Psoriatic Arthritis: A red, painful and swollen foot overlying a bone erosion.

PRESENTATION

Over the course of three days, intense pain, redness and swelling of the left fifth toe wrought great havoc to a 67-year-old man [Figure 1]. There was no antecedent trauma, insect bite, nor change in medication. He denied fever, chills or walking barefoot. He had known hypertension and vitamin D deficiency. As a child, he was diagnosed with psoriasis; skin lesions intermittently recurred into adulthood. Both his parents had gout and a grandfather had psoriasis.

Figure 1.

Left foot photograph depicting swelling and redness of the 5^th toe and dorsum of the foot.

ASSESSMENT

He was initially evaluated at an urgent care facility where a left foot x-ray showed early osseous destruction of the fifth toe metatarsal head [Figure 2]. This radiographic appearance together with erythema of the overlying skin raised concern for osteomyelitis, prompting hospitalization.

Figure 2.

Oblique plain radiograph of the left forefoot revealing focal erosion-like osseous destruction [black arrows] of the distal portion of the left fifth toe proximal phalanx.

On admission to the Medicine Service at the Johns Hopkins Bayview Medical Center, maximum temperature was 37°C, heart rate was 65 beats per minute, blood pressure 132/73 mm Hg, with oxygen saturation of 96% on room air. Heart, lung and abdominal examinations were unremarkable. There was diffuse erythema over the dorsum of the left foot. The left fifth toe was markedly swollen, exquisitely tender and erythematous [Figure 1]; toe motion was greatly limited due to pain. Ambulation was impaired. In addition, there were scattered erythematous plaques with scale, over the elbows, umbilicus and knees.

Laboratory studies revealed a leukocytosis, with WBC of 14.95 k/mm³ (78% neutrophils), mildly elevated acute phase reactants, and mild renal insufficiency [Table 1]. Liver function studies were normal.

Table 1.

Laboratory Studies on Admission

Tests	Patient’s values	Normal Values
White blood cell count	14.95	4.50–11.00 k/cu mm
Hemoglobin	15.1	13.9–16.3 g/dL
Creatinine	1.4 0	.60–1.30 mg/dL
Erythrocyte sedimentation rate	24	1–20 mm/h
C-reactive protein	1.21	<0.29 mg/dL

Given the clinical features of a warm, erythematous and swollen toe and foot, together with radiographic evidence of bony erosion, a diagnosis of osteomyelitis was invoked. Intravenous vancomycin and piperacillin-tazobactam were commenced. Further diagnostic clarity was sought with imaging and microbiology studies. Magnetic resonance imaging (MRI) examination of the forefoot was performed, demonstrating a 1.9 x 0.5 x 0.8 cm lesion in the great toe, hyperintense on the T2 weighted image, along with a small fluid collection ventral to the head of the fifth toe proximal phalanx, with peripheral rim enhancement [Figure 3A]. The Radiology service raised concern for a focal erosion at the head of the fifth toe adjacent to the fluid collection with bone marrow edema [Figure 3B]. These findings, consisting of bone erosion, with enhancement and marrow edema, were deemed consistent with osteomyelitis.

Figure 3.

Magnetic resonance imaging examinations of the left forefoot showing periosseous fluid collection (panel A, T2-weighted image) and peripheral soft tissue enhancement (panel B, post contrast VIBE sequence) surrounding the fifth proximal phalanx. This fluid collection was targeted for image-guided aspiration and biopsy.

To identify the putative organism and direct an extended course of antimicrobial therapy, aspiration of the fluid collection and adjacent bone biopsy were pursued in coordination with the Infectious Disease service. Two image-guided attempts at biopsy and aspiration were undertaken. First, ultrasound-guided aspirate of the left 5^th toe proximal interphalangeal joint was performed by the Radiology service; only 1 WBC/mm³ was identified with negative synovial fluid culture. Two days later, CT guided biopsy of the head of the left 5^th toe proximal phalanx, where imaging identified focal irregularity and erosion, yielded 8 WBC/mm³. Aspiration of the fluid collection, as before, was negative for microorganisms, with stains and cultures for bacterial, fungal and acid-fast bacillus pathogens. Blood cultures similarly remained sterile. Further, surgical pathology review of the biopsied left 5^th phalanx yielded minimal bone fragments, lacking histological features of osteomyelitis.

At this juncture, with bone biopsy and fluid aspirate negative for growth, and with the continued absence of fever, antibiotic therapy was discontinued. Next, in the effort to broaden the differential diagnosis, he was tested for rheumatoid factor, antinuclear antibodies and anti-cyclic citrullinated peptide antibodies, each of which was negative. However, HLA-B27 antigen returned positive. Given his sustained hemodynamic and clinical stability, he was discharged with scheduled outpatient follow-up care.

At home, he continued to experience toe pain. Within two weeks, he saw his primary care doctor. With the diminishing support for the original formulation of bacterial osteomyelitis, and given the family and personal history of cutaneous psoriasis since childhood, together with the positive HLA B27 assay, bone erosions, and examination notable for swollen toes suggesting dactylitis, consideration for a primary inflammatory arthritis was raised.

Rheumatology consultation yielded additional history of Achilles enthesitis several years earlier. Further, at that appointment, there was tenderness at the Achilles tendon insertion site at the calcaneus, consistent with enthesitis, and synovitis in the metatarsophalangeal and proximal interphalangeal joints. With a personal history of psoriasis and inflammatory joint findings, he was diagnosed with psoriatic arthritis.

DIAGNOSIS

The diagnostic challenges posed by this case underscore that an inflammatory arthritis may mimic an infectious process, including osteomyelitis, septic arthritis and cellulitis. In addition, new musculoskeletal symptoms in those with prior cutaneous psoriasis should trigger consideration for development of psoriatic arthritis. Further, an acute inflamed joint(s) should also prompt consideration of an acute microcrystalline arthropathy (i.e., a gout or pseudogout flare).

A key clinical feature of psoriatic arthritis is dactylitis, swelling of the whole digit resulting from inflammation of the tendon sheath. Dactylitis may affect fingers or toes. In addition, the presence of enthesitis, swelling and pain at a tendon insertion site (Achilles enthesitis being the most common) is a clinical clue to consider psoriatic arthritis and other seronegative spondyloarthropathies.

In the present case, the true diagnosis remained elusive until recognition of the clinical features of dactylitis and enthesitis. Approximately one-quarter of patients with cutaneous psoriasis will later develop psoriatic arthritis^1–4. While joint inflammation most often follows the presence of cutaneous features of psoriasis, synovitis may even precede the onset of skin disease^1–4. Notably, patient with psoriatic arthritis may have very mild skin disease with severe arthritis or vice versa. Erosive joint disease is common early in the course of psoriatic arthritis, even within months of disease onset as seen in this case³. Moreover, osseous destruction interpreted radiographically as erosion may occur in several primary inflammatory arthropathies (including but not limited to rheumatoid arthritis and B27-arthropathy/spondyloarthropathy), and are not specific to infectious bone disorders alone (i.e., osteomyelitis).

There are several distinct clinical phenotypes within the spectrum of psoriatic arthritis⁴. Five subsets have been recognized, including (i) predominant distal interphalangeal joint involvement; (ii) arthritis mutilans; (iii) symmetric rheumatoid-like polyarthritis; (iv) asymmetric oligoarthritis; and (v) patients in whom the predominant feature is sacroiliitis⁴. The fourth subset predominates, and prominently features dactylitis, as seen in the present case. Further, psoriatic arthritis may also result in uveitis, or ocular inflammation of the uveal tract.

In this specific case, during the first 48 hours of the hospital course, the clinicians involved in his care were confident the clinical features and radiographic findings supported the diagnosis of osteomyelitis. Yet, as the patient lacked responsiveness to antimicrobial therapy, the hazard of premature diagnostic closure became evident. Such a cognitive error, resulted in a delay in diagnosis and prolonged the patient’s debility. Ultimately, broadening the differential diagnosis for bone and joint pain with overlying erythema, swelling and radiographic evidence of bone erosion, enabled his physicians to explore other etiologies, which led to recognition of psoriatic arthritis as the cause of his symptoms.

Finally, recognition of the prior and ongoing psoriatic skin disease could have played a critical role in earlier resolution of this diagnostic dilemma^5,6. As such, there is apt to be a high prevalence of undiagnosed psoriatic arthritis among patients with psoriasis seeking care with active musculoskeletal features^5,6.

MANAGEMENT

After being diagnosed with psoriatic arthritis, the patient was started on therapy with weekly methotrexate and infliximab, a tumor necrosis factor-alpha inhibitor. This combination therapy was initiated as front line treatment with the goal of quickly improving his symptoms due to the presence of erosive disease. With treatment, his inflammatory markers normalized, swelling and pain decreased in the toe and fingers, and he experienced improvement in functional status. At a year and a half of follow-up, the patient had not developed any further erosive disease in his feet or other joints. His psoriasis remained in remission.