Diabetes mellitus (DM) has been diagnosed in approximately 14 million US citizens. It can produce a complex clinical picture due to its involvement in numerous different organ systems. The combination of diabetic peripheral neuropathy and compromised distal vascularity act synergistically puts these patients at high risk for pedal complications. Individuals with diabetes tend to develop ulcerations in the feet, which often lead to infection of the soft tissue and bones.

Cellulitis is often the first sign of a soft tissue infection of the foot. In most cases, this marker represents an isolated localized skin infection but may represent a more severe process. Cellulitis usually originates from minor cuts and abrasions but also comes from more severe puncture wounds or trauma. Group A streptococci is the most common bacterial contaminant responsible for soft tissue infections, and Staphylococcus aureus is the second most common. Each is present in natural skin flora (Fry, 1998).

The clinical presentation for cellulitis typically includes blanching erythema in the skin that is generally bright red and angry. On examination, a break in the skin is usually present; however, patients with venous stasis or lymphatic disease can develop cellulitis with no apparent tissue loss. Increased calor relative to the contralateral foot and leg is noted often. Dependent rubor, observed in patients with PVD, and gout are the 2 most common differential diagnoses mistaken for cellulitis in the foot. Complications include lymphadenitis and contiguous inoculation to adjacent osseous structures, joints, deep structures, and/or tissue planes.

Initial treatment for simple cellulitis as a result of an abrasion in a host who is not immunocompromised includes oral antibiotics using first-generation cephalosporins, aminopenicillins, or quinolones. Group A streptococci is the most common pathogen and is usually susceptible to penicillin V and cephalexin. In more severe cases, oxacillin 2 g administered intravenously every 4 hours or cefazolin 1 g administered intravenously every 8 h can be used (Joseph, 1996). Simple cellulitis usually responds well to antibiotics, rest, and elevation of the extremity. However, in more severe cases invasive treatment with debridement of necrotic tissue becomes necessary if septic embolization ensues.

Paronychia

Paronychia is a more common soft tissue infection with inflammation of the periungual area adjacent to the nail grooves and borders (see Images 5-7). It can be initiated by a traumatic event such has dropping objects on the toes or having them stepped on; more often, paronychia results from an ingrown toenail (onychocryptosis). Underlying onychomycosis also can be a predisposing factor, which results in paronychia (Habif, 1995).

Initial treatment should include antibiotic therapy and warm soaks to the affected digit. Antibiotic therapy should be directed toward the offending pathogens, which are commonly skin flora (Deery, 1998). When onychocryptosis is the underlying etiology, that portion of the nail should be removed to address the soft tissue reaction. A partial nail avulsion involves removing the border of the ingrown nail. Chronic recurrent paronychias can be treated in a surgical manner both chemically and via local excision.

Puncture wounds

Puncture wounds occur more than 50% of the time on the planter surface of the foot with more than 90% of these involving penetration of a nail (Baldwin, 1999). Other objects in this category include wood, metal, plastic, glass, and animal and human bites. Puncture wounds have the potential to inoculate deep spaces of the foot, including bones, joints, tendons, and deep fascia, and serious complications can arise. Therefore, the depth of penetration is one of the most important factors in determining if a wound will resolve without complex intervention. Degree of infection can depend on location, type of penetrating object, retained foreign bodies from pieces breaking off, and penetration through shoes and socks.

The most common organisms implicated in penetrating wounds are S aureus, beta-hemolytic streptococci, and various anaerobic bacteria. Pasteurella multocida is typical in dog and cat bites or claw puncture wounds. Viridans streptococcus is responsible for most problems related to human bites. Pseudomonas aeruginosa is usually responsible for infection when the injury is due to object penetration through shoes and socks (Patzakis, 1989).

Signs of erythema, edema, and pain at the site of injury usually suggest that an infectious process has begun. The most common presenting complaint in the emergency department (ED) is persistent pain (Lavery, 1996). A complete history can aid in determining the age of the wound in addition to the mechanism, type of contamination, and possibility of retained foreign body. Pain out of proportion to the injury strongly indicates that an infection has developed. Inoculation into a joint can lead to septic arthritis with rapid destruction of the articular structures. P aeruginosa is the most common pathogen associated with osteochondritis following puncture wounds of the foot. Foreign body penetration into bone can lead to direct extension osteomyelitis (Laughlin, 1997). If the wound can be probed directly to bone or joint capsule, inoculation is strongly indicated, and the wound needs to be debrided and explored in surgery.

Diagnostic imaging is indicated for all puncture wounds (Steele, 1998; Manthey, 1996). When foreign bodies become lodged within the foot, they need to be removed. If visible or readily accessible through the wound, an attempt can be made in the ED to retrieve the object. If the object is deeply imbedded, it should be addressed in surgery and retrieved under fluoroscopy. Deep penetrating injuries should be debrided to remove all contaminants and nonviable tissue. Superficial wounds can be irrigated with saline either in the ED or the physician’s office. Drains are often helpful for deep penetrating wounds in addition to open or closed systems.

Empiric antibiotic therapy should be directed toward the suspected pathogen, with S aureus being the most common bacterial contaminant. A complete history aids in antibiotic choice against other pathogens (eg, P multocida, P aeruginosa, S viridans). Puncture wounds that occur in a contaminated environment, such as farms and industrial areas, can result in polymicrobial infections, which include gram-negative and anaerobic bacteremia. Deep wound cultures and sensitivities help guide the antibiotic choice. Superficial infections usually can be treated adequately with oral antibiotics. Deep wounds or suspected bone and joint involvement should include intravenous therapy and appropriate surgical decompression or exploration.

Immunocompromise

Pedal infections in patients who are immunocompromised can be difficult to diagnose and treat due to comorbidities, which often alter the presentation and require treatment. This group of patients includes persons with HIV, systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and high-dose corticosteroid use, DM, and asplenia. These patients can have impaired host defenses and are at higher risk of acquiring infections.

Human immunodeficiency virus

Patients with HIV are more susceptible to fungal and viral pedal infections (Barbosa, 1998). Tinea pedis and onychomycosis often are observed in this population. Human papilloma virus, manifesting itself as verruca plantaris, occurs at a higher rate than in the normal population. Fungal infections usually can be managed with a variety of antifungal creams. Verrucae can be more challenging to treat. The condition often requires several modalities for complete resolution. Common therapies include salicylic acid application, cryotherapy, blistering agents, and surgical excision.

Systemic lupus erythematosus

Individuals with SLE are predisposed to infection due to their impaired humoral immunity and lowered T-lymphocyte–mediated immunity, which results from immunosuppressive therapy (Cunha, 1998). Lupus flares can be mistaken for an infection and the differential diagnosis must be evaluated carefully. SLE skin and soft tissue infections most commonly are caused by S aureus and less commonly by group A streptococci. The absence of a true leukocytosis may create a confusing environment for the diagnostician.

Rheumatoid arthritis

Patients with systemic arthropathies such as RA are often on long-term corticosteroid therapy, which suppresses cell-mediated immunity; thus, a higher risk of infection results (Anderson, 1997). Classic presentations include synovitis, pannus formation, and periarticular bone and cartilage destruction. These local manifestations of systemic disease can be confused with soft tissue infection. Approximately 20% of patients with RA have rheumatoid nodules. They frequently occur over pressure areas, which tend to rupture in the foot causing skin breakdown, erythema, and infection. Therefore, patients with RA presenting with possible pedal infections require a thorough workup to exclude the conditions discussed above.

Diabetes

Persons with diabetes are truly compromised due to their impaired host defenses, decreased perfusion to the lower extremities, and diminished sensation in the feet secondary to peripheral neuropathy. This group of patients is very challenging to treat; treatment is covered in greater detail in Diabetic Foot Infections.

Asplenia

The spleen is the primary site for immunoglobulin M (IgM) synthesis, which is the first early response of the body. This places patients with asplenia at higher risk for all infections. They are at a particular high risk for infection with encapsulated bacterial organisms (Cunha, 1998). It is important to determine if they have joint implants or internal fixation in the feet because of the organisms' affinity for seeding these areas. For fractures requiring open reduction and external fixation (ORIF), prophylaxis against these pathogens is appropriate. Typical pedal infections in these patients are clinically indistinguishable from those in healthy hosts, except that they are often more severe.

Antibiotic coverage should be directed towards the encapsulated pathogens and consist of third- or fourth-generation cephalosporins. Imipenem should be used in the patient who is allergic to penicillin. If hardware is present and has become colonized, it should be removed for complete resolution of the infection.

Peripheral vascular disease

The poorly perfused or ischemic foot is prone to more frequent and severe infections due to low oxygen tension (Fry, 1998). The inflammatory response to stress can be reduced. Decreased local perfusion, edema formation, and neutrophil infiltration are present. Small abrasions in these patients can become quite problematic because the lesions do not have sufficient blood supply to heal properly. Prolonged healing time leaves the patient more susceptible to infection and complications. Ischemic feet are also prone to ischemic-type ulcerations that have an extended healing time and an increased chance of infection.

The evaluation of a patient with PVD presenting with an apparent foot infection can be quite challenging, and one must exclude dependent rubor versus cellulitis. Dependent rubor is very common in this group and consists of an erythematous foot while dependent, which closely mimics cellulitis. They differ clinically in that dependent rubor does not have increased temperature, can be bilateral, and, most importantly, completely resolves with elevation of the extremity. Therefore, when a patient presents with absent pedal pulses or known PVD and has apparent cellulitis in the foot, the leg should be elevated to observe if the erythema subsides.

Injured tissue in the critically ischemic foot can proceed to gangrene because the metabolic requirements to heal the wound are much greater than those needed to maintain normal tissue viability. This event is typically nonemergent and is described as dry gangrene. Clinically, the lesion is dry and stable with no real local or systemic signs of infection. A small amount of erythema around the demarcation may be present between healthy and nonviable tissue, and, again, dependent rubor needs to be ruled out. If this wound remains stable and does not become grossly infected, allowing the digit to auto amputate is often the management of choice.

Conversely, gas gangrene is an emergent situation but has a different pathophysiology. It has a wet appearance, and the patient usually has local and systemic signs of infections requiring surgical decompression.

Antibiotics should be directed toward the offending organisms. Ischemic tissues may not respond to antibiotics and, therefore, require debridement. When attending to deep space infections, debridement in surgery is the only option for infection control. Superficial wound infections not probing to deep structures can respond to topical dressings and antibiotic therapy.

Trauma

Foot wounds associated with various traumatic events are inherently contaminated. Potential infecting organisms vary, depending on how, when, and where the injury occurred. A thorough history hopefully clarifies this information. If rapid signs of infection develop, group A streptococcal and clostridium species need to be excluded because they occur within hours of injury. Tetanus prophylaxis should be reviewed with all patients, and vaccine is administered when appropriate (Hoover, 2000).

Burn wounds

Burn wounds to the feet, like other parts of the body, are susceptible to becoming infected by various microorganisms. This represents a major complication for the newly exposed tissue. Common pathogens include P aeruginosa, S aureus, Escherichia coli, and Klebsiella and Enterococcus species (Ravathi, 1998). Treatment includes aggressive debridement of all nonviable tissue, administration of systemic antibiotics, and application of topical antimicrobial agents. Silver sulfadiazine cream 1% remains the most popular topical agent due to its effectiveness in controlling P aeruginosa as well as many other wound pathogens (Marone, 1998). Once wounds are stable, coverage of the area should be sought using synthetic skin equivalents, porcine xenografts, allografts, or autografts.

Degloving injuries

Degloving injuries occur due to a shearing force on the skin, causing it to be undermined and elevated through the subcutaneous plane. A flap of tissue is created with a margin remaining intact or becoming completely avulsed. This represents a potentially serious situation that can lead to significant infection.

Initial treatment consists of high-pressure irrigation and debridement of all necrotic tissue. The soft tissue defect is similar to a burn wound; thus, common pathogens can be expected. Because these wounds on the feet typically occur in contaminated environments such as motor vehicle accidents or farm-type injuries, anaerobic pathogens also should be expected.

As with burn injuries, topical and systemic antibiotics should be administered. If a tissue flap remains intact, it is replaced over the wound and allowed to demarcate. Once the margins are defined, the nonviable tissue is debrided, and plans for final reconstruction using split-thickness skin graft or flap coverage are initiated.

Crush injuries

Crush injuries are common with foot trauma, especially in industrial settings. The widespread use of safety boots has decreased the incidence and severity of these injuries, but they still occur on a regular basis. The degree of injury can include mild contusions, skin lacerations, fractures, disruption of vascular supply, or combinations thereof. Crush injuries to the digits commonly include a nail bed laceration with fracture of the underlying phalanx. These are treated as open fractures with local wound care, tetanus prophylaxis, and antibiotics. Industrial settings generally are contaminated; therefore, surgical debridement and delayed closure should be considered.

The vascular supply to the digits is easily compromised and often damaged in crush injuries. Decreased tissue perfusion and oxygen tension occur, causing an ischemic digit. Therefore, in addition to typical pathogens, anaerobic organisms should be suspected and prophylaxis against these organisms employed. Crush injuries to the midfoot and rearfoot have a lower incidence of soft tissue infection relative to the digits. Open fractures in these areas still have a heightened risk for osteomyelitis, but this is discussed later in the chapter (Oloff, 2001).

Gunshot wounds

Gunshot wounds generally are described as low or high velocity. The speed at which the bullet is traveling as it exits the barrel or enters the body determines which group it is classified under. It also serves as an indicator for the amount of damage induced by the projectile, with low velocity providing relatively more benign injuries (Fackler, 1996). Handguns, which create a low-velocity wound, cause most civilian gunshot wounds.

All gunshot wounds are contaminated with bacteria. The old myth that a bullet becomes sterilized by the high temperatures reached in the barrel is false (Thoresby, 1967). The newly inoculated bacteria multiply in proportion to the amount of devitalized tissue. The tissue surrounding the bullet path becomes devitalized with compromised blood flow. It takes a few days for new capillaries to grow into the disrupted area. The interim period of decreased blood results in poor delivery of antibiotics to necrotic regions. Boucree et al demonstrated S epidermidis to be the most common pathogen cultured in a large study of gunshot wounds to the foot. A wide range of gram-negative and gram-positive organisms as well as aerobes and anaerobes also have been observed (Boucree, 1995).

Early debridement and intravenous antibiotics are the recommended treatments of choice. All nonviable tissue should be debrided, and the original wound should be left open for 4-7 days. The wound is debrided as often as needed and finally is closed by delayed primary closure. All periwound erythema and edema must be resolved prior to reconstruction.

Lawn mower injuries

Upward of 60,000 lawn mower injuries occur annually in the United States with most of these involving the lower extremities (Love, 1988). Injuries can range from simple lacerations to more severe fractures and amputations. The setting for these injuries is inherently dirty so a wide variety of pathogens will be present. The combination of pathogens and severity of injury leads to a prolonged and expensive treatment course with an estimated annual cost of 475 million dollars.

A good history is important for determining the mechanism of injury, setting it took place in, and tetanus status. The injuries often occur after patients fall off of or get hit by a riding mower, so lesions away from the lower extremity need to also be excluded on examination. Lacerations should be inspected in depth to ensure all tendon, bone, and neurovascular structures are intact. These can easily be compromised because of their superficial nature.

After a complete history is obtained and thorough physical examination is performed, treatment should begin with broad-spectrum antibiotics covering both gram-positive and gram-negative organisms. These accidents are considered farm-style injuries; the most common organisms cultured are Enterobacter cloacae, P aeruginosa, Enterococcus species, coagulase-negative Staphylococcus, diphtheroids, and Staphylococcus aureus (Anger, 1995). In addition to antibiotic therapy, the patient is taken to the operating room for irrigation and debridement. The grass, water, dirt, and debris that frequently contaminate these wounds are removed, and all nonviable tissue is excised. As with most contaminated wounds, these are left open and packed until the wound converts to healthy tissue. Several staged debridements are commonly needed before the damaged limb is definitively reconstructed.

Surgical emergencies

Fortunately, only a small group of potentially life-threatening situations exist that require immediate and aggressive treatment to preserve a limb or a life. Necrotizing fasciitis and gas gangrene are the 2 most common pedal surgical emergencies. Local tissue damage, progressive gangrene, and potential systemic toxemia and/or death can result from these infections (Hill, 1998).

Necrotizing fasciitis

Necrotizing fasciitis is characterized by widespread necrosis of fascia and deeper subcutaneous tissues, with initial sparing of skin and muscle. Eventually, skin involvement is noted, with cellulitis evolving into cutaneous gangrene. The most common underlying risk factor is being a patient with DM. One recent study by Elliott reported that foot ulcerations and infections associated with diabetes were the second most common cause of necrotizing fasciitis; thus, 15.2% of cases of necrotizing fasciitis are due to foot ulcerations and infections associated with diabetes.

Surgical wounds and infections resulting from intravenous drug abuse or "skin popping" also can lead to necrotizing infections. Aerobic streptococci are typical pathogens in addition to Bacteroides species, staphylococci, and enterococci, which all play a role in the infectious process. E coli and Proteus species are facultative anaerobic gram-negative rods that often are cultured from these wounds.

Patients usually present with nonspecific inflammatory signs of pain, swelling, and erythema. Crepitus in the soft tissue is almost diagnostic for this condition but is not always present at the initial presentation. Blistering of the skin and soft tissue gas on radiographic examination is also highly indicative. The average time from onset of symptoms until the patient is observed in the ED is 4 days (Elliott, 1996).

The key to successful management of necrotizing fasciitis and associated low mortality rates is early diagnosis, broad-spectrum antibiotics, and prompt surgical intervention, which often involves multiple debridements. Antibiotics must be broad-spectrum to cover polymicrobial flora. Piperacillin and tazobactam and ticarcillin and clavulanate are used frequently and are very effective. Combination therapy with an antipseudomonal, third-generation cephalosporin, and clindamycin also are indicated. Aggressive debridement is mandatory to control local factors such as tissue necrosis and anaerobic bacterial overgrowth.

Gas gangrene

Gas gangrene or clostridial myonecrosis is considered a surgical emergency. It includes a rapid fulminating course, severe toxin-related systemic toxicity, a vast level of tissue destruction, and a high mortality rate. Rapid diagnosis and aggressive management are crucial with respect to limb preservation.

Six different species of Clostridia can be responsible for the soft tissue destruction; however, Clostridium perfringens is the most common. C perfringens produces 12 active tissue toxins responsible for the syndromes of gas gangrene. Clostridia organisms are saprophytes and are quite ubiquitous. Infections leading to gas gangrene require an opportunistic environment. Prerequisites include a wound, contamination with Clostridia organisms, and a depressed oxygen state at the site of inoculation. This accounts for the increased incidence of gangrene noted in patients with diabetes and patients with PVD. The decreased oxygen state also can be observed postoperatively from local edema and dressings.

Typical onset can be 12-24 hours but can be as short as 6 hours. Excruciating progressive muscular pain is the first and most important symptom. Systemic findings are more frequent and severe than in individuals with necrotizing fasciitis. Diaphoresis, low-grade fevers, tachycardia, and mental status changes are often observed with this population.

Skin initially has a pale white, shiny, and tense tone. Eventually, an orange discoloration develops with hemorrhagic bullae, leading to a brown serosanguineous discharge that is foul smelling. The smell is distinctly different from the typical foul smell of an anaerobic infection. Microscopic examination of the discharge exhibits a dense smear of club-shaped gram-positive rods, numerous RBCs, and rare polymorphonuclear leukocytes (PMNs). Crepitus is felt in the tissue with progression of the disease but is not always palpable early on. Radiographs demonstrate gas distributed in the myofascial bundles (Rabinowitz, 1999).

Successful management includes high-dose intravenous antibiotics with aggressive surgical debridement. The antibiotic of choice is penicillin G 10-12 million units per day. Chloramphenicol 4 g per day can be used in patients who have a penicillin allergy. Surgical debridement is performed on an emergent basis and confirms the diagnosis suspected clinically when necrotic muscle is found beyond the area of trauma. Multiple debridements are performed every 24-48 hours, often requiring fasciotomies and/or amputations.

Hyperbaric oxygen (HBO) therapy is an accepted adjunct to antibiotic and surgical treatments. High oxygen concentrations reduce the germination rate of activated C perfringens spores and inhibit the production of alpha-toxins (Dehaven, 1971; VanUnik, 1965). However, most studies demonstrate similar survival rates between patients treated with and without HBO. When used, it is performed with 100% oxygen at 3 standard atmosphere of pressure for 1-2 hours every 8-12 hours for a total of 6-8 treatments (Faglia, 1996; Hammarlund, 1996).

The incidence of osteomyelitis following gunshot wounds to the foot is relatively low. A retrospective study by Boucree et al demonstrated that 81 of 101 patients with gunshot wounds to the foot sustained fractures. Of these, only 4 had osteomyelitis and were associated with high-velocity gunshots. Contributing to the low infection rate was an aggressive treatment protocol. Low-velocity wounds demonstrated ideal uncomplicated healing following hospitalization, a minimum of 3 days of intravenous antibiotics using a first-generation cephalosporin, and irrigation and debridement as needed. Incision and drainage (I&D) usually was performed only if the patients exhibited clinical signs of infection after the 3 days.

In Boucree’s study high-velocity gunshot wounds were treated with a protocol with slight variations. These patients were hospitalized, received broad-spectrum intravenous antibiotics, and underwent multiple debridements. Three of the 4 patients with osteomyelitis had I&D only once, while the others had I&D a minimum of 3 times. The authors believed that the high-velocity wounds required repeat debridements because the amount of necrotic bone was not completely evident after 48 hours. Aggressive treatment with repeat debridement is important for prevention of infection (Boucree, 1995).

Foot infections in persons with diabetes are responsible for more hospital days than any other aspect of diabetes (Durham, 1991; Gibbons, 1984). Typically, the diabetic foot infection stems from an open ulceration or wound. Diabetic peripheral neuropathy with additional vascular disease can predispose these patients to this condition. Immunopathy, neuropathy, and vascular disease provide an environment that fails to support standard wound healing as observed in the nondiabetic population.

Diabetic peripheral neuropathy has several components involving both somatic and autonomic fibers. Type-A sensory fibers are responsible for light touch sensation, vibratory sensation, pressure, proprioception, and motor innervation to the intrinsic muscles of the foot. The muscles exhibiting early involvement are the flexor digitorum brevis, lumbricals, and interosseous muscles. These groups act to stabilize the proximal phalanx against the MT head preventing dorsiflexion at the MT phalangeal joint (MTPJ) during midstance in the gait cycle. With progression of the neuropathy, these muscles atrophy and fail to function properly. This causes the MTPJs to become unstable, allowing the long flexors and extensors to act unchecked on the digits.

Dorsal contractures develop at the MTPJs with development of hammertoe deformity, also known as intrinsic minus disease. The deformity acts to plantarflex the MTs, making the heads more prominent and increasing the plantar pressure created beneath them. It also acts to decrease the amount of toe weightbearing during the gait cycle, which also increases pressure on the MT heads. Normal anatomy consists of a MT fat pad located plantar to the MTPJs. This structure helps to dissipate pressures on the MT heads from the ground. When the hammertoe deformity occurs, the fat pad migrates distally and becomes nonfunctional. This results in elevated planter pressures that increase the risk of skin breakdown and ulceration due to shearing forces (Sumpio, 2000).

Type-C sensory fibers detect painful stimuli, noxious stimuli, and temperature. When these fibers are affected, protective sensation is lost. This manifests as a distal symmetric loss of sensation described in a "stocking" distribution and proves to be the primary factor predisposing patients to ulcers and infection (Levin, 1995). Patients are unable to detect increased loads, repeated trauma, or pain from shearing forces. Therefore, injuries such as fractures, ulceration, and foot deformities go unrecognized. Repeat stress to high-pressure areas or bone prominences that would be interpreted as pain in the patient without neuropathy, also go unrecognized. Gait patterns go unchanged, and the stresses eventually cause tissue breakdown and ulceration.

Autonomic involvement causes an interruption of normal sweating at the epidermal level, and causes arteriovenous (AV) shunting at the subcutaneous and dermal level. Hypohidrosis leads to a noncompliant epidermis that increases the risk of cracking and fissuring. AV shunting diminishes the delivery of nutrients, oxygen, and antibiotics to a certain tissue region. Skin and subcutaneous tissues become more susceptible to infection (Saltzman, 1999).

Vascular disease is a common manifestation of diabetes. Diminished perfusion to the feet decreases tissue resilience, leading to rapid tissue death and impeded wound healing. The oxygen supply, nutrients, and chemical mediators needed for the wound-healing repair process often are delayed or absent. Long-standing open wounds are prone to becoming infected, and the lack of adequate blood flow makes fighting infection and delivery of antibiotics difficult. The vessels typically are affected by atherosclerotic obstruction, with individuals who are diabetic being affected at an earlier age than persons with vasculopathy without diabetes. The larger arteries in the legs are affected with usual sparing of the arteries in the feet (Karnal, 1996).

The absence of palpable pedal pulses suggests some form of PVD. Other signs of a more progressive disease state include pallor on elevation, dependent rubor, absence of digital hair, thickened toenails, and slow capillary refill time to the digits. A history of pain in the feet at rest, claudication, impotence, and coronary artery diseases are also markers of vascular disease.

Charcot foot disease is known as a neuropathic osteoarthropathy and can be observed in other patients who are neuropathic in addition to patients with diabetes. The exact etiology is still unknown; however, the most common theory involves hyperperfusion of the foot. The autonomic component of the neuropathy leads to vasodilation and hyperperfusion. The perfusion causes demineralization of the bones. Weightbearing forces cause the bones to begin to fragment and fracture, leading to collapse of the arch. The long-term sequelae of a rockerbottom-shaped foot leads to high-pressure areas that are prone to ulceration.

Charcot ulcerations are typically mechanical in nature but can become infected within the soft tissue and osseous structures. A midfoot collapse with tissue loss and radiographic signs of osteomyelitis in addition to clinical signs of edema and erythema often lead to a confusing and difficult-to-diagnose condition. Charcot osteoarthropathies often are diagnosed as an osteomyelitis by plain radiographs. Scintigraphic studies help in determining the nature of these changes.

Further adding to the complexities of diabetic foot infections are the organisms observed in these wounds. They are typically polymicrobial with moderate-to-severe infections culturing 3-5 organisms (Frykberg, 1996). Gram-positive and gram-negative organisms as well as aerobes and anaerobes often are noted. The most common gram-positive organisms include Enterococcus faecalis, S aureus, S epidermidis, and group B streptococci. Common gram-negative organisms include Proteus species, E coli, Klebsiella species, and Pseudomonas species. Of the anaerobes, Peptococcus species and Bacteroides fragilis are observed most often.

Aerobic gram-negative bacilli and anaerobes also are found in addition to the gram-positive cocci but are rarely the sole cause of the infections. Methicillin-resistant S aureus (MRSA) is more prominent in the diabetic population (Brunner, 1999). MRSA does not produce a more severe infection, but the antibiotic selection becomes more limited. It is also not uncommon to see vancomycin-resistant Enterococcus (VRE) species infection. Strict isolation measures need to be implemented when these patients are hospitalized.

On examination, local signs of infection include erythema, edema, and calor. Loss of skin wrinkles due to edema is a significant indication of infection and abscess. Open wounds with drainage, purulent discharge, and odor are usually indications of infection. The skin around the lesion should be "milked" toward the ulceration to see if exudate is brought from the deep spaces to the surface through the lesion (see Image 10). Foul-smelling drainage is consistent with anaerobic infections. Drainage from Staphylococcus species infections is usually odorless with a creamy yellowish color. Streptococcus species infections are also odorless and have a milky yellow-white color. Proteus species infections smell like urea, and Pseudomonas species infections are green with a fruity putrid odor. Exudate from gram-negative organisms is typically clear. All abscesses and infections of soft tissue and bone should be considered surgical emergencies.

Infections of the lower extremities in the presence of vascular insufficiencies require cellulitis differentiated from dependent rubor by elevation of the extremity. These patients often have ischemic ulcers with eschar covering them. The lesions need to be palpated for fluctuance under the eschar. If present, the eschar requires debridement. Fluctuance also can be palpated deep to normal skin, which also requires debridement.

Life-threatening infections like necrotizing fasciitis and gas gangrene are more common in hosts with vascular compromise and should be considered among the differential diagnoses. They usually have systemic toxemia out of proportion to the degree of the apparent infection, with progressive necrosis of the skin and subcutaneous tissue. Local tissue changes can include minimal suppurative drainage and periwound necrosis. Palpation reveals pain out of proportion to the wound, induration, and, often, crepitus. Pain in the presence of neuropathy is a significant indicator of infection and/or underlying abscess formation.

A thorough evaluation of foot infections associated with ulcerations includes probing of the wound. This is important for determining the depth and extent of the lesion. The area should be prepared with Betadine and probed with the wooden end of a sterile cotton-tipped swab or metal probe. If the lesion is deep, assume that the infection has spread to that level. Once in the deep compartments of the foot, bacteria can migrate easily along fascial planes and tendon sheaths. If the lesion can be probed directly to bone, osteomyelitis is predicted and the lesion should be treated as such. Studies have demonstrated that palpating bone when probing the lesion is highly specific and has a high positive predictive value for osteomyelitis (Grayson, 1995).

Laboratory studies

Laboratory analysis should include a complete blood count with differential, chemistry panel, and coagulation studies.

Elevated white blood cell count in association with a left shift can be pathognomonic for a systemic infection. It is important to note that many patients with diabetes and limb-threatening infections fail to produce a true leukocytosis; therefore, laboratory values can be nonpredictive in this setting.

Renal values such as BUN and creatinine should be evaluated to establish a baseline prior to initiation of antibiotics, which can be nephrotoxic. Many individuals who are diabetic have chronic renal insufficiency or end-stage renal disease for which antibiotic dosing needs to be very specific. Patients with renal diabetes are often anemic and require Epogen or transfusions prior to surgery because blood loss can initiate or potentiate a cardiac event, especially in this population with global vascular diseases.

Coagulation studies are important if the patient is going to surgery for irrigation and debridement. Patients with PVD often have foot infections. Coronary artery disease and atrial fibrillation require anticoagulation with Coumadin; therefore, prothrombin time (PT) and international normalized ratio (INR) values are important prior to surgery. Fresh frozen plasma and vitamin K can be given prior to debridement if the INR is too high. Other useful lab studies are C-reactive protein, which provides a good gauge of treatment efficacy; erythrocyte sedimentation rate (ESR), which indicates an infectious or reactive process and is a good gauge for response to antibiotics; and albumin levels, which can pose a problem for wound healing if too low.

Wound cultures with susceptibilities are important parts in the diagnostic process. Organisms are identified and antibiotic sensitivities are noted. Initial Gram stains help with empiric antibiotic therapy. Superficial ulcerations should not be swabbed because the bacterial flora are contaminants and not true pathogens. Deep cultures taken in a controlled setting such as in surgery are much more accurate and truly represent the pathogens responsible for the infection. Overall, the general consensus currently is that superficial cultures should be avoided (Perry, 1991).

Other tests and diagnostic procedures

Tissue biopsy and quantitative counts are other means of identifying bacteria and diagnosing infections. Soft tissue culture swabs are preferred over biopsy for identifying pathogens. Biopsy is important in determining quantitative bacteria counts. Biopsy is also very useful for diagnosing bone infections. Bone biopsy remains the criterion standard in the diagnosis of osteomyelitis (Mader, 1996).

Imaging studies

Plain film radiography

Plain film radiographs remain the initial imaging modality for most foot infections including soft tissue, bone, or combinations thereof. Plain films provide useful information rapidly and inexpensively. Soft tissue infections such as cellulitis are observed on plain films as increased soft tissue densities. Radiographs can reveal soft tissue swelling, obscuration of fascial planes, and mottled opacification of subcutaneous fat. Puncture wound and possible retained foreign bodies should be evaluated with plain film radiographs. Radiopaque objects like metal, gravel, and glass can be identified easily with plain films.

Radiolucent objects such as plastic, rubber, or wood are not often observed on x-ray films and require ultrasound identification. The most important reason for ordering plain films, especially with infected diabetic ulcers, is to exclude gas in the soft tissues (Struk, 2001). Again, this represents an emergent situation and must be treated aggressively with surgical decompression.

Plain film radiographs can be useful in the evaluation of osteomyelitis. The initial manifestations of osteomyelitis occur as pathogens invade the periosteum via direct extension. This causes the periosteum to be lifted and periosteal new bone formation to occur. The progression of the infectious process results in cortical disruption and medullary involvement. Simple radiographs lack sensitivity and specificity with respect to early detection of osteomyelitis. Bone destruction and periostitis are usually not detectable for up to 2 weeks after the initiation of infection. Plain films identify periosteal reaction, cortical disruption, Brodie metaphyseal abscess, sequestrum, and involucrum. Radiographs are often more helpful with long-term bone infection during which overt bone changes are noted.

Ultrasound

Another useful and cost-effective radiologic modality used with foot infection is the ultrasound examination. This study is very helpful when looking at retained foreign bodies not observed on plain films and when looking for soft tissue abscesses. Radiopaque foreign bodies such as wood and plastic can be identified using ultrasound (Manthey, 1996). Abscesses are observed as complex and typically cystic structures containing internal echogenicity with through transmission. The echogenicity varies depending on the contents of the abscess. Ultrasound is particularly useful in the foot because it can help identify the exact location of the abscess within the many deep compartments and fascial structures.

Scintigraphy

Bony changes can be delayed in persons with vascular compromise; therefore, plain film radiographs may not be adequate during the acute phases. Scintigraphic studies can be useful in the diagnosis of bone infections in the foot, bone infection versus soft tissue infection, and bone infection versus Charcot osteoarthropathy. The 3 common nuclear medicine studies in this group include the radioactive forms of technetium, gallium, and indium.

Technetium Tc 99m methylene diphosphonate (MDP), when used in a 3-phase bone scan, can reveal infection 24-48 hours after the clinical onset. Three-phase bone scans assist in distinguishing cellulitis from osteomyelitis. Increased uptake of the scintigraphic tracer during the early phases of a 3-phase bone scan with little or no increase in uptake during the late phase is indicative of soft tissue infection without bone involvement. Uptake in all 3 phases is observed in persons with osteomyelitis. This test is typically very sensitive but not specific because increased uptake of tracer can be observed with several other conditions, including bone tumors and Charcot osteoarthropathy, and following trauma or surgery. The results need to be interpreted in conjunction with clinical findings and other diagnostic modalities.

Gallium Ga 67 can be used in conjunction with Tc-99m to increase the overall specificity of the study. Gallium accumulates in infected bone, making it a useful tracer. Although specificity is increased, false-positive results still occur because it can accumulate in infected soft tissue, hematomas, some tumors, and areas of increased bone turnover (osteoarthropathy, fracture, and postsurgery). False-negative results also may occur with this tracer when patients are taking antibiotics.

Leukocyte-labeled indium In 111 reveals infection at an earlier stage than Tc 99m MDP alone and is both sensitive and specific. It is often combined with a Tc 99m MDP scan to increase the sensitivity and specificity even more. Although it has many attributes, several problems also exist. In 111 uses PMN leukocytes and not lymphocytes, so it is not as sensitive for detecting chronic osteomyelitis. It has a low spatial resolution, so it is difficult to differentiate from other soft tissue infections. It is a time-consuming examination, requiring 3 days to complete. Finally, it is more expensive than an MRI.

A newer nuclear medicine study gaining favor uses leukocyte-labeled Tc 99m hexamethylpropylamine oxime (HMPAO). This offers a more cost-effective alternative to the In 111 labeled scan. The biodistribution and function are very similar to In 111, with the benefit of a more favorable radiation dosimetry, allowing for a larger dose of radioactivity to be administered with less systemic absorption. This provides a larger amount of local activity, and allows the radiologist better ability to interpret the structures involved. The test is completed in 3-4 hours rather than 3 days.

HMPAO has many uses but proves to be invaluable in patients in 2 specific situations: the postsurgical patient and the patient with recent fracture in whom osteomyelitis is suspected. Healing bone following surgery or fracture lights up on Tc 99m MDP 1-2 years after the incident. If osteomyelitis is suspected at a recent surgical site, differentiating the 2 using conventional Tc 99m MDP scintigraphy is not possible. However, Tc 99m HMPAO scans are more specific for the infectious process.

Another difficult situation involves the patient with a chronic ulceration secondary to a rockerbottom foot deformity from Charcot osteoarthropathy. The physician cannot determine definitively if bone infection is present with Tc-99m MDP scintigraphy because it lights up due to the osteoarthropathy as well as the infection. HMPAO has demonstrated promise in differentiating between the two, but larger studies must be performed (Blume, 1997).

Computed tomography scan

Computed tomography (CT) scan is beneficial in diagnosing both soft tissue and bone infections of the foot. It is very useful for detecting small areas of osteolysis in cortical bone, small foci of gas, and small foreign bodies that may be associated with infection. Abscesses are observed as fluid collections displaying walls of various thicknesses. Edema and cellulitis are observed as mild enhancements in the soft tissue with thickening of the fascia. Gas in the deep tissues associated with gas gangrene or necrotizing fascitis is displayed readily on CT scan exhibiting the seriousness of the infection.

CT scan is an imaging method with high spatial resolution that provides great detail of cortical bone in a cross-sectional display. For this reason, it is excellent for picking up cortical disruptions or changes in bone associated with osteomyelitis. On CT scan, sequestra appear as fragments of dense bone often surrounded by increased soft tissue or fluid density. It can depict increased intraosseous density, reflecting the accumulation of pus that replaces the fat in the marrow. Involucra and sequestra are also readily detectable with CT scan. Because of its great detail, CT scan works well for identifying subtle changes within the intricate soft tissue layers and bones of the foot.

Magnetic resonance imaging

The most advanced radiologic study used in foot infections is magnetic resonance imaging (MRI). This is ideal for identifying lesions within soft tissues and works well for detecting changes taking place within bones. MRI proves to be very sensitive in determining the presence and extent of inflammation. Because of this, it is helpful in delineating whether an infection is limited to soft tissue, joints, bones, or whether several structures are involved. This is beneficial for treatment given the complex anatomy of the foot. Viable tissue also is differentiated readily from necrotic tissue, which aids in surgical planning and helps preserve the viability of adjacent healthy tissues during debridement. However, it should be understood that MRIs tend to over represent the amount of involved tissue, especially bone. This must be taken into account when using MRI to help plan surgical debridement.

MRI provides an accurate visual image of soft tissue, joint spaces, tendon sheaths, and fascial planes, making it ideal for use in diagnosing or studying the extent of infectious processes or abscesses within these structures. It has the advantage of being able to demonstrate an abscess in any plane and can better delineate the extent of adjacent soft tissue changes when compared to other studies such as CT scan. MRI is also useful in determining the extent of necrotizing fascitis infections, which, again, can aid in surgical planning. The image demonstrates thickening and tracking of abnormally high signal intensity along deep fascial planes with the presence of small gas bubbles. General consensus exists that MRI is a superior study to CT scan when assessing for nonviable tissue or drainable fluid collection (Cook, 1996).

The use of MRI to assist in diagnosing osteomyelitis continues to gain popularity. One recent study demonstrated MRI to be significantly more sensitive and accurate, with equal specificity in comparison to plain radiographs, Tc 99m MDP, and Ga 67 scans (Weinstein, 1993). The sensitivity of MRI for diagnosis of early osteomyelitis stems from its ability to differentiate normal bone marrow from abnormal bone marrow. With bone infection, marrow is replaced by fluid and inflammatory cells, which are displayed as regions of reduced signal intensity on T1-weighted images and as increased signal intensity on T2-weighted images and short tau inversion recovery (STIR) sequences. Other changes displayed include erosions and perforations of the cortex, periosteal new bone formation, and soft tissue edema. Although highly sensitive, it remains less specific because several disorders, including diabetic osteoarthropathy, can produce similar abnormalities in signal intensity.

When present, secondary signs of infection, such as cellulitis, sinus tract formation from skin to bone, cortical disruption, and sequestrum formation, need to be looked for to rule in osteomyelitis. However, MRI is an excellent study for evaluating bone infections overall.

These wounds can be thought of as tumors and excised completely until only normal tissue remains (Bowler, 2001). To accomplish this, serial debridements often are required because determining the viability of tissue early in the infection is difficult. The surgical site is left open and local wound care is provided along with appropriate antibiotics. Lines of demarcation are allowed to form between healthy and unhealthy tissues for 2-3 days before the patient is taken back to surgery for further debridement. This may need to be repeated several times before the wound is converted completely to a healthy one that is free of necrotic material . Necrotic and devitalized tissues need to be removed because they provide a favorable environment for bacterial growth (Bowler, 2001). More aggressive infections, such as necrotizing fasciitis, may require debridements every 1-2 days. Once the infection resolves, reconstruction of the foot can begin.

The first layer of tissue encountered during surgery is the skin. An incision is made at the border where healthy tissue meets nonviable tissue. This border can be demarcated clearly as observed in individuals with dry gangrene, or it can be more obscure, which is the more common scenario. If the lines of demarcation are not obvious, the best way to differentiate between viable and nonviable tissue is to look for bleeding margins. The margins of healthy tissue actively bleed when an incision is made. The incision is created in the center of the questionable area and the skin is removed in concentric circles until bleeding edges are encountered.

Only when normal bleeding is encountered at the fresh skin edges can the physician be certain that the nonviable tissue has been removed (Attinger, 2000). The subcutaneous tissue layer is comprised mostly of fat but also contains nerves and blood vessels. Fat is relatively avascular compared to surrounding tissues, thus active bleeding is not the best indicator for determining its viability.

Healthy fat feels soft, is resilient, and has a shiny yellow color. In contrast, nonviable fat feels hard, is nonpliable, and has a grey-white coloration. Healthy nerves are white and shiny and should be spared if possible. Caution should be used to ensure that retained nerves do not become entrapped in granulation or scar tissue and that adequate soft tissue coverage is created during reconstruction to avoid painful neuropraxia. In persons in whom nerves need to be sacrificed, neuroma formation can be minimized by sewing the epineurium at the cut end together with 8-0 sutures or by implanting the cut end of the nerve into muscle or bone. Blood vessels needing to be removed should be cauterized at the open ends or tied off when appropriate.

Deep structures in the foot include tendon, tendon sheaths, muscle, bone, and the fascia that divides them into different planes. When debridement reaches these deep structures, careful planing is needed because foot function is altered. The surgeon should concentrate not only on removing all nonviable tissue but should have a medical care plan for the reconstruction and eventual long-term functionality of the foot. Healthy tendon is white and shiny, whereas infected tendon is dull, soft, stringy, and can be in the process of liquefying. Normal muscle is beefy red and bleeds when cut. Dead muscle tends to be dull, dark, and falls apart easily when handled. An effective test for determining muscle viability is to pinch it with an instrument or buzz it with the Bovie electrocautery device; muscle that is still viable contracts. Infected bone is soft and does not bleed when cut.

As with the other tissues, nonviable bone should be excised until a healthy bleeding margin is obtained. An additional small portion of what is deemed to be healthy bone should be removed and sent for culture and pathologic examination to ensure that all osteomyelitic tissue has been removed.