Chapter 19

Nephrology

Rajiv Sinha, Stephen D Marks

Learning objectives

By the end of this chapter the reader should:

• Understand the critical anatomy and embryology of the urinary tract

• Understand the relevant physiology of the kidney and understand how dysfunction relates to disease

• Know the commoner presentations of renal disease in children

• Have a logical rationale for the investigation of renal disease and disorders of the urinary tract

• Understand the principles of water balance and rehydration

• Know the causes of hypertension

Introduction

The kidneys and urinary tract are a critical organ system in enabling homeostasis. Their principle functions are those of water, electrolyte and acid–base balance. They also play an important role in hormone secretion, red cell production and the control of blood pressure. The functions of the renal tract are most clearly elucidated by examining the results of renal failure and renal disease (Fig. 19.1).

Fig. 19.1 Functions of the healthy kidney and consequences of renal dysfunction.

Anatomy and physiology of the kidney

Kidneys are retroperitoneal structures located in the paravertebral space; the right kidney is slightly lower than the left. At birth, average length of the kidney is 4.5–5.5 cm, whereas an adult kidney measures 10–11.5 cm in length and 5–7 cm in width. The anterior surface of the kidney lies in contact with the duodenum on the right side and the pancreas on the left side. Variable portions of colon may be in contact with the inferior pole of the anterior surface of the kidneys, and on the left side the spleen wraps the anterolateral aspects of the upper half of the kidney. The 12th rib and a portion of the 11th rib cover the upper third of the posterior surface of the left kidney, whereas the 12th rib may touch the upper pole of the right kidney. The outer surface of the kidney is covered by a thin but firm capsule. The anatomical position of the kidney explains the retroperitoneal approach for renal biopsy and the choice of lower pole of the right kidney.

A longitudinal cut section of the kidney and its relationship to the other structures in the urinary tract is shown in Figure 19.2.

Fig. 19.2 The major components of the urinary tract and bladder. The ureter and renal vessels are placed in the medial side of the kidney, known as the hilum, which opens into a central space, the renal pelvis. The pelvis of the kidney extends out of the kidney to form the ureter, while the intrarenal portion divides into numerous calyces (6–10 in number), which drain the renal pyramids. The papillae of the renal pyramids open directly into the calyces. The pelvi-ureteric junction and vesico-ureteric junction are narrower points along the ureter and hence are most prone to obstruction. (From Naish J, et al. Medical sciences, 2e. Saunders, 2014, with permission.)

Nephrons (Fig. 19.3) are the ultrastructural units of the kidney, of which we have around a million in each kidney. The nephron comprises a glomerulus connected to a tubule which in turn drains into a collecting duct; ultimately, they join and drain into the calyces at the renal pyramids. The glomerulus generates filtrate through ultrafiltration of blood brought in by the afferent arterioles. The ultrafiltrate accumulates in the Bowman’s space and thereafter traverses the tubules, where it is further modified into the final urine. During passage of tubular fluid down the renal tubule, solutes are reabsorbed by the highly selective transport mechanisms (Fig. 19.4). In general, most transport occurs in the proximal tubule, where the luminal membrane forms an elaborate ‘brush border’ of microvilli to provide extensive surface area for the reabsorptive processes. The brush borders are densely packed with mitochondria to supply the energy required for these active transports. Organic solutes such as low-molecular-weight proteins, sugars, and amino acids are avidly (>98%) reabsorbed in this segment. In addition, bulk transport of inorganic solutes and water is also accomplished in the proximal tubule. The subsequent tubules fine-tune the net reabsorption of these solutes and water. Dysfunction in the absorptive capacity of the tubules can give rise to various disorders of tubulopathy (see below). The glomerular filtration barrier consists of parietal epithelial cells with its foot process (podocytes), the glomerular basement membrane (GBM) and the capillary endothelial cells. Filtration of molecules across this structural barrier is limited by size, shape, and charge. Whereas charge selectivity is determined by negatively charged molecules present in the filtration barrier, size selectivity is determined by the GBM and by the slit diaphragm generated by interposing podocyte foot processes.

Fig. 19.3 Schematic diagram of a nephron. The blood supply originates from the renal artery (red) and leaves via the renal vein (blue). Blood passes through two capillary beds, the glomerular capillaries followed by the vasa recta. (From Naish J, et al. Medical sciences, 2e. Saunders, 2014, with permission.)

Fig. 19.4 Diagrammatic representation of the major transport sites across the nephron. Apart from macromolecules, the glomerular filtrate contains glucose, amino acids, electrolytes such as sodium, potassium, chloride, urea, bicarbonate and water. The PCT is the major site for reabsorption: 99% of the glucose is reabsorbed into the peritubular circulation, also essential nutrients and electrolytes. Bicarbonate reabsorption at the PCT affects renal acid–base homeostasis. Toxic metabolites, cations and anions are also secreted by the PCT. The loop of Henle is the major site for urine concentration according to the countercurrent hypothesis. Further sodium chloride and water reabsorption takes place in the DCT and CT, controlled by vasopressin and aldosterone. AA, amino acid; CT, collecting tubule; DCT, distal convoluted tubule; Glu, glucose; HCO₃⁻, bicarbonate; PCT, proximal convoluted tubule. (From Naish J, et al. Medical sciences, 2e. Saunders, 2014, with permission.)

Tubular disorders

The adult kidneys filter on average 150 litres of plasma per day containing 22.5 mol of sodium; more than 99% of filtered sodium is reabsorbed by the tubules, so that the final excretion is <1% (see Fig. 19.4). Disorders in sodium handling affect blood pressure (BP); sodium-losing disorders lead to hypotension and sodium-retaining disorders to hypertension. The Na⁺/K⁺-ATPase, which is present in all cells, is the driving force which generates a favourable electrochemical gradient for Na entry into the cell. This gradient also enables cotransport of other substances (such as glucose, amino acids, phosphate) into the cell. As sodium is the main determinant of intravascular volume, fractional excretion of sodium is normal in almost all renal salt-wasting disorders due to activation of renin–angiotensin–aldosterone system (RAAS). An overview of the different aetiologies of tubular disorder is given in Table 19.1.

Table 19.1

Causes of renal tubular disorders

Segment	Function	Disorder
Proximal tubule*	Glucose transport Phosphate transport Amino acid transport	Renal glycosuria Hypophosphataemic rickets Isolated, generalized aminoaciduria
Ascending limb of Henle	Sodium, potassium, chloride transport	Bartter syndrome
Distal tubule	Proton (H⁺) secretion Sodium chloride transport	Distal renal tubular acidosis Gitelman syndrome
Collecting duct	Water transport Sodium, potassium transport	Nephrogenic diabetes insipidus Pseudohypoaldosteronism Liddle syndrome

Fanconi syndrome

This syndrome describes a generalized proximal tubular disorder with at least initially well-preserved glomerular function. There is a long list of potential causes and it is helpful to split these into congenital, acquired and renal causes. The cardinal clinical features are growth faltering, polyuria and rickets in association with a normal plasma anion gap, metabolic acidosis, hypophosphataemia, hypokalaemia and generalized aminoaciduria. Supportive therapy may be required, which includes salt, water and nutritional supplementation as well as bicarbonate, electrolyte and phosphate replacement.

Congenital causes include a familial idiopathic form, cystinosis (see below), tyrosinaemia and galactosaemia. Fanconi syndrome may also be acquired following treatment with aminoglycosides, sodium valproate, 6-mercaptopurine or ifosfamide or following poisoning with agents like toluene or paraquat. It occasionally occurs after renal transplantation, in the recovery phase of acute tubular necrosis or following tubulointerstitial nephritis or in focal and segmental glomerulosclerosis, a histopathological type of nephrotic syndrome.

Nephropathic cystinosis is the commonest cause of Fanconi syndrome in Europe and North America. It is a disorder of lysosomal cystine transport of autosomal recessive inheritance, resulting in excessive intracellular accumulation of free cystine in many organs including the kidney, eyes and thyroids. Management includes specific therapy with mercaptamine, which prevents accumulation of lysosomal cystine.

Bartter and Gitelman syndromes

These syndromes occur as a result of disturbances in the thick ascending limb of loop of Henle (Bartter) and distal convoluted tubule (Gitelman), respectively, and typically result in a hypokalaemic, hypochloraemic metabolic alkalosis with salt wasting. The main problem is tubular loss of sodium and chloride and secondarily excess loss of potassium in the distal tubule associated with hyperreninaemia and hyperaldosteronism. In this part of the nephron, sodium reabsorption is linked to chloride reabsorption through the furosemide-sensitive sodium–potassium–chloride channel (NKCC2) in the loop of Henle (disrupted in Bartter syndrome) or the structurally similar thiazide-sensitive sodium–chloride channel (NCCT) in the early distal convoluted tubule (disrupted in Gitelman syndrome), respectively. Salt-wasting disorders from these parts of the nephron will always be associated with urinary chloride loss in excess of urine sodium loss because all chloride reabsorption is linked with sodium, with twice as much chloride as sodium reabsorbed via NKCC2. Moreover, sodium reabsorption but not chloride reabsorption can occur partly via the paracellular route and there is no capacity for chloride reabsorption more distally within the nephron.

Hypercalciuria occurs in Bartter syndrome because calcium reabsorption is a linked paracellular process. Hypomagnesaemia does not occur because of compensatory reabsorption in the early distal convoluted tubule (DCT). By contrast, Gitelman syndrome has hypocalciuria and hypomagnesaemia because of a compensatory mechanism in the early DCT which down-regulates cells expressing NCCT (and an apical magnesium channel) in favour of cells which reabsorb sodium and calcium. Gitelman syndrome generally presents in older children or even adults with muscle weakness and cramps, and short stature. It is not uncommonly diagnosed following investigation of growth, constipation or enuresis. Classical Bartter syndrome is generally a more severe disorder, presenting in early childhood with growth faltering, dehydration, hypotonia and lethargy. There is often a history of maternal polyhydramnios with the classical form.

Renal tubular acidosis

The kidney achieves acid–base balance through bicarbonate reabsorption and acid secretion. Kidneys normally reabsorb up to 90% of filtered bicarbonate in the proximal tubules. The distal collecting tubules are principally responsible for acid secretion. Buffers in the tubular lumen bind free hydrogen ions, allowing excretion of the daily acid load within limits of the minimal achievable urine pH of 4.5 to 5. Ammonia and, to a lesser extent, phosphate are the main urinary buffers. Ammonia (NH₃), which is formed from amino acid metabolism, can freely diffuse across tubular membranes, where it combines with protons to form ammonium (NH₄⁺), which becomes trapped in the tubular lumen.

Renal tubular acidosis (RTA) occurs in several ways: bicarbonate wasting in the proximal tubule (historically known as type 2 RTA), which almost always occurs as part of Fanconi syndrome; impairment in formation of ammonia results in type 4 RTA and in renal failure, where the acidosis is associated with hyperkalaemia; and failure to adequately secrete hydrogen ions is the primary defect in distal RTA (associated with hypokalaemia).

In childhood distal RTA, most cases are genetic. Autosomal recessive forms can be associated with (ATP6V1B1) or without (ATP6V0A4) sensorineural deafness. Both mutations code for subunits of the H-ATPase apical hydrogen ion transporter. An autosomal dominant form is caused by mutations of the SLC4A1 gene which encodes the chloride–bicarbonate exchanger on the basolateral membrane. Urine pH in distal RTA is always >5.5, in contrast to proximal RTA where it varies according to the plasma bicarbonate.

Initial correction of acidosis needs to take into account potassium and calcium, which will both decrease in response to alkali treatment. Maintenance treatment consists of sodium bicarbonate or citrate (sodium and/or potassium). Generally the doses of base required are less than for proximal acidosis. Children require lifelong follow-up and are at risk of nephrolithiasis and long-term deterioration in renal function from the nephrocalcinosis.

Embryology of the urinary tract (Fig. 19.5)

In higher forms of vertebrates, including man, kidneys evolve through three stages: pronephros, mesonephros and metanephros. Pronephric and mesonephric kidneys are temporary and metanephros persist as permanent kidneys. Whereas the urinary tract develops from the cloaca and the intermediate mesoderm, the definitive functional kidney develops from the metanephros through involution of pronephros and mesonephros.

• The pronephros is thought to be the primitive kidney, which is non-functional in the humans. The pronephros degenerates by the fifth week; its duct becomes the mesonephric duct.

• The mesonephric duct ultimately becomes the ureteric bud, which is essential for the development of metanephros. The ureteric bud subsequently branches to form the ureter, pelvis, calyces and the collecting system.

• The metanephros forms the glomeruli and upper part of the nephrons. Interaction between the ureteric bud and the metanephros is essential for the development of the kidneys. Defective communication between them leads to various congenital anomalies of the kidney and urinary tract (CAKUT) ranging from renal dysplasia to renal agenesis.

• Nephrons start to develop from the eighth post-conceptual week and continue until 34 weeks post conception.

• The upper portion of the bladder is derived from the allantois and the lower portion from the cloaca, which is also involved in the development of the rectum. As a result of this link, anorectal malformations are often associated with nephro-urological problems. The fibrous cord, the urachus, is formed from the remainder of the allantois and connects the bladder to the umbilicus.

• During fetal life, the kidneys are lobulated, and a lobular appearance is present even at birth, which affects the appearance of the kidneys on ultrasound in newborns.

Question 19.1

Changes on ultrasound screening

The parents of a newborn term infant ask to speak to a paediatrician. They are concerned that their new baby had some ‘changes’ identified on antenatal scanning at 28 weeks and would like to discuss these with you. Which of the following represent changes that are likely to adversely affect the baby’s health and/or renal function? Answer with true (T) or false (F).

A. An anterior–posterior diameter of 5 mm of the right renal pelvis

B. Lobulated appearance of kidneys

C. Multicystic dysplastic kidneys

D. Oligohydramnios

E. Visible bladder with a 5 mL volume

Answer 19.1

A. False; B. False; C. True; D. True; E. False.

Improvements in second and third trimester ultrasound (USS) screening have resulted in an increased number of antenatally-detected urinary tract abnormalities (AUTA). If a child has unilateral multicystic dysplastic kidney with normal contralateral kidney which develops normally with compensatory hypertrophy with normal ureter and bladder, then the child has an excellent prognosis. This is very different from children with bilateral renal anomalies. Abnormalities fall into two main categories:

• Abnormalities of renal parenchymal texture

• Abnormalities of drainage system

Congenital anomalies of the kidney and urinary tract are the most common abnormality detected on antenatal ultrasound scans, with an incidence of 1–4.5% of all pregnancies (Table 19.2). One of the most frequently detected abnormalities is dilatation of the renal tract. An anterior–posterior diameter (APD) of ≥7 mm at 18–20 weeks is considered significant. Early diagnosis improves the outcome because of early recognition and treatment of critical obstructions and urinary tract infections, preventing further renal damage and loss of renal function.

The fetal bladder is approximately 1 mL in volume by 20 weeks and increases to 35–50 mL by 40 weeks. It can be seen emptying every 30 minutes by 20 weeks’ gestation and it empties approximately once per hour by term. A typical fetus at term produces approximately 50 mL of urine per hour.

Fig. 19.5 Embryology of the kidneys between the critical sixth and ninth weeks of gestation. Ventral views of the abdominopelvic region of embryos and fetuses (sixth to ninth weeks) showing medial rotation and ‘ascent’ of the kidneys from the pelvis to the abdomen. A,B. Observe also the decrease in size of the mesonephroi. C,D. Note that as the kidneys ‘ascend’, they are supplied by arteries at successively higher levels, and that the hilum of the kidney (where the vessels and nerves enter) is eventually directed anteromedially. (From Moore K L, Persaud TVN, Torchia MG. Before we are born, 8th edition. Saunders, 2013, with permission.)

Table 19.2

Congenital anomalies of the kidney and urinary tract

Transient hydronephrosis (normal postnatal scan)	50%
Hydronephrosis with no evidence of obstruction or external pelvis	15%
Pelvi-ureteric junction obstruction (PUJO)	11%
Vesico-ureteric reflux (VUR)	9%
Megaureter (obstructed, refluxing, non-refluxing and non-obstructed or both refluxing and obstructed)	4%
Renal dysplasia	3%
Multicystic dysplastic kidney (MCDK)	2%
Duplex kidney ± ureterocoele	2%
Posterior urethral valves	1%
Others	5%

Hydronephrosis

Hydronephrosis is dilatation of the collecting system of the kidney, and hydroureter is dilatation of the ureter. Neither term implies obstruction. There is no clear definition or test for obstruction; its diagnosis requires a mixture of clinical and functional assessment including nuclear medicine imaging with DTPA/MAG3 scans (see section on imaging below). Hydronephrosis secondary to pelvi-ureteric junction obstruction (PUJO) is the commonest abnormality of the upper urinary tract. Renal ultrasound would show a dilated renal pelvis (isolated renal pelvis diameter >20 mm is highly suggestive of PUJO) with usually non-dilated ureter. It is more common in boys with overall incidence of 1 in 1000. Older children may present with acute loin or abdominal pain (Dietl’s crisis), haematuria, a palpable flank mass, infection (including pyonephrosis), nausea/vomiting, or pelvic rupture following minor trauma.

As there can be progressive renal damage, early diagnosis and assessment of the degree of renal dysfunction is important. Dynamic renogram such as DTPA or MAG3 will aid in this as they give split renal function which can be monitored. As only around 25% develop clinical problems, the need for surgical intervention has to be carefully evaluated. Surgery is considered if there is deterioration of renal function or increasing dilatation of the renal pelvis with thinning of renal cortex.

Functional development

In fetal life, excretion is performed primarily by the placenta, which receives 50% of fetal cardiac output whereas the fetal kidney accounts for only 2–4%. Hence, creatinine at birth reflects maternal creatinine level. The glomerular filtration rate (GFR) is 10–15 mL/min/1.73m² in the premature infant and 15–20 mL/min/1.73m² in the term infant. These values double over the first two weeks after birth and reach adult values of 80–120 mL/min/1.73m² by one year of age. The increase in GFR is achieved by recruitment of more glomeruli and not by new glomerulus generation.

Urine formation starts by the end of the first trimester. Fetal urinary concentrating activity is poor and fetal urinary sodium and phosphate levels decrease while the creatinine increases with increasing gestation, reflecting increasing maturation. The maximum urine concentrating capacity is low even at birth (up to 600 mOsm/kg) which explains the higher susceptibility of newborns and infants to dehydration. Adult capacity is achieved only by 1–2 years. In contrast, the diluting capacity of newborns is equivalent to that of adults and the urine osmolality can be lowered to 30–50 mOsm/kg.

For common developmental anomalies of the kidneys, see Box 19.1 and Figure 19.6.

Box 19.1

Common developmental anomalies of the kidneys (see also Fig. 19.6)

• Agenesis of the kidneys – usually due to failure of development of the ureteric bud.

• Renal hypoplasia – renal architecture is maintained, but fewer nephrons. Unless bilateral, is usually asymptomatic. May predispose to hypertension in later life.

• Multicystic dysplastic kidneys (Fig. 19.6A) – if the secreting and collecting parts fail to communicate, the kidney may become non-functional and transformed into multiple non-communicating cysts.

• Renal dysplasia – malformed and rudimentary tissues such as cartilage, or even calcified tissue within normal kidney. Chronic kidney disease (CKD) may develop in severe bilateral cases.

• Multiple kidneys – due to early splitting of the ureteric bud.

• Pelvic kidney – failure of ascent of the kidney due to persistence of sickle-shaped fold of peritoneum which projects from the lateral pelvic wall containing umbilical arteries.

• Fused kidney – lower poles of both kidneys united by an isthmus of kidney tissue. Horse-shoe kidney (Fig. 19.6B) lies in a lower level as its ascent is arrested by inferior mesenteric artery.

• Duplex kidney (Fig. 19.6C) – embryologically, duplication occurs when two separate ureteric buds arise from a single Wolffian duct. Duplication is variable. At one end of the spectrum, there is merely duplication of the renal pelvis, draining via a single ureter. At the other extreme, two separate collecting systems drain independently into the bladder or ectopically.

Fig. 19.6 Developmental anomalies of the kidneys. A. Multicystic dysplastic kidneys. B. Horse-shoe kidney. C. Duplex.

Investigating renal function

Question 19.2

Urine examination

The following (A–J) is a list of diagnoses:

A. Acute glomerulonephritis

B. Acute intermittent porphyria

C. Alport’s syndrome

D. Benign familial haematuria

E. Diabetic ketoacidosis

F. Myoglobinuria

G. Nephrotic syndrome

H. Primary hyperoxaluria

I. Urinary tract infection

J. Zellweger’s syndrome

Select the diagnosis from the list that BEST fits with the following urine findings. Each answer may be used once, more than once or not at all:

1. Macroscopic appearance: red/brown; urine dipstick: positive for blood (3+) and protein (2+), negative for leukocytes, nitrites, glucose and ketones; urine microscopy: no cells

2. Macroscopic appearance: brown/red; urine dipstick: positive for blood (3+), protein (2+), leukocytes (2+), negative for glucose and ketones; urine microscopy: granular casts

3. Macroscopic appearance: cloudy; urine dipstick: positive for leukocytes (2+), protein (+), ketones (+), negative for nitrites and glucose; urine microscopy: 70 white cells per high-powered field, occasional rod

Answer 19.2

1. F. Myoglobinuria

2. A. Acute glomerulonephritis

3. I. Urinary tract infection

See below for explanation.

Urine examination

Urine examination is an important but simple and non-invasive aid to diagnosis. It should ideally start with visual inspection, which may provide clues to haematuria (red urine) or pyuria (cloudy urine). However, as other conditions can mimic them, suspected haematuria or pyuria on inspection needs to be confirmed by urine microscopy. Testing urine by dipstick is a useful bedside test. It can be used for:

• Glucose, urobilinogen, bilirubin, ketones, pH and specific gravity.

• Haemoglobin – if dipstick positive, this usually indicates blood in the urine. Haematuria is confirmed by red blood cells on urine microscopy, and absence of red blood cells in the urine in presence of positive dipstick suggests either haemoglobinuria or myoglobinuria.

• Protein – can be detected but urine dipstick does not give an accurate quantitative estimation. Quantifying it requires spot urine albumin or protein : creatinine ratio or 24-hour urine protein estimation in an older child. A morning sample is preferable for testing, as this helps to exclude the false positives seen in children with orthostatic proteinuria. If significant proteinuria is seen on dipstick but insignificant albumin on quantitative estimation, leak of tubular proteins indicating tubular damage should be suspected.

• Leukocytes and nitrites – leukocytes are suggestive of urinary tract infection (UTI) but not diagnostic, as they can be secondary to fever. Most pathogenic bacteria produce nitrite, which has a high specificity but low sensitivity for diagnosing UTI. Nitrites may be negative in infants and younger children with UTI, as their increased urinary frequency does not allow sufficient time for nitrites to be produced. Urine cultures are therefore advisable to determine the bacteriological cause and antibiotic sensitivities in complicated UTI, as well as to confidently rule out UTI in children younger than three years. Urine microscopy can identify red and white blood cells and also RBC casts, which is considered to be diagnostic of glomerular involvement.

Glomerular filtration rate

Glomerular filtration rate (GFR) is a measure of renal function. Although inulin remains the reference solute for GFR estimation, it is mainly used as a research tool. In clinical practice, GFR is measured using radiopharmaceuticals, such as ^99mTc-DTPA, 125I-iothalamate and ⁵¹Cr-EDTA. Isotopic methods have the advantage that they do not require timed urine collection. In older children and adults, 24-hour urine collections are possible and can also be used. As these methods are too complex for routine clinical use, serum creatinine is used as a surrogate marker for renal function. However, this is not ideal as it is not an early indicator of renal damage. It also varies with age, muscle mass and nutritional status. Therefore, it can be inappropriately low in malnourished children or those with major amputation even in a stage of advanced renal failure.

Formulas for calculating GFR (estimated GFR) have been devised to correct for body size, such as the modified Schwartz formula:

K×height (cm)Plasma creatinine (μmol/L)

where K is a constant (varying between 33 and 40).

When testing for renal function, serum electrolytes are usually measured. They not only tell us about life-threatening complications (e.g. hyperkalaemia), but are useful in assessing the tubular function and volume status. For interpreting urinary loss of any electrolytes, a calculation of the fractional excretion is better than spot urinary value.

Imaging of the urinary tract

Ultrasound

Ultrasound scan (USS) is the most frequently used imaging modality in paediatric nephrology, as it is non-invasive and provides very good anatomical detail. Colour Doppler USS is useful for ascertaining renal blood flow, such as graft perfusion after renal transplantation, suspected renal vein or venous thrombosis or renal artery stenosis.

Micturating cystourethrogram

Micturating cystourethrogram (MCUG) requires bladder catheterization. Once the catheter is in place, a small amount of contrast medium is injected through the catheter to fill up the bladder. It allows very good images to be obtained of the anatomy of the lower urinary tract and is the ‘gold standard’ test for documenting and grading vesico-ureteric reflux (see below). It is also useful in documenting any bladder outlet obstruction, such as posterior urethral valves.

Its disadvantages are the small risk of infection, the requirement for radiation and the need for urethral catheterization that can be psychologically traumatic, particularly in the toddler age group.

Nuclear medicine

Dynamic scanning (^99mTc-DTPA/MAG3)

Dynamic scanning (MAG3 or DTPA scans) is useful for showing obstruction, such as pelvi-ureteric junction (PUJ) obstruction and is most often used after USS has shown dilated collecting system (hydronephrosis). It also gives an estimate of split differential function of both kidneys. MAG3 nuclear scan is supposed to be superior to ^99mTc-DTPA scan and can even be used to document reflux in older children who are continent and can control urination on demand.

Static scanning (^99mTc-DMSA)

Static scanning provides information about the renal parenchyma, most frequently used to identify renal scars. However, the radiation dose is higher than that in dynamic scanning.

Fluid and electrolyte homeostasis

Basic principles

Fluid within the body is distributed between the intracellular fluid (ICF) and extracellular fluid (ECF) compartments (Fig. 19.7). Solute composition differs between the two components and this is maintained by cell membrane pump activity, and solute size and electrical charge.

Question 19.3

Intravascular volume

A 6-year-old boy presents with a relapse of his nephrotic syndrome. He has had heavy proteinuria for 5 days and is very oedematous. He has been taking oral prednisolone for two days. Which one of the following measurements will give the BEST indication of intravascular volume? Select ONE answer only.

A. Blood pressure

B. Capillary refill time

C. Heart rate

D. Respiratory rate

E. Weight

Answer 19.3

C. Heart rate.

Volume assessment of the individual compartments can be very tricky in clinical practice and involves clinical assessment of all of the above parameters. For example, in a situation like nephrotic syndrome, there may be weight gain and oedema on examination. However, since there is hypoalbuminaemia and albumin is the primary intravascular osmotic component, the intravascular fluid volume may be low but the total ECF volume high. Conversely, in acute kidney injury, there can be weight gain and oedema in a situation where both the total ECF and the intravascular volume are high. Oedema can therefore occur with high or low intravascular fluid volume. In nephrotic syndrome, intravascular hypovolaemia is easy to miss as the child is oedematous. Pulmonary oedema may result in tachypnoea and significant hypovolaemia will eventually result in a delayed capillary refill time and low blood pressure, but elevation of the heart rate in an undistressed child is likely to be the most sensitive sign. Others include central abdominal pain (from mesenteric ischaemia) and a low urine output.

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