Emerging from the COVID-19 pandemic is an increased interest in the extrapulmonary or atypical manifestations of the SARS-CoV-2 coronavirus infection. This is especially true in pediatrics since a serious Kawasaki-like syndrome known as multisystem inflammatory syndrome in children has been described ^[1]. Transient synovitis (TS) in the setting of a COVID-19 infection has been reported only a few times in the literature [2-6]. TS is a common cause of hip pain in the pediatric population. The exact etiology is unknown, but it is commonly associated with viral infections. We report a case of TS occurring 5 weeks after a positive SARS-CoV-2 polymerase chain reaction (PCR) test. In addition, we provide a summary of the other six reported cases found through a literature review. Our goal is to raise awareness that TS can occur in the setting of a COVID-19 infection and to compare the reported cases to demonstrate differences in the clinical presentation of TS associated with COVID-19 infection.

A 4-year-old female with no medical history presented to the emergency department (ED) with one day of right hip pain and inability to bear weight to her right lower extremity. The patient and her mother denied any trauma. Five weeks prior, this patient had a fever of 101.1°F, upper respiratory symptoms, and a positive PCR test for SARS-CoV-2. The fever and upper respiratory symptoms resolved a few weeks before the onset of right hip pain. The patient was afebrile on presentation to the ED. On physical exam, the patient was resting with her right hip in flexion, external rotation, and abduction. There was pain with passive right hip motion. Peripheral white blood cell (WBC) count was 12.08 × 10⁹/L with 72% neutrophils, C-reactive protein (CRP) was 6.5 mg/dL, and erythrocyte sedimentation rate (ESR) was 14 mm/h. Radiographs of the right hip and femur demonstrated no osseous abnormalities. Ultrasound (US) of the right hip demonstrated a 6 mm effusion (Fig. 1).

Due to the presence of the effusion and the patient meeting three modified Kocher criteria (inability to bear weight, WBC >12 x 10⁹/L, CRP >1 mg/dL), there was a relatively high likelihood of septic arthritis ^[7], therefore the decision for hip aspiration was made. The patient was taken to the operating room and an US-guided right hip aspiration was successfully performed under anesthesia. 4 mL of straw-colored fluid was obtained and sent to the laboratory for analysis. Synovial fluid analysis demonstrated a cloudy appearance, 8,375 WBC’s (86% segmented neutrophils, 3% lymphocytes, 8% monocytes, 3% eosinophils), and 18,500 red blood cells. The gram stain was negative and ultimately the aerobic, anaerobic, and fungal cultures were negative for any growth. The patient was admitted to the floor for observation and scheduled non-steroidal anti-inflammatory drugs (NSAIDs). By hospital day 1 the patient was able to move her right hip and walk without pain. Given other etiologies of acute right hip pain were excluded, including septic arthritis, fracture, and slipped-capital-femoral-epiphysis, a diagnosis of TS was made. With her continued improvement on NSAIDs alone, the patient was discharged on hospital day 1 with a plan for close monitoring with short interval follow-up. The patient followed up in the pediatric orthopedic clinic 2 weeks later and remained asymptomatic, walking with a normal gait, and denying any pain. Given the resolved pain with NSAIDs only and no antibiotics, the patient was discharged from formal follow-up and instructed to follow-up as needed if new symptoms developed.

TS is a self-limiting condition and one of the most common causes of pediatric hip pain ^[8]. Despite this, it remains a diagnosis of exclusion as there is a broad differential for pediatric hip pain and urgent conditions, such as septic arthritis must be ruled out ^[9]. The exact etiology of TS remains unclear, but it has often been linked to a preceding viral upper respiratory infection (URI) [8, 10, 11]. Proposed pathophysiology of TS related to viral illness involves non-specific inflammation targeting the synovial lining ^[12]. It is extremely important to rule out septic arthritis when evaluating a pediatric patient with acute hip pain. The Kocher criteria, which include fever over 38.5°C, inability to bear weight, ESR >40 mm/h, and WBC >12,000 × 10⁹/L are utilized to assist in the diagnosis of septic arthritis. The probability of septic arthritis given the presence of one, two, three, or four of these criteria are 3%, 40%, 93%, and 99%, respectively ^[7]. A modified Kocher criterion that includes CRP >1 mg/dL has demonstrated probabilities of septic arthritis diagnosis when three, four, and five criteria are met to be 83%, 93%, and 98%, respectively [13, 14, 15]. US-guided hip aspiration is generally indicated if the intracapsular effusion is found in the US in addition to ≥2 Kocher criteria ^[9]. A synovial WBC >50,000 is a common threshold used by orthopedic surgeons to determine the need for surgical irrigation and debridement ^[16]. Especially in the setting of a viral illness, TS can be confidently diagnosed once imaging and joint fluid analysis have ruled out other serious causes of pediatric hip pain. The manifestations of COVID-19 are reported to be overall less severe in children ^[17]; however, there have been reported cases of a post-COVID-19 pediatric multisystem inflammatory syndrome, sparking an interest in extrapulmonary manifestations of COVID-19 ^[1]. Despite the association of TS with viral illness, there have been very few reported cases of TS related to COVID-19 infection. We conducted a literature search and identified five articles with a total of six reported cases of TS in the setting of COVID-19 infection (Table 1). All reported cases except Turazza et al. had associated URI symptoms. In the case of Turazza et al., a hospital-mandated SARS-CoV2 PCR test was incidentally positive, suggesting that hip pain could be an initial presenting symptom of COVID-19 infection. Interestingly, the timing of URI symptoms differed among cases. In our case, URI symptoms preceded coxalgia by 5 weeks. In the other cases, URI symptoms of COVID-19 preceded hip pain by 2 weeks, 1 week, 3 days, and 0 days. Before our case, there was only one report of COVID-19-associated TS in which an US-guided hip aspiration was performed ^[6]. In that case, the patient had persistent hip pain and effusion, prompting aspiration despite only one Kocher criteria being met. All patients in reported cases achieved complete symptom resolution at follow-up.

Our case differs from previously reported cases of TS in the setting of COVID-19 in that three of the modified Kocher criteria were met (inability to bear weight, WBC >12 × 10⁹ L, CRP >1 mg/dL), indicating our patient for a joint aspiration. Of the other cases, only one or two modified Kocher criteria were met. The joint aspiration in our case was reassuring, as the presence of 8,375 synovial WBCs is not only below the commonly used threshold for septic arthritis of 50,000, but also below a higher sensitivity diagnostic cut-off value of 17,500 as reported by Li et al. ^[18]. Further, the patient in our case had complete resolution of symptoms at her 2-week follow-up after a course of NSAIDs and rest. Inflammatory markers can be present in the setting of a current or recent COVID-19 infection ^[19], and this rise in inflammatory markers could distort a case of TS into appearing septic. Our case provides further evidence that TS can occur in the setting of COVID-19, and TS in the setting of COVID-19 can have varying clinical presentations and laboratory findings based on our literature review. Thus, it is important to consider COVID-19 as a cause of TS in the pediatric population. In our case, factors favoring a diagnosis of TS associated with COVID-19 included the delayed onset of hip pain following initial COVID-19 symptoms, rapid symptom resolution with NSAIDs alone, a synovial WBC count well below septic thresholds, absence of fever at presentation, and negative synovial fluid cultures.

We report a case of TS as a sequela of a COVID-19 infection and provide a review of literature that includes six previously reported cases. We performed an US-guided hip joint aspiration to rule out septic arthritis which was done in only one of the previously reported cases. It is important to consider COVID-19 as a cause of TS in pediatric patients with hip pain.

TS can occur secondary to COVID-19 infection. COVID-19 should be considered as a cause of TS, especially in cases in which the initial workup suggests septic arthritis.