You are called to see a patient with chronic schizophrenia on the ward. The nurse explains that he is normally well but has been more agitated recently and has been vomiting. You note an ataxic gait. The nurse tells you that he was recently started on an SSRI as the consultant thought he was depressed and that he has been spending a lot of time in the bathroom. You request some tests and discover that the patient has hyponatremia. The urine osmolarity is low as is the plasma osmolarity. Which of the following would you expect?
Exam Question Jul 2014
The history, examination findings, and lab results suggest a diagnosis of psychogenic polydipsia. In situations of excessive fluid intake (without other pathology) the blood becomes diluted (low plasma osmolarity) as does the urine (low urine osmolarity).
In SIADH the body holds onto water resulting in diluted blood (low plasma osmolarity) and concentrated urine (high urine osmolarity).
In nephrogenic diabetes insipidus the body is struggling to hold onto water and so you expect the blood to be concentrated (high plasma osmolarity).
In gastroenteritis the patient is usually dehydrated resulting in concentrated blood and urine.
Conns syndrome is an aldosterone producing adenoma. Aldosterone increases sodium reabsorption and potassium secretion and you get water retention (so diluted blood).
Nephrogenic diabetes insipidus13%Psychogenic polydipsia22%Syndrome of inappropriate antidiuretic hormone secretion21%Gastroenteritis22%Conns syndrome22%
Exam Question Jul 2014
The history, examination findings, and lab results suggest a diagnosis of psychogenic polydipsia. In situations of excessive fluid intake (without other pathology) the blood becomes diluted (low plasma osmolarity) as does the urine (low urine osmolarity).
In SIADH the body holds onto water resulting in diluted blood (low plasma osmolarity) and concentrated urine (high urine osmolarity).
In nephrogenic diabetes insipidus the body is struggling to hold onto water and so you expect the blood to be concentrated (high plasma osmolarity).
In gastroenteritis the patient is usually dehydrated resulting in concentrated blood and urine.
Conns syndrome is an aldosterone producing adenoma. Aldosterone increases sodium reabsorption and potassium secretion and you get water retention (so diluted blood).
Psychogenic polydipsia
Psychogenic polydipsia refers to fluid drinking that greatly surpasses physiological requirements. It is most commonly seen in schizophrenia.
If fluid intake exceeds the capacity for excretion, then the resultant hyponatremia (i.e., sodium depletion) may produce signs of water intoxication including:-
- Vomiting
- Agitation
- Ataxia
- Seizures
- Coma
It is best managed by fluid restriction.
