Unilateral Small Kidney

Sara M. O’Hara, MD, FAAP

DIFFERENTIAL DIAGNOSIS

Common

Congenital Hypoplasia/Dysplasia
Scarring
- Postinfectious Scarring
- Postinflammatory Scarring
- Obstructive Scarring
- Vesicoureteral Reflux (VUR) Scarring
Post-Traumatic
Multicystic Dysplastic Kidney (MCDK)

Less Common

Page Kidney
Renal Vein Thrombosis, Chronic
Other Vascular Insult

Rare but Important

Partial Resection

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Hypoplasia, dysplasia, and scarring are all different from aplasia
- Aplasia is congenital absence of renal tissue
- Associated ureteral agenesis with absence of ipsilateral trigone and ureteral orifice
- Contralateral hypertrophy
  - Seen commonly with aplasia
  - Less common with hypoplasia/dysplasia
  - Also seen following early surgical removal of 1 kidney
- Hypoplastic kidney < 1/2 size of contralateral kidney
  - Calyces and parenchyma are normal in proportion
  - Architecture should be normal, not scarred or dysplastic
  - Just smaller version of opposite kidney
- Dysplasia: Congenitally malformed parenchyma
  - Chronic scarring and fibrosis also sometimes called “dysplasia”
All other diagnoses are acquired, typically chronic or recurrent
Differential by location
- Prerenal: Arterial stenosis, shock, infarction
- Renal: Postinfectious, hypoplasia, dysplasia, MCDK, radiation
- Postrenal: Obstructive or vesicoureteral reflux (VUR) atrophy

Helpful Clues for Common Diagnoses

Congenital Hypoplasia/Dysplasia
- Hypoplasia results from insufficient branching of ureteric bud
- Nephrons formed are normal but deficient in number
- Renal parenchymal volume is diminished but does function
- Segmental hypoplasia more often associated with hypertension
  - Hypertension refractory to medical Rx, may require surgical/ablative Rx
  - Segmental hypoplasia (a.k.a. Ask-Upmark kidney) may actually be segmental scar
  - Patients are typically female and present with hypertension
- Dysplasia results from faulty formation of nephrons &/or collecting system
- Renal volume may be normal or decreased initially but tends to decrease with age
- Nephrons are poorly functioning or malfunctioning → salt wasting
Postinfectious Scarring
- After pyelonephritis, renal abscess, sepsis
- Patchy or global, affecting entire kidney
- Xanthogranulomatous pyelonephritis
  - Chronic pyelonephritis with granulomatous abscess formation and severe kidney destruction
- Controversy exists regarding
  - Increased scarring in infants and young children compared with older children
  - Increased scarring when antibiotic therapy is delayed
  - Prospective, randomized trials on subject are lacking
- Imaging
  - Ultrasound: Cortical thinning, volume loss, increased echogenicity
  - DMSA: Absent radiotracer in areas of scar and fibrosis, often crescentic
  - CT & MR: Poorly enhancing, thinned cortex, lobulated contour
  - IVP: Seldom performed in children
Postinflammatory Scarring
- May be seen after any “nephritis”
  - Glomerulonephritis
  - Radiation nephritis
  - Autoimmune
  - Henoch-Schönlein purpura
  - Hemolytic uremic syndrome
- Scarring can affect 1 kidney asymmetrically, even when both kidneys have nephritis
- Imaging shows smaller kidney, typically with global scarring
Obstructive Scarring
- Scarring and nephron damage from any downstream obstruction
  - Ureteropelvic junction obstruction
  - Ureterovesical junction obstruction
  - Urinary calculi
  - Bladder outlet obstruction, posterior urethral valves
  - Neurogenic bladder and other voiding dysfunction
  - Pelvic mass or inflammation
Vesicoureteral Reflux (VUR) Scarring
- Scarring has been shown with reflux, even in absence of infection
- Higher grades of reflux are more likely to cause scarring
- Higher grades of reflux are less likely to spontaneously resolve with age/somatic growth
Post-Traumatic
- Underlying causes vary
  - Vascular insult, infarction, emboli, venous infarct
  - Obstruction to urine flow, superimposed infection
  - Perinephric hematoma with compressive injury
Multicystic Dysplastic Kidney (MCDK)
- Severely dysplastic, nonfunctional tissue
- Enlarged, normal size, or small in newborn
- Over years, tissue involutes and atrophies
- Recognizable only by location in teenagers

Helpful Clues for Less Common Diagnoses

Page Kidney
- Hypertension and renal insufficiency caused by compression of kidney
- Typically due to subcapsular hematoma, though other perinephric masses (tumor or urinoma) also possible
- In 1939, Dr. Irvine H. Page (1901-89) demonstrated that wrapping cellophane tightly around animal kidneys can cause hypertension
Renal Vein Thrombosis, Chronic
- Initially causes renal enlargement
- Kidney atrophies over weeks to months
- Seen in thrombotic conditions, premature infants with umbilical catheters, sepsis
Other Vascular Insult
- Numerous other vasculitides can cause chronic scarring or atrophy of 1 kidney

Helpful Clues for Rare Diagnoses

Partial Resection
- Nephron-sparing surgery continues to gain popularity
- Any surgery done to remove segment of kidney results in remaining tissue being “small”
- Consider partial resection when 1 renal pole appears flattened or truncated

Image Gallery

Longitudinal harmonic ultrasound shows a small, echogenic right kidney with poor corticomedullary differentiation in a newborn with a prenatal history of suspected right renal aplasia/hypoplasia.