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Diabetes Insipidus II: Pathophysiology

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Water balance is normally regulated by the thirst center in the hypothalamus, ADH secretion, and the kidneys’ response to ADH.

ADH promotes water reabsorption in the collecting ducts by inserting aquaporin-2 channels, allowing urine to become concentrated. This pathway fails in diabetes insipidus: in central diabetes insipidus, ADH production or release is insufficient, while in nephrogenic diabetes insipidus, the kidneys are unresponsive to ADH.

Without effective ADH action, the collecting ducts remain impermeable to water, causing excessive free water loss in urine. This results in polyuria, typically more than three liters per day, and a rise in plasma osmolality, which triggers intense thirst, or polydipsia. In children, this may appear as nocturnal enuresis or bedwetting.

Continued water loss raises plasma osmolality above the normal range of 285 to 295 milliosmoles per kilogram. If fluid intake does not match losses, dehydration develops.

Overview

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Normally, water balance is maintained through three interconnected mechanisms: the hypothalamic thirst center, the synthesis and release of antidiuretic hormone (ADH, or vasopressin), and the kidneys' responsiveness to this hormone. ADH is synthesized in the hypothalamus, released from the posterior pituitary, and acts on the distal nephron, allowing water reabsorption and concentrated urine production.

Diabetes Insipidus and Its Types

In diabetes insipidus (DI), this regulatory system is disrupted. In central DI, damage to the hypothalamus or posterior pituitary reduces or abolishes ADH secretion. In nephrogenic DI, ADH is present, but the kidneys fail to respond because of receptor or post-receptor defects. In gestational DI, excess placental vasopressinase accelerates ADH breakdown. In all forms, collecting ducts remain impermeable to water, leading to the excretion of large volumes of dilute urine (polyuria) and excessive thirst (polydipsia).

Pathophysiology

Increased urine output raises plasma osmolality above normal (285–295 mOsm/kg), triggering thirst. If water intake cannot match urinary losses, dehydration develops, leading to hypernatremia, hypovolemia, and cellular dehydration. Typical findings include low urine osmolality (<200 mOsm/kg), low urine specific gravity (~1.003), and elevated serum sodium (>145 mEq/L).

Two feedback systems regulate water balance. Osmoregulation, via hypothalamic osmoreceptors, triggers ADH release when plasma osmolality rises. Baroregulation, via vascular stretch receptors, stimulates ADH release when blood volume or pressure falls. In diabetes insipidus, these mechanisms function but are ineffective due to absent ADH or renal unresponsiveness to ADH.

Clinical Features

Clinically, this explains the persistence of polyuria and polydipsia, along with symptoms of dehydration such as dry mucous membranes, poor skin turgor, irritability, and confusion. Severe hypernatremia may cause neurological complications, including seizures, coma, and circulatory collapse.

Transcript

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Diabetes Insipidus

Antidiuretic Hormone

Central Diabetes Insipidus

Nephrogenic Diabetes Insipidus

Gestational Diabetes Insipidus

Plasma Osmolality

Urine Osmolality