Erythema on both the dorsal and plantar foot surfaces

CASE DESCRIPTION

A 6-year-old girl with a 7-day history of fever was referred to our hospital owing to suspected cellulitis involving her left foot (Figure 1A). She had been receiving treatment for atopic dermatitis. Her initial symptoms consisted of fever and right foot pain. On day 3 of her illness, she developed left foot pain and on the next day erythema and swelling appeared on the dorsum of her left foot. She had no recent history of lower-extremity injury. On physical examination, she complained of pain on the left foot and right lower leg upon compression. No overlying skin findings were observed on the right lower leg. In addition to the left dorsal foot erythema, erythema on the plantar aspect of the left foot was observed (Figure 1B).

Figure 1.

Erythema on the dorsum (A) and the sole (B) of left foot.

DIAGNOSIS

Magnetic resonance imaging (MRI) revealed high signal intensity in the left fourth metatarsal bone and its surrounding soft tissues on T2-weighted imaging (Figure 2). This led to the diagnosis of osteomyelitis of the left fourth metatarsal bone. In addition, MRI also revealed high signal intensity confined to the distal part of the right tibia, which was compatible with right tibial osteomyelitis. The soft tissue surrounding the right distal tibia was spared. Methicillin-susceptible Staphylococcus aureus was isolated from same-day blood cultures performed upon admission. She was administered intravenous cefazolin for 2 weeks and oral cephalexin for 4 weeks. Her erythema resolved 11 days after the initiation of treatment. No adverse events were observed during 5 months of outpatient follow-up.

Figure 2.

Sagittal (A) and coronal (B) sections of magnetic resonance imaging reveal high signal intensity of the left fourth metatarsal bone and surrounding soft tissues, extending to the dorsum and sole of the foot. This indicates that the inflammation of the bone has spread to the soft tissues causing paired erythema.

DISCUSSION

Acute osteomyelitis affects approximately 8 out of 100,000 children per year (1). Without prompt diagnosis and treatment, acute osteomyelitis can result in long-term adverse outcomes. Most cases of paediatric osteomyelitis are caused by hematogenous seeding of bacteria, rather than by direct bacterial invasion from the adjacent soft tissue (2). Approximately 5% of patients suffer from infections of multiple bones, as was observed in the present case (3). Owing to its rich vascularization, the metaphysis of long bones, e.g., femur or tibia, is the most common site to be infected (2). The metatarsal bones are affected in only 1%–2% of cases. Of these, the first metatarsal is impacted most commonly (2,4).

Our patient presented with erythema of both the dorsal and ventral sides of the foot, a finding we have coined “sandwich erythema.” The distance of the fourth metatarsal to both the dorsal and volar aspects of the foot is small, which likely explains how concentric inflammation around the metatarsal could manifest as seemingly non-continuous erythema on the dorsal and volar aspects of the foot.

Metatarsal osteomyelitis can be initially misdiagnosed as cellulitis (5), as in our case, because of their similarities in terms of clinical presentation. Cellulitis has a predilection for the lower extremity (6) and presents with fever, pain, and erythema of the affected extremity. The presence of sandwich erythema may be a useful clinical clue to distinguishing metatarsal osteomyelitis from cellulitis of the foot. Since erythema due to cellulitis spreads contiguously (7), multifocal involvement would be unusual in otherwise healthy children (8). Therefore, the finding of “sandwich erythema” should lead to consideration of osteomyelitis and appropriate investigations, including MRI and blood cultures undertaken. It should also be noted, however, that while “sandwich erythema” can serve as a useful clue for metatarsal osteomyelitis, its absence does not preclude this diagnosis; many cases of metatarsal osteomyelitis would not be expected to manifest in this manner.