Cystic Abdominal Mass

Paula J. Woodward, MD

DIFFERENTIAL DIAGNOSIS

Common

Urinary Tract
- Multicystic Dysplastic Kidney (MCDK)
- Ureteropelvic Junction Obstruction
- Enlarged Bladder
- Urinoma
Gastrointestinal Tract
- Bowel Atresia
- Meconium Pseudocyst

Less Common

Ovarian Cyst
Lymphangioma
Enteric Duplication Cyst

Rare but Important

Choledochal Cyst
Neuroblastoma
Fetus-in-Fetu, Teratoma
Urachal Anomalies
Cloacal Malformation, Hydrocolpos

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Can the cystic mass be localized to a normal structure?
- Most abdominal cystic masses are from the urinary tract
- Gastrointestinal tract next most common
Is it a simple cyst or a complex cystic mass?
- Septations, internal echogenic debris
What are the wall characteristics?
- Thin-walled, thick-walled, calcified, “gut signature”
Is it constant or does it change appearance during the exam, between exams?

Helpful Clues for Common Diagnoses

Multicystic Dysplastic Kidney (MCDK)
- Multiple cysts of varying sizes with no discernible renal parenchyma
- Reniform shape is lost
- Variable in utero course: May involute, remain stable, or grow
- May be massive and cross midline
Ureteropelvic Junction Obstruction
- May present as large cyst if severe obstruction
- Look for communication with dilated calyces
Enlarged Bladder
- Posterior urethral valves most common cause
  - Look for “keyhole” appearance created by the dilated posterior urethra
- Prune belly syndrome and urethral atresia less common causes
- Hydronephrosis also commonly seen
Urinoma
- Spontaneous rupture of the renal collecting system into retroperitoneum
- Look for contained fluid collection adjacent to obstructed kidney
Bowel Atresia
- Can occur anywhere along gastrointestinal tract
- Has a tubular, “sausage-shaped” appearance
- Peristalsis within a cystic mass is pathognomonic
Meconium Pseudocyst
- Wall-off bowel perforation
- Irregular, thick walls
- Look for other signs of meconium peritonitis
  - Intraperitoneal calcifications
  - Dilated bowel
  - Ascites

Helpful Clues for Less Common Diagnoses

Ovarian Cyst
- Top consideration for a unilocular cyst in a 3rd trimester female fetus
- “Daughter cyst” sign
  - Small cyst along wall of dominant cyst
  - Highly specific (up to 100%) sign for ovarian origin (82% sensitive)
- May have occasional septations
- If appearance becomes complex, with internal echoes, then there is concern for torsion
- Occasionally found in upper abdomen
  - Supporting ligaments are lax, allowing for displacement
- May occasionally be bilateral
Lymphangioma
- Thin-walled cystic mass
- May be unilocular or multilocular, with one or multiple septations
- Can be very complex, insinuating around organs and extending out of abdomen
- Variable echogenicity of fluid, but usually anechoic
Enteric Duplication Cyst
- Solitary, thick-walled cyst
- Look for “gut signature”
  - Layered appearance with echogenic mucosa, hypoechoic muscular layer, echogenic serosa
  - Often difficult to see in utero
- Rarely bowel dilatation from obstruction

Helpful Clues for Rare Diagnoses

Choledochal Cyst
- Cystic dilatation of extrahepatic &/or intrahepatic bile ducts
- Unilocular, simple, right upper quadrant cyst is most common presentation in fetus
  - Round in axial plane and fusiform in longitudinal plane
- Following bile ducts into cyst confirms diagnosis
Neuroblastoma
- Arises from adrenal gland
- Approximately 50% are cystic
- Complex appearance with thick septations
- Cystic neuroblastoma has an excellent prognosis
Fetus-in-Fetu, Teratoma
- Overlapping features between these two entities
  - Fetus-in-fetu more developed and must have spinal elements
- Complex, with a large solid component encapsulated within a cyst
- Calcifications, including well-formed bones, most specific finding
- Majority reported in upper retroperitoneum
- Fetus-in-fetu theoretically result of inclusion of a monochorionic diamniotic twin within a host twin
Urachal Anomalies
- Includes cysts and patent urachus
- Communication with bladder confirms patent urachus
- Bladder may appear elongated with a figure 8 or “waisted” configuration
- May extend into base of umbilical cord
  - Associated with allantoic cord cysts
- May resolve as gestation progresses
Cloacal Malformation, Hydrocolpos
- Persistent cloaca
  - Failure of urorectal septum to reach perineum
  - Seen in female fetuses
  - Results in single perineal opening for urine, genital secretions, and meconium
  - Bladder, vagina, and rectum may all communicate in utero
- Variable presentation
  - Cystic mass in pelvis
  - Dilated pelvic bowel loops; may see enteroliths from mixing of meconium and urine
  - Hydronephrosis and lumbosacral anomalies may also be present
  - Abnormal genitalia with lack of normal labial/clitoral formation
  - Ascites reported in some cases

Image Gallery

Coronal oblique ultrasound shows that the left kidney

is enlarged and cystic, consistent with a MCDK. Most abdominal cystic masses in the fetus are related to the urinary tract.

Axial ultrasound shows a cystic mass filling the fetal abdomen. When large, a MCDK can have a confusing appearance. Only one normal kidney

is seen, which is the key to the diagnosis.

(Left) Coronal ultrasound shows classic bilateral ureteropelvic junction obstruction

in a 2nd trimester fetus. (Right) Axial ultrasound of the same fetus in the 3rd trimester shows dramatic progression, with massive left renal pelvis distention (calipers). When presenting late, this appearance may be confusing. Careful scanning may show dilated calyces.