What is the recommended imaging and treatment approach for a patient with cellulitis of the lower extremity, considering potential underlying conditions such as osteomyelitis, deep vein thrombosis, or necrotizing fasciitis, and possible past medical history of diabetes or immunocompromised status?

ER Imaging for Lower Extremity Cellulitis

Initial Imaging Approach

Plain radiographs should be the first-line imaging study for all patients presenting with lower extremity cellulitis in the emergency department. 1

When to Order Initial X-rays

Plain radiographs are indicated when any of the following are present:

• History of acute trauma to the affected extremity (6.24 times increased risk of positive findings) 2
• Presence of a chronic wound or ulceration (2.98 times increased risk of positive findings) 2
• Suspected underlying osteomyelitis (probe-to-bone test positive, exposed bone, chronic ulcer overlying bony prominence) 1, 3
• Need to exclude fracture, foreign body, or soft tissue gas 1
• Diabetic patients with foot wounds 1, 3

In uncomplicated cellulitis without trauma history or chronic wounds, plain radiographs are not necessary and should be avoided to reduce unnecessary testing and cost. 2

What X-rays Can Detect

Plain radiographs provide essential baseline information including:

• Soft tissue swelling and gas (may indicate necrotizing fasciitis) 1
• Radio-opaque foreign bodies 1
• Bone abnormalities (deformity, destruction, periosteal reaction) 1
• Alternative diagnoses (fracture, tumor, degenerative changes) 1

Important caveat: Early acute osteomyelitis (<14 days) may show only mild soft tissue swelling or appear completely normal on radiographs. 1

Advanced Imaging: When and What to Order

MRI - The Gold Standard for Suspected Complications

MRI is the imaging modality of choice when cellulitis is complicated or when deeper infection is suspected. 1

Order MRI when:

• Osteomyelitis remains suspected despite negative or equivocal radiographs 1, 4
• Soft tissue abscess is suspected 1
• Deep infection extent needs to be defined (fasciitis, myositis, tenosynovitis) 1
• Diabetic foot infection with probe-to-bone positive or exposed bone 1, 3
• Clinical concern for necrotizing fasciitis 1

MRI has 100% negative predictive value for excluding osteomyelitis - a normal marrow signal reliably rules out bone infection. 1

CT Imaging - Limited but Specific Indications

CT should be considered when:

• MRI is contraindicated (pacemaker, severe claustrophobia, non-MRI compatible implants) 1
• Soft tissue gas needs confirmation (CT is more sensitive than MRI for gas detection) 1
• Foreign body detection (CT superior to MRI for radiopaque foreign bodies) 1
• Chronic osteomyelitis evaluation (better visualization of sequestra and cortical bone) 1
• Guidance for abscess drainage 1

Ultrasound - Complementary Role

Ultrasound is useful for:

• Detecting and characterizing fluid collections/abscesses 1, 5
• Identifying radiolucent foreign bodies (wood, plastic) 1
• Guiding aspiration or drainage procedures 1
• Evaluating for joint effusions 1

Limitation: Ultrasound can underestimate disease extent and has limited visualization of deeper structures. 1

Risk Stratification for Osteomyelitis

High-Risk Features Requiring MRI

The incidence of osteomyelitis in patients with cellulitis varies dramatically based on clinical features:

• Uncomplicated cellulitis: 11.8% rate of osteomyelitis 6
• Complicated cellulitis (with ulceration): 43.9% rate of osteomyelitis 6
• Forefoot ulceration: 5.6 times increased likelihood of underlying osteomyelitis 6

Order MRI when these high-risk features are present:

• Ulceration present (strongest predictor) 6
• Diabetes mellitus 6
• Hyperlipidemia or atherosclerotic disease 6
• Chronic wound overlying bony prominence 1, 4
• Probe-to-bone test positive 1, 3

Critical Mimics to Exclude

Life-Threatening Conditions That Present Like Cellulitis

Always consider these high-mortality mimics that require urgent surgical intervention: 7

• Necrotizing fasciitis - Look for severe pain out of proportion, crepitus, skin necrosis, systemic toxicity; order CT or MRI urgently for soft tissue gas 1, 7
• Deep vein thrombosis - Consider in unilateral leg swelling; may require venous duplex ultrasound 7
• Septic arthritis - Joint effusion, severely limited range of motion; requires arthrocentesis 1, 7
• Pyomyositis - Muscle compartment involvement; MRI shows muscle edema and abscess 7, 5
• Flexor tenosynovitis - Kanavel's signs present; requires urgent surgical consultation 7

Special Populations

Diabetic Patients

All diabetic patients with new foot infections require plain radiographs at presentation to look for bone abnormalities, soft tissue gas, and foreign bodies. 1

Proceed to MRI if:

• Probe-to-bone test positive 1, 3
• Deep or large ulcer, especially if chronic 1, 3
• Ulcer overlies bony prominence 1, 4
• Plain radiographs show bone changes 1

Immunocompromised Patients

Lower threshold for advanced imaging (MRI) in immunocompromised patients due to:

• Higher risk of atypical organisms 1
• Rapid progression potential 7
• Increased risk of deep/necrotizing infections 7

Common Pitfalls to Avoid

• Relying on negative plain radiographs to exclude osteomyelitis - Early osteomyelitis (<2 weeks) is often radiographically occult; repeat films in 2-4 weeks or proceed directly to MRI if clinical suspicion is high 1, 4
• Ordering plain films in uncomplicated cellulitis without trauma or wounds - This adds unnecessary cost without clinical benefit 2
• Delaying MRI when high-risk features are present - In diabetic foot infections with ulceration, 43.9% have underlying osteomyelitis requiring different management 6
• Missing necrotizing fasciitis - Severe pain, rapid progression, systemic toxicity, and crepitus demand urgent CT or MRI and surgical consultation 1, 7
• Assuming all erythema and swelling is cellulitis - Always consider DVT, septic arthritis, and other mimics that require different treatment 7