What is the appropriate diagnosis and management for a patient with right lower quadrant (RLQ) pain and right thigh weakness that started a few days ago?

Diagnostic Approach: Psoas Abscess Until Proven Otherwise

A patient presenting with right lower quadrant pain combined with right thigh weakness requires urgent evaluation for psoas abscess or retroperitoneal pathology, as this combination of symptoms suggests iliopsoas muscle involvement rather than typical intra-abdominal pathology like appendicitis.

Critical Clinical Distinction

The key differentiating feature here is the right thigh weakness, which is not a typical presentation of appendicitis or other common RLQ pathology. This neuromuscular finding suggests:

• Psoas muscle involvement causing hip flexion weakness and pain with hip extension
• Retroperitoneal pathology affecting the lumbar plexus (L2-L4 nerve roots)
• Possible femoral neuropathy from compression or inflammation

While the ACR guidelines extensively address RLQ pain evaluation, they focus primarily on appendicitis and intra-abdominal causes 1. The combination with thigh weakness shifts the differential diagnosis significantly.

Immediate Diagnostic Imaging

Contrast-enhanced CT of the abdomen and pelvis with IV contrast is the imaging modality of choice for this presentation 1:

• CT has sensitivities ranging from 85.7% to 100% and specificities from 94.8% to 100% for acute abdominal pathology 1
• CT can identify retroperitoneal collections, psoas abscesses, and alternative diagnoses beyond appendicitis 1, 2
• In patients with RLQ pain, CT identified non-appendiceal diagnoses including right colonic diverticulitis (8%) and obstruction (3%) 1

MRI abdomen and pelvis may be appropriate if CT is contraindicated, particularly for visualizing soft tissue and neuromuscular involvement 3.

Primary Differential Diagnoses to Consider

Most Likely: Psoas Abscess

• Presents with RLQ/flank pain, fever, and hip flexion weakness
• Requires percutaneous catheter drainage and antibiotics 1
• CT will show fluid collection within or adjacent to the psoas muscle

Retroperitoneal Hematoma

• Can compress lumbar plexus causing thigh weakness
• May occur spontaneously in anticoagulated patients

Appendicitis with Retroperitoneal Extension

• Retrocecal appendicitis can irritate the psoas muscle
• However, true motor weakness is uncommon 1

Right Colonic Diverticulitis

• Can present with RLQ pain and fever
• Seen in 8% of patients with RLQ pain on CT 1

Pelvic Abscess with Nerve Compression

• Transabdominal, transgluteal, transrectal, or transvaginal percutaneous drainage may be appropriate depending on collection location 1

Management Algorithm

Step 1: Obtain urgent contrast-enhanced CT abdomen/pelvis with IV contrast 1

Step 2: If psoas or retroperitoneal abscess identified:

• Initiate broad-spectrum antibiotics immediately 1
• Percutaneous catheter drainage (PCD) is usually appropriate for collections >3 cm 1
• Surgical consultation for cases not amenable to drainage

Step 3: If appendicitis confirmed:

• Proceed to appendectomy as standard treatment 1
• For perforated appendicitis with abscess: PCD followed by delayed surgery or PCD only is usually appropriate 1

Step 4: If alternative diagnosis identified:

• Direct treatment at specific diagnosis (diverticulitis, inflammatory bowel disease, etc.) 1, 3
• In one study, 41% of patients with non-appendiceal CT diagnoses required hospitalization, and 22% underwent surgical or image-guided intervention 1

Critical Pitfalls to Avoid

• Do not assume appendicitis based solely on RLQ pain and fever - the thigh weakness is a red flag for alternative pathology
• Do not delay imaging - psoas abscesses can lead to sepsis and require prompt drainage 1
• Do not rely on ultrasound alone - while US is appropriate for typical appendicitis evaluation, it has limited sensitivity for retroperitoneal pathology 1
• Do not miss femoral neuropathy - assess hip flexion strength, knee extension, and sensory changes in the anterior thigh distribution

The combination of RLQ pain with thigh weakness occurring over several days strongly suggests a process involving the iliopsoas muscle or retroperitoneal space, making contrast-enhanced CT the essential first diagnostic step 1.