What does caliectasis (dilation of the calyces) and peliectasis (dilation of the renal pelvis) of the kidney mean?

Understanding Caliectasis and Peliectasis of the Kidney

Caliectasis (dilation of the calyces) and peliectasis (dilation of the renal pelvis) are descriptive terms for fluid-filled expansion of the kidney's collecting system, which modern guidelines now recommend replacing with the unified term "urinary tract dilation" (UTD) to standardize communication across specialties. 1

Modern Terminology and Classification

• The 2014 multidisciplinary consensus from 8 professional societies established that terms like "hydronephrosis," "pelviectasis," "caliectasis," and "pelvicaliectasis" should be replaced with the standardized "urinary tract dilation" (UTD) classification system. 1

• The UTD system provides objective grading based on multiple parameters including anteroposterior diameter (APD), central and peripheral calyceal dilation, parenchymal thickness and appearance, ureteral abnormalities, and bladder appearance. 1

• Postnatal normative values established that APD <10 mm represents physiologic dilation that typically requires no intervention. 1, 2

Clinical Significance

When Dilation is Physiologic (Normal Variant)

• Approximately 80% of mild urinary tract dilation detected prenatally resolves spontaneously and represents a normal physiologic variant. 2

• APD <10 mm postnatally with no calyceal dilation, normal parenchymal thickness, and no ureteral abnormalities is considered normal. 1, 2

When Dilation is Pathologic

• Pathologic dilation occurs when there is central or peripheral calyceal dilation, parenchymal thinning or abnormalities, ureteral dilation, or APD ≥10 mm postnatally. 2

• Common pathologic causes include vesicoureteral reflux, ureteropelvic junction obstruction, multicystic dysplastic kidneys, and posterior urethral valves. 2

• For obstructive lesions, APD ≥15 mm detected in the third trimester serves as a threshold predictive of ureteropelvic junction obstruction. 1

Diagnostic Approach

Prenatal Detection

• Gestational age-dependent thresholds apply: APD <4 mm is normal before 28 weeks, and APD <7 mm is normal after 28 weeks gestation. 1, 2

• UTD occurs in 1-2% of pregnancies, with most representing transient findings. 2

Postnatal Evaluation

• Renal bladder ultrasound should be performed after 48 hours of life, as earlier studies underestimate dilation due to physiologic neonatal third spacing. 2

• Complete resolution of third spacing occurs by day 7-10. 2

• The Society for Fetal Urology (SFU) grading system remains the most commonly used postnatal classification, ranging from grade 1 (urine barely splits sinus) to grade 4 (parenchymal compromise). 1

Management Based on Severity

Low-Risk UTD (UTD A1/P1)

• Follow-up ultrasound at ≥32 weeks gestation if detected prenatally, with postnatal follow-up to confirm resolution. 2

• No prophylactic antibiotics or invasive testing typically required. 1

Moderate to Severe UTD (UTD A2-3/P2-3)

• Serial ultrasound monitoring every 4 weeks prenatally is recommended. 2

• Specialty consultation with pediatric urology and/or nephrology is indicated. 1, 2

• Additional postnatal imaging including voiding cystourethrogram (VCUG) and functional imaging may be necessary. 1

Critical Pitfalls to Avoid

• Do not perform ultrasound evaluation too early after birth (<48 hours), as this underestimates the degree of dilation. 2

• Mild renal pelvic dilation (3-10 mm APD without caliectasis) is not predictive of vesicoureteral reflux and should not automatically trigger voiding cystourethrography. 3

• Ultrasound has >90% sensitivity for hydronephrosis but cannot always determine the underlying cause with certainty. 1

• False-negative ultrasound studies can occur with dehydration, early obstruction, or compression by tumors or fibrosis. 1

Prognostic Indicators

• Combined assessment of APD and diffuse caliectasis better predicts need for surgical intervention than APD measurement alone. 4

• APD ≥6-9 mm with diffuse caliectasis carries a hazard ratio of 19.5 for surgery versus 0.59 for APD ≥6-9 mm alone. 4

• The SFU grading system demonstrates high positive predictive value for both kidney function and need for surgical intervention in ureteropelvic junction obstruction. 1