Renal Pyelectasis or Extra Renal Pelvis

Bilateral renal pyelectasis

True dilation of the renal pelvis, meaning the collecting system is distended.

Ultrasound features

  • Measurable APRPD increase
  • Pelvis looks ballooned within renal sinus
  • May extend into calyces if more significant

It represents:

  • Transient physiologic dilation
  • Reflux
  • Obstruction (if progressive)

Bilateral extrarenal pelvis

Anatomic variant where the renal pelvis lies partially outside the renal sinus.

Ultrasound features

  • Pelvis appears prominent
  • Renal sinus itself is not expanded
  • No calyceal dilation
  • No cortical thinning
  • Looks like a “bulge” external to kidney

It represents:

  • Normal variant
  • Not obstruction
  • Not reflux by itself

Difference is Calyceal dilation

Feature Bilateral Pyelectasis Bilateral Extrarenal Pelvis
APRPD increased Yes May appear mildly enlarged
Calyces dilated Sometimes No
Renal sinus stretched Yes No
Parenchymal thinning Possible (if severe) No
Progression over time Possible Usually stable
Clinical significance Variable Usually benign

If calyces are visible and dilated → true hydronephrosis.

If pelvis looks prominent but calyces are normal → likely extrarenal pelvis.

Bilateral involvement

Bilateral pyelectasis

  • More common in males
  • Slight association with Trisomy 21 if isolated and mild
  • Needs follow-up scan

Bilateral extrarenal pelvis

  • Often symmetric
  • No aneuploidy association
  • Usually does not progress
  • Rarely needs follow-up beyond routine

Aneuploidy risk comparison

Finding T21 Association
Isolated mild pyelectasis Mild soft marker
Isolated extrarenal pelvis Not a soft marker

Resolution patterns

  • Mild pyelectasis → often resolves
  • Extrarenal pelvis → persists as an anatomic variant but harmless

Postnatal prognosis

Isolated bilateral pyelectasis (mild)

  • Excellent prognosis
  • Small % reflux
  • Rare surgery

Bilateral extrarenal pelvis

  • Essentially normal kidney variant
  • No functional impact

Red flags

  • Calyceal dilation
  • Parenchymal thinning
  • Oligohydramnios
  • Ureteral dilation
  • Thick bladder
  • Progressive enlargement

If none of these are present → conservative follow-up.

Key interpretation

If

  • Central renal sinus preserved
  • No calyceal dilation
  • Normal parenchyma
  • Stable size

Strongly favors extrarenal pelvis, which is far more benign than true bilateral pyelectasis.