Explain renal caliectasis.

Renal Caliectasis: Definition and Clinical Significance

Renal caliectasis refers to dilation of the renal calyces—the cup-shaped structures that collect urine from the renal pyramids—and represents a spectrum ranging from physiologic variants to pathologic obstruction requiring intervention. 1

Terminology and Classification

• The term "caliectasis" is encompassed within the broader category of urinary tract dilation (UTD), which was adopted by a multidisciplinary consensus in 2014 to replace inconsistent terms like hydronephrosis, pelviectasis, and pelvicaliectasis. 1

• Caliectasis specifically describes dilation of the calyces, which may occur with or without concurrent renal pelvic dilation. 1

• The Society for Fetal Urology (SFU) grading system classifies calyceal dilation postnatally: SFU grade 2 shows full pelvis with major calyces dilated, SFU grade 3 demonstrates uniformly dilated minor calyces with preserved parenchyma, and SFU grade 4 indicates parenchymal compromise. 1

Pathophysiology and Causes

Obstructive Causes

• Ureteropelvic junction (UPJ) obstruction is the most common cause of significant caliectasis in pediatric populations, with an anteroposterior diameter (APD) ≥15 mm in the third trimester predicting UPJ obstruction. 1

• Urolithiasis is the predominant cause of acute caliectasis in adults, with stones causing increased intrapelvic pressure that transmits to the calyces. 1, 2

• Ureteral obstruction from strictures, tumors (including cervical cancer compressing the ureter), or ureteroceles can produce secondary calyceal dilation. 3, 2

• Bladder outlet obstruction from posterior urethral valves (in male infants), prostatic hyperplasia, or neurogenic bladder causes bilateral caliectasis through backpressure. 3, 2

Non-Obstructive Causes

• Vesicoureteral reflux (VUR) accounts for approximately 30% of urinary tract abnormalities in infants with antenatal hydronephrosis and produces caliectasis without true obstruction. 3

• Megacalycosis is a rare congenital anomaly characterized by non-obstructive dilation of 20-25 calyces (versus normal 8-12) with semilunar rather than triangular configuration, flat pyramid tips, absent papillae impressions, normal renal pelvis and ureter, and preserved renal function. 4, 5

• Transient physiologic distension is the most common cause overall, with spontaneous resolution occurring in 64-75% of cases of mild antenatal urinary tract dilation. 3

Pressure-Dilation Relationship

• Significant calyceal dilation does not occur until intrapelvic pressure exceeds 25 cm H₂O (normal is 10-18 cm H₂O), with a correlation coefficient of 0.95 between maximal intrapelvic pressure and degree of calyceal dilation. 6

Diagnostic Approach

Initial Imaging

• Renal and bladder ultrasound (RBUS) is the first-line modality, detecting hydronephrosis/caliectasis with >90% sensitivity and evaluating for hydroureter, duplex systems, ureterocele, bladder abnormalities, and parenchymal thinning. 1, 3

• Normal APD thresholds are <4 mm before 28 weeks gestation, <7 mm after 28 weeks gestation, and <10 mm postnatally; values above these indicate pathologic dilation. 1, 3

• Color Doppler should be used to differentiate dilated calyces from prominent renal sinus veins and to assess ureteral jets in the bladder. 1

Advanced Imaging Indications

• Voiding cystourethrography (VCUG) is indicated when bilateral high-grade hydronephrosis, duplex kidneys with hydronephrosis, ureterocele, abnormal bladder appearance, or febrile UTIs are present—but should be avoided for isolated mild caliectasis without these risk factors. 3

• MAG3 renal scintigraphy (performed at ≥2 months of age) is warranted when caliectasis persists or worsens on follow-up ultrasound, when parenchymal thinning develops, or when differential renal function assessment is needed to distinguish obstructed from non-obstructed dilated systems. 3, 7

• CT urography provides comprehensive anatomic detail to identify the cause of obstruction and is particularly useful when malignancy is suspected or when ultrasound findings are equivocal. 7

• MR urography is an alternative to CT when radiation exposure is a concern or in patients with renal impairment, with T2-weighted sequences showing 77% sensitivity for detecting perirenal fluid in acute obstruction. 1, 7

Clinical Significance and Risk Stratification

Mild Caliectasis (APD <10 mm)

• Spontaneous resolution occurs in 64-75% of cases, with low risk of significant underlying pathology. 3

• Initial ultrasound follow-up at 1-6 months is recommended, with subsequent monitoring every 6-12 months if stable, and at least once every 2 years long-term to detect "flow uropathy." 3

• The presence of symptoms—including urinary tract infections, hematuria, flank pain, or voiding dysfunction—increases clinical relevance and may warrant further evaluation even when imaging shows only mild dilation. 3

Moderate-to-Severe Caliectasis (APD ≥15 mm)

• Higher risk of UPJ obstruction and posterior urethral valves, with increased likelihood of requiring surgical intervention. 1, 3

• Parenchymal thinning indicates chronic obstruction with risk of permanent nephron loss if not corrected. 2, 7

• Bilateral severe caliectasis with elevated serum creatinine constitutes a medical emergency requiring urgent decompression via percutaneous nephrostomy or retrograde ureteral stenting. 2, 7

Management Principles

Conservative Management

• For isolated mild caliectasis without symptoms or risk factors, observation with serial ultrasound is appropriate. 3

• Prophylactic antibiotics (trimethoprim-sulfamethoxazole first-line; alternatives include nitrofurantoin, cefadroxil, or amoxicillin-clavulanate) are advised for patients at increased UTI risk based on severity and presence of VUR. 3

Indications for Urology Referral

• Referral is warranted when functional imaging demonstrates obstruction, differential renal function declines, obstructive symptoms develop, or febrile UTIs recur despite prophylaxis. 3

• In male infants with severe bilateral caliectasis, immediate bladder catheterization at birth and urgent VCUG are mandatory to rule out posterior urethral valves. 3

Urgent Intervention

• Pyonephrosis (infected obstructed kidney) requires urgent decompression before definitive treatment to prevent sepsis. 2

• Percutaneous nephrostomy is preferred in severe obstruction or when retrograde access is difficult; retrograde ureteral stenting may be appropriate in stable patients. 7

Critical Pitfalls to Avoid

• Do not delay postnatal imaging in male infants with severe bilateral urinary tract dilation; urgent bladder decompression is essential to prevent irreversible renal injury from posterior urethral valves. 3

• A distended bladder can cause artifactual caliectasis; ensure proper bladder emptying (by voiding or catheterization) before interpreting ultrasound findings. 1, 7

• Chronic caliectasis can be asymptomatic until significant renal damage occurs; progressive dilation leads to acute kidney injury and permanent nephron loss if not corrected. 2

• Avoid routine VCUG for isolated mild caliectasis (APD <10 mm) without additional risk factors to prevent unnecessary radiation exposure and catheterization. 3

• Always image both kidneys to identify bilateral disease or a solitary kidney, which would alter management urgency. 7

• Megacalycosis mimics obstructive hydronephrosis but requires no surgical treatment; distinguish it by the presence of 20-25 semilunar calyces, normal renal pelvis/ureter, and preserved renal function. 4, 5

• A "phantom calyx" (non-visualizing calyx on urography) may herald serious disease including tuberculosis, tumor, calculus, ischemia, or trauma and requires further investigation. 8