What is the differential diagnosis for a hypoechoic lesion in the left iliac fossa?

Differential Diagnosis of Hypoechoic Lesion in Left Iliac Fossa

A hypoechoic lesion in the left iliac fossa most commonly represents sigmoid diverticulitis in older patients, tubo-ovarian abscess or pelvic inflammatory disease in women of childbearing age, or less commonly colonic malignancy, epiploic appendagitis, psoas abscess, or peritoneal pathology including rare entities like hydatid cysts. 1

Primary Diagnostic Considerations by Patient Demographics

In Older Adults (>50 years):

• Sigmoid diverticulitis with abscess formation is the leading diagnosis, presenting as a hypoechoic inflammatory mass with or without fluid collection 1
• Colonic malignancy (adenocarcinoma of sigmoid or descending colon) appears as a hypoechoic mass with irregular borders and possible bowel wall thickening 1
• Sigmoid volvulus may present with emptiness of the left iliac fossa rather than a mass, though associated inflammatory changes can appear hypoechoic 2

In Women of Childbearing Age:

• Tubo-ovarian abscess or pelvic inflammatory disease are primary considerations, appearing as complex hypoechoic masses with internal debris 1
• Ovarian cysts (including endometriomas, hemorrhagic cysts) present as hypoechoic or anechoic lesions 3
• Ectopic pregnancy with hematoma formation can appear hypoechoic 1

Less Common but Important Differential Diagnoses

Inflammatory/Infectious Etiologies:

• Epiploic appendagitis presents as a small, oval hypoechoic lesion adjacent to the colon 1
• Psoas abscess appears as a hypoechoic collection within or adjacent to the psoas muscle 1
• Pyelonephritis or renal abscess of the left kidney may extend into the iliac fossa 1
• Primary peritoneal hydatid cyst is rare (2% of all hydatid cysts) but should be considered in endemic areas or immigrants, appearing as a large cystic hypoechoic lesion with daughter cysts 4

Colonic Pathology:

• Inflammatory or infectious colitis (Crohn's disease, ulcerative colitis, infectious colitis) with bowel wall thickening appearing hypoechoic 1
• Colonic lymphoma can present as a hypoechoic mass in the bowel wall 5

Other Considerations:

• Lymphadenopathy from various causes (infectious, inflammatory, malignant) appears as hypoechoic rounded structures 3
• Metastatic disease to peritoneum or lymph nodes can appear hypoechoic 5
• Hematoma (spontaneous or post-traumatic) in the abdominal wall or retroperitoneum 3

Critical Diagnostic Approach

Initial Imaging Strategy:

• Ultrasound is appropriate as first-line imaging for superficial or suspected gynecological pathology, with sensitivity for detecting masses but limited specificity for characterization 1, 6
• CT scan with IV contrast is the definitive imaging modality for complete assessment of disease extent, complications (abscess, fistula, perforation), and guiding percutaneous drainage 1

Key Imaging Features to Assess:

• Location and layer of origin (bowel wall, peritoneal, retroperitoneal, gynecological) 7
• Internal characteristics (solid vs cystic, presence of septations, debris, or daughter cysts) 7, 3
• Size, shape, and border characteristics (well-circumscribed vs infiltrative) 7
• Vascularity on Doppler examination helps differentiate inflammatory from neoplastic processes 8

Tissue Diagnosis Considerations:

• Hypoechoic lesions require tissue sampling when malignancy is suspected, as hypoechogenicity alone is not diagnostic 7
• CT-guided fine-needle aspiration or drainage can provide both therapeutic benefit and microbiological/cytological diagnosis 1
• Surgical exploration may be necessary when imaging is non-diagnostic or when clinical suspicion remains high despite negative imaging 4

Common Pitfalls to Avoid

• Do not rely on ultrasound alone for definitive diagnosis—while useful for detection, it has limited specificity for characterizing hypoechoic lesions in the left iliac fossa 1, 3
• Do not assume all left iliac fossa masses are diverticular disease—maintain high suspicion for malignancy, especially with atypical features or in patients without known diverticular disease 1
• In women of reproductive age, always consider gynecological pathology first before attributing symptoms to gastrointestinal causes 1
• Consider rare diagnoses like hydatid cysts in appropriate epidemiological contexts, as they can mimic more common pathology 4