Chapter 24-18: Psychogenic Polydipsia

TEXTBOOK PRESENTATION

Psychogenic polydipsia typically occurs in patients with a psychiatric history and unexplained hyponatremia. Patients are unaware of (or do not admit) to excessive water intake. Excessive water ingestion and hyponatremia are also seen occasionally in patient who ingest excess water for medical procedures.

DISEASE HIGHLIGHTS

1. In most other causes of hyponatremia, ADH is elevated (either appropriately or inappropriately), allowing water to be reabsorbed from the distal tubule, which concentrates the urine. In contradistinction, the increased water intake in psychogenic polydipsia suppresses ADH, increasing free water excretion and results in a dilute urine.

2. Hyponatremia develops only when massive water ingestion is sufficient to overcome maximal urinary free water excretion and then dilutes the serum sodium, which usually requires > 8–10 L/day fluid intake. (Less water intake can cause hyponatremia in patients with reduced kidney function who are unable to excrete large volumes of free water.)

3. Therefore, urine osmolality is usually maximally dilute (≈ 40–100 mOsm/L) which is the key to diagnosis.

4. Reported in 6–20% of chronically ill, hospitalized psychiatric patients. (SIADH may also be seen in psychiatric patients.)

5. Other causes of voluntary water intoxication include college students (and others) drinking excessive water as a challenge and, rarely, patients who drink water far in excess of what has been medically recommended.

6. Complications are secondary to both hyponatremia and marked polyuria (incontinence, hypocalcemia, hydronephrosis (from massive urinary output), and HF.

EVIDENCE-BASED DIAGNOSIS

1. The water restriction test limits the patient’s access to water. Since water excretion is normal, the patient excretes the excess water promptly and hyponatremia resolves rapidly. However, this must be done carefully in patients with marked hyponatremia (Na ≤ 120 mEq/L) to avoid over-rapid correct of hyponatremia and ODS.

2. Urine osmolality

   1. Mean urine osmolality 144 ± 23 mOsm/L vs 500 mOsm/L in SIADH and 539 mOsm/L in hypovolemic patients.

   2. Surprisingly, not all patients with psychogenic polydipsia have a maximally dilute urine. Several problems can aggravate the hyponatremia in psychogenic polydipsia and complicate the diagnosis.

      1. Psychotic episodes may cause a transient release of ADH or an increased renal responsiveness to ADH.

      2. In addition, nausea or psychiatric medications can induce concomitant SIADH (including selective serotonin reuptake inhibitors and phenothiazines). This accentuates the hyponatremia and can produce a higher than expected urine osmolality.

3. The urine sodium is often low (despite euvolemia) due to the dilution of the sodium in the urine by the massive excretion of water (mean 18 mEq/L). This can incorrectly suggest hypovolemia. However, the fractional excretion of sodium (FE_Na) is a more accurate measure of volume and sodium handling and is > 0.5% in 66% of patients.

4. CNS tumors may trigger polydipsia and cause hyponatremia. CNS imaging is recommended before making the diagnosis of psychogenic polydipsia.

TREATMENT

1. For severe neurologic symptoms (eg, seizures, coma), hypertonic saline can be used.

2. In other patients, careful free water restriction allows ...