Why Does Kidney Swelling Happen? Causes and Differential Diagnosis

Hydronephrosis is the appearance of the renal pelvis and calyces widened with urine. Ultrasound shows this picture but does not say why: urine may not be able to leave the kidney properly, it may be leaking back from the bladder, or the collecting system may simply be naturally roomy with no problem at all.

• Prenatal hydronephrosis is found in roughly 1–5% of pregnancies. In most of these babies, no serious problem emerges after birth; in a portion of them, a cause requiring treatment or follow-up is found. Differential diagnosis is the work of safely telling these two groups apart.

The approximate distribution reported in prenatal hydronephrosis series is as follows; the figures vary by center and diagnostic criteria:

• Temporary (physiological) widening, roughly 40–80%: the most common cause. The collecting system is naturally roomy or still maturing; it most often resolves in the first years.
• UPJ obstruction (ureteropelvic junction obstruction; informally, kidney outlet narrowing), roughly 10–30%: the most common pathological cause. There is a narrowing or a non-contracting segment at the point where the renal pelvis opens into the ureter; the ureter is normal.
• Kidney reflux (vesicoureteral reflux, VUR), roughly 10–20%: urine leaks back from the bladder toward the ureter and kidney; the ureter can also appear wide.
• Lower ureter narrowing / primary megaureter (UVJ obstruction), roughly 5–15%: narrowing where the ureter enters the bladder; the ureter is markedly widened.
• Ureterocele, ectopic ureter, duplex collecting system, roughly 5–7%: anatomic variants usually affecting the upper half of the kidney.
• Multicystic dysplastic kidney, roughly 4–6%: not true hydronephrosis, but a non-functioning cystic kidney; it can be confused with hydronephrosis on ultrasound.
• Posterior urethral valves (PUV), roughly 1–2%: a membrane in the urethra in male infants; it comes with two-sided widening and a thickened bladder wall and carries urgent importance.

• This page's focus is UPJ obstruction; kidney reflux, posterior urethral valves (PUV), and duplex collecting system are among the other causes and are covered on their own pages.

A good ultrasound narrows down the cause in most cases. Your doctor looks for answers to these questions in the report:

• Is the ureter wide? The ureter is normal in UPJ obstruction; a wide ureter suggests reflux, UVJ obstruction, or PUV.
• One-sided or two-sided? Two-sided widening raises the possibility of a bladder- or urethra-related cause (PUV, neurogenic bladder).
• Is the bladder wall thick, does the bladder empty properly? A thick, irregular bladder wall points to outflow obstruction.
• Is there a duplex system in the kidney, or separate widening in the upper pole? Ureterocele and ectopic ureter look like this.
• Is the kidney tissue normal, thinned, or cystic? Parenchymal thinning suggests ongoing obstruction; multiple cysts suggest multicystic dysplastic kidney.

• A one-sided picture where only the renal pelvis and calyces are widened, and the ureter and bladder look normal, is the typical appearance of UPJ obstruction; even so, this appearance can overlap with temporary widening, and the distinction is made over time and, if needed, with MAG3.

Serial ultrasound: time is the main way to distinguish temporary widening from the others. The first post-birth ultrasound is done at least 48 hours after birth; if normal or mild, it is repeated at 4–6 weeks. The large majority of mild widening resolves or stays stable in the first 2–3 years.

Most of differential diagnosis proceeds calmly over the course of weeks; but some pictures should be evaluated without delay. See a doctor the same day in the situations below:

• In a newborn, severe two-sided widening, a thickened bladder wall, or a history of low amniotic fluid (oligohydramnios) in pregnancy: suspected PUV.
• Fever (38°C / 100.4°F or higher) together with known hydronephrosis: possible infection in an obstructed kidney.
• In a solitary kidney or two-sided widening, a drop in urine output, swelling, or weakness.
• A baby who cannot feed, is drowsy, or has visible blood in the urine.

• If these symptoms are absent, differential diagnosis proceeds through planned ultrasound and, if needed, VCUG and MAG3 in sequence, carried out by the same team from diagnosis through follow-up. Once the UPJ obstruction path is clarified, the next question becomes follow-up or surgery.