Arginine Vasopressin Disorders

Arginine vasopressin disorders are a group of conditions of deficient arginine vasopressin (antidiuretic hormone) secretion or action, or renal resistance to adequately secreted hormone. Arginine vasopressin deficiency was historically termed central or cranial diabetes insipidus. Arginine vasopressin resistance was historically termed nephrogenic diabetes insipidus.

Both produce an inability to concentrate urine appropriately, with polyuria and polydipsia. They are the physiological converse of syndrome of inappropriate antidiuretic hormone secretion.

Accurately distinguishing the central and nephrogenic forms, and excluding the important differential of primary polydipsia, is essential. Pathophysiology and treatment differ fundamentally.

Pathophysiology and classification

• Arginine vasopressin deficiency (central diabetes insipidus) reflects inadequate hypothalamic synthesis or posterior pituitary release of arginine vasopressin. Causes are pituitary or hypothalamic tumour (particularly craniopharyngioma and germinoma, and metastatic disease to this region), traumatic brain injury, pituitary surgery, and infiltrative conditions (including Langerhans cell histiocytosis and neurosarcoidosis).
• In a proportion of cases the cause is idiopathic or autoimmune destruction of the relevant hypothalamic neurons.
• Arginine vasopressin resistance (nephrogenic diabetes insipidus) reflects renal tubular resistance, at the collecting duct, to adequately secreted arginine vasopressin. Genetic causes are mutations affecting the vasopressin receptor or the aquaporin water channel, typically presenting in infancy.
• More commonly in adults the causes are acquired: lithium therapy, hypercalcaemia, hypokalaemia and certain other medications. Lithium is a particularly important and common cause, given widespread use in bipolar disorder.
• Primary polydipsia is an important differential rather than a true arginine vasopressin disorder. It reflects excessive primary fluid intake, from a psychiatric compulsive drinking pattern or, less commonly, an organic hypothalamic thirst regulation abnormality.
• The resulting mild volume expansion and hypo-osmolality appropriately suppress arginine vasopressin secretion. The renal concentrating mechanism itself remains intact, in contrast to the genuine arginine vasopressin disorders.

• Polyuria is the shared presentation of the arginine vasopressin deficient and resistant forms. It is often marked, with urine output frequently exceeding several litres per day in significant disease.
• Polydipsia reflects an appropriate thirst response to the ongoing renal water loss.
• Where fluid replacement is not accessible, or the thirst mechanism is itself impaired, there is risk of significant dehydration and hypernatraemia.
• Nocturia, from polyuria continuing overnight, is common and often particularly troublesome.
• Seek a relevant history: preceding pituitary or hypothalamic pathology, head trauma, and medication exposure (particularly lithium). In the rare genetic nephrogenic forms, which typically present in infancy, seek a family history.
• Where the cause is a pituitary or hypothalamic mass lesion, headache, visual disturbance and features of anterior pituitary hormone deficiency may accompany the arginine vasopressin deficiency symptoms, given the anatomical proximity.
• Features favouring primary polydipsia over a genuine arginine vasopressin disorder include a relevant psychiatric history (particularly certain psychotic disorders associated with compulsive water drinking) and the pattern of biochemical and urinary response on formal water deprivation testing.

• Initial assessment confirms genuine polyuria (generally defined as urine output exceeding 3 litres per day in an adult) with paired serum and urine osmolality. A low urine osmolality with a normal or elevated serum osmolality supports a concentrating defect needing specific evaluation to distinguish the possible causes.
• A formal water deprivation test is performed under close medical supervision, given the risk of significant dehydration. It involves fluid restriction with serial monitoring of weight, serum osmolality and urine osmolality, then administration of desmopressin (a synthetic arginine vasopressin analogue) to assess the urinary concentrating response.
• Adequate urinary concentration with fluid restriction alone supports primary polydipsia. Arginine vasopressin secretion and renal responsiveness are intact once the physiological stimulus for concentration is present.
• Inadequate concentration with fluid restriction alone, but a significant, appropriate concentrating response to desmopressin, supports arginine vasopressin deficiency (central diabetes insipidus). Renal responsiveness is intact once exogenous hormone is provided.
• Inadequate concentration with fluid restriction and a similarly blunted response even to desmopressin supports arginine vasopressin resistance (nephrogenic diabetes insipidus). The kidney is resistant to the hormone regardless of its source.
• Copeptin is a stable surrogate marker for arginine vasopressin secretion, as native arginine vasopressin is technically difficult to measure reliably. Copeptin measurement, particularly with a hypertonic saline stimulation test, is increasingly used as a more direct, physiologically based alternative to the water deprivation test.
• This approach improves diagnostic accuracy, particularly in distinguishing partial arginine vasopressin deficiency from primary polydipsia, a scenario that has been historically challenging with the traditional water deprivation test.
• Magnetic resonance imaging of the pituitary and hypothalamus is essential in confirmed or strongly suspected arginine vasopressin deficiency, given the frequent underlying structural cause.
• Loss of the normal posterior pituitary "bright spot" on T1 weighted imaging, reflecting loss of arginine vasopressin containing neurosecretory granules, is a supportive but non-specific finding.
• Assess for and correct any contributing cause of nephrogenic arginine vasopressin resistance as part of the diagnostic evaluation. This includes reviewing lithium therapy and correcting hypercalcaemia or hypokalaemia where present.
• Where an acquired, reversible cause is identified and addressed, the renal concentrating defect may improve or resolve.

• Arginine vasopressin deficiency (central diabetes insipidus) is treated with desmopressin. This synthetic arginine vasopressin analogue acts selectively at the renal V2 receptor, avoiding the pressor, V1 receptor mediated effects of native arginine vasopressin.
• Desmopressin is given orally, intranasally or subcutaneously according to patient preference and clinical context. Dose is titrated to symptomatic response, with regular monitoring of serum sodium.
• Excessive dosing risks dilutional hyponatraemia, because sustained water retention occurs without the normal physiological arginine vasopressin regulatory feedback.
• Any identified structural cause of arginine vasopressin deficiency (pituitary or hypothalamic tumour, or other infiltrative or structural pathology) needs targeted treatment according to the specific diagnosis, alongside symptomatic desmopressin.
• Arginine vasopressin resistance (nephrogenic diabetes insipidus) does not respond to desmopressin, as the kidney is resistant to the hormone regardless of its source. Management focuses on any reversible cause: discontinuing or reducing lithium where clinically feasible given the psychiatric indication, and correcting hypercalcaemia or hypokalaemia.
• Symptomatic measures include a low sodium, low protein diet, which reduces the osmotic solute load requiring renal excretion and so the obligate urine volume.
• Thiazide diuretics paradoxically reduce urine output. They produce mild volume depletion, which increases proximal tubular sodium and water reabsorption.
• In more severe, refractory cases a potassium sparing diuretic or nonsteroidal anti-inflammatory drug may be added.
• Lithium induced nephrogenic diabetes insipidus needs individualised discussion of the risks and benefits of discontinuing lithium. Lithium is often highly effective, sometimes essential, for the underlying bipolar disorder, and discontinuation risks significant psychiatric relapse. The decision requires close collaboration between endocrinology and psychiatry.
• Amiloride, a potassium sparing diuretic, directly blocks the lithium mediated nephrotoxic effect at the renal collecting duct. It may allow lithium to continue alongside improved renal concentrating capacity in appropriately selected patients.
• Primary polydipsia, once confirmed on formal testing, is managed with behavioural strategies to gradually reduce fluid intake and, where relevant, treatment of any underlying psychiatric condition driving compulsive drinking.
• Avoid desmopressin in primary polydipsia. Suppressing the intact renal water excretion in a patient who continues excessive fluid intake risks severe, potentially life threatening dilutional hyponatraemia.
• Ensure adequate, ready access to fluid. Educate patients on maintaining adequate intake and on vigilance during intercurrent illness or any situation limiting access to fluid (surgery, travel, or acute illness with reduced oral intake).
• For patients on desmopressin, educate on allowing a scheduled period without dosing to permit some physiological water excretion. Completely uninterrupted desmopressin coverage risks progressive dilutional hyponatraemia.