Causes

• Endocrine:
  • Diabetes mellitus.
  • Cranial diabetes insipidus.
  • Cushing's syndrome, including pituitary adenomas (Cushing's disease) and ectopic ACTH secretion (most often due to small-cell carcinoma of the lung and bronchial carcinoid tumours).
• Renal:
  • Chronic kidney disease.
  • Relief of chronic urinary obstruction.
  • Early chronic pyelonephritis.
  • Nephrogenic diabetes insipidus.
  • Fanconi's syndrome.
• Iatrogenic:
  • Diuretic therapy.
  • Alcohol.
  • Other drugs - eg, lithium, tetracyclines.
• Metabolic:
  • Hypercalcaemia (eg, osteoporosis treatment, multiple bony metastases, hyperparathyroidism).
  • Potassium depletion (eg, chronic diarrhoea, diuretics, primary hyperaldosteronism).
• Psychological:
  • Psychogenic polydipsia (compulsive water-drinking).
  • Polyuria is an uncommon but serious complication of psychotic illness and, if untreated, may lead to hyponatraemia, coma or even death.
• Other causes: sickle cell anaemia, paroxysmal supraventricular tachycardia.

Presentation

Thirst, increased fluid intake: thirst will usually accompany polyuria. In psychogenic polydipsia (most often seen in adolescents), thirst with compulsive water drinking is the predominant symptom.

Nocturia will usually be a feature of true polyuria, and secondary enuresis in children can occur.

Associated general symptom - eg, weight loss, malaise, headache and shortness of breath - increase the likelihood of significant pathology.

Weight loss: rapid onset of weight loss is a feature of type 1 diabetes. Weight loss is also a feature of chronic renal failure and may also be due to dehydration in diabetes insipidus.

Symptoms suggestive of diabetes insipidus: daily urine output may be greater than 10 litres. A pituitary tumour may cause headaches, visual disturbance and symptoms of pituitary hormone excess or deficiency. Other brain tumours in the region of the pituitary may be also be responsible - eg, craniopharyngiomas.

Past medical history: chronic renal failure may be a consequence of diabetes mellitus, connective tissue disease such as systemic lupus erythematosus (SLE), renal vascular disease, pyelonephritis or obstructive uropathy. Cranial diabetes insipidus may be due to pituitary surgery or irradiation, infection (meningitis, cerebral abscess), sarcoidosis or head injury.

Family history: family history of diabetes mellitus or renal problems - eg, polycystic kidneys or nephrogenic diabetes insipidus.

Signs

• General - eg, dehydrated (acute presentation of type 1 diabetes, diabetes insipidus, chronic renal failure) or anaemia (chronic renal failure).
• Blood pressure and pulse: tachycardia and postural hypotension with dehydration, bradycardia and a raised blood pressure may occur if raised intracranial pressure.
• Eyes: papilloedema if raised intracranial pressure, diabetes eye complications (retinal haemorrhages, exudates, new vessel formation, cataracts). A pituitary tumour may cause a visual field defect.
• Abdomen: palpable kidneys in renal disorders.
• Neurology: peripheral neuropathy in diabetes mellitus or chronic renal failure.

Investigations

• Urine tests:
  • Urinalysis for diabetes (glucose, ketones) and signs of renal disease (proteinuria). Specific gravity is very low in diabetes insipidus and psychogenic polydipsia.
  • Urine osmolality: early morning urine sample in conjunction with a plasma osmolality. A high plasma osmolality and inappropriately low urine osmolality occur in diabetes insipidus; both plasma and urine osmolalities are correspondingly low in psychogenic polydipsia.
  • Quantification of proteinuria: 24-hour urine collection; albumin-creatinine ratio (ACR) for microalbuminuria.
  • Urine electrophoresis: Light-chain immunoglobulins (Bence Jones' protein): myeloma may be the cause of hypercalcaemia.
• Blood tests:
• Renal function, electrolytes: calcium, potassium, abnormalities suggesting chronic kidney disease.
• Fasting (preferable) or random glucose.
• FBC, ESR: anaemia found in chronic renal failure and collagen vascular diseases. Bone marrow infiltration may be evident in myeloma. ESR raised in collagen vascular diseases, myeloma and malignancy.
• Serum protein electrophoresis: for monoclonal immunoglobulin band in myeloma.
• Pituitary function tests.
• Autoantibody screen: if collagen vascular disease is a possible cause of renal failure.
• Serum lithium concentration if relevant.
• Further investigations:
  • These investigations (and some of the urine and blood tests) are more likely to be part of secondary care investigations.
  • Imaging of the renal tract: ultrasound, X-ray, CT scan of the abdomen.
  • Renal biopsy.
  • Lateral skull X-ray: may show an enlarged pituitary fossa with pituitary tumours. Calcification is common with craniopharyngiomas.
  • MRI or CT scan of brain: for pituitary or other brain tumour.
  • Water deprivation and desmopressin test: may be performed under supervision in secondary care and is useful in distinguishing cranial and nephrogenic diabetes insipidus.

Management

• Admit to hospital if significantly dehydrated, whatever the likely cause. Fluid balance and electrolyte disturbance will need to be corrected.
• Further management will depend on the results of initial investigations and any confirmed cause of polyuria.