Disorders of the Neurohypophysis¶
Chapter 393 | Part 12: Endocrinology and Metabolism
KEY CLINICAL POINTS¶
- AVP (arginine vasopressin) is critical for water reabsorption in the kidneys via V2 receptors, while oxytocin regulates milk letdown and uterine contractions.
- Diabetes insipidus (DI) includes central (AVP deficiency) and nephrogenic (renal resistance to AVP) forms, with polyuria, polydipsia, and hypernatremia as key features.
- Copeptin levels >21.4 pmol/L without water deprivation confirm AVP resistance, while >4.9 pmol/L after hypertonic saline identifies AVP deficiency.
- Desmopressin (DDAVP) is the treatment for central DI, while thiazide diuretics and low-sodium diets manage nephrogenic DI.
- Primary polydipsia is characterized by excessive fluid intake, often linked to psychiatric disorders or lifestyle factors, with normal serum sodium levels.
1. DEFINITION & OVERVIEW¶
The neurohypophysis (posterior pituitary) stores and secretes AVP and oxytocin. AVP regulates water balance by promoting renal water reabsorption, while oxytocin mediates milk ejection and uterine contractions. Disorders include diabetes insipidus (DI), inappropriate antidiuresis (SIAD), and TSH-secreting adenomas.
Table 393-1: Etiology of Polyuria–Polydipsia Syndromes¶
| BASIC DEFECT | ACQUIRED CAUSES | HEREDITARY CAUSES |
|---|---|---|
| AVP Deficiency | Trauma, neoplasia, vascular disorders, infections, drug/toxin exposure, osmoreceptor dysfunction | Autosomal dominant (AVP mutations), autosomal recessive (WFS1, PCSK1), X-linked (AVPR2 mutations) |
| AVP Resistance | Lithium, hypercalcemia, hypokalemia, infiltrative lesions, genetic mutations (AQP2) | Autosomal recessive/dominant, X-linked |
| Primary Polydipsia | Psychosis, compulsive drinking, health enthusiasts | NA |
| Gestational AVP Deficiency | Placental vasopressinase metabolism | NA |
1.1 Neurohypophysis Structure¶
Composed of axons of hypothalamic magnocellular neurons (paraventricular and supraoptic nuclei). These neurons produce AVP (antidiuretic hormone) and oxytocin, stored in neurosecretory vesicles and released via exocytosis.
1.2 AVP Synthesis and Secretion¶
AVP is synthesized as a precursor (pre-pro-vasopressin) with neurophysin and copeptin. Cleavage produces AVP, neurophysin II, and copeptin. Secretion is regulated by osmotic and hemodynamic stimuli.
2. EPIDEMIOLOGY¶
Central DI is rare (incidence ~1/100,000), while nephrogenic DI is more common in patients with lithium use or genetic mutations. Primary polydipsia is prevalent in psychiatric disorders (e.g., schizophrenia) and lifestyle-driven cases.
2.1 Risk Factors¶
Central DI: trauma, tumors, infections, medications (lithium). Nephrogenic DI: lithium, hypercalcemia, genetic disorders. Primary polydipsia: psychiatric illness, compulsive behavior.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
AVP deficiency (central DI) results from hypothalamic/pituitary damage. AVP resistance (nephrogenic DI) involves renal insensitivity to AVP. Primary polydipsia stems from altered thirst mechanisms or psychiatric conditions.
3.1 AVP Deficiency¶
Caused by hypothalamic injury, tumors, or genetic mutations (e.g., AVPR2). Leads to impaired renal water reabsorption and hypernatremia.
3.2 AVP Resistance¶
Renal tubule dysfunction (e.g., AQP2 mutations) or drug-induced (lithium). AVP fails to concentrate urine despite normal plasma osmolarity.
4. CLINICAL FEATURES¶
Polyuria (>40–50 mL/kg/24h), polydipsia, hypernatremia, and dehydration. SIAD presents with hyponatremia, headache, confusion, and neurological symptoms. Primary polydipsia has normal serum sodium with excessive fluid intake.
4.1 Diabetes Insipidus¶
Central DI: polyuria, polydipsia, hypernatremia. Nephrogenic DI: polyuria, normal sodium, and hyponatremia with low urine osmolality.
4.2 Inappropriate Antidiuresis (SIAD)¶
Hyponatremia (<135 mmol/L), hypoosmolality, and symptoms like confusion, seizures, or cerebral edema. Often due to ectopic AVP secretion or psychopharmacologic agents.
5. DIFFERENTIAL DIAGNOSIS¶
Differentiate DI from primary polydipsia (normal sodium), SIAD (hyponatremia), and other causes of polyuria (e.g., renal disease, diabetes mellitus). Assess urine osmolality and AVP/copeptin levels.
5.1 Polyuria-Polydipsia Syndromes¶
Central DI: low urine osmolality, high plasma osmolarity. Primary polydipsia: normal sodium, high urine volume with low osmolality. Nephrogenic DI: low urine osmolality despite AVP.
6. INVESTIGATIONS & DIAGNOSIS¶
Measure serum/osmolar urine concentrations, AVP/copeptin levels, and perform water deprivation tests. MRI of the pituitary and renal function tests aid in diagnosis.
Table 393-2: Management of Adipsic AVP Deficiency¶
| Step | Action |
|---|---|
| 1 | Replace AVP with DDAVP (desmopressin). |
| 2 | Monitor fluid intake/output as inpatient to achieve eunatremia. |
| 3 | Weigh patient daily and record eunatremic weight. |
| 4 | Recommend 1.5–2 L/day fluid intake with adjustments for losses. |
| 5 | Regular plasma sodium measurements to prevent hyponatremia. |
6.1 Diagnostic Tests¶
Water deprivation test: assess urine osmolality after fluid restriction. Desmopressin test: evaluate response to synthetic AVP. Copeptin levels >21.4 pmol/L confirm AVP resistance.
6.2 Imaging¶
MRI of the sella turcica and suprasellar regions to identify tumors or structural abnormalities causing DI.
7. MANAGEMENT & TREATMENT¶
DDAVP (desmopressin) for central DI, thiazide diuretics/amiloride for nephrogenic DI, and fluid restriction for primary polydipsia. Monitor for hyponatremia and renal function.
7.1 AVP Deficiency¶
DDAVP (nasal, oral, or IV) to restore antidiuresis. Avoid excessive fluid intake to prevent hyponatremia.
7.2 AVP Resistance¶
Thiazide diuretics + low-sodium diet. Avoid lithium if possible. Monitor for acute renal failure.
7.3 Primary Polydipsia¶
Gradual fluid restriction, treat underlying psychiatric conditions, and use GLP-1 analogs to reduce thirst.
8. PROGNOSIS & COMPLICATIONS¶
Central DI is treatable with DDAVP, but nephrogenic DI may require lifelong management. SIAD can cause severe hyponatremia, leading to seizures or cerebral edema. Hyponatremia correction must be gradual to avoid osmotic demyelination.
8.1 Complications¶
Hyponatremia: confusion, seizures, cerebral edema. Hyponatremia correction: 8–12 mmol/L/day to prevent osmotic demyelination.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: gestational AVP deficiency due to placental vasopressinase. Elderly: increased risk of SIAD and hyponatremia. Monitor for drug interactions (e.g., lithium, NSAIDs).
9.1 Pregnancy¶
Gestational AVP deficiency may resolve postpartum. Monitor for polyuria and hypernatremia during labor.
10. KEY POINTS & CLINICAL PEARLS¶
- AVP deficiency (central DI) is treated with DDAVP; AVP resistance (nephrogenic DI) with thiazides.
- Copeptin >21.4 pmol/L confirms AVP resistance.
- SIAD requires careful hyponatremia correction to avoid cerebral edema.
- Primary polydipsia is managed with fluid restriction and psychiatric evaluation.