Motor coordination disorders caused by cerebellar dysfunction are well known. However, the less known cerebellar neuropsychological disorders also merit attention, since they occur more frequently than one might imagine.
We describe a 66-year-old patient with severe cerebellar damage caused by hemorrhagic stroke and associated with cognitive impairments (including impaired executive function, reasoning and judgment). A review of the literature on these neuropsychological disorders revealed a set of clinical, anatomical and functional imaging arguments that prompted us to broaden our vision of the cerebellum’s role by acknowledging the presence of a cognitive component as well as the well-known motility component. In fact, there is good evidence of altered executive function (including mental flexibility, scheduling capacities and verbal working memory) in cerebellar patients. Visuospatial capacities are also affected, with disorders of visual memory and construction abilities having been reported. In terms of language, we noted reports of hypospontaneity and agrammatism with syntax problems. Memory (especially verbal memory), learning (both associative and procedural), judgment and reasoning also seem to be affected. In terms of emotion, various types of abnormal behavior and psychiatric disorders have been described and range from depression to true psychoses. Even though these data are controversial and must be confirmed, they prompt us to reconsider and deepen our understanding of the cerebellum’s role and the functioning and improve our approach to (and management of) patients with cerebellar damage.
Les désordres de la coordination motrice secondaires à une dysfonction du cervelet sont bien connus. Les troubles neuropsychologiques le sont moins et méritent l’attention car il s’agit d’une association clinique sans doute moins inhabituelle qu’il n’y paraît.
L’observation d’un patient de 66 ans, victime d’un accident vasculaire cérébelleux et présentant des troubles des fonctions cognitives, notamment des fonctions exécutives, du raisonnement et du jugement, est rapportée. La revue de littérature des troubles neuropsychologiques liés aux lésions cérébelleuses faite à cette occasion, comporte un ensemble d’arguments cliniques, anatomiques et d’imagerie fonctionnelle qui nous a invités à envisager chez ce patient une participation du cervelet dans ses altérations de fonctions cognitives . En effet, des troubles des fonctions exécutives, incluant la flexibilité mentale, les capacités de planification et la mémoire de travail ont été rapportés. Les capacités visuospatiales sont également concernées, notamment la mémoire visuelle, les capacités constructives et plus globalement la capacité à appréhender l’espace environnant. Concernant le langage, il a été rapporté une hypospontanéité et des difficultés syntaxiques. La mémoire selon plusieurs modalités, les capacités d’apprentissage, de raisonnement, de jugement sont également altérées. Des troubles du comportement, pouvant parfois avoir une présentation psychiatrique ont également été rapportés chez certains patients cérébelleux. Ces constatations, bien que fragiles et controversées, nous invitent à approfondir notre connaissance du fonctionnement du cervelet et par là même à améliorer notre approche du patient cérébelleux.
Cerebellar damage has long been regarded as a quasi-exclusive alteration in motor coordination, with the following set of symptoms: gait disorders, ataxia, posture disorders, impaired limb coordination (dysmetria, dyschronometria, adiadochokinesia, etc.), difficulty generating strong accelerations over short periods of time, hypotonia, dysarthria and swallowing disorders. This set of symptoms appears to result from alterations in the rate, rhythm and strength of contractions of agonistic, antagonistic and synergistic muscle groups. Nevertheless, in the 19th century, a number of clinical observations had already suggested that the cerebellum was involved in the cognitive process . A number of arguments on several levels (including anatomical, clinical, neuropsychological and functional imaging evidence) have reinforced the concept of cerebellar participation in cognitive processes. Starting from a clinical observation, we performed a literature review on the various types of cognitive damage observed in cerebellar patients.
Mr. Z. (a right-handed, 66-year-old company director with an unremarkable personal medical history) had suddenly experienced difficulty in walking and standing. Following a clinical examination in our hospital’s Emergency Department, he was diagnosed as having a static, mildly invalidating cerebellar syndrome and a few moderate, cerebellar kinetic signs in the right arm. The computed tomography (CT) brain scan performed on admission to the Emergency Department revealed a right-side, paramedian, cerebellar, hemorrhagic stroke but without any associated subtentorial abnormalities. The arteriogram showed a vermian arteriovenous malformation (AVM) in the region of the superior right cerebellar artery, which was subsequently embolized with partial success. Later, given the risk of rebleeding, it was decided to fully excise the right vermian AVM. Unfortunately, this operation resulted in a major kinetic and static cerebellar syndrome and Mr. Z. was thus hospitalized in our Physical Medicine and Rehabilitation Service. Our initial clinical examination confirmed the presence of a severe, kinetic and static cerebellar syndrome. The dysmetria (in all four limbs, though predominantly left-side) was very invalidating. Sitting balance was unstable and a standing position was impossible. Hypotonia of the trunk was severe, with large oscillations and incontrollable lurching. Significant dysphagia was aggravated by paresis of the left cranial nerves IX-X, although the other pairs were not affected. Severe cerebellar dysarthria made the patient’s speech unintelligible but there were no other motor command disorders or sensory impairments. A sleep apnoea syndrome became apparent during hospitalization but was not to be considered as the main cause of the cognitive alteration observed, which was more than just a simple vigilance disorder with day-time drowsiness; it required the use of a respiratory aid at night. Lastly, we made the following observations in terms of higher functions: significant attentional fluctuations; disorders of judgment and reasoning, which were especially noticeable in view of the fact that the patient had a high educational level and had been performing a high-responsibility job; a moderate executive function disorder, characterized by inhibition difficulties; significant cognitive slowing was also noted, particularly in terms of visuospatial analysis.
One year after the stroke, Mr. Z. was still suffering from significant dysmetria (mainly in the arms), which interfered with activities of daily living. Walking was only possible with two helpers and a walking frame. His Functional Independence Measure was 60 out of 126. Despite persistent, marked dysarthria, Mr. Z.’s speech had become intelligible. In terms of higher functions, some progress had been made and notably included better intentional coding in anterograde episodic memory. Nevertheless, significant attentional fluctuations persisted (possibly aggravated by the sleep apnoea syndrome, despite the use of a respiratory aid), combined with significant cognitive slowing and moderate impairment of executive function, reasoning and judgment.
In this clinical case, the motor component of the cerebellar damage was particularly intense; however, we also observed cognitive impairments for which a cerebellar origin was confirmed after other etiologies had been ruled out.
Arguments in favor of cerebellar involvement in cognition
Several anatomical arguments indicate that the cerebellum’s role extends beyond the motor sphere. The cerebellum’s large number of neurons relative to its volume (accounting for about half of the brain’s neurons) is the first aspect which, despite its non-specific nature, prompts such a hypothesis question . Likewise, the increase in the size of the cerebellum (and that of the dentate nucleus in particular) during evolution – in parallel with development of the frontal cortex – is a troubling observation that can legitimately be correlated with the progressive increase in the cognitive capacities of animal species. In preterm babies, a relationship between cerebellar volume and subsequent cognitive performance has also been established, independently of motor capacities . A third, very important argument relates to the cerebellum’s connections and particularly its many projections towards non-motor, associative areas of the brain. In fact, various neuronal circuits linking the brain and cerebellum have been described; this resulted in the concept of a cerebrocerebellar pathway connecting the cerebellum to brain areas involved in instinctive behavior, mood and the highest functions of cognition and reasoning, via a two-way circuit within the brain stem (the pontine nuclei and the red nucleus, etc.). Hence, there is certainly a link from the cerebellum to the motor cortex via the red nucleus and the thalamic motor nuclei (the ventroposterolateral and ventrolateral nuclei, etc.) but also to other areas of the associative cortex (the posterior parietal, superior temporal and prefrontal cortex, supplementary motor area and the cingulate and parahippocampal gyri) via the non-specific thalamic nuclei (the intralaminar and dorsomedial nuclei, etc.). The reciprocity of these cerebellum-brain projections is ensured by afferent fibres from the various cortical areas, relayed by the pontine nuclei. The dentate nucleus constitutes an important cerebellar area for the operation of this circuit and appears to be organized into a dorsal part (projecting towards the frontal cortex motor areas) and a ventral part (the “neo-dentate” part, projecting towards the cognitive areas of the prefrontal cortex and areas 9 (planning, working memory) and 46 in particular ).
Neuropsychology: clinical descriptions
Several series of patients with cerebellar damage and neuropsychological disorders have been published ( Table 1 ). A cerebellar cognitive and affective syndrome has notably been described by Schmahmann in patients with predominantly ischemic and hemorrhagic vascular lesions .
|Authors||Number of cases||Type of damage||Deficits observed|
|Schmahmann et al.,||20||Vermian resection (1), bilateral stroke (2), right stroke (7), left stroke (4), infection (3), cerebellar atrophy (3)||Attention, executive function, visuospatial abilities, language, memory|
|Drepper et al.,||9||Idiopathic cerebellar ataxia||Non-motor associative learning|
|Levisohn et al.,||19||Medulloblastoma (11), astrocytoma (7), ependymoma (1)||Executive function, visuospatial abilities, langage, modulation affective|
|Silveri et al.,||1||Right cerebellar haematoma||Right hemisensory neglect|
|Witt et al.,||16||Cerebellar degeneration|
|Brandt et al.,||31||Degeneration (olivopontocerebellar atrophy, spinocerebellar ataxia)||Executive function (word fluency, attention)|
|Gottwald et al.,||21||Metastases (7), meningioma (5), haemangioblastoma (2), angioma (2), haematoma (4), ganglioma (1)||Executive function, attention, working memory|
|Molinari et al.,||39||Left cerebellar damage (stroke, tumour, malformation) (12), right damage cerebellar (13), idiopathic cerebellar ataxia (14)||Visuospatial abilities, mental rotation of objects|
|Quintero et al.,||18||Vermian medulloblastoma (7), vermian astrocytoma (3), hemispherical (7), undocumented (1)||Alteration in procedural learning|
|Ravizza et al.,||15||Left stroke (6), right stroke (4), left (2) or right (2) resected tumour, left resected subarachnoid cyst (1)||Verbal working memory|
|Tavano et al.,||27||Congenital malformation limited to the cerebellum (adults and children)||Affective disorders, autism, social behaviour, language, executive function, visuospatial abilities|
Executive function disorders
Impairment of executive functions can be explained by damage to the connections between the cerebellum and the prefrontal cortex. These executive functions are related to the ability to structure various cognitive tasks for a specific purpose.
Lack of mental flexibility (perseveration of gesture or while drawing, inhibition difficulties, etc.) has been reported, together with impaired planning and decreased attentional abilities , as witnessed by difficulty in learning gestural sequences, whereas simple gestures are performed correctly. The cerebellum thus plays a role in the ability to associate various motor acts making up a sequence. Verbal fluency also appears to be perturbed (independently of dysarthria ), with a more marked impairment of literal fluency than categorical fluency and mainly in right hemispheric lesions .
The spatial component of working memory appears to be relatively undamaged. In contrast, verbal working memory undergoes much more marked impairments, which again appear to be independent of motor articulation disorders . From a topographic point of view, functional imaging suggests that the superior part of the right cerebellum is more involved in articulation control and the initial encoding, whereas the inferior part may be involved in data consolidation and recall .
Cognitive disorders in the strictest sense are often quite minor and, once again, are accentuated by attention disorders or mood disorders. Nevertheless, it appears that cerebellar damage can lead to an overall decrease in intellectual performance , judgment disorders (e.g. belief of absurd stories) and abstract reasoning . Furthermore, during normal aging, cell loss in and atrophy of the vermis dorsal have been observed; there may be a decrease in IQ which spares verbal performance levels and may be correlated with loss of grey matter in the posterior lobe of the right cerebellum .
Instrumental function disorders
Visuospatial and time perception disorders
Visuospatial disorders may be linked to difficulty in mentally manipulating objects in space–capacities that may well involve connections with the parietal lobe.
In terms of perception, simultagnosia has been observed in some patients by applying using tasks such as the Minnesota Paper Form Board Test and the Differential Aptitude Test. This disorder is found for complex tasks requiring assessment of the three-dimensional aspect, whereas simpler, two-dimensional tests (such as the Benton Line Orientation Test) are less affected. However, a cerebellar role in the judgment of line orientation has also been suggested . These tasks appear to be lateralized and primarily involve the right side of the cerebellum . Hence, alterations in spatial construction have been observed via the Rey Complex Figure Test. This is a question of difficulty in planning and judging the figure’s overall organisation (resulting in fragmented copying), rather than a motor coordination disorder. Visual memory is also altered, as witnessed by the accentuated difficulties in differed recall of this same figure, which are more significant than expected – even when taking into account the change in the initial copying .
Certain authors have mentioned a possible organizational change in the spatial representation and/or visual exploration of the environment; these processes may occur more slowly and less efficiently, due to a lack of control of eye saccades and visually guided limb movements. A few cases of hemisensory neglect have thus been described, including a case occurring after a right cerebellum hemorrhage with modifications in the extracorporeal and corporal space, combined with visual (but not tactile) extinction .
Furthermore, time perception appears to be altered in patients presenting cerebellar atrophy or acute, unilateral damage. The cerebellum could be an internal system for time judgment, involved in representing an event’s time schedule, the interval between the beginning and the end of a movement and the duration of a stimulus. Manual tapping tests (temporal productions) are perturbed on the ipsilesional side in cases of damage to the superior part of the cerebellum (the superior semilunar lobe, the anterior and posterior quadrilateral lobes and the central lobule), whereas time perception tests in the strictest sense are not significantly affected. Even though perturbed temporal production has been seen in motor tasks, the disorder could be more related to the temporal component of the task than the motor component , especially if the damage is lateralized. One illustration of this was reported in a music learning disorder, which was possibly linked to difficulty in analyzing variations in inter-note time intervals . The cerebellum has thus been described as a probable site for interpreting spatial and temporal relationships between sensory stimuli .
True language disorders have been reported and may be explained by the cerebellum’s connections with the frontal cortical areas (the left inferior frontal gyrus) and the left lateral temporal area. In the main, verbal hypospontaneity with brief, delayed answers has been observed , prompting the suggestion that the cerebellum has a role in planning speech production at a “pre-articulatory” stage . The main disorders are syntax problems or a decreased ability to distinguish grammatically correct phrases from those that are incorrect (particularly for subject-verb agreements) and even agrammatism (with use of the infinitive verb instead of conjugated forms), due to asynchrony between application of the rules of syntax and the availability of grammatical morphemes . Dysprosodia (with a sharp, childish or whining tone) and hypophonia have often been noted. More rarely, there have been reports of anomia, naming difficulties and trouble deducing the verb derived from a given noun . It seems that the cerebellum helps to adjust to the context (with a view to composing fluent language) syllables learnt and stored stereotypically in the premotor cortex .
Altered speech perception has also been reported and appears to affect the temporal component used to judge the duration of a syllable or a sound, which is sometimes essential for distinguishing between two similar words, which only differ by the pronunciation .
Memory and learning
Various memory disorders have been noted but are sometimes difficult to study because they can be accentuated by concomitant executive function and (above all) attentional disorders. Hence, cognitive tasks requiring sustained attention are often impaired . In terms of verbal memory (part of “sectorial” memory), changes in delayed recall have been observed and may be related to restrictions on verbal working memory, which would doubtless hinder encoding . The disorder may well be situated within the repetition system, which implies the silent manipulation of verbal information and in which the involvement of the right cerebellum has been suggested . In contrast, there are no marked deficiencies in declarative (semantic and episodic) memory.
Concerning so-called “associative” cognitive learning, cerebellar patients took longer to learn an association between six pairs of colors and the numbers from one to six. This test is useful in that it minimizes the potential involvement of verbal and motor damage in the observed difficulties .
Concerning memory and implicit (i.e. non-declarative) learning, it is clear that the cerebellum has a role in mastering motor tasks (in relation with the basal ganglia) and conditioning (in relation with the hippocampus and the cortex). Procedural learning (in serial reaction time tasks, for example) is perturbed (mainly in the initial phase) in cerebellar patients. Nevertheless, these patients do improve and manage to perform the task after a certain number of trials, which contrasts with patients presenting associated striatal lesions. Furthermore, it appears that damage to the left cerebellum tends to interfere with procedural learning involving the left hand. Performance can be improved by inhibiting the healthy tissue (i.e. the right cerebellum and the left prefrontal cortex) with transcranial magnetic stimulation and thus reducing the asymmetry of the cerebellar afferents to the two prefrontal lobes .
In contrast, no alterations (independently of motricity, executive function, visuospatial capacities, working memory and attention) in implicit cognitive learning (e.g. probabilistic classification, such as the weather prediction task) have been observed in cerebellar patients, in contrast with the situation in Parkinsonian patients. This argues in favor of the preponderant role played by the basal ganglia in these learning processes, in addition to dysfunction of the prefrontal cortex and its afferents .
Mood, emotion and behavior
A number of mood disorders have been recorded: emotional blunting, apathy and a lack of empathy or, in contrast, disinhibition with hyperfamiliarity, impulsive acts, irritability, inappropriate jokes and even regressive, childish behavior . These two facets of behavioral disorders have been interpreted as either exaggeration (“hypermetria”) or diminution (“hypometria”) of responses to the internal and external environment . More generally, cerebellar lesions seem to be responsible for difficulties in adapting behavior to social situations . An illustration of this is pathological, out-of-context laughter and crying – a process that is usually regulated by the cerebellum . Apparently psychotic symptoms (such hallucinations) may be related to cerebellar neuron damage. In the particular case of schizophrenia, a correlation between the severity of cerebellar atrophy and the duration of negative and productive symptoms has been identified .
However, in most cases, depression or anxiety are the most common disease-linked symptoms . Indeed, this has been correlated with an increase in vermian blood flow and metabolism, which could be useful in a differential diagnosis versus Alzheimer’s-type disorders, in which cerebellar metabolism is decreased .
Cerebellar damage in children
Cerebellar agenesis gives rise to executive function disorders, stereotypic, autistic-type activities and obsessive rites . The impairments observed in congenital damage are more severe and less specific than for acquired lesions and so it is more difficult to relate a clinical presentation to a precise lesion site . Furthermore, it appears that one of the earliest signs of acquired cerebellum damage in children is irritability (combined with attention disorders), which is often well controlled by carbamazepine administration. A “posterior fossa” syndrome has thus been described after resection of a median line tumour; it combines initial mutism (progressing into dysarthria), buccolingual apraxia and behavioral changes (regression, apathy, reduction in spontaneous movements, etc.) . This syndrome is considered by some to be an acute manifestation of a cerebellar cognitive affective syndrome .
The cerebellum may also be involved in other manifestations. Abnormality of the posterior vermis (which can be hypo- or hyperplastic) has been found in a few cases of autism . Cerebellar development disorders could also be linked to written language disorders and dyslexia, in the absence of a lower IQ or attention disorders .
Functional imaging also provides evidence by revealing activation of the cerebellum during non-motor experimental procedures. Many examples can be cited, such as the cerebellum’s involvement in silent countdown tests and activation of the right cerebellum in a right-handed person during verbal fluency or semantic pairing tests . A large number of cognitive fields have thus been tested – particularly language, during substitution of a verb by the corresponding noun or the formulation of synonym. Studies focusing on memory have also been performed; they show that encoding involves many structures, including the cerebellum (and notably its right, posterior-superior part ). In elderly subjects with intellectual decline (predominantly concerning executive function and particularly verbal working memory), functional imaging evidences lower cerebellar activation than that shown by younger subjects in a similar task . These activations or activation defects do not necessarily correspond to cognitive activity per se but rather involve sensorimotor integration, with the adaptation of cerebellar function controlled by the prefrontal or cingulate cortex according to the task’s complexity and the level of attention required .
An attempt at functional synthesis: integration of motricity and cognition
In view of these many clinical observations, it appears that operation of all or part of the cerebellum is involved in almost the full spectrum of cognitive psychological functions.
Certain authors have advanced the concept of functionally distinct anatomical units. Hence, the neocerebellum is supposedly involved in the cognitive-affective domain and stands out clearly from areas which regulate motricity . More exactly, several levels of organisation in the cerebellum have been described :
an anterior-posterior level of organisation, with an anterior part linked to sensorimotor function and a posterior part (the neocerebellum) involved in cognitive and affective functions;
a medial-lateral level of organisation, with the vermis and fastigial nucleus (involved in affective states, social & emotional behavior and affective memory) on one hand and the hemispheres and the dentate nucleus (involved in the cognitive process) on the other;
lateralization: motor disorders may be more severe when the right cerebellum is damaged. The right hemisphere also appears to be especially involved in verbal intelligence, complex linguistic tasks and sequential auditory memory, whereas the left hemisphere seems to be more focused on (predominantly visuospatial) non-verbal tasks and sequential visual memory . A case of left cerebellar hypoplasia with damage to cerebellar connections to the right hemisphere (diaschisis) presented impaired visuospatial functions but generally intact verbal aptitudes, i.e., a similar picture to that seen in minor hemispheric syndrome . According to certain authors, deficits are more severe when the lesion is within the right cerebellum, which one could thus qualify as “dominant” .
Several hypotheses have been put forward to explain the cerebellum’s role in cognitive function.
The cerebellum may participate in the recognition of sensory stimuli by modulating activation of the primary sensory cortex. It may thus be the area where habitual and new stimuli are compared; cerebellar damage may thus result in an inability to “preconsciously” detect unusual stimuli. According to certain authors , these observations have implications which extend beyond the sole domain of sensitivity. The cognitive and behavioral anomalies found in cerebellar patients may be related to impairments in analyzing afferent sensory stimuli, which could influence the predictive control of cognitive processes.
Other authors have suggested applying the same operating procedure to the cognitive field as has been used in movement . The cerebellum’s role can thus be perceived as modulating neurological function on the motor, affective and cognitive levels and thus enabling harmonious activity . The concept of a constant, unequivocal cerebellar influence on all these various systems also implies the potential for general dysfunction; this would correspond to the observed dysmetria and may express itself as several sub-ensembles, including “dysmetria of thought” in the affective and cognitive domains .
Although the theories that are derived from the above-mentioned clinical descriptions are rather attractive, they are nevertheless controversial in several respects. The observations cover a small number of patients and it is often difficult to unambiguously circumscribe the involvement of motor disorders in the clinical symptoms. It is also difficult to be sure that neuronal lesions are strictly limited to the cerebellum , especially in the case of degenerative diseases or adjunct radio/chemotherapy. Lastly, from an anatomical point of view, certain authors have observed that most of the cerebellar afferents come from cortical zones involved in motor control, which thus appear to be preponderant and may cast doubt on the significance (though not the existence) of cerebellar involvement in non-motor processes .
In conclusion, all these various elements argue in favor of a degree of caution when considering cerebellar involvement in cognitive functions but offer interesting perspectives for investigating certain impairments observed in these patients and which are doubtless yet to be fully evaluated. The physician-patient relationship can thus be transformed by providing the patient and his/her careers with better information on these little-known disorders. In addition to improve cognitive abilities, the value of providing cerebellar patients with rehabilitation care relates to the functional component, which is linked to the motor damage but also to the interaction with certain cognitive components, such as visuospatial capacities and perception of the surrounding space. The reverse also appears to be true: motor rehabilitation work could also help improve the patients’ cognitive and emotional status .
L’atteinte cérébelleuse a longtemps été rattachée à une altération quasi exclusive de la coordination motrice, dont la sémiologie est faite de : trouble de la marche, de l’équilibre (ataxie) et de la posture, trouble de la coordination des membres supérieurs et inférieurs (dysmétrie, dyschronométrie, adiadococinésie …), difficulté à générer des accélérations importantes sur des durées brèves, hypotonie, dysarthrie, troubles de la déglutition. Cette sémiologie semblant résulter d’une altération du taux, du rythme et de la force des contractions des chaînes musculaires agonistes, antagonistes et synergiques. Néanmoins, dès le 19 e siècle, certaines observations cliniques ont laissé entrevoir la possibilité d’une participation du cervelet dans les processus cognitifs . Certains arguments de plusieurs ordres ont permis de renforcer l’idée d’une participation du cervelet aux processus cognitifs : des constatations anatomiques, cliniques ou neuropsychologiques et d’imagerie fonctionnelle. À partir d’une observation clinique, nous avons réalisé une revue de la littérature sur les différentes atteintes cognitives observées chez des patients cérébelleux.
M. Z., patient de 66 ans, droitier, occupant un poste de directeur dans une grande entreprise, sans antécédent particulier ressentit un jour brutalement une instabilité à la marche et à la station debout. Il consulta aux urgences et à l’examen clinique, il avait été objectivé un syndrome cérébelleux statique peu invalidant et quelques signes cérébelleux cinétiques discrets au membre supérieur droit. Le scanner cérébral, réalisé en urgence montrait un accident vasculaire hémorragique cérébelleux paramédian droit, sans anomalie sus-tentorielle associée pouvant entrer en compte dans le tableau clinique. L’artériographie mit en évidence une malformation artérioveineuse (MAV) vermienne dans le territoire de l’artère cérébelleuse supérieure droite qui sera embolisée avec une efficacité partielle. Par la suite, devant les risques de resaignement, il a été décidé de programmer une exérèse complète de la MAV vermienne droite. Celle-ci se fera malheureusement au prix d’un syndrome cérébelleux statique et cinétique majeur. C’est dans ce contexte que M. Z. a été hospitalisé en médecine physique et de réadaptation. L’examen clinique initial objectivait un syndrome cérébelleux cinétique et statique sévère. La dysmétrie des quatre membres prédominant à gauche était très invalidante. L’équilibre assis était instable, la station debout impossible. L’hypotonie du tronc était majeure avec de grandes oscillations et des embardées incontrôlables. Il existait une dysphagie importante, aggravée par une parésie du IX et du X gauches, sans autre anomalie des paires crâniennes. La dysarthrie cérébelleuse majeure rendait la parole inintelligible ; il n’existait pas d’autre déficit de la commande motrice ni trouble sensitif. Un syndrome d’apnée du sommeil sera objectivé au cours de l’hospitalisation nécessitant un appareillage nocturne, mais insuffisant pour rendre compte de l’altération cognitive du fait de l’atteinte beaucoup plus large qu’un simple trouble de la vigilance avec hypersomnie diurne. Enfin sur le plan des fonctions supérieures, étaient constatés : d’importantes fluctuations attentionnelles ; des troubles du jugement et du raisonnement, d’autant plus spécifiques que ce patient avait un haut niveau scolaire et qu’il occupait un poste à responsabilités ; un trouble des fonctions exécutives modéré marqué notamment par des difficultés d’inhibition ; un ralentissement idéatoire significatif a également été noté, portant en particulier sur l’analyse visuospatiale.
Un an après l’AVC, il persistait une dysmétrie majeure, surtout aux membres supérieurs, gênant la réalisation des gestes de la vie quotidienne ; la marche n’était possible qu’avec deux aides humaines et un déambulateur. La mesure d’indépendance fonctionnelle était de 60/126. Malgré la dysarthrie qui restait marquée, la parole était devenue intelligible. Sur le plan des fonctions supérieures, quelques progrès ont été enregistrés, marqués notamment par une meilleure capacité d’encodage intentionnel en mémoire épisodique antérograde. Il persistait néanmoins d’importantes fluctuations attentionnelles (possiblement aggravées par le syndrome d’apnée du sommeil malgré l’appareillage), associées à un ralentissement idéatoire significatif, un trouble modéré des fonctions exécutives, du raisonnement et du jugement.
Dans cette observation, la composante motrice de l’atteinte cérébelleuse était particulièrement sévère, mais on observait des dysfonctionnements cognitifs dont l’origine cérébelleuse avait été évoquée après que les autres étiologies aient été infirmées.
Arguments en faveur du rôle du cervelet dans la cognition
Plusieurs arguments anatomiques semblent orienter vers un rôle élargi du cervelet, s’étendant au-delà de la sphère de la motricité. Le grand nombre de neurones contenus dans le cervelet par rapport à son volume (environ 50 % des neurones du cerveau) est un premier élément qui, quoique peu spécifique, invite à se poser cette question . De même, l’importance du développement du cervelet (en particulier du noyau dentelé) au fur et à mesure de l’évolution des espèces et en parallèle du cortex frontal est une constatation troublante, semblant devoir être à mettre en corrélation avec l’accroissement proportionnel des capacités cognitives des espèces animales. Une relation a également été établie chez l’homme entre le volume du cervelet (étudié chez des prématurés) et les performances cognitives, à niveau de motricité égal, par ailleurs . Un troisième argument, dont l’importance semble majeure, est lié aux connexions du cervelet, en particulier à ses nombreuses projections vers les aires associatives cérébrales, autres que motrices. En effet, différents circuits neuronaux entre cerveau et cervelet ont été décrits, aboutissant au concept de système cérébro-cérébelleux, unissant le cervelet à des zones du cerveau impliquées dans les comportements instinctifs, l’humeur et les plus hauts degrés de la cognition et du raisonnement, en un circuit bidirectionnel faisant relais dans le tronc cérébral (noyaux du pont, noyau rouge…). Une liaison est ainsi assurée entre cervelet et cortex moteur via le noyau rouge et les noyaux moteurs du thalamus (ventro-postérolatéral, ventrolatéral …), mais également avec d’autres zones du cortex associatif (pariétal postérieur, temporal supérieur, préfrontal, aire motrice supplémentaire, gyrus cingulaire et parahippocampique), via les noyaux non spécifiques du thalamus (intralaminaires, dorsomédial…). La réciproque de ces projections cérébello-cérébrales est assurée par des fibres partant des différentes aires corticales, avec un relais par les noyaux du pont. Le noyau dentelé constitue une zone importante du cervelet dans le fonctionnement de ce circuit. Il semble s’organiser en une partie dorsale, qui se projette vers les aires motrices du cortex frontal et une partie ventrale (« neo-dentate »), se projetant vers les aires cognitives du cortex préfrontal : aire 9 (planification, mémoire de travail) et 46 en particulier .
Neuropsychologie : descriptions cliniques
Plusieurs séries de patients porteurs de lésion cérébelleuse et présentant des troubles neuropsychologiques ont été publiées ( Tableau 1 ). Un syndrome cognitivo-affectif cérébelleux a notamment été décrit par le Dr Schmahmann après l’étude de patients présentant pour la plupart des lésions vasculaires ischémiques ou hémorragiques .