Ankle Redness, Swelling, and Tenderness: Differential Diagnoses and Diagnostic Approach

Immediate Life- and Limb-Threatening Diagnoses to Exclude

The American College of Radiology recommends considering septic arthritis, osteomyelitis, soft tissue infection, and Charcot neuro-osteoarthropathy in patients with ankle redness, swelling, and tenderness, as these conditions require immediate exclusion due to their potential for severe consequences 1, 2, 3
Septic arthritis requires at least two of the following: local swelling, erythema, local tenderness, local warmth, or purulent discharge, and the presence of fever, elevated white blood cell count, or elevated C-reactive protein suggests infection 3, 4
Joint aspiration with culture is often necessary for definitive diagnosis of septic arthritis, as imaging cannot reliably distinguish infected from noninfected joints 1, 2
Osteomyelitis may present with redness, swelling, and tenderness overlying bone, and early acute osteomyelitis may show only mild soft tissue swelling on radiographs 1, 2
Soft tissue infection may be indicated by soft tissue swelling, effacement of fat planes, gas, and foreign bodies on imaging 1, 2
Charcot neuro-osteoarthropathy should be suspected in patients with diabetes and neuropathy presenting with a unilateral red, warm, swollen foot, and temperature difference ≥2°C between the affected and unaffected foot strongly suggests active CNO 3

Trauma, including fracture and sprain, should be considered in the differential diagnosis, and the Ottawa Ankle Rules can be applied to determine the need for radiographs 6
Gout should be considered in patients with acute onset, severe pain, and history of hyperuricemia or prior attacks 3
Deep venous thrombosis must be excluded with appropriate clinical assessment and D-dimer/ultrasound if indicated 3

Initial clinical assessment should include measurement of skin temperature using infrared thermometry and palpation for point tenderness 3
Plain radiographs of the ankle should be obtained as the first imaging study for initial evaluation 1, 2, 6
Advanced imaging, such as MRI or CT, may be necessary if initial radiographs are normal but symptoms persist 1, 2, 3

Delaying immobilization in diabetic patients with neuropathy and a hot swollen foot can have devastating consequences 3
Assuming the absence of pain rules out serious pathology in diabetic patients with neuropathy can lead to delayed diagnosis 3
Relying solely on imaging to distinguish infected from noninfected joints can lead to misdiagnosis 1, 2

Immediate subspecialist referral is recommended for suspected active Charcot neuro-osteoarthropathy to prevent progression to deformity and ulceration 3
Referral is also recommended for suspected osteomyelitis, septic arthritis, or other conditions that require urgent intervention 1, 2, 3