What is the differential diagnosis for a painful lump on the forearm that appeared in 2 days and feels like a bruise?

Differential Diagnosis for Painful Forearm Lump Appearing in 2 Days

A painful lump on the forearm that appeared rapidly over 2 days and feels like a bruise most likely represents traumatic soft tissue injury (hematoma or deep muscle contusion), but requires urgent evaluation to exclude soft tissue sarcoma, infection, or occult fracture.

Immediate Diagnostic Approach

Initial Imaging

• Plain radiographs of the forearm are the first-line imaging study to exclude fracture, particularly if there was any trauma, even minor 1, 2, 3
• Radiographs can identify occult fractures, avulsion injuries, and joint effusions that may present as soft tissue swelling 2, 3

Urgent Ultrasound Consideration

• If the lump is increasing in size or there is no clear trauma history, obtain urgent ultrasound within 2 weeks to assess for soft tissue sarcoma 1
• The British Sarcoma Group guidelines specifically recommend urgent direct access ultrasound for unexplained lumps that are increasing in size 1
• Ultrasound effectively discriminates benign from malignant soft tissue masses and is highly user-dependent 1

Primary Differential Diagnoses

Most Likely: Traumatic Causes

• Deep muscle contusion/hematoma: Direct trauma causing bleeding into muscle tissue, presenting with pain, swelling, and bruise-like tenderness 4, 5
• Muscle tear: Forceful contraction or direct blow causing muscle fiber disruption with subsequent hematoma formation 5
• Occult fracture with soft tissue injury: May present as soft tissue swelling before bruising becomes visible 2, 3

Concerning: Soft Tissue Mass

• Soft tissue sarcoma: While rare, any unexplained lump warrants evaluation, particularly if deep-seated, larger than expected, or without clear trauma 1
• Most soft tissue masses in primary care are benign lipomas, but atypical presentations require imaging 1

Other Considerations

• Infection (cellulitis, abscess): Would typically present with warmth, erythema, and systemic symptoms 1
• Spontaneous hematoma: Consider if patient is on anticoagulation or has bleeding disorder 6
• Thrombophlebitis: Superficial vein inflammation with overlying tenderness 1

Clinical Red Flags Requiring Urgent Referral

Sarcoma Concern

• Lump continues to increase in size over days 1
• Deep-seated mass (below fascia) 1
• Size greater than 5 cm 1
• No clear history of adequate trauma to explain the size 1

Infection Concern

• Fever, spreading erythema, or systemic symptoms warrant evaluation for osteomyelitis or septic arthritis 1
• Rapid progression with warmth and redness suggests infectious process 1

Advanced Imaging Algorithm

If Radiographs Are Normal but Clinical Concern Persists

For suspected occult fracture:

• CT without contrast is the next appropriate study if fracture is suspected but radiographs are normal 1, 2, 3
• CT identifies occult fractures with superior sensitivity compared to plain films 1

For suspected soft tissue injury:

• MRI without contrast is highly sensitive for detecting soft tissue pathology including muscle tears, hematomas, and bone marrow edema 2, 3
• MRI provides superior soft tissue detail compared to other modalities 2
• Contrast is unnecessary for initial evaluation of simple hematomas or muscle injuries 2

For uncertain soft tissue mass:

• If ultrasound findings are uncertain and clinical concern persists, refer to sarcoma specialist within 2 weeks 1
• MRI of the affected region provides the most accurate information for diagnosis of soft tissue tumors of the extremity 1

Common Pitfalls to Avoid

• Do not dismiss rapidly appearing lumps without imaging, especially if increasing in size, as soft tissue sarcomas can present acutely 1
• Do not assume all painful lumps are simple bruises without obtaining appropriate history about trauma mechanism 4, 5
• Do not order advanced imaging before plain radiographs unless there is clear indication 1, 2, 3
• Do not miss occult fractures that may only manifest as soft tissue swelling initially 2, 3
• Avoid inadequate treatment of deep muscle contusions, which can lead to complications like muscle shortening 4

Initial Management While Awaiting Imaging

• Rest, ice, compression, and elevation (RICE protocol) for suspected traumatic injury 4, 5
• Restrict activity to prevent further injury 5
• Protect from further trauma during evaluation period 5
• Do not apply heat in the acute phase, as this may worsen bleeding 4