The Diabetic Foot: Complications, Part 2

Infection in the Diabetic Foot M E Edmonds MB BS MRCP Lecturer, Diabetic Department King’s College Hospital, Denmark Hill, London, SE5 9RS

Introduction The most important complication of

the diabetic foot is infection. The foot is the most frequent site for infection in diabetics. Sepsis is responsible for considerable tissue destruction in both neuropathic and ischaemic feet. It is the responsibility of all who care for the diabetic patient to detect infection early and treat it aggressively.

Predisposing factors Impaired resistance to infection

It is commonly stated that patients with diabetes mellitus are more susceptible to infection, but this is controversial. The reaction of the diabetic to an infection may be altered by virtue of defects in the host defence mechanism. A number of abnormalities in white cell function have been reported and their ability to kill bacteria is reduced, especially in the presence of ketosis. Furthermore, high concent- rations of glucose support the growth of bacteria.

Neuropathy Neuropathy leads to loss of pain

sensation. Pain is one of the cardinal symptoms of infection and its loss will reduce the ability of the diabetic to detect minor injuries and the initial stages of sepsis. Furthermore, its absence in the diabetic foot will allow infections to involve deep tissues before medical attention is requested.

lschaemia Reduced blood supply compromises

the integrity of the tissues. Minor injuries

are difficult to heal and are quickly followed by necrosis. Ischaemic tissue supports the growth of micro-organisms and predisposes to infection.

Bacteriology Infection develops in the foot through defects in the covering skin, which allow the penetration of micro-organisms. Bacteria may gain entry from any ulcerated area, from fissures in the skin and from the nail bed after fungal infection. The bacteria can be divided into two groups: aerobic and anaerobic organisms. The aerobes need oxygen to survive whereas the anaerobes can flourish in deep tissue without oxygen.

Aerobic Bacteria The most common organism infecting

the diabetic foot is the Staphylococcus aureus. Staphlococcus aureus is carried on the nose and skin as part of the normal flora, but diabetics have an impaired response to infection by this organism.

Streptococci are responsible for rapidly spreading sepsis in the diabetic foot. These organisms release special substances which cause increased capillary permeability and swelling of the foot. They also lead to thrombosis of the small vessel of the feet, with resulting tissue destruction and eventual gangrene - so called diabetic gangrene (Fig I). Occasionally, both staphylococci and streptococci are present together and combine to produce a rampant cellulitis that extends rapidly through the foot, producing marked necrosis within only a few hours (Fig 2).

Anaerobic Organisms Anaerobic organisms flourish in deep-

seated infections. Organisms such as Bacteroides often remain localised, but others can spread rapidly. The latter include Clostridial organisms, such as Clostridium perfringens, which produce gas in the tissues and lead to ‘gas gangrene’, and non-Clostridial organ- isms such as anaerobic streptococci.

Both aerobic and anaerobic organisms can rapidly infect the bloodstream and result in life-threatening bacteraemias.

Fungi Fungal infections are less common

than bacterial sepsis and d o not cause systemic upset. However, infections of the toe nails (tinea unguium) and inter- digital spaces (tinea pedis) by such fungi as Trichophyton and Candida albicans can serve as portals of entry for bacterial organisms.

Presentation The trophic ulcer, which develops

most commonly at the tips of the toes or under the metatarsal heads, is the most frequent site of infection in the neuro- pathic foot. Ulceration usually develops under callous tissue. Fissuring in the callous allows skin organisms to enter and this leads to sepsis, with discharge of pus from the ulcer. When the ulcer is at the tip of the toe, that digit becomes red a n d swollen a n d sausage-shaped. Oedema then spreads to the foot and first appears on the dorsum of the webspace, as the lymphatic channels drain from the toe in this direction. Occasionally, a

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discrete abscess may form over the dorsum of the foot, which becomes red and oedematous (dorsal foot phlegmon). If the small vessels in this area become occluded, the skin and subcutaneous tissue becomes necrotic and there is extensive tissue destruction.

Infection of the perforating ulcer beneath the metatarsal heads quickly tracks to underlying fascia, tendon bone and joint, with the development of septic arthritis. Abscesses form in the deep tissues of the foot, and swelling and erythema develop in the region of the

longitudinal arch. Widespread infection leads t o

cellulitus of the foot and streaks of inflamed lymphatic channels may spread up the leg. Lymph node enlargement can develop a t the groin. In the majority of. these infective conditions, pain and tenderness are absent although patients often notice a burning sensation or dull ache especially when pus is present and u n d e r tens ion . Indeed t h e f i rs t abnormality noted by the patient may be a putrid odour from the foot, especially when anaerobes are present.

Figure 1. Gangrene of 3rd toe in Neuropathic foot with palpable pulses

Infection in the Diabetic Foot

R a m p a n t infect ion leading t o thrombosis of small vessels results in tissue necrosis and gangrene. Gas gangrene leads to skin necrosis and subcutaneous gas which can be detected by palpation of the foot.

Systemically, the patient may develop a high fever and become ill. His diabetes becomes difficult to control. Often, the initial presentation of sepsis in the foot may be an unexplained rise in blood glucose. In these circumstances, a careful examination of the feet will usually reveal the cause of loss of diabetic control.

RBC Se&s

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Management - ‘Finding the organism’

Every effort must be made to detect the organism responsible for the infection, before antibiotic therapy is administered. Often, treatment of the infected diabetic foot is a protracted exercise and it is vital at the outset to know the infecting organism. A bacteriological swab should be taken from the floor of an ulcer, after the callous has been removed. If surgical drainage is required, a swab should be taken from the deep tissues. Aerobic and anaerobic cultures should be made. If there is evidence of cellulitis, blood cultures should also be performed.

Treatment Infection in the diabetic foot should be

treated aggressively. Control of the diabetes should be meticulous.

A superficial ulcer, with slight discharge of pus, may be treated as an outpatient case. Oral antibiotics should be prescribed according to the organism isolated, until the ulcer has healed. The patient can be instructed to carry out daily dressings of the ulcer. A simple non- stick dressing should be used, after cleaning the ulcer and surrounding tissue with an antiseptic such as Savlodil.

If cellulitis is present, the limb is th rea tened a n d urgent h o s p i t a l admission should be arranged. The limb should be rested and intravenous anti- biotics administered. These comprise benzylpenicillin 1 mega unit six-hourly, flucloxacillin 500mg six-hourly and metronidazole Ig per rectum eight- hourly. Blood glucose may need to be controlled with an insulin pump.

In the neuropathic foot, it is important that all necrotic tissue be removed and abscess cavities drained surgically. If gangrene has developed in a digit, a ray amputation to remove that toe and part of the associated metartarsal is necessary and is usually very successful. Post- operatively, wounds should be irrigated four times daily with 2% Milton solution and antibiotics continued until complete healing has occurred. Rampant infection in an ischaemic foot is life threatening and usually a below-knee amputation is required. This is mandatory if gas gangrene is present.

Tinea pedis should be treated with a

FIBC SeriQS Infection in the Diabetic Foot

Figure 2. Cellulitis and Necrosis in Neuropathic foot

topical antifungal agent, active against severe infections, patients need urgent dermatophytes. Whitfield's ointment or admission to hospital, intravenous clotrimazole are often successful. Foot infections threaten both limbs antibiotics and early surgery to drain pus. Candida infections should respond to and life of diabetic patients. Early and Only thus will thenurnberofarnputations topical miconazole. aggressive treatment is necessary. In be reduced in the diabetic foot.

Conclusion

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