Soft Tissue & Bone Infections
Diabetic foot infections are the infectious diseases physician’s bread and butter. Patients with diabetes often have an associated immunodeficiency, neuropathy, and/or vasculopathy, all of which increase their risk for foot infections and osteomyelitis.
The biggest risk factors for infection include: an ulcer present for >1 month, PAD in the involved extremity, previous amputation, recurrent foot ulcers, posttraumatic foot wounds, walking barefoot, and the ability to probe to bone. The IDSA published guidelines on diabetic foot infections in 2012 and developed a classification system of diabetic foot infection severity, which are summarized in the table below.
|
Severity of infection |
Definition |
Treatment |
|
Uninfected |
No symptoms or signs of infection |
Local wound care, pressure off-loading. No antibiotics. |
|
Mild |
At least 2 of the following: - Purulence
Infection does not extend deeper than the subcutaneous tissues, erythema cannot extend more than 2 cm beyond the ulcer. |
Short course of oral antibiotics targeting streptococci and Staph. (Patients are not usually admitted for mild infections). |
|
Moderate |
At least 2 of the above, but erythema extends more 2 cm beyond the ulcer or deeper involvement (abscess, joint or muscle infection, fasciitis or osteomyelitis) present. Patient does not meet SIRS criteria. |
Deep tissue specimens (not superficial swabs) should be sent for culture PRIOR to start of antibiotics. Abx therapy to include coverage for gram-negative organisms, e.g., ceftriaxone. |
|
Severe |
SIRS criteria are met, or other evidence of systemic infection (confusion, acidosis, severe hyperglycemia, etc.) |
If possible, obtain blood cultures as well as deep wound cultures prior to empiric therapy with vancomycin + ceftriaxone + flagyl. |
A few key points from this table:
Empiric coverage for Pseudomonas is not recommended, unless there is culture data or other risk factors for true infection with Pseudomonas (such as a high local prevalence rate).
Notice that osteomyelitis falls under the “Moderate” category... Remember, most patients with diabetic foot infections (including osteomyelitis) are NOT septic, so don’t feel the need to pull the antibiotic trigger if imaging is suggestive of osteomyelitis! You have time. Get those deep tissue or bone cultures first.
Keep in mind that while all wounds are colonized with bacteria, the presence of infection is defined by ≥2 classic findings of inflammation (purulence, warmth, pain, swelling, and erythema) per the guidelines. Most diabetic foot infections are polymicrobial, with gram-positive cocci (GPCs) and Staphylococci being the most common causative organisms. Aerobic gram-negatives are often copathogens in chronic infections, and anaerobes may be copathogens in chronic, ischemic/necrotic wounds, or if a large abscess is present. Although there is no guideline for empiric Pseudomonas coverage, if your hospital has a high rate of Pseudomonas, then empiric coverage for a severe diabetic foot infection is reasonable.
A new diabetic foot infection should always be imaged to determine the extent of infection (with MRI being the most sensitive modality).
We’ll discuss osteomyelitis in detail after the next case, but let’s say you have a patient in whom you’re worried about diabetic osteomyelitis (OM):
Step 1: Even though diabetic osteomyelitis is usually considered “moderate” in severity, before you start any antibiotics, get blood cultures. (As we discussed above, hematogenous seeding accounts for ~20% of OM, and you may be able to avoid more invasive testing if a blood culture is positive!) A pre-treatment CRP and ESR are also useful to have.
Step 2: Get a deep culture, preferably of bone and culture if possible. (A bone biopsy is not absolutely required.) Remember, most patients with OM are not septic, so you have time to gather data. Superficial cultures or cultures from sinus tracts correlate poorly with deep cultures from bone.
Step 3: NOW you can consider starting empiric antibiotics if the infection is severe (you want to make sure to cover Staph as well as possible GPCs and GNRs, so this is typically vancomycin and ceftriaxone) while you wait on your cultures to result… and then narrow!
Taking care of diabetic foot infections requires a multidisciplinary approach, which is much easier said than done. Most diabetic foot infections require some surgical intervention, ranging from minor (debridement) to major (resection, amputation). Patients may require vascular procedures, wounds must be properly dressed and off-loaded of pressure, glycemic control needs to be optimized… in addition to regular follow-up.