Salt-losing nephropathy

Bartter syndrome

Most classic renal causes of severe polyhydramnios.

Mechanism

Defective tubular salt reabsorption leads to:

• massive fetal polyuria
• increased urine output
• severe polyhydramnios

Often:

• starts mid-trimester
• progresses rapidly

Ultrasound features

Typical pattern:

• polyhydramnios (often severe)
• normal or mildly enlarged kidneys
• kidneys may become echogenic
• bladder usually normal or large

Usually bilateral renal involvement, not unilateral.

Congenital nephrotic syndrome

Congenital nephrotic syndrome of the Finnish type; Diffuse mesangial sclerosis

These are glomerular disorders, not obstructive ones.

Mechanism of polyhydramnios

Occurs due to:

• fetal protein loss
• reduced plasma oncotic pressure
• increased urine production

Also:

• placentomegaly common
• maternal serum AFP may be elevated

Ultrasound features

• bilateral echogenic kidneys
• kidneys often enlarged
• polyhydramnios
• placentomegaly

bilateral involvement is typical.

Salt-losing nephropathy

• both kidneys echogenic
• progressive severe polyhydramnios
• kidneys not obstructed
• bladder normal
• placenta enlarged

Case

• unilateral PUJO
• progressive echogenicity
• CMD loss
• normal opposite kidney
• polyhydramnios

Most likely contralateral compensatory hyperfiltration

Feature	PUJO progression	Bartter	Mesangial sclerosis
Laterality	Unilateral	Bilateral	Bilateral
Echogenicity	Unilateral	Bilateral	Bilateral
Polyhydramnios	Mild–moderate	Severe	Moderate–severe
Bladder	Normal	Large	Normal
Placenta	Normal	Normal	Enlarged

Bartter

• very early severe polyhydramnios
• normal renal pelvis
• normal corticomedullary differentiation initially
• rapidly rising AFI

Congenital nephrotic syndrome

• placentomegaly
• bilateral large echogenic kidneys
• elevated maternal AFP
• early fetal edema

Unilateral PUJO → echogenic kidney + CMD loss + polyhydramnios - progressive unilateral obstructive nephropathy with preserved contralateral function

Salt-losing nephropathy or mesangial sclerosis: much less likely unless kidneys become bilaterally echogenic or polyhydramnios becomes severe and disproportionate.

Congenital mesoblastic nephroma

Most common fetal renal tumor.

• Tumor-induced polyuria
• Increased renal perfusion
• Occasionally hypercalcemia-related diuresis

Result: Often rapidly progressive polyhydramnios

Kidney appearance:

• Large unilateral renal mass
• Solid, homogeneous or slightly heterogeneous
• Often replaces normal kidney
• Kidney contour distorted

Doppler features: Marked vascularity

Mesoblastic nephroma

Typical sequence:

• Solid renal mass appears
• Kidney enlarges
• Mass grows
• Polyhydramnios develops

When mesoblastic nephroma SHOULD be suspected

• Kidney massively enlarged
• Solid renal mass seen
• Polyhydramnios rapidly worsening
• Doppler shows hypervascular lesion
• No clear obstructive pathway

Mesoblastic nephroma is unlikely when a kidney evolves from hydronephrosis to echogenic dysplastic appearance with CMD loss, because tumors present as solid enlarging masses rather than obstructive deterioration patterns.