Reset Osmostat

Reset osmostat is a form of chronic mild hyponatraemia in which the osmotic threshold for vasopressin release is set at a lower plasma osmolality. Water handling is otherwise normal.

Sodium is stable, usually 125 to 135 mmol/L, and varies little from day to day. It accounts for a minority of patients labelled as having the syndrome of inappropriate antidiuresis (about 10 to 20%).

Associated conditions are tuberculosis and other chronic lung disease, malignancy, psychiatric illness, quadriplegia, malnutrition, pregnancy, HIV and cerebral disease.

Pathophysiology.

The hypothalamic osmoreceptors defend a lower set point, so vasopressin is suppressed at a lower osmolality and the kidney still dilutes and concentrates urine. The set point resets as part of the underlying illness or as a variant in normal people.

Thirst, vasopressin secretion and urine dilution therefore all operate, around a lower sodium.

• Usually asymptomatic, with low sodium found on routine tests.
• Features of the underlying disease. No oedema, normal blood pressure and no dehydration.

Red flags.

• Sodium falling under 125 mmol/L or new symptoms (confusion, seizure), which suggest a different cause.

• Stable mild hyponatraemia (125 to 135 mmol/L) on repeated tests, with low plasma osmolality, in a euvolaemic patient with normal thyroid, adrenal and kidney function.
• Water load test (20 mL/kg orally over 15 to 30 minutes): urine osmolality falls under 100 mOsm/kg and over 80% of the load is excreted in 4 hours. Plasma sodium stays stable at its new lower level. A hypertonic saline test shows vasopressin release at a lower osmolality.
• A drop in sodium after the load is not seen, unlike the syndrome of inappropriate antidiuresis.
• Look for the underlying cause with chest imaging, medicines review and cancer assessment as indicated.

Differential diagnosis.

• Syndrome of inappropriate antidiuresis (urine does not dilute after a water load).
• Adrenal insufficiency, hypothyroidism, diuretic effect and cerebral salt wasting.
• Psychogenic polydipsia and low solute intake.

• Explain the benign nature of the condition. No treatment is needed because sodium is stable and the person is well.
• Treat the underlying disease. Avoid attempts to force sodium to normal with fluid restriction, hypertonic saline, urea or vaptans, which fail or cause harm because vasopressin responds normally to the new set point.
• Avoid drugs and fluids that lower sodium further.

Special groups. Pregnancy: sodium falls by about 5 mmol/L normally, so interpret with a lower reference range.

Monitoring. Sodium every 6 to 12 months and during any intercurrent illness.

Referral and escalation. Endocrinology if sodium falls under 125 mmol/L, symptoms develop or the diagnosis is uncertain.