What the photo shows

• A large, full-thickness ulcer on the plantar surface of the mid and forefoot, over a weight-bearing area.
• A black necrotic edge, with slough and dark devitalised tissue in the base.
• White fibrous strands in the base that look like exposed tendon or plantar fascia, so the wound goes deep.
• A raised, peeling flap of skin at the margin.
• Dark discolouration near the toe web and the edge of the wound, which suggests early gangrene.
• No obvious surrounding cellulitis is visible, but a photo can't exclude it.

Why it likely happened

1. Neuropathy. Loss of sensation lets a small wound, blister or pressure point go unnoticed. The patient keeps walking on it, and the injury progresses.
2. Repetitive plantar pressure. Causes include poorly fitting footwear, walking barefoot, foot deformity or callus. A callus that breaks down leads to ulceration.
3. Ischaemia. Peripheral arterial disease slows healing and causes the necrosis and gangrene seen here. Check pulses and ABI, and assess for critical limb ischaemia.
4. Infection. Deeper infections are usually polymicrobial, with S. aureus, streptococci, gram-negative bacilli (Pseudomonas is over-represented) and anaerobes. Infection spreads along tendon sheaths and can reach bone.
5. Contributing factors. These include poor glycaemic control, delayed presentation, smoking, CKD and malnutrition.

Differentials to exclude

• Leprosy with plantar trophic ulcers, which is relevant in India.
• Non-diabetic critical limb ischaemia or gangrene.
• Necrotizing soft tissue infection. Look for crepitus, rapid spread, severe pain or systemic toxicity.
• Pressure injury or an old traumatic or burn wound.
• Vasculitis, or a malignant ulcer (Marjolin's ulcer), if the ulcer is long-standing.

Suggested work-up and management

• Bedside assessment: Check glucose and HbA1c, FBC, CRP, renal function and blood cultures, and look for sepsis. Do a probe-to-bone test, check pulses and ABI, and consider Doppler or angiography.
• Imaging: Take X-rays for gas, foreign body and osteomyelitis. MRI is the most sensitive test for bone involvement (Bailey & Love, p. 627 to 628).
• Antibiotics: For severe infection, start broad-spectrum IV cover that includes Pseudomonas and anaerobes. Rosen's gives piperacillin-tazobactam plus vancomycin as an example regimen. Take deep tissue cultures, because superficial swabs are unreliable.
• Surgery: Urgent surgical debridement of the necrotic tissue and any collections. Early vascular review for revascularisation. Amputation of the toe or part of the foot may be needed if the tissue is non-salvageable, and up to one-third of patients with deep infection end up having one (Rosen's, p. 2539 to 2540).
• Supportive care: Tight glycaemic control, strict offloading and non-weight-bearing, and regular wound care.