Diabetic Foot Infections


Diabetic Foot Infections (DBI)

  1. Pathophysiology
    1. Neuropathic Ulcers
      • result from diabetic peripheral neuropathy and loss of sensation in the toes and feet
      • puncture wounds, lacerations, abrasions may go unnoticed in diabetics and result in injury, infection, and tissue loss
      • neuropathic arthropathy (Charcot’s foot) leads to muscle atrophy and malformation of the joints of the foot and toes, resulting in more surface ulceration and trauma

      Charcot Foot
    2. Ischemic Ulcers
      • arterial occlusive disease often coexists in patients with diabetes
      • diminished or absent pulses, hair loss, and a cool foot suggest an ischemic ulceration
      • ischemic ulcers commonly arise from multilevel occlusive disease
      • one or both tibial arteries are usually involved, as well as the aortoiliac or femoropopliteal segments
      • most diabetics ulcers result from a combination of neuropathic and ischemic components

  2. Microbiology
    • most DBIs are polymicrobial, with 5 – 7 different organisms involved
    • MRSA is a common pathogen
    • additional common pathogens include enterococci, gram-negative rods, pseudomonas, and anaerobes (clostridium species and anaerobic streptococci)

  3. Clinical Presentation
    • wide range of presentations, from chronic non-healing wounds to necrotic, infected, gangrenous wounds
    • erythema, tenderness, or edema suggests a deep space infection or osteomyelitis
    • elevated blood glucose levels should also prompt a search for infection

    Chronic Diabetic Plantar Foot Ulcer
  4. Evaluation
    1. Pulse Examination
      • used to determine the contribution of ischemia to the ulceration
      • if pulses are palpable at the ankle, the ulcer is likely to be neuropathic; if the femoral, popliteal, and pedal pulses are absent, then a significant ischemic component is present

    2. Noninvasive Studies
      • important complement to the physical exam
      • help to determine the degree of ischemic component present in the ulcer, and whether revascularization will improve wound healing

      1. Ankle-Brachial Index (ABI)
        • ABI < 0.9 is diagnostic of arterial occlusive disease
        • ABI < 0.4 is consistent with critical limb ischemia and poor wound healing
        • ABI > 1.3 suggests calcified non-compressible vessels and does not reflect perfusion – additional studies will be necessary

      2. Toe-Brachial Index (TBI)
        • more reliable indicator of perfusion in diabetics, because the small vessels are not calcified
        • normal TBI is 0.7 to 0.8
        • an absolute pressure > 45 mm Hg is required for wound healing in diabetics

      3. Transcutaneous Oxygen Measurement
        • measures the absolute value of oxygen tension
        • a normal value at the foot is 60 mm Hg
        • values < 20 mm Hg indicate severe ischemia, and revascularization will be necessary for wound healing

        Transcutaneous Oxygen Measurement
    3. Imaging
      1. X-rays
        • plain films are used to evaluate for bony deformity, foreign bodies, and gas in the soft tissues
        • findings suggestive of osteomyelitis include cortical erosion and periosteal reaction

        Gas Gangrene - Diabetic Foot
      2. MRI
        • superior to plain films for diagnosing osteomyelitis
        • findings characteristic of osteomyelitis include cortical destruction, bone marrow edema, and soft tissue swelling

    4. Lab Tests
      • hemoglobin A1c levels are an indicator of diabetic control
      • albumen > 3.5 predicts the ability to heal
      • total lymphocyte count > 1500 is also a predictor of healing potential
      • ESR and C-reactive protein are markers of inflammation and can be useful for monitoring response to therapy

  5. Management
    • chronic ulcers, infected ulcers, and gangrene of the foot all require a separate approach

    1. Chronic Ulcers
      • if no deep space infections are present, then minor wound debridement should be sufficient
      • topical proteases (Santyl) and wet-to-dry dressings may replace or augment surgical debridement
      • wound VACS are another useful healing adjunct

      1. Offloading
        • goal is to prevent further mechanical damage to injured tissue
        • specialized orthotic shoes are available for neuropathic or malformed feet
        • bed rest, foot elevation, and restricted weightbearing are additional offloading strategies in more serious wounds

        Offloading Devices
    2. Infected Ulcers
      • antibiotics, debridement, amputation, and revascularization may all play a role in management

      1. Antibiotics
        • broad-spectrum antibiotics should initially be chosen, since the infections are polymicrobial
        • cultures should be obtained in the OR by tissue biopsy
        • the diagnosis of osteomyelitis will require a bone biopsy
        • moderate to severe infections are treated for 3 weeks

      2. Debridement
        • source control of infection is the key step in management
        • devitalized tissue is poorly vascularized, and not penetrated by antibiotics
        • all deep space infections must be widely opened
        • multiple procedures may be necessary
        • in ambulatory patients, preserving as much viable tissue as possible is the second goal, after eradicating all infected and dead tissue

      3. Revascularization
        • if the patient has an ulcer with a significant ischemic component, and the patient is ambulatory, then revascularization should be considered once the infection is under control
        • goal is to restore pulsatile flow to the foot, usually by saphenous vein bypass to the anterior or posterior tibial arteries
        • adequate inflow must also be established at the level of the common femoral and deep femoral arteries
        • even after revascularization, diabetic foot ulcers are slow to heal – only 25% are healed within 6 months of surgery

      4. Amputation
        • many patients will not be candidates for attempted limb salvage
        • if the patient has no potential for ambulation, then amputation should be the primary procedure
        • if the patient has significant medical comorbidities, then amputation should be the primary procedure
        • extensive osteomyelitis will also often require an amputation

      5. Healing Adjuncts
        • hyperbaric oxygen therapy facilitates healing of diabetic ulcers
        • wound VACs also facilitate healing by improving local wound oxygenation, granulation, and contraction

    3. Gangrene
      1. Classification
        1. Dry Gangrene
          • not associated with infection
          • emergency amputation is not indicated
          • if left alone, autoamputation will occur

          Dry Gangrene - Foot
        2. Wet Gangrene
          • associated with bacterial infection
          • surgical emergency because sepsis and multiorgan failure may result

          Wet Gangrene - Foot
      2. Amputation
        • extensive gangrene or osteomyelitis of the foot will require some level of amputation
        • radical debridement will leave the patient with a nonfunctional foot

        1. Toe amputation
          • reserved for gangrene of the middle or distal phalanges
          • usually performed as a disarticulation between the proximal phalanx and metatarsal
          • for a great toe amputation, preservation of the proximal phalanx aids in balance and gait
          • can be done with a fish-mouth or racquet-type incision
          • not indicated if the metatarsal head or forefoot is infected

          Toe Amputation Incisions
        2. Ray Amputation
          • toe amputation with partial resection of the metatarsal
          • indicated when there is not enough viable tissue to cover a toe disarticulation
          • usually performed with a racquet-type incision

          Ray Amputation Incisions
        3. Transmetatarsal Amputation
          • indicated for forefoot gangrene that results in significant tissue loss
          • plantar flap is usually created longer than the dorsal flap

          Transmetatarsal Amputation Incision
        4. Below Knee Amputation
          • indicated when foot salvage is not possible
          • usually performed with a long posterior flap
          • anterior incision is made 10 cm distal to the tibial tuberosity and extended medially and laterally for two thirds of the circumference of the calf
          • posterior incision is made 12 – 15 cm below the anterior incision
          • the posterior muscle flap is created just deep to the tibia, and includes the soleus and gastrocnemius muscles
          • neurovascular bundles should be suture ligated and divided
          • a splint or knee immobilizer should be applied to prevent knee contracture
          • with a prosthesis, the energy cost of ambulation increases by 30% - 60%
          • in a septic patient, a guillotine amputation can be performed just above the ankle, and then revised to a formal below knee amputation after recovery

          Below Knee Amputation Incision
        5. Above Knee Amputation
          • indicated for extensive infection of the lower leg
          • also indicated in nonambulatory patients if healing of more distal amputations is unlikely
          • in an ambulatory patient, the longer the stump, the better
          • usually done with a fish-mouth incision
          • energy cost of ambulation increases by 60% - 100%, which, in practical terms, means that most diabetic patients will be nonambulatory

          Above Knee Amputation Incision






References

  1. Sabiston, 20th ed., pgs 1767 – 1771
  2. Cameron, 13th ed., pgs 1044 - 1054, 1101 - 1104
  3. UpToDate. Clinical Manifestations, Diagnosis, and Management of Diabetic Infections of the Lower Extremities. Amy C. Weintrob, Md, Daniel J. Sexton, MD. Jan 19, 2021. Pgs 1 – 32