CAKUT stands for congenital anomalies of the kidney and urinary tract. These are malformations that arise when the developing kidney or drainage system forms abnormally before birth. As the posterior urethral valves note set out, posterior urethral valves belong to the group of CAKUT.
CAKUT is the reason a first febrile UTI in a child is investigated thoroughly. As the imaging note set out, a first febrile urinary infection is followed by an ultrasound-first imaging ladder. The infection may be the first indication that the tract was never structurally normal, and detecting the anomaly early prevents years of unrecognised damage. As the posterior urethral valves note set out, pressure from a blocked tract damages a developing kidney, and urine that stays in place is easier to infect.
How does a kidney end up malformed?
The kidney and its drainage tube are not made in one step. They form from a close exchange between the ureteric bud and the metanephric mesenchyme, and the two tissues have to signal to each other in the right order and place. An error in that exchange malforms the kidney or its drainage system. So CAKUT is not one disease but a spectrum of malformations of kidney size, shape, position, number, drainage and cyst formation, and each fault depends on which step went wrong.
The anomalies range from a kidney too small or never formed to a drainage system blocked or letting urine flow backwards. They are usually grouped by what is wrong.
What goes wrong with size and shape?
The filtering units are made from mesenchyme that the bud induces. In renal hypoplasia, fewer of them are formed, so the kidney is small, with a reduced number of nephrons (filtering units) but normal architecture. In renal dysplasia the nephrons do not form properly because the dialogue between bud and mesenchyme goes wrong. The tissue stays disorganised, often with primitive ducts and cysts, and is usually poorly functional. A multicystic dysplastic kidney is a non-functioning kidney replaced by cysts.
Shape can also be affected. In renal fusion, such as horseshoe kidney, the two kidneys are joined. They are fused at their lower poles, and as they ascend, the joined isthmus is trapped under the inferior mesenteric artery. Both kidneys therefore stay low.
What goes wrong with position and number?
Position depends on how the kidney ascends and rotates, and number on how many ureteric buds form. The kidney ascends from the pelvis during development. When ascent or rotation is arrested, the result is renal ectopia: a kidney located in the pelvis, iliac fossa or across the midline.
Normally each side forms a single ureteric bud. In duplication, the drainage develops as two, giving a duplex collecting system. When no bud forms, the mesenchyme is never induced and no kidney develops. This is renal agenesis, the congenital absence of one kidney, and an anomaly of the Wolffian duct can lead to it.
What goes wrong with drainage and cysts?
The last two groups concern how urine leaves and how the tissue is laid down. Drainage anomalies include ureteropelvic and ureterovesical junction obstruction, megaureter, posterior urethral valves and vesicoureteral reflux. As the posterior urethral valves note set out, obstruction raises the pressure behind it. As the reflux note set out, reflux is backflow of urine from the bladder towards the kidney, and it harms the kidney by pressure and by infection.
Cystic disease includes autosomal recessive and autosomal dominant polycystic kidney disease, nephronophthisis and hereditary nephrotic syndromes.
How common is CAKUT, and can it be caught early?
Published prevalence estimates vary widely with how broadly CAKUT is defined and whether stillbirths and autopsy series are included. The range is roughly 0.3 to 17 per 1,000 births, and 3–6 per 1,000 live births is the figure most often quoted. CAKUT accounts for a substantial share of anomalies detected on antenatal ultrasound.
CAKUT is the clearest pediatric example of prevention changing an outcome. A urinary tract dilated on antenatal ultrasound can be imaged after birth, defined anatomically, and corrected before it produces infection or obstruction.
Why does CAKUT cause kidney failure in children?
In adults, CKD is most often the endpoint of diabetes, hypertension and obesity. These are decades of acquired metabolic and vascular injury acting on kidneys that developed normally. In children those exposures have not accumulated. So the leading cause in children is CAKUT and its consequences: dysplasia, obstruction, reflux and the scarring left by recurrent febrile UTI. Registry data put CAKUT at roughly 30–40% of kidney failure in children, in contrast to the diabetes, hypertension and obesity that dominate adult CKD. Glomerulonephritis becomes increasingly common as children get older. Broader estimates attribute close to half of CKD presenting within the first three decades of life to CAKUT.
This gives a diagnostic rule. Because the adult causes are acquired and the child’s are structural, when a young child has kidney disease the first question is what was malformed, not whether the child has adult risk factors.
What is a congenital solitary kidney?
One of the faults above is a kidney that is missing or does not work. A congenital solitary kidney means the child is born with only one functioning kidney. The other kidney is absent (unilateral renal agenesis) or present but non-functioning, for example a multicystic dysplastic kidney that has involuted or shrunk away.
A solitary kidney is not simply a normal kidney with a spare removed. The kidney that formed alone has only the nephrons it was built with, and its growth is how it makes up for the missing second kidney.
How common is a solitary kidney?
The prevalence is commonly quoted as about 1 in 1,500 newborns. Published ranges are wider, roughly 1 in 450 to 1 in 3,200, depending on criteria and population. Unilateral renal agenesis alone is estimated at about 1 in 2,000, and multicystic dysplastic kidney at about 1 in 4,300. Most cases are now suspected antenatally.
How does the single kidney cope?
A solitary kidney is a reduced nephron endowment system. Nephron endowment is the number of nephrons with which the kidneys start life. When part of nephron mass is lost early, the surviving nephrons hyperfiltrate (each filters more than usual) to maintain total filtration. So the single-nephron filtration rate rises while total filtration is kept up.
A sustained single-nephron load is the proposed mechanism for later focal glomerulosclerosis (scarring in some filtering units), proteinuria (protein leaking into urine) and hypertension. The proposed explanation is that the high filtration load on each unit injures the podocytes. A solitary kidney means reduced nephron endowment, which is why blood pressure, urinalysis and kidney growth are monitored over years.
Does a normally grown kidney cope?
The answer depends on whether the kidney grew to make up for the missing nephrons. A single kidney that grew normally has a total nephron mass closer to that of two average kidneys, so function usually remains in range. The concern is reduced reserve rather than predictable decline. Modest attention to hydration and diet is reasonable.
What if the kidney did not grow?
Compensation depends on growth, so a kidney that does not enlarge keeps a higher load on each nephron. When a single kidney did not grow adequately, its endowment never reached the expected level, and the trajectory is towards CKD even if current function looks normal. Follow-up is therefore closer.
Risk factors repeatedly associated with kidney injury are:
- low birth weight or prematurity; prematurity is among the causes of nephron loss, so these children may start with fewer filtering units;
- ipsilateral CAKUT, a further anomaly on the side of the single kidney;
- absence of compensatory hypertrophy, meaning the single kidney has not enlarged as expected.
What else tends to be malformed?
A single kidney often has a second anomaly in the same tract. About one-third of children with a solitary kidney have additional CAKUT in the same urinary tract, which may involve the kidney they depend on. Beyond the urinary tract, a reported 20–30% of children with CAKUT have an associated malformation outside it, so a broader examination belongs in the postnatal assessment. Associated anomalies are not only urological: the kidney and the Müllerian structures share an early origin in the urogenital ridge, so an error there can affect both. Other extrarenal malformations are reported in a minority of children and should be sought clinically rather than assumed absent.
Why do genital malformations come with kidney anomalies?
The genital tract and the kidney share an early origin. Müllerian anomalies are malformations of the uterus, fallopian tubes, cervix or upper vagina. They occur in over 30% of patients with unilateral renal agenesis, and so in over 30% of girls with it. An anomaly of the Wolffian duct leads to renal agenesis and to uterine anomalies on the same side.
When are genital malformations found?
The same malformation can wait until puberty to show itself. Obstructive genital malformations may only declare themselves at puberty with pain or absent menstruation. The obstructed tract collects menstrual blood behind the blockage, so the problem appears when periods start. In girls, pelvic ultrasound before puberty is therefore part of the assessment, and it is part of the workup.
Should a child with one kidney play contact sport?
Contact-sport restriction for children with a solitary kidney has been traditional, on the reasoning that an abdominal blow could cost the child their only kidney.
What does the injury evidence show?
The traditional concern can be tested against injury data, and the evidence is less alarming than the tradition. A national high-school athletic injury surveillance database study recorded only 18 kidney injuries across a very large number of athlete-exposures. That kidney injury rate is roughly 4 per million. Head, neck and spine injuries were far more common. A 2025 systematic review of 25 studies including 24,424 patients reached a similar conclusion and estimated the risk of losing a kidney to sport-related trauma as low. Kidney injury from sport is rare.
Who decides?
Guidance favours an individual answer. The American Academy of Pediatrics recommends a “qualified yes” to contact and collision sports for athletes with a single kidney, with individual clinical judgement about restrictions. Canadian guidance takes a comparable position. Surveys show most physicians still advise against contact sports, and national medico-legal frameworks differ. This is a decision to make with the family, using the actual injury data rather than the assumption that the kidney is the organ most at risk. Most guidance permits participation with individual assessment, though practice and regulation vary.
How is a child with one kidney followed?
The answer to sport depends on how the kidney is watched over time. A single kidney must be confirmed with neonatal ultrasound and examined for other anomalies, and its compensatory enlargement documented, which is the growth by which it makes up for the missing kidney. The follow-up schedule is used to detect hyperfiltration injury early. That injury builds gradually through the long term, towards hypertension and then CKD, and is asymptomatic by design.
What is checked, and how often?
The checks are blood pressure, urinalysis, ultrasound and creatinine.
- Blood pressure at least annually, because hypertension is often the earliest measurable sign of renal injury.
- Urinalysis at least annually. Proteinuria or albuminuria signals glomerular damage, because glomerular proteinuria is caused by increased permeability of the glomerular capillary wall.
- Renal ultrasound every 2–4 years, more often in early childhood. It tracks kidney growth and detects new dilatation or scarring.
- Serum creatinine when clinically indicated. It confirms reduced function when blood pressure, urinalysis or ultrasound suggest it.
In short: blood pressure and urinalysis at least annually, ultrasound every 2–4 years, creatinine when indicated.
How much monitoring does a normal solitary kidney need?
A normal solitary kidney needs less than the others. In a solitary kidney with normal compensatory growth and no other anomaly, the Italian Society of Pediatric Nephrology consensus recommends urinalysis at diagnosis. The same consensus does not require routine blood tests or genetic testing. This reflects how much long-term monitoring can be done in primary care when the kidney is otherwise normal.
What else can damage a kidney that is already malformed?
Monitoring tracks function, but a malformed kidney can also be damaged by infection. A tract that refluxes silently can present with a first febrile UTI, then a second, then scarring, then reduced kidney function in adolescence. Reflux, and the scarring left by recurrent febrile UTI, are among the leading causes of childhood CKD. Which children go on to scar, and why does a second infection matter more than the first?