This hyponatremic patient with nausea, malaise, headache, and confusion; a low urine specific gravity (normal is approximately 1.010); and appropriately dilute urine (<100 mOsm/kg) is likely suffering from primary polydipsia. Patients with schizophrenia are at an increased risk of developing primary polydipsia.
Primary polydipsia is the excessive ingestion water to the point of causing hyponatremia, which can often be profound. Urine osmolality and specific gravity will be low. The urine concentration of sodium will also be very low. If you withhold water from the patient and the urine osmolality and specific gravity both increase, this confirms the diagnosis.
Goldman et al. examined the mechanisms of altered water metabolism in a small number of psychotic patients with polydipsia and hyponatremia. The authors noted an abnormality in renal water excretion manifested as impairments in maximal urinary dilution and in free water clearance that cannot be attributed to inadequate suppression of plasma vasopressin. In addition to augmented sensitivity to the actions of vasopressin, they also found that the osmotic threshold for vasopressin secretion was reduced in these patients.
Ali et al. describe the case of a 66-year-old male found to have psychogenic polydipsia. In their review of the disorder, they note that hyponatremia develops when free water intake exceeds free water excretion. They report that psychiatric patients appear to have a greater desire for water at any given level of plasma osmolality.
Illustration A is an algorithm for evaluating hypotonic hyponatremia from Kugler and Hustead. Note the use of urine osmolality as a way to differentiate psychogenic polydipsia from other causes.
Answer 1: RTA will cause a hyperchloremic, normal anion gap metabolic acidosis due to renal tubular dysfunction. Hyponatremia is not typically a component of RTA.
Answer 2: Several medications can cause nephrogenic DI or SIADH (syndrome of inappropriate ADH secretion) including some antipsychotics. However, serum sodium is usually normal or even elevated in the former and urine osmolality and specific gravity are usually very high in the latter.
Answer 4: Acute renal failure is unlikely to cause isolated hyponatremia. The absence of volume overload makes this very unlikely.
Answer 5: Central diabetes insipidus is a condition in which there is insufficient production of ADH, usually caused by trauma to neurohypophyseal stalk (from neurosurgery or head injury). It can be distinguished from primary polydipsia by a fluid deprivation test.
Goldman MB, Luchins DJ, Robertson GL. Mechanisms of altered water metabolism in psychotic patients with polydipsia and hyponatremia. N Engl J Med. 1988 Feb 18;318(7):397-403. PubMed PMID: 3340117.
PMID:3340117 (Link to Abstract)
Ali N, Imbriano LJ, Maesaka JK. The Case | A 66-year-old male with hyponatremia. Psychogenic polydipsia. Kidney Int. 2009 Jul;76(2):233-4. doi: 10.1038/ki.2009.150. PubMed PMID: 19564861.
PMID:19564861 (Link to Abstract)
Kugler JP, Hustead T. Hyponatremia and hypernatremia in the elderly. Am Fam Physician. 2000 Jun 15;61(12):3623-30. Review. PubMed PMID: 10892634.
PMID:10892634 (Link to Abstract)