The differential diagnosis of diabetes insipidus includes psychogenic polydipsia and a water deprivation test is sometimes difficult to perform due to patients confusion, compliance or extreme thirst. We present a simple, cost effective alternative to the water deprivation test.
67-year-old male was admitted with a 3 week history of polyuria, polydipsia and extreme thirst. On systemic enquiry he complained of left sided pleuritic chest pain for few months prior to presentation. Past medical history included COAD and stable ischaemic heart disease. Medications included bronchodilators, statin, nitrates and aspirin. Clinical examination revealed bilateral expiratory wheeze, no clubbing and an irregular pulse. Chest X-ray showed left parahilar consolidation and features consistent with chronic airway disease.
His renal functions, liver functions, full blood count, thyroid function, calcium, CRP and plasma glucose were normal. Serum osmolality was 296 mOsm/Kg, Urine osmolality 100 mOsm/Kg; Urinary spot sodium 38 mmol/L. Urine volume was reported as 8 to 12 litres on different occasions. The differential was diabetes insipidus or psychogenic polydipsia. A daytime water deprivation test could not be performed as he was not able to stop drinking.
Therefore an overnight water deprivation test was performed. Serum osmolality, urine osmolality and electrolytes were measured at 10 pm (baseline) and 6 am next day morning after overnight sedation. Serum osmolality rose from 292 to 320 mOsmol/Kg, serum sodium from 140 to 150 mmol/L and urine osmolality remained stable. MRI of pituitary showed hypothalamic and cerebellar metastases. A diagnosis of Cranial Diabetes Insipidus was made and he responded promptly to desmopressin. CT scan of the chest and abdomen demonstrated metastatic lung cancer and fine needle aspiration of the lymph nodes confirmed metastatic adenocarcinoma.
This case illustrates the difficulty in performing routine daytime water deprivation test in some patients and demonstrates an alternate investigation.