1. Studies were performed in five hyponatraemic (plasma sodium 129 ±1.6 mmol/l; plasma osmolality 268 ±3.0 mosmol/kg) quadriplegic patients in order to elucidate its aetiology. Five age- and sex-matched healthy subjects served as controls.

2. Daily urine volumes were high (4454 ± 624 ml) in the quadriplegic patients secondary to habitually increased fluid intake.

3. All quadriplegic patients had suppressed plasma arginine vasopressin levels (< 0.8 pmol/l) and were able to form dilute urine after a water load (20 ml/kg). However, free water clearance and the ability to excrete the water load were frequently impaired, and these defects were associated with reductions in both osmolar clearance and delivery of filtrate to the distal diluting sites of the nephron.

4. During hypertonic saline (5%, w/v, NaCl) infusion, plasma arginine vasopressin rose progressively before plasma osmolality reached the normal range, consistent with a resetting of the osmostat.

5. We conclude that hyponatraemia in quadriplegic patients is related to an intrarenal defect in water excretion and resetting of the osmostat coupled with increased fluid intake.

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