Foot Abscesses With No Accompanying Wound: Clinical Presentation and Pathogens

Acute bacterial infections of the skin and soft tissue are common and often pose serious complications, most commonly caused by Streptococcus species and Staphylococcus aureus.^1-3 Patients with soft tissue infections comprise 1% to 14% of emergency department visits and 4% to 7% of hospital admissions.⁴ An infection incidence rate of 24.6 per 1000 people per year has been reported, with a higher incidence among males and persons aged 45 to 64 years.⁵ In adults, foot infections are usually precipitated by a break in the cutaneous skin barrier such as interspace fissures, traumatic penetrating injuries, or ulcerations.^6,7 It is uncommon for foot infection to occur when the integument is intact. Lavery et al⁸ reported that only 1 in 150 patients with foot infection did not have some type of wound as part of the presenting concern. The objective of the current study is to report the clinical presentation and pathogens of patients with a foot abscess and no accompanying wound.

Approval of this study was granted by the institutional review board at the University of Texas Southwestern Medical Center. The authors retrospectively evaluated 20 patients who were admitted to the hospital with a foot abscess and no wound. Patients younger than 18 years and older than 90 years were excluded. Demographic data including race and ethnicity, sex, and age; medical history; laboratory values; surgical cultures; and clinical presentation were collected. 

The diagnosis of DM was based on American Diabetes Association criteria.⁹ The Infectious Diseases Society of America Diabetic Foot Infection Classification was used to define foot infection.¹⁰ This definition includes clinical features of purulence or at least 2 local clinical signs of infection. Sensory neuropathy was defined as reduced sensation to vibrational stimuli using a tuning fork or inability to sense at least 1 testing point using a Semmes-Weinstein 10 g monofilament. Peripheral vascular disease was defined as an ankle-arm systolic blood pressure ratio of less than 0.90. 

Descriptive statistics, t test, and Fisher exact test were used for analysis. Dichotomous variables were reported as number (percent) and continuous variables as mean ± standard deviation. 

Twenty patients were identified. Fifteen were male (75%), 10 (50%) had DM, and 16 (80%) were White. Patients presented to the hospital a mean of 7.8 ± 4.8 days after initial onset of foot symptoms. Surgery was performed within a mean of 2.0 ± 0.9 days from admission. The mean number of surgeries per patient during admission was 2.4 ± 1.0. 

Ten patients (50%) had DM. When comparing demographics of patients with DM, the only significant difference was the incidence of neuropathy (80% DM vs 10% no DM; P = .01). Laboratory values on admission are reported in Table 1. Patients with DM presented with a significantly higher erythrocyte sedimentation rate than patients without DM (66.60 mm/hour ± 46.05 vs 43.30 mm/hour ± 26.18; P = .02).

Deep intraoperative tissue cultures showed no polymicrobial infections. Seven patients had methicillin-sensitive S aureus (35%), 4 had Streptococcus agalactiae (20%), 3 had methicillin-resistant S aureus (15%), 1 had Streptococcus pyogenes (5%), 1 had Escherichia coli (5%), 1 had Streptococcus dysgalactiae (5%), and 2 had no growth (10%). Results are shown in Table 2 and Figure 1. 

Patients with DM had an equal percentage of S aureus and Streptococcus infections (50% for each) (Figure 2). None of the Staphylococcus infections were due to resistant pathogens. In contrast, there were more S aureus than Streptococcus infections among patients who did not have DM (25% and 10%, respectively). The 1 E coli and 2 no growth cultures occurred in patients without DM.

Results of this study show that only half the subjects who presented with abscesses and no wounds had DM. This was unexpected, because the majority of foot infections in the literature are due to wound infections in patients with DM.¹¹ All foot abscesses in the current study were monomicrobial, in contrast to the polymicrobial results commonly reported in the foot infection literature.^1,2 Kalan et al¹² conducted a longitudinal, prospective study investigating the role of colonizing microbiomes in diabetic wound healing. They identified polymicrobial growth among diabetic foot infections and found that contaminants significantly affected wound severity and healing. It should be noted, however, that cultures of uninfected wounds do not predict outcomes.^13,14

In the current study, the most common findings on presentation were subjective pain, with or without erythema. Patients received antibiotics for a mean of 2 days prior to incision and drainage in the operating room. It may be that the abscesses consolidated after administration of intravenous antibiotics. 

The authors of the current study did not identify any literature on abscesses without accompanying wounds. Since there is no prior literature outlining physical examination findings and outcomes for this condition, it is difficult to draw comparisons. A study by Venkata et al¹⁵ evaluated foot infections in patients with and without DM. In patients with DM the most common site of infection was the dorsum of the foot (40%), and in patients without DM the most common site was the toes (40%).^5,15 Truong et al¹⁶ compared outcomes of foot infections secondary to puncture injuries in patients with and without DM. That study concluded that patients with DM took longer before presenting to the hospital (mean, 20.1 days ± 36.3 vs 18.8 days ± 34.8; P = .09; 95% CI, 13–26.5). This result was not statistically significant.

This study has limitations. Owing to the retrospective nature of the study design, operational definitions for some variables may not be consistent throughout the medical documentation. However, since all the foot and ankle infections managed in the hospital of the study authors are evaluated by the limb salvage service, patients included in this cohort were evaluated based on a systematic format. The study population may not represent the general population, however, because Parkland Hospital is a safety-net hospital, serving a great number of patients who have a low income or no income, who are unfunded, and who are underfunded.¹⁷ 

Foot abscesses without wounds are not frequently seen in practice. The authors of the current study did not identify any published reports describing clinical presentation or microbiological data associated with this pathology. Half of the subjects in the current study did not have DM. All the abscesses in this series had infection caused by a single pathogen, predominantly Staphylococcus and Streptococcus.