eMedicine Specialties > Infectious Diseases > Skin and Soft-Tissue Infections

Diabetic Foot Infections

Author: Burke A Cunha, MD, Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital
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Updated: Aug 12, 2009

Introduction

Background

Foot infections are the most common problems in persons with diabetes. These individuals are predisposed to foot infections because of a compromised vascular supply secondary to diabetes. Local trauma and/or pressure (often in association with lack of sensation because of neuropathy), in addition to microvascular disease, may result in various diabetic foot infections. For additional information, see Medscape’s Diabetic Microvascular Complications Resource Center.

The spectrum of foot infections in diabetes ranges from simple superficial cellulitis to chronic osteomyelitis. Infections in patients with diabetes are difficult to treat because these patients have impaired microvascular circulation, which limits the access of phagocytic cells to the infected area and results in a poor concentration of antibiotics in the infected tissues. For this reason, cellulitis is the most easily treatable and reversible form of foot infections in patients with diabetes. Deep skin and soft tissue infections are also usually curable, but they can be life threatening and result in substantial long-term morbidity.

In terms of the infecting microorganisms and the likelihood of successful treatment with antimicrobial therapy, acute osteomyelitis in people with diabetes is essentially the same as in those without diabetes. Chronic osteomyelitis in patients with diabetes mellitus is the most difficult infection to cure. Adequate surgical debridement, in addition to antimicrobial therapy, is necessary to cure chronic osteomyelitis.

Patients with diabetes also can have a combined infection involving bone and soft tissue called fetid foot. This extensive, chronic soft tissue and bone infection causes a foul exudate and usually requires extensive surgical debridement and/or amputation.

Individuals with diabetes may also have peripheral vascular disease that involves the large vessels, in addition to microvascular and capillary disease that results in peripheral vascular disease with gangrene. Dry gangrene is usually managed with expectant care, and gross infection is usually not present. Wet gangrene usually has an infectious component and requires surgical debridement and/or antimicrobial therapy to control the infection.

Except for chronic osteomyelitis, infections in patients with diabetes are caused by the same microorganisms that can infect the extremities of those without diabetes. Gas gangrene is conspicuous because of its low incidence in patients with diabetes, but deep skin and soft tissue infections, which are due to gas-producing organisms, frequently occur in patients with diabetes. In general, people with diabetes have infections that are more severe and take longer to cure than equivalent infections in other people.

Pathophysiology

Diabetes mellitus is a disorder that primarily affects the microvascular circulation. In the extremities, microvascular disease due to "sugar-coated capillaries" limits the blood supply to the superficial and deep structures. Pressure due to ill-fitting shoes or trauma further compromises the local blood supply at the microvascular level, predisposing the patient to infection. The infection may involve the skin, soft tissues, bone, or all of these tissues.

Diabetes also accelerates macrovascular disease, which is evident clinically as accelerating atherosclerosis and/or peripheral vascular disease. Most diabetic foot infections occur in the setting of good dorsalis pedis pulses; this finding indicates that the primary problem in diabetic foot infections is microvascular compromise. Impaired microvascular circulation hinders white cell migration into the area of infection and limits the ability of antibiotics to reach the site of infection in an effective concentration. Diabetic neuropathy may be encountered in conjunction with vasculopathy. This may allow for incidental trauma that goes unrecognized (eg, blistering, penetrating foreign body).

In chronic osteomyelitis, a sequestrum and involucrum form; these represent islands of infected bone. Bone fragments that are isolated have no blood supply. Administered antibiotics do not penetrate the devascularized infected bone fragments; they can enter the area of osteomyelitis only via the remaining blood supply. Therefore, antibiotic therapy alone cannot cure patients with chronic osteomyelitis without surgical debridement to remove these isolated infected elements. Surgical debridement is essential to remove the infected bony fragments that the antibiotics cannot reach so that affected areas can be treated with antimicrobial therapy.

Frequency

International

Diabetic foot infections range from cellulitis to chronic osteomyelitis, and, globally, they are the most common skeletal and soft tissue infections in patients with diabetes.

Mortality/Morbidity

Mortality is not common, except in unusual circumstances. The mortality risk is highest in patients with chronic osteomyelitis and in those with acute necrotizing soft tissue infections.

Race

The incidence of diabetic foot infections is similar to that of diabetes in various ethnic groups.

Sex

No important sex differences exist.

Age

Diabetic foot infections most frequently affect elderly patients.

Clinical

History

Patients may or may not have a history of trauma or previous infection.

Physical

Findings after physical examination may include the following:

• Cellulitis
  • Cellulitis may involve tender and erythematous nonraised skin lesions on the lower extremity that may or may not be accompanied by lymphangitis.
  • Lymphangitis suggests a group A streptococcal etiology.
  • If bullae are present, Staphylococcus aureus is the most likely pathogen, but group A streptococci occasionally causes bullous lesions.
  • No ulcer or wound exudate is present in patients with cellulitis.
• Deep skin and soft tissue infections
  • Patients with deep skin and soft tissue infections may be acutely ill, with painful induration of the soft tissues in the extremity.
  • These infections are particularly common in the thigh area, but they may be seen anywhere on the leg or foot.
  • Wound discharge is usually not present. In mixed infections that may involve anaerobes, crepitation may be noted over the afflicted area.
  • Extreme pain and tenderness indicate the possibility of a compartment syndrome, which may be diagnosed with the aid of a CT scan. Similarly, extreme pain may be an indication of infection with clostridial species (ie, gas gangrene).
  • The tissues are not tense, and bullae may be present.
  • If a discharge is present, it is often foul.
• Acute osteomyelitis
  • Unless peripheral neuropathy is present, the patient has pain at the site of the involved bone.
  • Usually, fever and regional adenopathy are absent.
• Chronic osteomyelitis
  • In chronic osteomyelitis, the patient's temperature is usually less than 102°F.
  • Discharge is commonly foul.
  • No lymphangitis is observed.
  • Pain may or may not be present, depending on the degree of peripheral neuropathy.
  • The deep penetrating ulcers and sinuses are usually located between the toes or on the plantar surface of the foot.
  • In patients with diabetes, chronic osteomyelitis usually does not occur on the medial malleoli, shins, or heels.
  • Importantly, deep penetrating foot ulcers or deep sinus tracts are diagnostic of chronic osteomyelitis.

Causes

The microbiologic features of diabetic foot infections vary according to the tissue infected.

• In patients with diabetes, superficial skin infections such as cellulitis are caused by the same organisms as those in healthy hosts, namely group A streptococci and S aureus. However, in unusual epidemiologic circumstances, organisms such as Pasteurella multocida (eg, from dog or cat bites or scratches) may be noted and should always be considered.
• Group B streptococcal cellulitis is uncommon in healthy hosts and not uncommon in patients with diabetes. In people with diabetes, group B streptococci may cause urinary tract infections and catheter-associated bacteriuria in addition to cellulitis, skin and/or soft tissue infections, and chronic osteomyelitis. Such infections may be complicated by bacteremia.
• In patients with diabetes, deep soft tissue infections can be associated with gas-producing gram-negative bacilli. Clinically, these infections appear as necrotizing fasciitis, compartment syndrome, or myositis. Gas gangrene is uncommon in persons with diabetes.
• Acute osteomyelitis usually occurs as a result of foot trauma in an individual with diabetes. The distribution of organisms is the same as in an individual without diabetes who has acute osteomyelitis.
• In chronic osteomyelitis, the pathogens are group A and group B streptococci, aerobic gram-negative bacilli, and Bacteroides fragilis, among others.
  • Pseudomonas aeruginosa is generally not a pathogen in chronic osteomyelitis in patients with diabetes.
  • P aeruginosa is frequently cultured from samples obtained from a draining sinus tract or deep penetrating ulcers in patients with diabetes. However, these organisms are superficial colonizers and are generally not the cause of the bone infection.
  • Because Pseudomonas organisms are water-borne, superficial ulcers may be contaminated by bacteria in wet socks or dressings. To the author's knowledge, no well-documented cases of biopsy-proven P aeruginosa infection have been reported in patients with chronic osteomyelitis.
  • Bone biopsy performed under aseptic conditions in the operating room reveals that chronic osteomyelitis in patients with diabetes is not due to P aeruginosa.
  • B fragilis is an important bone pathogen in chronic osteomyelitis in patients with diabetes.
  • Other pathogens implicated in chronic osteomyelitis in patients with diabetes include Escherichia coli, Proteus mirabilis, and Klebsiella pneumoniae.
• Fetid foot represents a combined deep skin and soft tissue infection caused by pathogens involved in chronic osteomyelitis.

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