Nondiabetic Foot Infections

Adolph Samuel Flemister Jr, MD

Dr. Flemister or an immediate family member serves as a board member, owner, officer, or committee member of the American Orthopaedic Foot and Ankle Society and the New York State Society of Orthopaedic Surgeons.

ABSTRACT

Foot and ankle infections in nondiabetic patients are relatively uncommon. Recognition of these infections requires a thorough history and physical examination, prompt ordering of appropriate laboratory and imaging studies, and a high index of suspicion. Prompt management with antibiotics and débridement to remove infected tissue are essential in the case of many deep infections. The biomechanics and weight-bearing characteristics of the foot and ankle should be considered during surgical débridement.

Introduction

Although majority of foot and ankle infections occur in patients with diabetic neuropathy, sometimes these infections develop in patients who do not have diabetes.¹^,² Foot or ankle infection can result from penetrating trauma, repetitive microtrauma, a compromised postoperative wound, or hematogenous spread. Foot or ankle infection can be devastating, regardless of whether the patient has diabetes, and potentially can lead to limb loss and permanent dysfunction. Prompt diagnosis and treatment are essential to prevent needless morbidity and prolonged disability.

History and Physical Examination

Patients with a foot or ankle infection have some combination of pain, swelling, and erythema, with or without an obvious wound. It is important to determine the duration of symptoms, the severity of pain, and the presence of constitutional symptoms such as fevers, chills, and malaise. Chronic renal insufficiency, diabetes mellitus, HIV disease, organ transplantation, or inflammatory arthropathy has the potential to compromise the patient’s immune system and ability to fight infection. Current tobacco use affects soft-tissue healing and may affect blood flow.³ It is important to determine whether the patient has a history of gout because the signs and symptoms of gout can mimic those of infection. The physical examination begins with the patient’s vital signs, including temperature, heart rate, and blood pressure. The patient’s overall mental status must be assessed. Tachycardia, hypotension, fever, and altered mental status should alert the provider to the possibility of early sepsis. The affected limb should be inspected for overall alignment, deformity, swelling, and erythema.

Erythema not resolved with limb elevation suggests the presence of an infection, such as cellulitis. The skin and interdigital areas of the foot must be closely inspected for calluses, blisters, fissures, and open wounds. The size and depth of a wound should be assessed as well as the presence, amount, and consistency of drainage. The risk of developing osteomyelitis is increased in a wound that can be probed to bone.⁴ Exposed tendons or ligaments should be identified. Fluctuant areas should be identified by palpation, with specific attention to gaslike crepitus, regardless of the presence of a wound. Careful inspection and palpation of the surrounding joints are essential to assessing joint range of motion and stability. Diminished range of motion or instability that compromises normal foot mechanics leads to overloading of other areas of the foot, and this repetitive microtrauma increases the risk of skin breakdown. Swollen and painful joints with suspected effusion suggest septic arthritis and may require aspiration.

It is important to perform a thorough neurovascular examination. Although diabetes mellitus is the common known cause of peripheral neuropathy in the United States, neuropathy can develop in patients without diabetes as a result of idiopathic factors or other known causes, such as alcohol abuse, chemotherapy, a viral infection, or a vitamin deficiency. It is essential to determine the patient’s gross sensation to light touch and the ability to feel a Semmes-Weinstein 5.07 monofilament.⁵ Motor function should be carefully evaluated. If pulses are not palpable, the ankle-brachial index and toe pressure should be measured. An ankle-brachial index of less
than 0.45 and an absolute toe pressure of 40 mm Hg or less means that the patient is at risk for poor wound healing and therefore requires a more extensive vascular workup.⁶

Imaging Studies

Plain radiography of the foot and ankle is the primary screening tool for infection. If possible, weight-bearing studies should be obtained to best evaluate bone and joint alignment. Radiographs may show soft-tissue gas or densities representing localized edema, gas gangrene, or abscess. Bony changes such as erosion, periosteal reaction, and frank destruction are late changes and indicate osteomyelitis. It is important to realize that negative plain radiographs do not eliminate the possibility of osteomyelitis.⁴ Bony change may not appear radiographically during the first 2 to 4 weeks of the acute stage of osteomyelitis (Figure 1).

CT is more sensitive than plain radiography for detecting early-stage bony erosion or destruction. CT also shows presence of air in the soft tissues and is used to locate abscesses. CT is most commonly used if MRI is contraindicated. Ultrasonography also is useful for detecting soft-tissue abscesses and can be used for image-guided aspiration.

MRI is an effective imaging study for evaluation of soft tissues, and it is the study of choice for detection of a fluid pocket such as an abscess. MRI also readily detects bone edema. However, bone marrow edema, shown by increased signal with T2 weighting and decreased signal with T1 weighting, is a nonspecific finding that also is seen in fracture, tumor, Charcot arthropathy, or bony overload related to poor mechanics. The sensitivity and specificity of MRI for detecting osteomyelitis varies among studies.⁴^,⁷^,⁸^,⁹

FIGURE 1 AP radiographs of the great toe showing a dorsal proximal interphalangeal ulcer in a 60-year-old patient (A) and destructive osteomyelitis 3 wk later (B).

Nuclear imaging can provide valuable information about the presence or absence of osteomyelitis.⁷^,⁹^,¹⁰ Triple-phase technetium Tc-99m bone scanning detects even subtle bony destruction but can provide a false-positive finding of infection in the presence of fracture, Charcot arthropathy, or stress-related bony changes. The sensitivity and specificity of nuclear testing for chronic osteomyelitis can be improved by combining the triple-phase scan with an indium-111-labeled white blood cell (WBC) scan.¹⁰

Laboratory Studies

Laboratory studies are an important adjuvant in the diagnosis and treatment of infection. A complete WBC count with differential, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) level should be obtained.¹¹ In an acute infection, the WBC count normally is elevated; an increase in neutrophils with left shift is noted. Patients who are older than 65 years or who are immunocompromised may have little or no elevation of the WBC count. The ESR and CRP level are general inflammatory markers that typically are elevated with infection but can be elevated with any inflammatory process or for several weeks after surgery. The ESR and CRP level are useful for monitoring the patient’s response to treatment because typically they return to normal with effective treatment of the infection.¹¹ The CRP level declines rapidly, but the ESR tends to remain somewhat elevated for several weeks, even with effective treatment.¹¹

The patient’s nutritional status can have a substantial effect on the response to treatment. Albumin, prealbumin, and transferrin levels all are measures of the patient’s nutritional status and should be obtained as part of the infection workup. A total lymphocyte count higher than 1,500 is a useful indicator of general health. A patient found to be nutritionally depleted should take nutritional supplements.¹²

The preferred tissue for culturing contains débrided soft tissue or fluid rather than tissue from a simple swab.¹³ Depending on the clinical indications, fluid aspirated from a septic joint should be sent for Gram staining and cultured for aerobic and anaerobic bacteria as well as fungi and acid-fast bacilli. The fluid itself should be sent for WBC count and the presence of crystals. A WBC count higher than 50,000 mL generally indicates an infection but may have to be repeated if this is early in onset of septic joint. The presence or absence of crystals is used to rule out a gouty process.³^,¹⁴

Nail Disorders

Infection surrounding the toenail bed is classified as an infected ingrown nail (paronychia), felon, or onychomycosis. An ingrown toenail is caused by a deformity of the nail bed or improper trimming. The irritated surrounding soft tissues become colonized with bacteria. Numerous bacteria have been implicated in such infections, including Staphylococcus aureus, Streptococcus, and Pseudomonas.¹⁵ Patients have a red, swollen, and draining area adjacent to the medial or lateral nail fold. Initial radiographs are taken to evaluate for osteomyelitis. In most patients, an early infection responds to local nail débridement. The use of oral antibiotics is not helpful. A resistant or recurrent infection may require partial or complete nail removal for permanent ablation.¹⁵^,¹⁶

Felon is a deeper infection occurring in the tissue septi of the distal pulp of the toe, with S aureus is the most common organism. Patients have a red, swollen, fluctuant area at the distal aspect of the toe. Surgical drainage is essential, followed by culture-specific antibiotic therapy. The wound should be left open and packed as needed, depending on its size. Commonly, these infections respond to an antibiotic that covers gram-positive organisms.

Onychomycosis is one of the most common diseases of the nails. The nails become thickened, discolored, and often brittle. The pathogens most often responsible for onychomycosis are dermatophytes, including Trichophyton rubrum and Trichophyton mentagrophytes.¹⁷ Candida and molds are less common causal agents. Onychomycosis can be difficult to treat and often several months of treatment are required. The systemic antifungal agents carry a risk of liver toxicity, but the use of topical medications may lead to a recurrence after discontinuance. In a healthy patient, onychomycosis rarely is more than a cosmetic concern.¹⁷ The thickened nails may catch on clothing and occasionally cause an ingrown toenail, but frequent débridement usually is sufficient for controlling such issues.

Soft-tissue Infections

Cellulitis is an infection of skin and subcutaneous tissues. One common cause is contamination of an obviously open wound or small nondetectable wounds secondary to microtrauma. The incidence of cellulitis increases with patient age, and commonly seen in patients with compromised skin from lymphedema, chronic venous stasis, chronic steroid use, or chronic edema.¹⁸ The foot and ankle are inherently predisposed to cellulitis because of their weight-bearing position, multiple bony prominences, and shoe wear requirements, all of which increase the risk of microtrauma to the surrounding soft tissues. Cellulitis appears as erythema, swelling, increasing pain, and induration. Patients may have fever or other constitutional symptoms. Lymphadenopathy may occur proximally at the knee or groin. Initial outlining of the involved area can be helpful in guiding the treatment response.

Methicillin-susceptible S aureus and streptococci are the most common organisms responsible for cellulitis. These organisms often reside in the interdigital toe spaces.¹⁹ Usually cellulitis will respond to oral or intravenous antibiotic therapy within the first 1 to 2 days, depending on the severity of the condition, with antibiotics effective against methicillin-susceptible S aureus and streptococci. The patient should be closely followed as the symptoms resolve because fluctuant areas or painful joints may become evident, indicating the concomitant presence of an abscess or septic arthritis.²⁰

Necrotizing fasciitis is an aggressive, rapidly spreading soft-tissue infection that travels along fascial planes. Necrotizing fasciitis is most common in the lower extremities and often starts with a traumatic wound. The foot is particularly at risk for this type of infection. The infection often is polymicrobial, involving both gram-negative and gram-positive species as well as aerobic and anaerobic organisms.²¹^,²² The commonly found organisms include group A and α-hemolytic streptococci, S aureus, Escherichia coli, and Pseudomonas. Many of these organisms secrete toxins, causing septic shock followed by multiple organ failure.²¹^,²²^,²³ Patients who are immunocompromised, such as those with diabetes mellitus, are particularly at risk.

In the early stages of necrotizing fasciitis, the patient has vague muscle joint aches and pains. The symptoms can rapidly deteriorate, however, and the patient may have signs of systemic toxicity such as hypotension.²¹ The affected limb rapidly becomes swollen and erythematous. Fluid-filled bullae may be present, signs of necrosis may ensue, and sepsis may progress rapidly.²¹^,²² A delayed diagnosis resulting from the insidious onset of symptoms can further compromise the outcome.

Emergency surgical débridement of all necrotic tissue including subcutaneous tissues, fascia, and skin is essential to eradicate the infection.²¹^,²²^,²³ Multiple repeat débridement is required in a temporally staged manner. Amputation of the limb may be required to avoid mortality. Reconstructive procedures including skin grafting often are necessary after the infection has been eradicated.²¹^,²²

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