2.2
Diabète insipide II : physiopathologie
Normalement, l'équilibre hydrique est maintenu par trois mécanismes interconnectés : le centre de soif hypothalamique, la synthèse et la libération de…
L’équilibre hydrique est normalement régulé par le centre de soif dans l’hypothalamus, la sécrétion d’ADH et la réponse des reins à l’ADH.
L’ADH favorise la réabsorption de l’eau dans les canaux collecteurs en insérant des canaux aquaporine-2, permettant à l’urine de se concentrer. Cette voie échoue dans le diabète insipide : dans le diabète insipide central, la production ou la libération du TDAH est insuffisante, tandis que dans le diabète insipide néphrogénique, les reins ne répondent pas au TDAH.
Sans action efficace de l’ADH, les conduits collecteurs restent imperméables à l’eau, provoquant une perte excessive d’eau libre dans l’urine. Cela entraîne une polyurie, généralement plus de trois litres par jour, et une augmentation de l’osmolalité plasmatique, qui déclenche une soif intense, ou polydipsie. Chez les enfants, cela peut apparaître sous forme d’énurésie nocturne ou d’énurésie nocturne.
La perte d’eau continue élève l’osmolalité plasmatique au-dessus de la plage normale de 285 à 295 milliosmoles par kilogramme. Si l’apport en liquides ne correspond pas aux pertes, la déshydratation se développe.
View the full transcript and gain access to JoVE Core videos
Q1: How does ADH normally regulate water balance in the body?
ADH, synthesized in the hypothalamus and released from the posterior pituitary, acts on the distal nephron to promote water reabsorption. ADH inserts aquaporin-2 channels into collecting duct cells, allowing water to pass through and urine to become concentrated. This mechanism maintains plasma osmolality within the normal range of 285 to 295 milliosmoles per kilogram.
Q2: What is the difference between central and nephrogenic diabetes insipidus?
In central diabetes insipidus, damage to the hypothalamus or posterior pituitary reduces or abolishes ADH secretion. In nephrogenic diabetes insipidus, ADH is produced normally, but the kidneys fail to respond due to receptor or post-receptor defects. Both forms result in collecting ducts remaining impermeable to water, causing excessive urine loss.
Q3: Why does diabetes insipidus cause excessive thirst and urination?
Without effective ADH action, collecting ducts cannot reabsorb water, leading to polyuria—typically more than three liters per day of dilute urine. This excessive water loss raises plasma osmolality above normal, triggering the hypothalamic thirst center and causing intense polydipsia. If fluid intake cannot match urinary losses, dehydration develops rapidly.
Q4: What laboratory findings are typical in diabetes insipidus?
Diabetes insipidus presents with low urine osmolality below 200 milliosmoles per kilogram, low urine specific gravity around 1.003, and elevated serum sodium above 145 milliequivalents per liter. Plasma osmolality rises above the normal range of 285 to 295 milliosmoles per kilogram. These findings reflect the kidneys' inability to concentrate urine and the resulting hypernatremia.
Q5: How do osmoregulation and baroregulation control ADH release?
Osmoregulation occurs when hypothalamic osmoreceptors detect elevated plasma osmolality and trigger ADH release to conserve water. Baroregulation activates when vascular stretch receptors sense decreased blood volume or pressure, also stimulating ADH secretion. In diabetes insipidus, these feedback systems function normally but remain ineffective due to absent ADH or renal unresponsiveness to ADH.
Q6: What are the clinical symptoms of severe diabetes insipidus?
Severe diabetes insipidus causes dehydration symptoms including dry mucous membranes, poor skin turgor, irritability, and confusion. Severe hypernatremia may trigger neurological complications such as seizures, coma, and circulatory collapse. In children, nocturnal enuresis or bedwetting may be an early presenting sign of the condition.
Q7: How does gestational diabetes insipidus differ from other forms?
Gestational diabetes insipidus occurs when excess placental vasopressinase accelerates ADH breakdown during pregnancy, reducing hormone availability. Like other forms, it results in collecting ducts remaining impermeable to water and causes polyuria and polydipsia. This type is typically reversible after delivery when placental vasopressinase production ceases.