Introduction to Diabetic Foot

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.

A diabetic foot is a foot that exhibits any pathology that results directly from diabetes mellitus or any long-term (or “chronic”) complications of diabetes mellitus. Presence of several characteristic diabetic foot pathologies is called diabetic foot syndrome.

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.

Etiology
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.

Risk factors
Two main risk factors that cause diabetic foot ulcer are Diabetic Neuropathy and micro as well as macro ischemia. Diabetic patients often suffer from diabetic neuropathy due to several metabolic and neurovascular factors. Type of neuropathy called peripheral neuropathy causes loss of pain or feeling in the toes, feet, legs and arms due to distal nerve damage and low blood flow. Blisters and sores appear on numb areas of the feet and legs such as metatarso-phalangeal joints, heel region and as a result pressure or injury goes unnoticed and eventually become portal of entry for bacteria and infection.

Microbial Organisms involved in Infection
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.

Deep soft-tissue infections in diabetic persons can be associated with gas-producing, gram-negative bacilli.

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.

Patient education
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.

Clinical Presentation
Local trauma and/or pressure (often in association with lack of sensation because of neuropathy), in addition to microvascular disease, may lead to a diabetic foot infection. However, patients may not necessarily have a history of trauma or have suffered a previous infection.

Cellulitis
Cellulitis may involve tender, erythematous, nonraised skin lesions on the lower extremity that may or may not be accompanied by lymphangitis.

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.

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 osteomyelitisIn chronic osteomyelitis, the patient’s temperature is usually less than 102°F. Discharge is commonly foul. No lymphangitis is observed, and pain may or may not be present, depending on the degree of peripheral neuropathy.

The deep, penetrating ulcers and deep sinus tracts (which are diagnostic of chronic osteomyelitis) 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.

Treatment and Management
Cellulitis is the easiest diabetic foot infection to cure, because it does not pose the same circulatory limitations that the more serious infections do, making it easier for medications to reach the infection site. In contrast, chronic osteomyelitis, which is the most difficult diabetic foot infection to cure, requires surgical debridement before antibiotic therapy can be effective. The patient may participate in activities as tolerated. However, weight bearing may be contraindicated.

Infections in patients with diabetes are difficult to treat because they 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.

Adequate surgical debridement, in addition to antimicrobial therapy, is necessary to cure chronic osteomyelitis. Immobilization is important in acute or chronic osteomyelitis.

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.