How to make a diagnosis of Charcot Neuro-osteoarthropathy (CNO) in an elderly diabetic female patient with hypertension (High Blood Pressure), hyperlipidemia (Elevated Lipid Levels), and previous knee replacement?

Diagnosing Charcot Neuro-Osteoarthropathy in an Elderly Diabetic Patient

Suspect active Charcot neuro-osteoarthropathy immediately when this patient presents with a unilateral warm, swollen, or red foot with intact skin, and initiate knee-high immobilization/offloading while pursuing diagnostic confirmation. 1

Clinical Presentation to Recognize

Always consider active CNO when three elements coexist: diabetes mellitus, peripheral neuropathy, and clinical signs of inflammation (increased temperature, edema, and/or redness) in one foot compared to the contralateral foot, with intact skin. 1

• Pain may be absent or surprisingly mild due to sensory neuropathy, so do not rely on pain as a diagnostic criterion 1
• The diagnosis is frequently missed or delayed because the presentation mimics cellulitis, DVT, gout, or osteoarthritis 2, 3
• This patient's comorbidities (hypertension, hyperlipidemia, previous knee replacement) do not exclude CNO and may actually increase risk through vascular compromise 3

Immediate Clinical Assessment

Measure skin temperature differences between both feet using infrared thermometry at corresponding anatomic points. 1, 4

• A temperature difference >2°C between the affected and unaffected foot at the same anatomic point strongly suggests active CNO 4
• Use the highest temperature on the affected foot or ankle and compare it to the identical location on the contralateral foot 1
• Document these measurements with a standardized approach to allow accurate comparison over time 1

Critical action: Initiate knee-high immobilization/offloading immediately upon clinical suspicion, before imaging confirmation. This is a Strong recommendation despite low-quality evidence because withholding offloading risks irreversible joint destruction, deformity, ulceration, and amputation. 1, 4

Imaging Algorithm

Step 1: Plain Radiographs (First-Line)

Obtain bilateral weight-bearing plain X-rays of both feet and ankles for comparison. 1, 4

• Foot views required: anteroposterior (AP), medial oblique, and lateral projections 1
• Ankle views required: AP, mortise, and lateral projections 1
• Weight-bearing films are strongly preferred because they reveal dynamic abnormalities (joint malalignment, subluxation, fracture displacement) not visible on non-weight-bearing films 1, 4
• If the patient cannot bear weight due to immobilization, non-weight-bearing radiographs are acceptable but less informative 1

Step 2: MRI if Radiographs Are Normal

If plain X-rays appear normal but clinical suspicion remains high (warm, swollen foot with temperature difference >2°C), perform MRI immediately. This is a Strong recommendation with Moderate evidence. 1, 4

• MRI is the gold standard for diagnosing early-stage active CNO before radiographic changes appear 1, 4
• MRI can distinguish CNO from osteomyelitis, which is critical in diabetic patients 1

Step 3: Alternative Imaging if MRI Unavailable

If MRI is unavailable or contraindicated (e.g., pacemaker, previous knee replacement with certain metal components), consider nuclear imaging (bone scintigraphy), CT scan, or SPECT-CT to support the diagnosis. 1

Laboratory Testing: What NOT to Order

Do not use C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), white blood count, alkaline phosphatase, or other blood tests to diagnose or exclude CNO in a patient with intact skin. 1

• These inflammatory markers lack diagnostic accuracy for CNO and may mislead clinicians toward infectious etiologies 1
• Blood tests are only useful if infection is suspected (ulcerated skin, purulent drainage) 1

Diagnostic Confirmation

Active CNO is confirmed when clinical inflammation (warmth, swelling, temperature elevation >2°C) occurs together with imaging abnormalities showing bone and/or joint involvement. 4

• The site of maximum skin temperature difference correlates with radiographic abnormalities in 92% of cases at diagnosis 1
• Skin temperature serves as a proxy measure of underlying active disease process 1

Common Diagnostic Pitfalls

• Assuming all foot swelling in diabetics is infection: CNO presents with intact skin, whereas infection typically involves ulceration or purulent drainage 1, 2
• Waiting for radiographic changes before treatment: Early CNO may have normal X-rays; MRI is required for confirmation, but offloading must begin immediately based on clinical suspicion alone 1, 4
• Attributing symptoms to osteoarthritis or recent knee surgery: The unilateral warmth and temperature asymmetry distinguish CNO from degenerative joint disease 1, 2
• Delaying specialist referral: Prompt referral to a podiatrist, orthopedic surgeon, or endocrinologist experienced in CNO management is essential to prevent deformity 2, 5

Summary of Diagnostic Approach

1. Clinical suspicion: Diabetic patient with neuropathy + warm/swollen foot with intact skin 1, 4
2. Temperature measurement: >2°C difference between feet 4
3. Immediate offloading: Knee-high immobilization device while awaiting imaging 1, 4
4. Bilateral weight-bearing X-rays: AP, oblique, lateral views of feet and ankles 1, 4
5. MRI if X-rays normal: Strong recommendation to confirm or exclude CNO 1, 4
6. Avoid blood tests: CRP, ESR, WBC do not help diagnose CNO with intact skin 1