What are the possible causes and treatments for a painful lump on the forearm that appeared suddenly over 2 days and feels like a bruise?

Painful Forearm Lump Appearing Over 2 Days

For a painful lump on the forearm that appeared suddenly over 2 days and feels like a bruise, you should obtain plain radiographs (X-rays) as the initial imaging study, including anteroposterior (AP), lateral, and oblique views to exclude fracture, hematoma with underlying injury, or other acute pathology. 1, 2

Initial Clinical Assessment

The rapid onset (2 days) and bruise-like pain suggest several key possibilities that require immediate evaluation:

• Traumatic hematoma - Most common cause of acute painful swelling with bruise-like quality, even without recalled trauma 2
• Occult fracture - May present with localized swelling and tenderness without obvious deformity 1
• Compartment syndrome - Rare but critical diagnosis requiring urgent recognition, presents with severe pain, swelling, and decreased range of motion 3
• Soft tissue mass - Less likely given the acute 2-day timeframe, but glomus tumors can present with focal tenderness and pain 4, 5

Diagnostic Imaging Algorithm

Step 1: Plain Radiographs (Initial Study)

Obtain standard three-view radiographs of the forearm immediately to assess for:

• Fractures or avulsion injuries at tendon/ligament attachment sites 1
• Joint effusions that may indicate occult fracture 1
• Soft tissue swelling patterns 2
• Any calcifications or bony abnormalities 6

The American College of Radiology designates radiographs as the appropriate initial imaging modality for acute forearm pain, as they provide good delineation of bony anatomy and are essential before considering advanced imaging 6, 2

Step 2: Advanced Imaging (If Radiographs Normal or Indeterminate)

If radiographs are normal but clinical concern persists:

• Ultrasound can be performed urgently (within 2 weeks) to assess soft tissue masses and differentiate benign from concerning lesions 6
• Ultrasound is particularly useful for identifying hematomas, confirming extravasations, and evaluating soft tissue pathology 6, 4

If fracture is suspected but radiographs are indeterminate:

• CT without contrast is the next appropriate study for identifying occult fractures 1

If vascular compromise is suspected (expanding hematoma, neurological deficit, or signs of compartment syndrome):

• CT angiography should be obtained urgently, with 96.2% sensitivity and 99.2% specificity for detecting vascular injuries 2

Critical Red Flags Requiring Urgent Evaluation

Watch for signs of compartment syndrome, which requires emergency fasciotomy:

• Severe pain out of proportion to examination 3
• Decreased range of motion of wrist and fingers 3
• Numbness or paresthesias 3
• Tense, swollen forearm compartments 3

Do not delay imaging if these signs are present - compartment syndrome can occur spontaneously without obvious trauma and requires immediate surgical decompression 2, 3

Management Based on Imaging Results

If hematoma is identified:

• Most hematomas resolve with conservative management (rest, elevation, ice) 6
• Surgical evacuation is indicated only if the hematoma is compromising vascular structures or causing compartment syndrome 6
• Avoid cannulation or aspiration of acute hematomas to prevent infection 6

If soft tissue mass is identified on ultrasound:

• Consider urgent referral (within 2 weeks) if ultrasound findings are suggestive of soft tissue sarcoma or uncertain with persistent clinical concern 6
• Most forearm masses are benign (lipomas most common), but increasing size warrants evaluation 6

If radiographs and ultrasound are normal:

• Reassure the patient and treat symptomatically with rest, elevation, and analgesics
• Schedule follow-up in 1-2 weeks to reassess if symptoms persist

Common Pitfalls to Avoid

• Never skip plain radiographs before ordering advanced imaging - this can lead to missed diagnoses and inappropriate management 2
• Do not assume all acute swelling is simple hematoma - occult fractures may only manifest as joint effusion on radiographs 1
• Do not miss compartment syndrome - this is a surgical emergency that can occur without obvious trauma and requires high clinical suspicion 3
• Remember that not all fractures are visible on initial radiographs - clinical correlation is essential, and repeat imaging or CT may be needed if suspicion remains high 1, 2