Diabetic Foot Infections 

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 that run the spectrum from simple, superficial cellulitis to chronic osteomyelitis.

The radiograph below demonstrates a foot lesion in a patient with diabetes.

Infections in patients with diabetes are difficult to treat because these individuals 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. In addition, diabetic individuals can not only have a combined infection involving bone and soft tissue called fetid foot, a severe and extensive, chronic soft-tissue and bone infection that causes a foul exudate, but they may also have peripheral vascular disease that involves the large vessels, as well as microvascular and capillary disease that results in peripheral vascular disease with gangrene.^{[1, 2, 3, 4, 5]}

Except for chronic osteomyelitis, infections in patients with diabetes are caused by the same microorganisms that can infect the extremities of persons 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 these infections.

In general, foot infections in persons with diabetes become more severe and take longer to cure than do equivalent infections in persons without diabetes.

Staging in diabetic foot infections is applicable only in cases of chronic osteomyelitis that require surgery.

Go to Diabetes Mellitus, Type 1; Diabetes Mellitus, Type 2; Diabetic Foot; and Diabetic Ulcers to see more complete information on these topics.

In chronic osteomyelitis, a sequestrum and involucrum form; these represent islands of infected bone. Bone fragments that are isolated have no blood supply.

Bacteremia may accompany cellulitis, skin or soft-tissue infections, and/or acute osteomyelitis, but this is not a complication per se. If chronic osteomyelitis is left untreated for years, it may lead to complications such as amyloidosis or squamous cell carcinoma at the site of drainage through the skin. Bacteremia and septic shock rarely, if ever, occur as a result of chronic osteomyelitis.

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, which may involve the skin, soft tissues, bone, or all of these combined.

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 blood 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). Go to Diabetic Neuropathy for more complete information on this topic.

Microbial characteristics

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 Staphylococcus aureus. In unusual epidemiologic circumstances, however, 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 but not uncommon in patients with diabetes. In diabetic individuals, 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.

Furthermore, as previously mentioned, deep soft-tissue infections in diabetic persons 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 that in an individual without diabetes who has acute osteomyelitis. In chronic osteomyelitis, however, the pathogens include group A and group B streptococci, aerobic gram-negative bacilli, and Bacteroides fragilis.

Other pathogens implicated in chronic osteomyelitis in patients with diabetes include B fragilis, Escherichia coli, Proteus mirabilis, and Klebsiella pneumoniae.

Pseudomonas aeruginosa is generally not a pathogen in chronic osteomyelitis in these individuals. Although P aeruginosa is frequently cultured from samples obtained from a draining sinus tract or deep penetrating ulcers in patients with diabetes, 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 authors' knowledge, however, no well-documented cases of biopsy-proven P aeruginosa infection have been reported in patients with chronic osteomyelitis.

Fetid foot represents a combined deep-skin and soft-tissue infection caused by pathogens involved in chronic osteomyelitis.

Globally, diabetic foot infections are the most common skeletal and soft-tissue infections in patients with diabetes. The incidence of diabetic foot infections is similar to that of diabetes in various ethnic groups and most frequently affect elderly patients. There are no significant differences between the sexes.

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.

The prognosis for cases of cellulitis, skin and/or soft-tissue infections, and acute osteomyelitis depends on the adequacy of antimicrobial therapy and surgical debridement. For cases of chronic osteomyelitis, the prognosis is directly related to the vascular supply in the affected limb and the adequacy of surgical debridement.

Patients with diabetes must be careful to avoid foot trauma and to properly care for their feet to minimize the possibility of infection. In addition, they must understand that chronic osteomyelitis cannot be cured with antibiotics alone and that adequate surgical debridement is necessary.

Patients who are unwilling to undergo the surgical procedure must understand the long-term complications of chronic osteomyelitis. They should be advised that if the infection is not adequately treated with sufficient surgical debridement and/or amputation, systemic complications, including bacteremia and/or systemic infection, amyloidosis, and squamous cell carcinoma at the affected site, may occur over time.

Long-term suppressive therapy may decrease the incidence of septic complications, but it does not affect the long-term complications, which may include amyloidosis or squamous cell carcinoma at the drainage site.

For patient education information, see eMedicine's Diabetes Center, as well as Diabetic Foot Care.