What are the differential diagnoses for anterior forearm pain with erythema, tenderness, and pain on pronation, supination, and elbow flexion?

Differential Diagnoses for Anterior Forearm Pain with Erythema, Tenderness, and Pain on Rotation and Flexion

The most likely differential diagnoses include distal biceps tendon tear, pronator teres syndrome (median nerve compression), musculotendinous injury to the flexor-pronator mass, and less commonly, septic arthritis or deep soft tissue infection requiring urgent evaluation.

Primary Musculotendinous Considerations

Distal Biceps Tendon Pathology

• Distal biceps tendon tear should be strongly considered given pain with both pronation and supination, as these movements stress the biceps tendon at its radial tuberosity insertion 1.
• Partial tears of the long head of the biceps with intact short head represent the most common injury pattern, while traumatic ruptures have significantly higher association with short head involvement 1.
• Pain with elbow flexion against resistance is characteristic, and the anterior location with erythema may indicate acute inflammatory response to tendon injury 1.

Pronator Teres Syndrome

• Median nerve compression at the pronator teres classically presents with aching discomfort in the anterior forearm, with distinctive tenderness over the proximal pronator teres muscle 2.
• Pain is characteristically aggravated by resisted pronation of the forearm and resisted elbow flexion, matching the described presentation 2, 3.
• Symptoms typically worsen with cyclic stress and repetitive pronation-supination movements 4.
• The presence of erythema is atypical for pure nerve compression but may coexist with muscular inflammation 5.

Flexor-Pronator Muscle Strain

• Acute strain or tear of the flexor-pronator mass originating from the medial epicondyle can cause anterior forearm pain with both internal rotation (pronation) and flexion 1.
• Erythema and tenderness suggest acute inflammatory response to muscle injury 6.

Brachioradialis Muscle Pain

• Brachioradialis muscle pathology can produce sharp, shooting pain during activity and aching pain at rest in the lateral-to-anterior forearm 6.
• Pain is exacerbated by elbow flexion (especially with neutral hand position) and pronation-supination movements 6.
• This diagnosis is commonly underdiagnosed or misdiagnosed as other conditions 6.

Critical Urgent Considerations

Septic Arthritis or Deep Infection

• The presence of erythema with pain mandates immediate consideration of infection, including septic arthritis or deep soft tissue infection 7.
• Red-flag symptoms suggesting infection require immediate in-person evaluation to assess for osteomyelitis or septic arthritis 7.
• Fever, warmth, severe swelling, or systemic symptoms would elevate concern significantly.

Ligamentous Injuries

Collateral Ligament Complex Injuries

• Medial collateral ligament (MCL/UCL) injury can occur with elbow trauma and produces pain with valgus stress and flexion 1.
• Lateral collateral ligament complex injury may present with pain on external rotation and instability 1.
• Ligamentous injuries frequently accompany elbow dislocations and can involve both medial and lateral stabilizers 1.

Diagnostic Approach Algorithm

Initial Clinical Assessment

• Obtain mechanism of injury: direct trauma, fall on outstretched hand, repetitive overhead activities, or insidious onset 7.
• Assess for red flags: fever, severe swelling, inability to move joint, visible deformity, or systemic symptoms requiring immediate referral 7.
• Localize tenderness: proximal pronator teres (4-5 cm distal to elbow crease) suggests pronator syndrome 2, 3, while anterior cubital fossa tenderness suggests biceps pathology 1.

Provocative Testing

• Resisted pronation with elbow flexed: pain indicates pronator syndrome or flexor-pronator strain 2, 3.
• Resisted supination (thumbs-up position): pain suggests distal biceps tendon pathology 1.
• Resisted elbow flexion: biceps tendon injury if anterior pain; brachioradialis if lateral-anterior pain 1, 6.

Imaging Strategy

• Plain radiographs are the mandatory first-line imaging to exclude fracture, dislocation, or avulsion injury 1, 7.
• MRI without contrast is the preferred advanced imaging for suspected tendon, ligament, or muscle injury when radiographs are normal or indeterminate 1.
• Ultrasound represents an equivalent alternative to MRI for evaluation of distal biceps tendon tears and can assess for fluid collections suggesting infection 1.
• The flexion-abduction-supination (FABS) MRI view may provide additional benefit for challenging cases of high-grade partial versus complete biceps tendon tears 1.

Common Pitfalls to Avoid

• Do not dismiss erythema as purely inflammatory—always exclude infection first, as delayed diagnosis of septic arthritis leads to significant morbidity 7.
• Pronator syndrome is frequently misdiagnosed as carpal tunnel syndrome; distinguish by localizing tenderness to the proximal forearm rather than wrist 3, 5, 4.
• Electrophysiological testing is often normal in pronator syndrome, so diagnosis relies primarily on clinical examination 2, 3, 5.
• Ultrasound has inferior accuracy compared to MRI for partial biceps tears (45.5% vs 86.4%), so MRI is preferred when surgical planning is needed 1.
• Brachioradialis muscle pain is commonly overlooked and may mimic lateral epicondylitis, radial tunnel syndrome, or cervical radiculopathy 6.