Le nombre de sujets atteints de démence augmente avec le vieillissement des populations. Il s'agit donc, dans les pays développés, d'un problème important de santé publique. C'est pourquoi la recherche médicale concernant les origines de ces maladies se poursuit sans relâche. Des traitements médicamenteux sont aujourd'hui disponibles, dont l'efficacité est néanmoins limitée ; ils permettent de retarder le placement du patient en institution. La recherche s'oriente vers des traitements précoces qui pourraient retarder l'évolution de la démence.
Voir : maladie de Creutzfeldt-Jakob, dengue.
démodex
Parasite présent dans les follicules pileux.
Le démodex, un acarien, pourrait être à l'origine de la blépharite et de l'acné rosacée.
Demons-Meigs (syndrome de)
Syndrome caractérisé par l'association d'une tumeur ovarienne bénigne (souvent un fibrome) et d'un épanchement de liquide dans le péritoine (ascite) et dans la plèvre (hydrothorax).
Synonyme : syndrome de Meigs.
L'origine du syndrome de Demons-Meigs est inconnue. L'ascite se traduit par un gonflement de l'abdomen, et l'épanchement pleural par une gêne respiratoire. Le diagnostic de la tumeur repose sur l'examen clinique et est confirmé par échographie. La radiographie du thorax met en évidence l'hydrothorax, et l'échographie, l'ascite. Intarissables, ces épanchements doivent être ponctionnés fréquemment.
Le traitement consiste en l'ablation chirurgicale de la tumeur, qui entraîne la guérison immédiate.
démyélinisation
Perte de la gaine de myéline qui entoure certaines fibres nerveuses.
Les gaines de myéline, de nature lipidique et protéique, ont pour rôle d'accélérer la vitesse de transmission des influx nerveux le long des axones des cellules nerveuses. La démyélinisation perturbe donc le fonctionnement de celles-ci. Un tel phénomène est observé dans différentes maladies (syndrome de Guillain-Barré, sclérose en plaques). La cause de cette affection est souvent mal connue, mais ses effets évoquent des anomalies du système immunitaire, créant ce que l'on appelle « une susceptibilité personnelle ». Un ou des agent(s) environnemental(aux) agissent comme cofacteur(s) dans le déclenchement de la maladie. Les symptômes, neurologiques, dépendent de la maladie : paralysies, perte de sensibilité, troubles sensoriels.
Le diagnostic est confirmé par un électromyogramme du système nerveux périphérique, qui traduit l'effondrement des vitesses de conduction nerveuse. Généralement, le traitement ne peut que ralentir momentanément la maladie ou soulager les symptômes. Cependant, dans un certain nombre de cas, la démyélinisation régresse spontanément.
dengue
Maladie infectieuse due à un virus du groupe des arbovirus.
Synonyme : fièvre rouge.
La dengue est transmise à l'homme par la piqûre d'un moustique, Aedes ægypti. C'est une maladie endémique survenant par épidémies dans de nombreuses régions chaudes du globe. Longtemps cantonnée en Asie tropicale (Sud-Est), cette infection ne cesse de s'étendre dans le monde depuis 1970, y compris dans des régions moins chaudes. Elle se rencontre aujourd'hui dans les îles de la zone tropicale du Pacifique, en Amérique centrale, dans les Caraïbes, en Amérique du Sud (Brésil) et, à un moindre degré, en Afrique. Un autre moustique susceptible de véhiculer l'agent de la dengue, l'Ædes albopictus, s'est par ailleurs installé en Amérique du Nord et dans certaines régions de l'Europe (Italie, Albanie). Deux milliards d'individus sont exposés à la maladie, et 300 millions de cas sont recensés chaque année.
SYMPTÔMES ET SIGNES
La dengue se déclare habituellement, entre le cinquième et le huitième jour après la piqûre, par un état grippal avec une fièvre élevée et des douleurs diffuses. Une éruption érythémateuse cutanée apparaît au deuxième jour. Après une rémission d'une journée, les symptômes reprennent puis la guérison survient en une dizaine de jours, laissant le malade dans un état de grande fatigue.
Une autre forme de la maladie, la dengue hémorragique, provoque des hémorragies cutanées, viscérales et digestives, et peut être mortelle ; elle nécessite des soins intensifs.
TRAITEMENT ET PRÉVENTION
Purement symptomatique, le traitement vise à réduire la fièvre et les douleurs. La prévention consiste essentiellement à se protéger des moustiques. Des recherches sont engagées pour la mise au point d'un vaccin.
Voir : fièvre hémorragique virale.
déni
Refus inconscient de reconnaître une réalité extérieure traumatisante.
En psychanalyse, le terme de déni fut employé pour la première fois en 1924 par Sigmund Freud pour désigner un mécanisme de défense du moi du petit enfant contre l'angoisse provoquée par la découverte de la différence des sexes. Refusant d'admettre la réalité, l'enfant continue à croire à l'existence d'un pénis féminin, puis attribue son absence à une castration.
Selon Freud, le mécanisme du déni s'apparente à celui de la psychose, tous deux étant marqués par un rejet de la réalité. Si le déni est normal chez le tout petit enfant, la persistance d'une croyance au phallus maternel lorsque l'enfant grandit peut se transformer en fétichisme. Dans cette forme de déviation sexuelle, qui traduit la persistance d'un conflit infantile inconscient, l'individu ne trouve sa satisfaction érotique que par l'intermédiaire d'un objet (chaussure, gant, etc.) appartenant au sexe opposé et qui tient pour lui le rôle de phallus maternel : cela lui permet ainsi de concilier le déni et la reconnaissance de la différence sexuelle.
D'une manière générale, le terme déni est utilisé pour signifier le refus de la réalité, d'une maladie ou d'un handicap. La dénégation est la mise en acte du déni. En cas de psychose, la dénégation est un refus de la réalité associée à une contestation des dires du médecin ou de l'entourage.
densitométrie osseuse
Mesure de la densité osseuse par évaluation du contenu minéral osseux, essentiellement du calcium.
Synonyme : ostéodensitométrie.
La densitométrie osseuse est d'abord indiquée pour quantifier le risque fracturaire de l'ostéoporose ménopausique, par réduction de la masse osseuse ; mais elle s'applique aussi à d'autres formes d'ostéoporose, ou à des maladies osseuses comme l'ostéomalacie, l'hyperparathyroïdie, la fluorose osseuse. Elle se pratique actuellement après l'âge de 65 ans, pour dépister les sujets à risque d'ostéoporose.
TECHNIQUES
La technique de référence est l'absorptiométrie biphotonique qui utilise les rayons X à faible dose (l'ostéodensitomètre est un appareillage spécial) et, plus précisément, les mesures d'absorption de deux photons d'énergie différente pour calculer la « masse osseuse ». Les résultats sont chiffrés et comparés à la normale pour le sexe et l'âge. Des indications de traitement peuvent en dépendre (diphosphonates).
Le scanner à rayons X donne des évaluations de la densité osseuse plus approximatives, moins adaptées à l'évaluation de la masse osseuse et ne permettant pas de caractériser le risque fracturaire.
La mesure de l'atténuation d'un faisceau d'ultrasons à travers le calcanéum est une méthode simple et peu coûteuse.
Voir : absorptiométrie biphotonique.
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