What is the differential diagnosis for acute forearm swelling that spares the elbow and is painful to touch?

Differential Diagnosis for Acute Forearm Swelling Sparing the Elbow with Pain

The most critical diagnosis to rule out immediately is acute compartment syndrome, which presents with severe pain out of proportion to examination, tense swelling, and pain with passive finger extension—this is a surgical emergency requiring urgent fasciotomy. 1, 2, 3

Life-Threatening and Urgent Diagnoses

Acute Compartment Syndrome

• Presents with severe pain out of proportion to clinical findings, tense forearm swelling, and excruciating pain with passive finger extension or flexion 1, 2, 3
• Can occur spontaneously without trauma, including bilaterally in rare cases 1, 2
• Associated with markedly elevated creatine kinase (>37,000 in documented cases) 2
• Requires emergency fasciotomy of flexor and extensor compartments within hours to prevent permanent muscle and nerve damage 1, 2, 3
• May be precipitated by drug-induced rhabdomyolysis (angiotensin II antagonists like losartan), though etiology is often unclear 2

Deep Vein Thrombosis

• Must be considered in any acute arm swelling, particularly in patients with risk factors 4
• Requires urgent ultrasound evaluation with Doppler imaging 4

Necrotizing Soft Tissue Infection/Gas Gangrene

• Rare but critical differential when soft tissue gas and extensive swelling are present 5
• Requires immediate surgical consultation if suspected 6

Traumatic/Musculoskeletal Causes

Occult Fracture with Soft Tissue Injury

• Begin with plain radiographs (AP, lateral, and oblique views) as the initial imaging study 6, 7
• Radial head/neck fractures account for 50% of elbow fractures and may present with forearm pain 7
• Joint effusion on radiographs (posterior and anterior fat pad elevation) may indicate occult fracture even without visible fracture line 8, 7
• If radiographs are normal but clinical suspicion remains high, CT without contrast is the next appropriate study for detecting occult fractures 6, 7

Tendon, Ligament, or Muscle Injury

• If radiographs are normal and soft tissue injury is suspected, MRI without contrast is the appropriate next imaging study 6, 8, 7
• MRI without contrast is highly sensitive for detecting bone marrow edema, ligament tears, and tendon injuries 8
• Biceps tendon tears may cause forearm pain radiating from the elbow region 7

Vascular Causes (in specific populations)

Arteriovenous Access Complications (dialysis patients)

• Includes stenosis, draining vein complications, thrombosis, steal syndrome, and aneurysms 4
• Requires ultrasound with Doppler evaluation as initial imaging 4

Critical Clinical Approach

Immediate Assessment

• Assess for compartment syndrome signs: pain with passive stretch, tense swelling, paresthesias, and pain out of proportion to examination 1, 2, 3
• Check distal pulses (note: pulses may be present in early compartment syndrome) 1
• Measure compartment pressures if diagnosis is uncertain (though this is primarily a clinical diagnosis) 3

Imaging Algorithm

1. Plain radiographs (AP, lateral, oblique) are the mandatory first imaging study 6, 7
2. If fracture suspected but radiographs normal/indeterminate: CT without contrast 6, 7
3. If soft tissue injury suspected with normal radiographs: MRI without contrast 6, 8, 7
4. If vascular pathology suspected: ultrasound with Doppler 4

Common Pitfalls to Avoid

• Never delay fasciotomy if compartment syndrome is suspected—this is a clinical diagnosis that does not require imaging confirmation 1, 2, 3
• Do not assume normal pulses exclude compartment syndrome; pulses may remain palpable until late stages 1
• Avoid ordering advanced imaging (CT/MRI) before obtaining plain radiographs first 6, 7
• Do not miss joint effusion on radiographs, which may be the only indication of occult fracture 8, 7
• Remember that MRI with contrast offers no advantage for simple evaluation and is unnecessary for initial assessment 8

Laboratory Evaluation

• Check creatine kinase immediately if compartment syndrome or rhabdomyolysis is suspected 2
• Review medication list for drugs associated with rhabdomyolysis (angiotensin II antagonists, statins) 2