Child with 3-year history of joint pain and swelling

Case

In December 2015, an 8-year-old boy presented to a pediatric clinic with a 2-week history of painful left knee swelling and difficulty walking. There was no history of injury or trauma. The family reported that he had a mild upper respiratory tract infection 6 weeks previously. The patient was otherwise healthy, without a relevant medical or surgical history such as a history of inflammatory bowel disease. He had a risk of tick exposure, as his family reported frequent camping trips in a Lyme-endemic region. Neither the patient nor his parents recalled him having a tick bite or rash consistent with erythema migrans (EM). He lived in southeastern Ontario and had not traveled outside the region in the previous year. His family history was relevant for juvenile idiopathic arthritis (JIA) and rheumatoid arthritis.

On examination, the child was afebrile, appeared well, and had no difficulties walking. Findings of neurologic and ocular examinations were normal. Examination of the left knee revealed a large effusion with tenderness to palpation but without warmth or erythema. There was full active range of motion, with mild pain at the extremes of flexion and extension. The remainder of the physical examination findings were normal and he had no EM rash. The boy underwent bilateral knee ultrasound scans for suspected reactive arthritis (ReA), which revealed large left and small right effusions with associated regional inflammatory synovitis. Bloodwork results showed mildly elevated inflammatory markers: an erythrocyte sedimentation rate of 45 mm/h and a C-reactive protein level of 21.4 mg/L (Table 1). Test results for antinuclear antibodies were negative. He was diagnosed with presumed viral ReA and was treated with nonsteroidal anti-inflammatory drugs, resulting in gradual pain relief. However, the boy continued to experience recurring joint swelling every few weeks.

In December 2017, the boy returned to his family physician with a history of recurring painful knee swelling in flares lasting 5 to 14 days before resolving. Occasionally his elbow, ankles, and shoulder joints were involved. There was never small joint involvement. On repeat examination he was afebrile and able to walk. The family reported history of a recurring eczematous rash on his abdomen and buttocks. Both knees were tender to palpation and there was mild pain on passive flexion and extension. The remaining physical examination findings were normal.

The boy was referred to a pediatrician for possible JIA or psoriatic arthritis. He was seen in February 2018, 26 months after initial symptom onset. His mother reported that he had become more fatigued over the past year and that his joint symptoms worsened with activity. Findings of a complete physical examination were unremarkable.

A workup was initiated by the consultant pediatrician. Bilateral knee x-ray scans showed no abnormalities. Results of a complete blood count with differential count were normal. Lyme serology tests were done and the results were reactive for immunoglobulin (Ig) M and IgG on enzyme-linked immunosorbent assay, with reactive IgG Western blot results (Table 1). The boy was diagnosed with LA and prescribed amoxicillin for 21 days. He was referred to the pediatric rheumatology department for further evaluation.

In April 2018, the patient was seen in the pediatric rheumatology department and was still experiencing episodic oligoarthritis of the large joints, with limitations to function and ongoing fatigue. Given his ongoing symptoms, doxycycline was prescribed for 28 days. At follow-up in July 2018, all symptoms had resolved and examination findings were normal.

Differential diagnosis

One of the challenges in diagnosing pediatric LA is differentiating it from other causes of infectious or inflammatory arthritis, namely septic arthritis, acute ReA, and nonsystemic-onset JIA. The main clinical features of each are listed in Table 2.^2-6