naming impairment (anomia) is a rough measure of aphasia severity. Assessment must be performed with common objects (coin, ring, spoon, pencil, wristwatch), to avoid the confounding effect of cultural factors and aging upon naming. Patients’ responses vary from word-finding difficulties (with pauses and tip-of-the-tongue phenomenon), paraphasias, use of supraordinal responses (fruit for apple), and descriptions of use (circumlocutions). Occasionally, patients present “category-specific impairments,” a type of naming or recognition difficulty that affects predominantly a specific category of names: it may impair the names of living entities more than artifacts, actions but not objects, or proper names but not common names. These unusual cases are more common in lesions of the left temporal lobe and pole following posterior cerebral artery infarcts [5], suggesting that this region is a hub in the semantic system.

Speech fluency is analyzed during conversation and can be prompted by asking the patient to tell a personal event or a recipe or to describe a picture. Speech is usually classified as fluent or non-fluent (Table 15.1) [4], a distinction that is made easier if the listener concentrates on the effort, speech rate, and the number and duration of pauses and manages to ignore its content, as if listening to a foreign language. Fluent speech “sounds” normal as opposed to non-fluent speech that is interrupted by long pauses and hesitations and is produced with effort.

Verbal comprehension is tested by simple verbal commands (“close your eyes,” “raise your arm,” etc.). Speech comprehension is supported by a ventral language stream that maps acoustic, phonetic, and lexical representations to word meaning. This stream connects the temporal lobe to the prefrontal cortex [6] and can be disrupted either by cortical lesions or by the interruption of the subcortical fiber tracts connecting those areas. Poor lexical (words/nouns) comprehension is usually associated with temporal lobe lesions, while inferior frontal/opercular stroke tends to impair the understanding of sentences and syntax. The comprehension of discourse in real time, on the other hand, requires a much larger network that integrates incoming information with context, requiring working memory and top-down control [7]. It is easily broken in aphasia.

Finally, one should ask the patient to repeat words, pseudowords (pronounceable strings of syllables that do not belong to the lexicon), and sentences. Repetition evaluates the integrity of the dorsal language stream, which maps speech sounds to articulation (Table 15.2). Repetition is impaired in lesions of the posterior temporo-parietal region and the periventricular white matter fiber tracts [8]. Transcortical aphasias are characterized by a disproportionate capacity to repeat, compared to other language abilities and sometimes these patients repeat compulsively what they hear, a phenomenon called echolalia. In conduction aphasia, in contrast, patients have outstanding difficulty in repeating pseudowords or even words they can otherwise produce.

Difficulty in any of these four tasks may vary from mild (occasional difficulty) to severe, and the classification of aphasia varies accordingly (Table 15.3).

Effective language recovery, in adults, depends mostly upon the reorganization of the intact areas of the left hemisphere [9, 10].

The initial severity of language impairment is the main predictor of aphasia recovery.

Certain brain lesions may impair the ability to read (alexia or acquired dyslexia) or to write (agraphia/dysgraphia). Both conditions are commonly found in aphasia, but may occur in isolation following lesions of the left hemisphere.

The study of patients with reading or writing disorders has contributed to the understanding of the cognitive processes subserving those abilities and to the building of theoretical models of written language processing. They have shown that there are separate pathways to process particular categories of words (regular vs. irregular; meaningful words vs. functional words, such as “to,” “if,” “so”), non-words, or specific tasks (copying vs. writing spontaneously). This information has been incorporated into the assessment and classification of these disorders [11].

Alexia and agraphia can be classified as central or peripheral, depending on whether the impairment affects the central processing or its afferent or efferent pathways.

The best-known peripheral alexia is “pure alexia” (also called “alexia without agraphia” or “letter-by- letter reading”). This is a rather counterintuitive syndrome, whereby patients are unable to read, but can write to dictation or spontaneously. There is an inability to associate visually presented written words with their sound or meaning, but this difficulty is overcome through the tactile and auditory modalities (patients can read words spelled aloud), showing that the central reading processing is intact. Patients may be able to read single letters or small words, but reading becomes rather difficult, laborious, or impossible as word size increases, as they try to read letter by letter. This syndrome results from lesions in the “word form area” in the left fusiform gyrus due to left temporo-occipital infarcts involving the posterior splenium.

In central dyslexias, the impairment is independent of the presentation modality (visual, auditory, or tactile) and involves writing and spelling. There are two main types of central dyslexia (Table 15.4). In “deep dyslexia” patients may grasp the meaning of some written words, including irregular words, producing semantic paraphasias (orange for lemon) when reading aloud, but are unable to read function words or non-words, which are deprived of meaning. In contrast, in “surface dyslexia” patients can read aloud regular words and pseudowords (because they can convert letters to their corresponding sound), but have difficulty reading irregular words or accessing their meaning. These opposite types of impairment have shown the existence of two pathways for reading: a fast whole-word recognition with access to meaning (used when one reads frequent and meaningful words) and a step-by-step conversion that is useful for reading new or infrequent words.

When alexia occurs in the context of aphasia, reading impairment for words is more often associated with frontal lesions, while alexia for non-words is more likely to occur in posterior lesions [12].

Acquired writing disorders are also classified as central and peripheral. In “central agraphias” writing impairment is similar across different output modalities (handwriting, spelling, or typing) and can be of a “deep type” (phonological dysgraphia) with preserved access to meaning, or a “surface type” (or “lexical agraphia,” whereby sound-to-grapheme conversion is preserved and there is a particular difficulty writing irregular words). There are also patients with a predominant defect of the “graphemic buffer.” This is a short-term memory system that enables the writer to keep the word “on line” as it is being written in real time. Those cases are characterized by a difficulty in writing long words. In contrast, peripheral agraphia is a selective damage in the selection of letters or letter drawing that can be overcome by typing or the use of anagrams and is associated with normal spelling.

Deep forms of dyslexia and dysgraphia are associated with large left hemisphere strokes, while surface types result from more limited lesions. It is possible that reading and writing/spelling rely on identical cognitive processes, but in reverse order (the “shared components hypothesis”) and share the same neural network that includes the angular, supramarginal, and fusiform gyrus (BA 37) and BA 22 and 44/45, as suggested in a study performed in acute stroke patients [13].

Neglect

Neglect is an inability to attend to, orient, or explore the hemispace opposite to the side of brain damage. The patient directs his or her attention towards the space homolateral to the lesion, acting as if the contralateral side of the space does not exist. Since the right hemisphere is dominant for spatial attention, this syndrome is usually observed following right hemisphere stroke (affecting some 36–80% of acute stroke patients) [14] and in the left-hand side of space.

Neglect has a negative impact on daily living activities and on functional recovery, because the patient cannot be expected to focus on a symptom that consists exactly of lack of awareness.

Attention relies on a large network with two main processing pathways (Table 15.5) [15]: a ventral network, clearly lateralized to the right hemisphere, is responsible for stimulus detection, reorientation, and arousal, while a bilateral dorsal pathway controls spatial attention and eye movements to the contralateral space, within an egocentric frame. Although the ventral system does not map spatial representation its damage causes hemispatial neglect, possibly because it induces a dysfunction and imbalance of the dorsal system [15]. Since there are many anatomical regions that participate in those systems neglect may occur with lesions at different sites: anterior cingulate gyrus (responsible for its motivational aspects), frontal-parietal and superior temporal regions (mapping the perception of space and intentional/exploratory aspects of attention), as well as subcortical structures, such as the thalamus and the striatum. Extensive lesions, advanced age, defective arousal, and associated leukoaraiosis increase the severity of neglect.

Neglect can produce different symptoms that must be looked for to be detected. It can affect different compartments of space: personal space (forgetting to dress, groom the left side of the body), peri-personal or “hand reach” space (failing to detect or orient to surrounding objects or persons), the distant space (“at eye reach”), leading to spatial disorientation, or the representational space (mental imagery). It may occur spontaneously or only during competing sensory stimulation (extinction phenomena) and in any sensory modality (visual, tactile, auditory). In its most severe form it comprises anosognosia or denial of illness.

The most common tests used to diagnose neglect are performed in the peri-personal space and require the patient to draw, copy, or cross out lines or other stimuli (cancellation tasks) or to read or write. A qualitative analysis of the defect allows us to further classify the defect as viewer-centered or “egocentric neglect” (involving the angular gyrus) whereby the patient ignores all stimuli on the contralateral half of the space, and stimulus-centered or “alocentric neglect” (right superior temporal cortex) characterized by poor attention to the contralateral side of individual stimuli [16]. Viewer-centered neglect is much more common, easily noticed, and diagnosed than stimulus-centered neglect. The latter may go unrecognized because the defects are much more subtle and require a careful comparison of both sides of a stimulus. Neglect symptoms that occur following left hemisphere lesions (on the right side of the space) are object-centered type.

Memory Disturbances

Memory is not a unitary function. It consists of five independent systems and involves three processes (encoding, storing/consolidation, and retrieval). Both depend on specific neural networks that may dissociate following a brain lesion.

Classification of memory systems (Box 15.1) [16] depends upon three main vectors: duration of memory traces (fractions of seconds, seconds, or “for life”), content (explicit knowledge or motor routines), and access to consciousness (explicit or implicit).

According to the processes affected amnesia is further subdivided in reference to a specific time event into anterograde (patients cannot encode/consolidate new information) and retrograde (the difficulty lies in retrieving information that was already stored).

Amnestic strokes, i.e. infarcts presenting amnesia for recent events as the main clinical feature, usually involve the mesial cerebral hemispheres. Amnestic strokes can result from posterior cerebral artery, posterior communicating artery, anterior and posterior choroidal artery, anterior cerebral and anterior communicating artery thrombosis or embolism. Infarcts in the territories of the last two arteries can also be secondary to subarachnoid hemorrhage and its complications and to the surgical and less often to the endovascular treatment of aneurysms located in these arteries. Single case reports or small case series of amnestic stroke have been reported following infarcts of the inferior genu of the internal capsule, the mammillothalamic tract, the fornix, or the retrosplenium [10]. Anterolateral and medial thalamic hemorrhages, caudate and intraventricular hemorrhages, and venous infarcts due to thrombosis of the deep venous system also produce memory defects.

A quarter of posterior cerebral artery infarcts result in memory defects (Box 15.1) [11]. These amnestic strokes usually have mesial temporal involvement and the damage extends beyond the hippocampus to the entorhinal cortex, perirhinal cortex, collateral isthmus, or parahippocampal gyrus. The memory defect is more frequent and severe after left-sided and especially after bilateral infarcts. Left posterior cerebral artery infarcts cause either a verbal amnesia or a global amnesia, while right lesions produce visuospatial memory defects, including deficits in the memory for familiar faces or locations and topographical amnesia. Confabulations appear to be more likely if there is a dual lesion (temporo-occipital and thalamic).

Infarcts restricted to the hippocampus due to occlusion of the middle or the posterior hippocampal arteries, either complete, lateral, or dorsal, also produce persisting memory defects, affecting mostly learning. Small, dot- or comma-shaped infarcts cause a syndrome of transient global amnesia [17].

In thalamic infarcts [18], memory defects (Table 15.6) are also a distinct feature of anterior, dorsomedial, and, in the variant types, anteromedian and central infarcts. Combined polar and paramedian infarcts also cause a severe and persistent amnesia. Left thalamic infarcts can produce “pure amnesia” in the form of a verbal or global amnesia. Memory disturbances are more frequent and severe after left than after right thalamic infarcts. Right thalamic infarcts cause visual and/or visuospatial amnesia. Following unilateral infarcts (left or right) a complete or partial recovery of memory disturbances can be expected. Bilateral infarcts produce global and severe amnesia and a persistent deficit, with slow and limited improvement. In thalamic amnesia confabulations, intrusions, and perseveration are frequent. Distractibility, alternating good and poor performance, and better performance on first attempts are also characteristic.

Memory defects are a frequent clinical feature of subarachnoid hemorrhage due to ruptured anterior communicating artery aneurysms and may also follow posterior communicating artery aneurysm rupture. They are a frequent and disabling long-term sequela: the Australian Cooperative Research on Subarachnoid Haemorrhage Study Group (2000) [19] found problems with memory in 50% of survivors. Recently, hippocampal atrophy was found on neuroimaging studies in subarachnoid hemorrhage survivors [20].

Amnesia following rupture of anterior communicating artery aneurysms is characterized by a severe anterograde and a moderate retrograde amnesia (Table 15.7). There is a high susceptibility to interference, false recognitions, confabulations, and anosognosia. Amnesia is related to damage to the anterior cingulum, subcalosal area, and basal forebrain. Temporal error contexts are associated with ventromedial prefrontal cortex damage, but for spontaneous confabulations to occur there must be additional orbitofrontal deficit [21]. The brain has a mechanism to distinguish mental activity representing ongoing perception of reality from memories and ideas. Confabulations can be traced to fragments of previous actual experiences. Confabulators confuse ongoing reality with the past because they fail to suppress evoked memories that do not pertain to the current reality. The role of the anterior limbic system is the suppression of currently irrelevant mental associations. It represents “now” in human thinking.

Executive Deficits

Executive functions are classically assigned to the prefrontal lobes. Three types of prefrontal lobe functions are usually considered: (1) dorsolateral (executive/cognitive), including working memory, programming/planning, concept formation, monitoring of actions, and external cues and metacognition; (2) orbital (emotional/self-regulatory), consisting of inhibition of impulses and of non-relevant sensorial information and motor activity; and (3) mesial (action regulation), including motivation. These functions are served by three prefrontal–subcortical loops: dorsolateral, lateral orbital, and anterior cingulate, whose dysfunction produces three distinct clinical syndromes composed respectively of executive deficits, uninhibited behavior, and apathy. Executive difficulties manifest as difficulty deciding, leaving decisions to proxy, and being stubborn or rigid. Examples of uninhibited behavior include inappropriate familiarity, being distractible and shouting when constrained, and manipulation or utilization behavior. Recent models propose four main executive functions: dual task coordination, switch retrieval, selective attention and holding, and manipulation of information stored in long-term memory, so-called working memory; and three executive processes: updating, shifting, and inhibition [22]. Box 15.2 lists instruments that can be used to evaluate executive functions.

There are few systematic studies of executive functioning and other “frontal” syndromes in stroke patients. About one-third of acute stroke patients show either disinhibition or indifference and 30–40% display executive deficits in formal testing [23, 24]. Among patients with subarachnoid hemorrhage, one-half to two-thirds have executive deficits [25]. Stroke in some specific locations can cause executive deficits, disinhibition, or apathy. Examples are middle cerebral artery infarcts with frontal lobe or striatocapsular involvement; uni- or bilateral anterior cerebral artery infarcts; anterior or paramedian thalamic infarcts; striatocapsular, thalamic, intraventricular, or frontal intracerebral hemorrhages; subarachnoid hemorrhage due to rupture of anterior communicating artery aneurysms; and thrombosis of the sagittal sinus or of the deep venous system.

Executive deficits due to lesions in the prefrontal lobe occur in about one-third of stroke patients and can be divided into three distinct clinical syndromes:

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  executive deficits – corresponding to the dorsolateral prefrontal lobe

  
  
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  uninhibited behavior – corresponding to the lateral orbital prefrontal lobe

  
  
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  apathy – corresponding to the anterior cingulate prefrontal lobe.

Visual Agnosia

The human brain has two parallel visual systems: a ventral occipito-temporal stream, whose main function is the recognition of visual stimuli (the “what” system), and a dorsal occipito-parietal stream, whose main function is the spatial localization of visual stimuli (the “where” system) [26]. The paradigm of human dysfunction of the ventral system is visual agnosia, while that of the dorsal system is Balint’s syndrome.

Visual agnosias are disorders of visual recognition and are one of the clinical manifestations of posterior cerebral artery infarcts and occipito-temporal hemorrhages. Agnosias can be seen in patients improving from cortical blindness. Visual agnosias can be classified following the type of stimuli that is defectively recognized or following the impaired functional step in the processing of information from the visual system to the semantic and the language systems (Table 15.7). Apperceptive visual object agnosia is characterized by the presence of perceptual defects in visuoperceptive tasks and a defective perception of elementary perceptual features (color, shape, contour, brightness). The most distinctive feature of patients with apperceptive visual agnosia is visual matching errors when trying to match identical visual stimuli. Their naming errors are morphological, based on visual similarity. They perform better with real objects than with drawings. There are two varieties of apperceptive visual agnosia: form and integrative agnosia. Patients with form agnosia cannot perceive contours, although they can perceive brightness, color, or luster. They have a better recognition of moving than of static objects. In contrast, patients with integrative agnosia perceive single contours, but cannot integrate them in a coherent structure of the object, and produce predominantly visual similarity errors. Apperceptive visual agnosia is due to bilateral occipital or occipito-temporal lesions.

In associative visual object agnosia the distinctive feature is the intact perception. Although minor errors can be detected in complex perceptual tasks, the perception of elementary perceptual features (color, shape, contour, brightness) is correct, as is the matching of visual stimuli. Naming errors are semantic-related, perseverations, or confabulatory.

A variety of associative visual agnosia is semantic access agnosia (visuo-verbal or visuo-semantic disconnection). Patients with this type of agnosia show not only intact naming in other modalities (tactile, auditory), but also a correct use of objects. They may be able to select the correct name of an object in multiple-choice tasks and can sort objects by semantic categories. They may also be able to describe or pantomime the use of visually presented objects and have a superior naming of actions than of objects. Associative visual agnosia results from left or bilateral occipito-temporal lesions. In the literature the term “optic aphasia” is also found. It refers to a syndrome closely linked to visual agnosia and to transcortical sensory aphasia, and is often found during recovery from those. Patients have a disproportionate difficulty in naming stimuli presented visually, but otherwise do not display other features of visual agnosia (Figure 15.1).

Figure 15.1 Processing of visual stimuli and visual agnosias.

Testing for color agnosia deserves a note. A careful check for achromatopsia in the whole or part of the visual field should precede other tasks. Color perception is checked by asking the patient to match identical colors. To test the visual–verbal connection we ask the patient to name colors and to point to named colors. To evaluate whether there is color anomia and to ensure that language is intact we ask for color names in responsive naming (e.g. “Tell me the names of the colors of the national flag”). Finally, we can test visual–semantic connections by showing the patient drawings of stimuli which are painted in the correct and the wrong colors (e.g. blue banana) and asking the patient whether the colors are correct. Functional and lesion localization studies found that the V4v, V8, and V4a areas and the lingual gyrus are the human brain “color areas” [27]. Strokes causing color agnosia are left posterior cerebral infarcts with inferior temporal involvement. Color agnosia is more frequent than object agnosia.

Prosopagnosia is defined as an inability to recognize visually familiar faces, i.e. faces known by the patient, despite preserved visual perception. Recent studies using functional imaging indicate that the human brain areas activated by personally familiar faces (family, friends, etc.), famous familiar faces (media, politicians, sports people, etc.), and even of one’s own child vs. familiar unrelated children are in part distinct. Current cognitive models consider a core system necessary for the recognition of visual appearance (the system which is disturbed in prosopagnosia), and an extended system relative to person knowledge and to emotion related to or triggered by the perception of a face [28]. Prosopagnosia should not be confused with visuoperceptive deficits in tests using unknown faces, nor with the common complaint of prosopanomia (difficulty in recalling the names of known persons). Patients with prosopagnosia retain their ability to recognize people through other cues, such as voice, gait, size, and clothes. They may also be able to recognize faces by facial features, e.g. moustache, scar, or accessories, e.g. spectacles, rings. They may be able to identify gender, ethnicity, age, and emotional expression. They have a normal semantic knowledge about people. Functional and anatomical studies identified the occipital face area, the fusiform face area, and the superior temporal sulcus as the areas crucial in processing information relative to human faces [29]. Prosopagnosia can be found in 4–7% of posterior cerebral artery infarcts, either bilateral inferomedial or less commonly right inferomedial [30].

Hyperfamiliarity for unknown faces has also been reported.

Related posts:

Chapter 4 – Imaging for Prediction of Functional Outcome and for Assessment of Recovery Chapter 8 – Cardiac Diseases Relevant to Stroke Chapter 12 – Intracerebral Hemorrhage Chapter 19 – Acute Therapies for Stroke Chapter 22 – Management of Acute Ischemic Stroke and its Late Complications Chapter 21 – Intensive Care of Stroke

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