Presentation of a patient with polyuria

Abstract

Introduction: Polyuria in children is defined as urine production of more than 4 mL/kg/h. The cause of polyuria can be osmotic diuresis (e.g. diabetes mellitus), and primary or secondary disorders of the kidney's ability to concentrate urine (central or nephrogenic diabetes insipidus, chronic kidney disease, obstructive uropathy, etc.). Primary polydipsia is also a possible cause of polyuria.

Case: A 17-year-old girl was examined in the nephrology clinic due to polyuria that had been present for the past year. Along with polyuria, nocturia is also present without dysuria and without enuresis. In addition to the above, she was thirsty during the day. She did not lose weight. Family history is unremarkable. Pregnancy and childbirth were normal, as was her previous growth and development.

On examination, her vital signs were normal, her somatic status was normal, and her blood pressure was 47.c. for body weight and 32nd centile for height and without clinical or laboratory signs of dehydration. A random urine sample showed urine of normal volume mass (1.09 kg/L), normal pH and sediment, without glucosuria. Ultrasound of the kidneys and urinary tract was normal.

Polyuria was objectified by a 48-hour drinking and urination diary, which determined a daily urine volume of 13,360 L (11.13 ml/kg/h), of which 7,475 L was voided during the night period of measurement. The measured bladder capacity was 157% of the expected volume for age - 850mL.

Further diagnostic workup was performed through hospitalization. Upon admission, vital parameters were normal again, normal somatic status, no clinical signs of dehydration and normal volume status. The laboratory work-up indicated a normal acid-base status of pH 7.333, normal glycemia (4.4 mmol/L), normal serum concentration of electrolytes and parameters of renal function (eGFR 144 (mL/min/1.73m2).

The blood count was normal, without signs of acute infection. A random urine sample was of reduced volumetric mass (1,001 kg/L), no glucosuria, no proteinuria, and regular sediment. Urine osmolality was 49 mOsmol/kg, and serum 288 mOsmol/kg.

A thirst test was performed, which determined an increase in urine osmolality up to 956 mOsm/kg in a state of fluid deprivation with maintained serum osmolality, thus confirming the ability to concentrate urine. This girl was diagnosed with primary polydipsia, which caused polyuria, and was advised to limit water intake to physiological needs.

Discussion: In this case, an initial laboratory work-up, which indicated normal blood glucose values ​​and the absence of glucose in the urine, ruled out diabetes as the cause of polyuria and polydipsia. A urinary tract infection was ruled out by urine analysis.

Ultrasound diagnostics ruled out the presence of renal dysplasia or obstruction of the urinary system, which could lead to polyuria in the stage of tubule damage.

A normal acid-base status ruled out the presence of renal tubular acidosis, and the normal values ​​of electrolytes in the portion of urine additionally confirmed that there were no disturbances in the function of the proximal and distal tubules and Henle's loop.

What remained to be assessed was the kidney's ability to concentrate urine, which mostly takes place at the level of the collecting ducts, i.e. their sensitivity to ADH. A thirst test was performed, which proved an orderly increase in urine osmolality, and thus the diagnosis of primary polydipsia was made.