Assessment of the kidneys and urinary tract

The glomerular filtration rate (GFR) is low in the newborn infant and is especially low in premature infants; the GFR at 28 weeks’ gestation is only 10% of the term infant. In term infants, the corrected GFR (15–20 ml/min per 1.73 m²) rapidly rises to 1–2 years of age when the adult rate of 80 to 120 ml/min per 1.73 m² is achieved (Fig. 18.1). The assessment of renal function in children is listed in Table 18.1 and the radiological investigations of the kidneys and urinary tract in Table 18.2.

Anomalies detectable on antenatal ultrasound screening

Absence of both kidneys (renal agenesis) – as amniotic fluid is mainly derived from fetal urine, there is severe oligohydramnios resulting in Potter syndrome (Fig. 18.2a, 18.2b), which is fatal.

Multicystic dysplastic kidney (MCDK) – results from the failure of union of the ureteric bud (which forms the ureter, pelvis, calyces and collecting ducts) with the nephrogenic mesenchyme. It is a non-functioning structure with large fluid-filled cysts with no renal tissue and no connection with the bladder (Fig. 18.3). Half will have involuted by 2 years of age, and nephrectomy is indicated only if it remains very large or hypertension develops, but this is rare. Since they produce no urine, Potter syndrome will result if the lesion is bilateral. Other causes of large cystic kidneys are autosomal recessive polycystic kidney disease (ARPKD) (Fig. 18.4), autosomal dominant polycystic kidney disease (ADPKD) (Fig. 18.5) and tuberous sclerosis. In contrast to a multicystic dysplastic kidney, in these disorders some or normal renal function is maintained but both kidneys are always affected. Autosomal dominant polycystic kidney disease has an incidence of 1 in 1000; the main symptoms in childhood are hypertension and haematuria and it causes renal failure in late adulthood. It is associated with several extra-renal features including cysts in liver and pancreas, cerebral aneurysms and mitral valve prolapse.

Abnormal caudal migration may result in a pelvic kidney or a horseshoe kidney (Fig. 18.6), when the lower poles are fused in the midline. The abnormal position may predispose to infection or obstruction to urinary drainage.

Premature division of the ureteric bud gives rise to a duplex system, which can vary from simply a bifid renal pelvis to complete division with two ureters. These ureters frequently have an abnormal drainage so that the ureter from the lower pole moiety often refluxes, whereas the upper pole ureter may drain ectopically into the urethra or vagina or may prolapse into the bladder (ureterocele) and urine flow may be obstructed (Fig. 18.7).

Failure of fusion of the infraumbilical midline structures results in exposed bladder mucosa (bladder extrophy). Absence or severe deficiency of the anterior abdominal wall muscles is frequently associated with a large bladder and dilated ureters (megacystis-megaureters) and cryptorchidism, the absent musculature syndrome (prune-belly syndrome) (Fig. 18.8).

Obstruction to urine flow may occur at the pelvi-ureteric or vesicoureteric junction, at the bladder neck (e.g. due to disruption of the nerve supply, neuropathic bladder) or at the posterior urethra in a boy due to mucosal folds or a membrane, known as posterior urethral valves. The consequences of obstruction to urine flow are shown in Figures 18.9a–18.9d. At worst, this results in a dysplastic kidney which is small, poorly functioning and may contain cysts and aberrant embryonic tissue such as cartilage. In the most severe and bilateral cases Potter syndrome is present. Renal dysplasia can also occur in association with severe intrauterine vesicoureteric reflux, in isolation or in certain rare, inherited syndromes affecting multiple systems.

Some congenital abnormalities of the kidneys and urinary tract

Urinary tract obstruction

Antenatal treatment

The male fetus with posterior urethral valves may develop severe urinary outflow obstruction resulting in progressive bilateral hydronephrosis, poor renal growth and declining liquor volume with the potential to lead to pulmonary hypoplasia. Intrauterine bladder drainage procedures to prevent severe renal damage have been attempted but results have been disappointing. Early delivery is rarely indicated.

Postnatal management

An example of a protocol for infants with antenatally diagnosed anomalies is shown in Figure 18.10. Prophylactic antibiotics may be started at birth to try to prevent urinary tract infection, although practice varies between centres. As the newborn kidney has a low GFR, urine flow is low and mild outflow obstruction may not be evident in the first few days of life. The ultrasound scan should therefore be delayed for several weeks. However, bilateral hydronephrosis in a male infant warrants an ultrasound shortly after birth to exclude posterior urethral valves, which always requires urological intervention such as cystoscopic ablation (Case History 18.1).

Case History

18.1 Posterior urethral valves

Bilateral hydronephrosis was noted on antenatal ultrasound at 20 weeks’ gestation in a male fetus. There was poor renal growth, progressive hydronephrosis and decreasing volume of amniotic fluid (Fig. 18.11a) on repeated scans. After birth, prophylactic antibiotics were started. An urgent ultrasound showed bilateral hydronephrosis with small dysplastic kidneys. The bladder and ureters were grossly distended. The plasma creatinine was raised. A micturating cystourethrogram (MCUG) (Fig. 18.11b) showed vesicoureteric reflux, a dilated posterior urethra and posterior urethral valves which was treated endoscopically. Renal function initially improved but then progressed to chronic renal failure. He had a renal transplant at 10 years of age.

Clinical features

Presentation of UTI varies with age (Box 18.1). In infants, symptoms are non-specific; fever is usually but not always present, and septicaemia may develop rapidly. The classical symptoms of dysuria, frequency and loin pain become more common with increasing age. Serious illness from septicaemia is described in the child with a fever in Chapter 14. Dysuria alone is usually due to cystitis, or vulvitis in girls or balanitis in uncircumcised boys. Symptoms suggestive of a UTI may also occur following sexual abuse.

Collection of samples

The commonest error in the management of UTI in children, and especially in infants, is failure to establish the diagnosis properly in the first place. If the diagnosis of a UTI is not made, the opportunity to prevent renal damage may be missed, or, if incorrectly diagnosed, may lead to unnecessary invasive investigations.

For the child in nappies, urine can be collected by:

In the older child, urine can be obtained by collecting a midstream sample. Careful cleaning and collection are necessary, as contamination with both white cells and bacteria can occur from under the foreskin in boys, and from reflux of urine into the vagina during voiding in girls.

Ideally, the urine sample should be microscoped to identify organisms and cultured straight away. This is indicated in all infants and children <3 years old with a suspected UTI. If this is not possible, the urine sample should be refrigerated to prevent the overgrowth of contaminating bacteria. Urinary white cells are not a reliable feature of a UTI, as they may lyse during storage and may be present in febrile children without a UTI and in children with balanitis or vulvovaginitis. Dipsticks can be used as a screening test. Urine culture should still be performed unless both leucocyte esterase and nitrite are negative or if the clinical symptoms and dipstick tests do not correlate (Table 18.3).

Antenatally diagnosed renal or urinary tract abnormality

Increases risk of infection.

Incomplete bladder emptying

Contributing factors in some children are:

Vesicoureteric reflux

Vesicoureteric reflux (VUR) is a developmental anomaly of the vesicoureteric junctions. The ureters are displaced laterally and enter directly into the bladder rather than at an angle, with a shortened or absent intramural course. Severe cases may be associated with renal dysplasia. It is familial, with a 30–50% chance of occurring in first-degree relatives. It may also occur with bladder pathology, e.g. a neuropathic bladder or urethral obstruction, or temporarily after a UTI. Its severity varies from reflux into the lower end of an undilated ureter during micturition to the severest form with reflux during bladder filling and voiding, with a distended ureter, renal pelvis and clubbed calyces (Fig. 18.12). Mild reflux is unlikely to be of significance, but the more severe degrees of VUR may be associated with intrarenal reflux (IRR), the backflow of urine from the renal pelvis into the papillary collecting ducts; intrarenal reflux is associated with a particularly high risk of renal scarring if UTIs occur. The incidence of renal defects increases with increasing severity of reflux. There is considerable controversy as to whether renal scarring is a congenital abnormality already present in children with reflux and which predisposes to infection or if children with reflux have normal kidneys at birth which are damaged by UTIs and that preventing UTIs in these children prevents scars. Reflux tends to resolve with age especially lower grades of VUR.

Reflux with associated ureteric dilatation is important, as:

Infection may destroy renal tissue, leaving a scar, resulting in a shrunken, poorly functioning segment of kidney (reflux nephropathy). If scarring is bilateral and severe, chronic renal failure may develop. The risk for hypertension in childhood or early adult life is variously estimated to be up to 10%.