Diagnosis of abnormalitiesof theurinary system

Contemporary aspects of diagnosis and treatment of abnormalitiesof theurinary system in newborns and infants

Authors: Gel’dt V. G., Kuzovleva G. I., Moscow Scientific Research Institute of Pediatry and Pediatric Surgery, Ministry of Health of RF

(The lecture, is published in abridged version)

Fetal excretory organ in antenatal period is placenta. Its functioning stops after birth, resulting in increased load on the kidney, while activation of glomerular filtration becomes insufficient.  Transient renal failure, which manifests itself in the form of physiological azotaemia, arises.

Newborns have a reduced glomerular filtration rate (reaching 30-50 % of older children rate) and low renal concentration ability. In the first days of life hypotonic with respect to plasma urine with a low specific weight is secreted. Primary urine contains a minor amount of urea, glucose and protein. This urine requires large volume of liquid for excretion of waste products of nitrogen metabolism. This causes the relative polyuria and hyposthenuria, typical for this age.

The high probability of renal tissue lesion in newborns is determined by the presence of signs of morpho-functional immaturity of kidneys. This is clinically confirmed by the frequent appearance of protein, erythrocytes and tube casts in urine. It is known that at the time of the birth kidney contains a complete set of nephrons - from 800,000 to 1 million; they are located very compactly in a small in size kidney (up to 50 glomeruli per HPF, while up to 5 - in adults). Formation of glomeruli is usually fully completed at the age of 1.5 months.

Diuresis and urination are of especially interest. Fetus begins to urinate at 12-13 weeks of gestation by accumulating the required volume of amniotic fluid. At birth urine bladder contains a small amount of urine; and it is excreted in small amount during the first days of life.

The first urination occurs at an average in 12 hours in 67% of healthy newborns, after 12 hours - in 25%, after 24 hours - in 7%, and about 0.6% of healthy newborns do not urine even after 48 hours. However, it is difficult enough to determine the interval between birth and first urination, because the first miction of  newborn, occurring shortly after birth, may be unnoticed. Urinary frequency ranges from 2 to 6 times in the first and second days of life, and from 5 to 25 times a day hereafter.

Evaluation of urine output in infants during the first days and weeks of life is difficult because of the excretory system liability at this age. According to Tsybulkina E. K. (1981), diuresis is 8.8 ml/kg for the first 24 h, 19 ml/kg - for the third 24 h, 49 ml/kg - for the fifth 24 h, 61ml/kg - for the seventh 24 h. At the age of 10-60 days infant excretes 250-450 ml of urine during 24 h.

The absence of urination in the first 72 hours of life leads to anuria. The reasons for decreasing and complete stopping of diuresis may be different extra renal, renal and subrenal factors.

Among extra renal factors leading role belongs to the secondary hypovolemia while prolonged labor; they areaccompanied by fetal hypoxia or asphyxia, which cause nephron destruction and decrease renal perfusion. Prolonged labor leads to increased degradation of proteins of fetus, increase of phosphates excretion, reduction of glomerular filtration rate, decrease of urine output and reduction of electrolytes excretion. Eventually, all of this is the reason for a possible oliguria and anuria in the first days of life. Postnatal asphyxia owing to respiratory distress syndrome can cause similar renal dysfunction.

Hypovolemia also develops as a result of transplacental hemorrhage at placental abruption, or as a result of microcirculatory disorders of the renal capillaries in course of craniocerebral injury, congenital cardiovascular insufficiency, enterocolitis and after heart and blood vessels surgeries.

Due to infrequent urination urates and calcium oxalate may be deposited in renal tubule that leads to uric acid infarct. With an increase of diuresis salts are washed out. In rare cases, profuse discharge of salts is accompanied by hematuria due to injure of pelvis and ureters soft mucous membrane. At the age of 7-8 days the amount of urine increases, and the number of urination ranges up to 15-25 times a day and persists for first three months of life. The clear reducing trend is noted only after 9 months.

Functional particularity of the lower urinary tract in first weeks and months of life is urinary bladder emptying is performed in two steps; it was detected in 43% of newborns. This number reduces to 19% in the age of 6 months while urinating children older than 11 months  is not registered at all (our data).

During first months of life mean effective bladder volume is 20 ml; it reaches 36 ml by six months, and should be increased threefold by one year.

Another functional particularity of the lower urinary tract is large amount of residual urine. It exceeds 10 ml at 40% of newborns. Residual urine volume is decreased by one year and, as a rule, does not exceed 4.5 ml that is 9% of bladder volume. So, newborns and infants suffer from functional instability of the bladder that decreases with age and reaches some maturity by one year.

Diagnostics of abnormalities of urinary system development in newborns is staged. At the present day it should be based on information obtained from studying of family anamnesis, gestation course, data of fetal ultrasonography, complex examination. That complex examination should include laboratory tests and special methods. Every diagnostics stage has its’ objective; achieving the objective makes it possible to arrange indications for further stage.