What could be causing a throbbing sensation in the right abdominal rectus muscle?

Throbbing Sensation in Right Abdominal Rectus Muscle

A throbbing sensation in the right rectus abdominis muscle is most likely a benign musculoskeletal condition originating from the abdominal wall itself, not from intra-abdominal pathology, and can be diagnosed clinically with specific physical examination maneuvers.

Clinical Diagnosis

The key to diagnosing abdominal wall pain (rectus syndrome) is performing specific physical examination tests:

• Carnett's sign: Pain that increases when the abdominal muscles are tensed (patient lifts head or legs while supine) indicates abdominal wall origin rather than visceral pathology 1
• Localized tenderness: Pain should be reproducible at a circumscribed site with digital palpation 1
• Movement-related pain: Pain elicited by physical movement suggests musculoskeletal origin 1

Differential Diagnosis to Exclude

While abdominal wall pain is most likely, you must systematically exclude serious intra-abdominal pathology:

Vascular Causes (Most Critical)

• Abdominal aortic aneurysm with pulsatile mass: A true throbbing sensation could represent transmitted pulsations from an underlying AAA, though this typically presents in the midline rather than laterally in the rectus muscle 2
• Rectus sheath hematoma: Presents with palpable abdominal mass and pain, particularly in patients on anticoagulation therapy; CT establishes diagnosis in 100% of cases 3

Musculoskeletal Causes (Most Likely)

• Rectus abdominis muscle strain or tear: Common with exercise or forceful muscle contractions; ultrasound shows increased muscle size, loss of linear architecture, and decreased echogenicity 4
• Rectus syndrome: Somatic pain from rectus abdominis musculature, often with small painful pits in the abdominal wall; can occur at the semilunar line, within the rectus muscle itself, or along the linea alba 5, 1

Intra-abdominal Causes (Less Likely Given Location)

• Appendicitis: While right-sided, typically presents in the right lower quadrant rather than the rectus muscle specifically 6, 7
• Cholecystitis: Presents with right upper quadrant pain, not typically in the rectus muscle 6

Diagnostic Approach

Step 1: Clinical examination with Carnett's sign

• If positive (pain increases with muscle tensing), diagnosis of abdominal wall pain is established 1
• If negative or equivocal, proceed to imaging

Step 2: Imaging selection based on clinical concern

• Ultrasound first for young patients or when muscle pathology suspected: Can identify rectus muscle tears, hematomas, and exclude appendicitis without radiation 4
• CT abdomen/pelvis with IV contrast if clinical examination is concerning for intra-abdominal pathology or if patient has risk factors for serious disease (anticoagulation, older age, fever, peritoneal signs) 6, 2, 3

Red Flags Requiring Immediate Imaging

Proceed directly to CT imaging if any of the following are present:

• Anticoagulation therapy: High risk for rectus sheath hematoma, which requires CT diagnosis and may need transfusion 3
• Hemodynamic instability or signs of blood loss: Suggests hemorrhage 3
• Fever with elevated WBC: Suggests infectious/inflammatory intra-abdominal process 6
• Peritoneal signs: Suggests visceral pathology requiring urgent intervention 2
• Older age (>50) with new-onset symptoms: Higher risk of serious pathology including vascular disease 2

Management

For confirmed abdominal wall pain/rectus syndrome:

• Conservative management is effective in 90% of cases: NSAIDs, activity modification, and reassurance 3
• Rectus nerve block provides both diagnostic confirmation and therapeutic benefit for intractable cases 5
• Avoid unnecessary investigations once diagnosis is established 1

For rectus sheath hematoma:

• Conservative treatment effective in 90% of cases, particularly spontaneous hematomas in anticoagulated patients 3
• Blood transfusion required for type III hematomas (large, extending below arcuate line) 3
• Surgery reserved only for severe cases with hemodynamic compromise 3

Common Pitfalls

• Mistaking abdominal wall pain for visceral pathology: This leads to repeated costly investigations and unnecessary anxiety; always perform Carnett's sign 1
• Missing rectus sheath hematoma in anticoagulated patients: This diagnosis should be at the top of your differential in any anticoagulated patient with abdominal pain and palpable mass 3
• Assuming all right-sided abdominal pain is appendicitis: Rectus muscle pathology can mimic appendicitis but has completely different management 4