Hydronephrosis is the swelling or dilation of one or both kidneys, occurring when urine cannot drain properly into the bladder. This backup causes the kidney’s collecting system—the renal pelvis and calyces—to become distended and enlarged. Hydronephrosis is not a disease itself but a sign of an underlying issue, such as a blockage or urine flowing backward toward the kidney. The condition is often detected using ultrasound imaging during routine prenatal screening (antenatal hydronephrosis). Postnatal follow-up confirms the findings and determines the severity. The degree of swelling varies widely, from minor fluid collection that resolves spontaneously to severe obstruction that can lead to kidney damage if untreated.
Why Standardized Grading is Essential
A standardized system for classifying hydronephrosis ensures consistency in medical communication across different physicians and institutions. Without a common language, a description of “mild swelling” from one doctor might be interpreted differently by another, leading to inconsistent patient care. This structured classification system allows doctors to accurately predict the likely course of the condition, known as the prognosis.
It helps stratify patient risk, separating those likely to resolve spontaneously from those at higher risk for complications like infection or loss of renal function. This triage system is fundamental to guiding the initial management plan, determining whether a case requires observation or active intervention. This consistency is important in conditions that may resolve naturally, where the decision to intervene must be based on objective evidence of severity or progression.
The Criteria of the SFU Grading System
The Society for Fetal Urology (SFU) developed a widely recognized system that classifies hydronephrosis into four distinct grades based on the visual appearance of the kidney’s internal structures on ultrasound. This qualitative assessment focuses on the dilation of the renal pelvis and calyces, along with the appearance of the surrounding renal tissue, or parenchyma. Although other systems exist, the SFU grading remains a traditional standard for evaluating the severity of the condition.
SFU Grade I
SFU Grade I represents the mildest form, characterized by dilation of the renal pelvis only, where the urine barely separates the renal sinus. The calyces, the small cup-like structures that collect urine, appear normal and are not dilated. The renal parenchyma remains fully intact and undamaged.
SFU Grade II
SFU Grade II involves a slightly greater degree of dilation, including the pelvis and some of the calyces, although the overall pelvicalyceal pattern remains recognizable. The dilation is mild, and the major calyces may show some fluid accumulation. Crucially, the renal parenchyma still shows no sign of thinning or atrophy.
SFU Grade III
SFU Grade III is classified as moderate hydronephrosis, where both the renal pelvis and the calyces are moderately dilated. The normal concave shape of the calyces becomes blunted, and the papillae may appear flattened. The kidney tissue is generally preserved, without significant thinning of the outer cortex.
SFU Grade IV
SFU Grade IV indicates the most severe form, featuring gross dilation of the entire collecting system, where the calyces and pelvis appear ballooned and lose their distinct borders. The hallmark of this grade is parenchymal compromise, meaning the severe pressure from the fluid has caused the outer renal cortex to thin.
Interpreting Each Grade and Management Pathway
Grades I and II: Observation and Spontaneous Resolution
Hydronephrosis classified as SFU Grade I or Grade II is generally considered low risk and often follows a conservative management pathway centered on watchful waiting. These milder degrees of dilation have a high likelihood of resolving spontaneously, with reported resolution rates ranging between 64% and 73%. The primary management involves regular follow-up ultrasounds, typically scheduled every three to six months until the dilation resolves completely. The goal is to monitor the kidney to ensure the dilation does not progress and that kidney function remains stable.
Grade III: Increased Monitoring and Prophylaxis
SFU Grade III hydronephrosis signifies a higher risk level, requiring a more proactive approach to monitoring and management. Because the calyces are blunted, indicating a more significant buildup of pressure, the chance of spontaneous resolution is lower than in the mild grades. Management often includes more frequent ultrasound monitoring, initially every one to three months, to quickly detect any progression. Prophylactic antibiotics may be recommended in some cases to prevent urinary tract infections (UTIs), a serious complication due to the stagnant urine. Further diagnostic tests, like a specialized X-ray or nuclear scan, may be used if there is concern about an underlying obstruction or reduced kidney function.
Grade IV: Specialist Referral and Intervention
SFU Grade IV hydronephrosis is the highest-risk classification because it involves severe dilation and has already caused thinning of the renal parenchyma. This degree of tissue compromise is a strong indicator of a true obstruction that is causing damage to the kidney’s functional capacity. Patients with Grade IV are significantly more likely to require surgical intervention than those with lower grades.
These cases necessitate immediate referral to a specialist, such as a pediatric urologist. The diagnostic workup often includes a diuretic renal scan, like a MAG3 scan, to precisely evaluate the kidney’s function and the rate at which urine drains. Surgical intervention, such as a pyeloplasty to correct an obstruction, is often indicated if the kidney’s differential function drops below 40% or if drainage significantly worsens over time.