#410 Diabetic Foot Infections

October 2, 2023 | By Monee Amin

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A Guide to Seeing Them and Stopping Them in Their Tracks

Be a step ahead of diabetic foot infections! Learn who really needs that MRI, when to call your vascular colleagues, and when you really need that pseudomonas and anaerobic coverage. We are joined by Dr. Andrew Webster, infectious disease physician and antimicrobial stewardship director at the Atlanta VA.  Emory Division of Infectious Diseases

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Show Segments

• Intro
• Picks of the Week
• Case
• Definitions
• Risk Stratifications
• Diagnostic Work-up
• Vascular evaluations
• Treatment
• Outro

Diabetic Foot Infection Pearls

1. Consistent classification of severity of  diabetic foot infections is important for management. These include  no infection, mild infection (superficial soft tissue), moderate infection (involves deeper structure(s), and severe infection (signs of sepsis and systemic manifestations).
2. X-ray can be specific if osteomyelitis is detected, but x-ray findings can lag behind.  MRI is useful when x-ray and other available features and labs are ambiguous.
3. The minimum evaluation for vascular disease in a patient with diabetic foot infection is a good history and physical (e.g., history of claudication, co-morbidities, tobacco use). Ultimately, this is still not enough to exclude PAD. Ideally, studies like ankle-systolic pressures or toe pressure index would be used to evaluate all patients with diabetic foot infections.
4. The WIfI scoring system (wound, ischemia and foot infection) can help predict who would benefit from revascularization and who is high risk for needing amputation.
5. The results of a superficial wound swab do not correlate with the pathogen causing infection so it is best to get a tissue culture at the time of debridement.
6. Signs of gangrene, undrained abscess or evidence of gas on imaging should prompt anaerobic coverage.
7. There are four principles that promote good wound healing. They are :  removal of  necrotic tissue (i.e., debridement), appropriate antibiotics, achieving adequate blood flow, and creating a good wound healing environment (e.g., offloading, tobacco cessation, good glycemic control).

Diabetic Foot Infections Show Notes

Defining and Classifying the Diabetic Foot Infection

A diabetic foot infection can be defined as a soft tissue defect or injury (often a diabetic foot ulcer or trauma) associated with classical signs of infection (i.e., redness, warmth, tenderness, swelling) in a patient with diabetes mellitus.  It is important to classify these infections into one of four categories of severity (IWGDF 2019):  1. No infection 2. Mild infection (confined to the superficial soft tissues and <2cm of erythema from the edge of the wound) 3. Moderate infection (involvement of the deeper structures and >2cm of erythema from the edge of the wound) 4. Severe infection (signs of sepsis or systemic inflammation)

Physical Exam Findings

Neuropathy and peripheral vascular disease are very common features among patients presenting with a diabetic foot infection (Curbsiders Episode #42).  Once a patient is presenting to the hospital with an infection however, the utility of these findings are less useful. Advanced complications of diabetes may also make the physical exam and diagnosis of a diabetic foot infection challenging as patients with significant neuropathy may not have pain and patients with significant peripheral vascular disease may not have typical erythema. Other markers that could be useful include ulcers that have been present more than thirty days and wounds that do not heal despite adequate wound care.

Imaging and Markers of Osteomyelitis

All patients presenting with a diabetic foot infection should have an X-ray of the affected foot (IDSA Guidelines Lipsky, et. al, 2012).  X-ray can be specific for the presence of osteomyelitis but sensitivity is limited by the fact that findings can lag behind by weeks.  MRI is useful when investigating for osteomyelitis or soft tissue abscess, particularly when other data (e.g., clinical features or labs) are ambiguous. Clinical features and/or labs that can be markers for osteomyelitis include: 1. X-ray with the aforementioned limitations, 2. Probe to bone test (relies on experience of performing clinician) 3. Elevated ESR has a sensitivity and specificity in the 80s for detecting osteomyelitis (Majeed, et. al, 2019). All of these can help determine the need for an MRI. 

Vascular Evaluation

The history and exam are not enough to exclude clinically significant peripheral vascular disease but should be the minimum that is performed.  Patients with normal exams may still have clinically significant vascular disease. Non-invasive vascular exams are preferred for patients in locations with the available resources, including ankle-systolic pressures, ABIs (may have false negatives), toe pressure index or transcutaneous oximetry (APMA/SVM Guidelines Hingorani, et. al, 2016).  The WIfI (Wound, Ischemia, and Foot Infection) scoring system uses the results of the non-invasive vascular studies and other factors to help predict who would benefit from revascularization and who is at greater risk for amputation (JVS Practice Guideline Mills, Sr., et. al, 2014).  Clinicians can synthesize this information to discuss severity of illness and possible treatment plans with their patients.

Cultures and Antibiotics

Cultures

The results of a superficial wound swab do not correlate with the pathogen causing infection. Clinicians should avoid superficial swab specimens and opt for a specimen obtained from deep tissue, often at the time of debridement (IDSA Guidelines Lipsky, et. al, 2012).

Principles of Antimicrobial Therapy

In a clinically stable patient, it is acceptable to hold empiric antibiotics until cultures are obtained.  Dr. Webster often bases the decision to delay empiric antibiotics based on the patient’s clinical exam instead of a defined length of time. Antibiotics should be started in the setting of severe infection and/or a significant clinical extent of the infection.

Severity and classification of the infection can serve to aid in choice of empiric antibiotics.  A patient presenting with a mild diabetic foot infection, would typically include coverage for Staphylococcus and Streptococcus spp. For patients with severe infection, the recommendation is to proceed with broad coverage (i.e. anti-MRSA and/or anti-pseudomonal coverage). Risk factors for MRSA include: prior MRSA infection, prior antibiotic use, previous hospitalization, and residence in long-term care facility. Risk factors for pseudomonas include: warmer climates, immunocompromised patients, and recent antibiotic exposure. Anaerobic coverage is clearly indicated in presence of gangrene, an undrained abscess, or with

evidence of gas on imaging.  

Duration of antibiotics for diabetic foot infection is based on the extent of infection and type of debridement done.  In patients with mild diabetic foot infection, two weeks of therapy is often adequate.  For patients with moderate diabetic foot infection, duration can vary based on whether there is associated osteomyelitis or not and the extent of debridement. Patients that have infection extending down to bone with an adequate debridement can receive 4 weeks of therapy.  If debridement can not be performed or is incomplete, extending the duration of therapy to 6 weeks is a common approach.  If all infected bone is clearly removed, as in the case of most amputations, then 48 hours of antibiotics post-operatively is sufficient

Oral antibiotics were found to be non-inferior to intravenous antibiotics for complex orthopedic infections (OVIVA Trial Li, et. al, 2019). However, Dr. Webster cautions applying this study to diabetic foot infections as it looked at complex orthopedic infections.  He does use oral antibiotics to treat even more serious infections such as osteomyelitis, particularly when he has good culture data. 

Role of Offloading and Other Recommendations

There are at least 4 things that patients need to heal their wound:  1. Debridement  2. Appropriate antibiotics 3. Adequate blood flow 4. A good “wound healing environment” (i.e., offloading, following post operative instructions, patient understanding of weight bearing advice, tobacco cessation, and glycemic control) per Dr. Webster.

Anticipatory Guidance at Hospital Discharge

Dr. Webster focuses on the 4 things mentioned previously in addition to patient education about watching for signs of infection, signs of wound dehiscence and anticipatory guidance about any antibiotics the patient is being discharged on.  Counseling patients about expected side effects of antibiotics or potential complications such as C. difficile also plays an important role.  Emphasizing glycemic control to patients should not be overlooked as part of the discharge process as well.

Follow-up and a Multi-disciplinary Approach

A multi-disciplinary approach to follow-up is important for patients with diabetic foot infection.  They often need follow-up with surgery, longitudinal wound care, infectious diseases and potentially others.

Links

1. The Baseball 100 (book)
2. Project Hail Mary (book)
3. Chemistry (album)
4. Loving Sports When They Don’t Love You Back (book)

Goal

Listeners will gain a comprehensive overview of diabetic foot infection, including its causes, presentation, diagnosis, and treatment.

Learning objectives

After listening to this episode listeners will…  

1. Define diabetic foot infection.
2. Describe the clinical presentation of diabetic foot infection.
3. Discuss the diagnosis and management of diabetic foot infection.
4. Review the utility  for superficial wound cultures in patients presenting with diabetic foot infection.
5. Identify which patients should undergo vascular evaluation and what the preferred approach would be.

Disclosures

Dr. Webster reports no relevant financial disclosures. The Curbsiders report no relevant financial disclosures. 

Citation

Blackburn Jr., R. MD, Webster, A MD, Amin, M MD, Trubitt M MD. “#410 Diabetic Foot Infections”. The Curbsiders Internal Medicine Podcast. thecurbsiders.com/category/curbsiders-podcast Final publishing date October 2, 2023.