Diabetic Foot Infections & Ulcers

Epidemiology & Risk Factors

Pathogenesis: Triad of peripheral neuropathy (loss of protective sensation to minor trauma) + peripheral artery disease (PAD) (impaired healing) + immunopathy (hyperglycemia-induced impaired neutrophil function).
Risk Factors:
- Longstanding DM2/DM1 w/ poor glycemic control (HbA1c > 8-10%).
- Prior foot ulceration or lower extremity amputation.
- Diabetic peripheral neuropathy (distal symmetric polyneuropathy).
- Foot deformities (Charcot foot, hammer/claw toes, hallux valgus).
- Chronic kidney disease (CKD), smoking, or underlying PAD.
Microbiology:
- Superficial/Mild: Monomicrobial (S. aureus, S. pyogenes).
- Deep/Chronic/Severe: Polymicrobial (S. aureus, MRSA, Streptococci, Gram-negative bacilli [P. aeruginosa], anaerobes [Bacteroides]).

History:
- Painless skin break/ulcer (due to loss of pain sensation).
- Delayed recognition; foot deformities causing unnatural pressure points.
- Intermittent claudication or rest pain (if concomitant PAD).
Physical Exam:
- Location: Weight-bearing bony prominences (e.g., plantar 1st/5th metatarsal head, heel, tips of toes).
- Ulcer Appearance: Punched-out lesion surrounded by hyperkeratotic callus.
- Infection Signs: Erythema (>2 cm suggests moderate/severe infection), warmth, purulence, foul odor, crepitus (gas-forming anaerobes).
- Systemic Signs: Fever, chills, tachycardia, altered mental status (may be absent even in severe infection due to blunted immune response).
- Charcot Joint: Loss of sensation -> repeated trauma -> collapse of foot architecture -> “rocker-bottom” foot.

Screening/Bedside Tests:
- 10-g Semmes-Weinstein Monofilament Test: Evaluates loss of protective sensation (distal polyneuropathy).
- Probe-to-Bone (PTB) Test: Insert sterile metal probe into ulcer base; feeling hard, rigid bone strongly correlates w/ osteomyelitis (high PPV).
Key Labs:
- Inflammatory Markers: ESR > 70 mm/hr strongly supports osteomyelitis. Elevated CRP/WBC.
- Microbiology: Deep tissue culture obtained after debridement (superficial wound swabs are WRONG as they reflect colonizers).
- Blood cultures if septic/systemic signs present.
Imaging:
- Initial: Plain X-ray of foot to rule out subcutaneous gas, foreign body, cortical erosion, or periosteal reaction (osteomyelitis findings take 2-3 weeks to appear).
- Confirmatory / Gold Standard Imaging: MRI of the foot (most sensitive test for acute/early osteomyelitis and soft tissue abscess).
Vascular Assessment:
- Ankle-Brachial Index (ABI): Assess for PAD. Note: Calcification (Mönckeberg) can falsely elevate ABI (> 1.4); use Toe-Brachial Index (TBI) or arterial duplex US instead.

Venous Stasis Ulcer:
- Diff: Located over medial malleolus, irregular shallow borders, hyperpigmentation (hemosiderin deposition), stasis dermatitis, pain improved with leg elevation.
Arterial / Ischemic Ulcer:
- Diff: Located at distal extremities/tips of toes, extremely painful, cold leg, shiny pale skin w/ loss of hair, absent distal pulses, pain worsened by elevation.
Uninfected Neuropathic Ulcer:
- Diff: Ulcer w/ callus border, lack of surrounding erythema (< 0.5 cm), no purulence, normal ESR/CRP, negative PTB test.
Acute Charcot Arthropathy (Non-infected):
- Diff: Warm, red, edematous foot without open skin ulcer or purulent discharge; negative PTB test and ESR usually normal/mildly elevated.

Immediate Emergency / Systemic Stabilization:
- Hemodynamic support (IVF, broad-spectrum IV Abx) if septic.
Antibiotic Therapy:
- Mild Infection (superficial, local erythema < 2 cm): Oral Abx covering S. aureus & Streptococcus (e.g., Cephalexin, Augmentin; add Bactrim/Doxycycline for MRSA).
- Moderate/Severe or Deep Infection / Osteomyelitis: IV broad-spectrum empiric coverage against MRSA + Gram-negatives + Anaerobes (e.g., Vancomycin + Piperacillin-Tazobactam [Zosyn] or Vancomycin + Cefepime + Metronidazole).
- Adjust Abx based on deep tissue cultures; treat osteomyelitis for 4-6 weeks.
Surgical Intervention:
- Debridement: Operative clearance of necrotic tissue, callus, and infected bone (essential for healing).
- Drainage: Emergency incision & drainage for abscesses or gas gangrene.
- Amputation: Reserved for refractory osteomyelitis, extensive tissue necrosis, or non-revascularizable ischemia.
Offloading & Wound Care:
- Offloading: Pressure relief w/ Total Contact Casting (TCC) or specialized orthotic boots (gold standard for neuropathic ulcers).
- Moist wound environment dressings; hyperbaric oxygen therapy (refractory non-healing ulcers w/ severe ischemia).
Adjunctive Measures:
- Glycemic control (optimizing HbA1c < 7%).
- Vascular consult for revascularization (endovascular angioplasty or surgical bypass) if severe PAD present (ABI < 0.5 or non-healing ulcer).

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