Arthritis or an Adjacent Fascial Response? A Case Report of Combined Pyomyositis and Aseptic Arthritis

Pyomyositis, accompanied by aseptic arthritis, has been previously documented in several publications. However, none of the authors in the mentioned case reports offered a pathophysiological explanation for this unusual phenomenon or proposed a treatment protocol. We present a case of a healthy, 70-year-old male who was presented to the emergency department 4 days after tripping over a pile of wooden planks and getting stabbed by a nail to his thigh. The right thigh was swollen. Unproportional pain was produced by a light touch to the thigh. A laboratory test and a CT scan were obtained. The working diagnosis was pyomyositis of the thigh and septic arthritis of the ipsilateral knee. The patient underwent urgent debridement and irrigation of his right thigh. An arthroscopic knee lavage was performed as well. Intraoperative cultures from the thigh revealed the growth of Streptococcus pyogenes and Staphylococcus aureus. Cultures from synovial fluid were sterile; thus, septic arthritis was very unlikely. The source of the knee effusion might have been an aseptic inflammatory response due to the proximity of the thigh infection. Anatomically, the quadriceps muscle inserts on the patella, and its tendon fuses with the knee capsule, creating a direct fascial track from the thigh to the knee. The inflammatory response surrounding the infection may have followed this track, creating a domino effect, affecting adjacent capillaries within the joint capsule, and causing plasma leakage into the synovial space, leading to joint effusion. Our suggested treatment is addressing the primary infection with antibiotics and considering adding anti-inflammatory therapy, given our suspicion that this process has an inflammatory component.


Introduction
Pyomyositis is an acute bacterial infection of skeletal muscles, with or without abscess formation.It was frst described in 1885 in Japan [1,2] and was previously known as tropical pyomyositis, earning its name from early 20thcentury studies that showed a signifcantly higher incidence in tropical climates, attributed to the consistently hot and humid weather [3].However, in the 21st century, similar cases are increasingly being reported in temperate countries [2,4].Ngor et al. [5], in a recent systematic review, found that half of the case series originated from high-income temperate countries.Other studies found an increase in pyomyositis incidence in the USA [6,7] and Australia [8].Some authors suggest that in tropical countries, it mainly afects young and healthy individuals, while in temperate countries, it afects immunocompromised adults [6,9].
Previous studies found a correlation between pyomyositis and other conditions, with diabetes mellitus (DM), HIV, and malignancy having the highest correlation [5,[9][10][11].Yu et al. [10], on a single-center retrospective study, found that among patients diagnosed with pyomyositis, 40% sufered from DM, 25% sufered from malignancies, and 10% suffered from autoimmune disease or asthma with long-term steroid usage.According to Ngor et al. [5], individuals with HIV infection were fve times more likely to be diagnosed with pyomyositis than those uninfected.Meesiri [12] suggested an association between pyomyositis and systematic lupus erythematosus, while other authors suggested a correlation between hematological malignancies and pyomyositis [13][14][15].Most patients show a full recovery with no long-term complications.Complications are often due to late diagnosis and include osteomyelitis, septic arthritis, deep vein thrombosis, and pneumonia [4].More serious complications include sepsis and septic shock, meningitis, acute respiratory distress syndrome (ARDS), and acute kidney failure (AKI) [4].Te mortality rate is unclear and varies greatly in the literature [5].One recent publication presented a fatal case of a 6-year-old female with a history of pelvic contusion who developed pyomyositis of the iliopsoas muscle.Due to a delayed diagnosis, the patient deteriorated to septic shock and died approximately 16 hours after admission [16].Similarly, another recent case report described a 16-year-old, otherwise healthy female who presented with abdominal pyomyositis.A delayed diagnosis of approximately three weeks resulted in her death [17].Te common pathogen is Staphylococcus aureus, which occurs in both temperate and tropical areas and has the most common location in the thigh muscles [5,7,[18][19][20].Te precise underlying mechanisms of pathophysiology remain unclear.Some authors suggest that the typical mode of dissemination is hematogenic and less frequently via adjacent structures [7,21,22].History may reveal blunt or penetration trauma, exertional exercise, or prolonged vascular insufciency [6,9,23].However, Bickels et al. [24] reported trauma in less than 5% of cases.Agarwal et al. [9] suggested that trauma to muscle tissue results in iron release from myoglobin.Tis iron is then used by bacteria to grow and proliferate.Moreover, local hematoma may provide a favorable environment for bacteria to bind.Patients may complain of fever, chills, and myalgia.Physical examination reveals swelling with or without overlying erythema.On palpitation, tenderness with wooden induration is seen [9,23].Pyomyositis is sometimes misdiagnosed as cellulitis [25,26] or may present secondary to cellulitis [27].
Lab tests reveal leucocytosis with a shift to the left.Creactive protein (CRP) and erythrocyte sedimentation rate (ESR) are usually elevated [6,9,23].Serum muscle enzymes, such as creatine phosphokinase (CPK), tend to remain within normal levels despite muscle destruction [9,23,28,29].Blood cultures are sterile in 70-90% of patients [9].When present, the abscess may be aspirated.Verma et al. examined 40 patients and revealed positive cultures in 42.5% of abscess aspirations; among them, 100% showed growth of Staphylococcus aureus [30].In their study, Chattopadhyay et al. investigated 12 patients who underwent abscess aspiration.Teir fndings indicated that 50% of the cultures were positive for Staphylococcus aureus, while 17% of the cultures yielded negative results [18].Section et al. examined 33 pediatric patients with tissue cultures and found that 50% were positive for Staphylococcus aureus and 36% were sterile [19].
On imaging investigation, plain radiographs are usually normal and can be used to rule out other pathologies such as fractures, malignancy, or advanced osteomyelitis [9].Ultrasound (US) can serve as the frst imaging method as it is available, noninvasive, and spares radiation.It demonstrates heterogenic tissue with hypoechoic areas [9] and can differentiate cellulitis from pyomyositis [31].Point of care US (POCUS) is particularly recommended for children because more advanced imaging techniques often require longer scanning times and sedation [32,33].A computed tomography (CT) scan may demonstrate muscle enlargement with heterogeneous attenuation and focal edema [34] although it cannot distinguish between an abscess and a swollen muscle [9].Magnetic resonance imaging (MRI) is the most sensitive and specifc mode of imaging, and it remains the gold standard [6,7,9,21].In the initial stage of pyomyositis, MRI shows an increased muscle volume and an increased signal on T2-weighted images, along with a loss of normal muscular architecture and variable enhancement on the postcontrast sequences [35,36].MRI demonstrates the extent of involvement, the location of fuid collection [6], the presence of an abscess [21], and coexisting pathologies such as cellulitis, osteomyelitis, and arthritis [6,21,34].MRI can further assist in diferentiating pyomyositis from necrotizing fasciitis.Pyomyositis is characterized by difusely hyperintense fuid-sensitive signals and intramuscular abscesses, whereas necrotizing fasciitis presents a peripheral band of hyperintensity.Fascial enhancement occurs in both conditions but is thicker and uneven in pyomyositis [35].Maravelas et al. [7] demonstrated that nearly 9% of patients exhibited concurrent pyomyositis and septic arthritis.Teir fndings suggested that joint infection could either lead to secondary pyomyositis or serve as a complication of primary pyomyositis.Gordon et al. [34] demonstrated that among 32 patients, 8 patients had evidence of fuid in the adjacent joint on CT or MRI.Te author did not further detail whether the fuid was infected or sterile.
Pyomyositis with aseptic arthritis is a rare condition that was previously described in some case reports [37][38][39][40][41][42][43].Patients present with or without the abovementioned clinical presentation of pyomyositis and an efusion of the proximate joint.Te laboratory reveals high infammatory markers, a positive culture of the muscle tissue, and a negative culture of the joint fuid.Blood culture results can either yield positive or negative fndings.

Case Report
An otherwise healthy 70-year-old male, with a history of penicillin allergy, was presented to the emergency department (ED) with a blunt contusion to his thigh after tripping over a pile of wooden planks.On physical examination, the patient could walk with full weight and without limitations.On inspection, there was no evidence of contusion, soft tissue swelling, hematoma, limb deformation, or open wounds.Ranges of motion of the hip joint, knee, and ankle were normal.He was neurovascularly intact.Te only positive sign was that on palpation, there was mild tenderness of the right thigh anteriorly.No fractures, soft tissue swelling, or gas were seen on plain radiographs.
Te patient was discharged home with analgesics.Te patient returned to the ED four days later, complaining of severe pain in his right thigh and an inability to move the right hip joint and knee.He also reported a fever 2 Case Reports in Rheumatology with shivering chills in the previous 48 hours.He denied any further injury since his previous presentation; although, after taking a thorough history, he reported a possible penetrating injury to his thigh from a screw presented on one of the planks.On physical examination, the right thigh was swollen, and a pinpoint wound was seen on its lateral aspect without any redness or discharge from the wound.Te patient's knee was set in an extended position, and he could not fex it due to severe pain.When inspecting the right knee, no signifcant warmth, joint swelling, or signs of infection were seen, but unproportional pain and tenderness were produced by a light touch of the thigh and knee.Te neurovascular examination revealed no defcit.On laboratory exams: a leucocyte count (WBC) of 28,800 c/uL (normal values: 4,500 to 11,000 c/uL) and a CRP of 330 mg/L (normal values: less than 8 mg/L).Plain radiographs did not reveal any fractures or abnormalities in the bones.
A CT scan of the thigh was performed before and after an IV iodinated contrast injection.
Te precontrast scan demonstrated a mild hypodensity of the right quadriceps muscle, specifcally the vastus intermedius.Tere was evidence of increased right thigh circumference compared to the left (Figures 1, 2).
On the postcontrast scan, along the vastus intermedius of the quadriceps muscle, there was evidence of fuid dissecting between muscle fbers with a peripheral enhancement.Te vastus lateralis was partially involved as well.Te regional fascia was involved as well; fuid was seen along the soft tissues of the thigh, up to the iliopsoas muscle.No gas was evident in the soft tissues.
Moreover, the CT scan demonstrated suprapatellar effusion in the right knee (Figure 1).
Based on clinical and radiographic fndings, the working diagnosis was thigh muscle pyomyositis or necrotizing fasciitis with knee septic arthritis.As the patient presented with acute, severe systemic sepsis and an inability to move the knee, a decision was made to transfer the patient as soon as possible to the operating room.Terefore, we decided not to aspirate the knee in the ED but rather to perform a complete arthroscopic lavage.Hence, the patient underwent urgent fasciotomy, debridement, and irrigation of his right thigh with an arthroscopic knee lavage.
Arthrocentesis of his right knee was performed and revealed 15 ccs of clear synovial fuid.Cultures were taken from both the right thigh and the right knee.
Te patient was admitted to the orthopedic department and was treated empirically with antibiotics (IV Clindamycin, because of the medical history of an allergic reaction to penicillin) after blood cultures were taken.Due to immobility, prophylactic anticoagulation treatment was initiated.Analgesics were administered as well, with nonsteroidal anti-infammatory drugs (NSAIDs) included.Gram stain of the synovial fuid was negative, WBC from the knee was 74,000 c/uL, and glucose was 55 mg/dl (blood glucose was 186 mg/dl).
On postoperation day (POD) 1, blood examination did not demonstrate any signifcant improvement in acutephase reactants.Te surgical wound on POD1 is shown in Figure 3.
On POD2, the patient underwent a second debridement of the thigh wound and a second arthroscopic knee lavage.Te second debridement was performed to exclude an ongoing deep soft tissue infection ("second look").Te second knee lavage was performed due to the high leukocyte and low glucose count from the frst knee lavage to rule out the relatively remote possibility of septic arthritis with a negative synovial fuid culture.
On POD3, blood exams demonstrated a slight improvement in acute-phase reactants (WBC of 24,000 c/uL [PMN 81%] and CRP of 224 mg/L).Moreover, culture results had arrived: thigh wounds cultured with cultivated Streptococcus pyogenes and methicillin-sensitive Staphylococcus aureus (MSSA).Blood cultures and knee synovial cultures were negative.Later, the treatment was changed to cefazolin and clindamycin to give the best therapy for MSSA as well as group A Streptococcus.
Te patient was still in pain and could not bear weight or fex his right knee.Physiotherapy treatment was initiated.
On POD 11, after a plastic surgeon consultation, the patient underwent a third superfcial tissue debridement and an approximation of wound margins.Te debridement of the thigh wound was performed to remove excess fbrin tissue from the wound edges for quicker wound healing.
Due to an improvement in the patient's clinical presentation and laboratory exams, the antibiotic course was stopped, and the patient was discharged home.
Te patient reported a signifcant clinical improvement two weeks after his outpatient clinic visit.On examination, full knee range of motion (ROM) without pain on weight bearing was inspected.On blood exams, infammatory markers were no longer elevated (WBC was 8.7 c/uL, and CRP was 4.6 mg/L).

Discussion
We presented a case of pyomyositis of the thigh, with both MSSA and Streptococcus pyogenes that grew from the muscle, and a unique presentation of arthritis that was not diagnosed on the frst inspection, as it was "masked" by the signifcant swelling of the ipsilateral thigh.Tis arthritis was frst suspected as septic, but as no organisms grew on aerobic and anaerobic cultures, gram stain negative, and inconclusive synovial fuid chemistry from two diferent knee lavages, we conclude that the synovial fuid in access is sterile.
Staphylococcus aureus was described in the literature as the most common bacterial cause of pyomyositis, mostly in tropical countries [44,45].Pyomyositis induced by Streptococcus pyogenes was also described in several case reports [46,47].It is extremely aggressive, and muscle necrosis may occur [48].Te combination of Staphylococcus aureus and Streptococcus pyogenes as a cause of pyomyositis was not described previously in the literature, as far as we know.Upon literature review, there were 7 publications presenting a total of 10 cases of pyomyositis with aseptic arthritis [37][38][39][40][41][42][43].Tese reports included 4 children and 6 adults; among them, 3 sufered from diabetes.Five cases involved the hip joint, three involved the knee, one involved the shoulder, and one involved the elbow.Te joint culture was negative in all 10 cases, and the muscle culture was positive for Staphylococcus aureus in 7 cases (Table 1).None of the authors of the aforementioned case reports provided a pathophysiological explanation for this unusual phenomenon.
Similar to the abovementioned cases, in our case, septic arthritis was primarily suspected, but a defnitive diagnosis could not be established since the synovial fuid was sterile on two diferent knee arthroscopic lavages.Synovial cultures have a sensitivity of 75-95% [49].Te synovial fuid WBC count of more than 50,000 cells/mcl has high specifcity for septic arthritis, while a low glucose level is 85% specifc for septic arthritis.However, since our patient presented in this case had a rapid arthroscopic lavage, clear-appearing synovial fuid, and sterile cultures with gram stain negative, we presume that he did not have septic arthritis, although this diagnosis cannot be ruled out.
So why does pyomyositis of the thigh cause a sterile efusion of the adjacent joint?Case Reports in Rheumatology Te frst one is reactive arthritis.Reactive arthritis is described as a sterile joint infammation triggered by an infection that develops days to weeks after the acute infection [50].Our patient presented with both the onset of myositis and arthritis, so the timeline is inadequate for reactive arthritis.
Furthermore, reactive arthritis usually follows urogenital or gastrointestinal infections [50,51].No cases of reactive arthritis following soft tissue infections were found at the time of writing this article.After conducting a literature review and convening a multidisciplinary meeting involving our orthopedic, rheumatology, and infectious disease units to discuss the patient, we concluded that the likelihood of reactive arthritis occurring in our patient was low.
Te quadriceps tendon inserts on the patella, and its fbers fuse with the knee capsule [57,58].Hence, there is a clear track between the thigh compartments and the knee joint.
During an infection, infammatory mediators afect vascular permeability, which results in an increase in interstitial fuid [59].Following the above-mentioned fascial track, this infammatory response may lead to a domino efect, where infammation spreads along adjacent structures.Eventually, afecting capillaries within the joint capsule and causing plasma leakage into the synovial space-therefore joint efusion [60].In other words, the knee efusion might have been a local response due to the fascial continuity of the afected areas.
In the last two decades, the study of fascia has gained signifcant attention in the anatomical felds, shifting the traditional focus from isolated muscle or capsule structures to the integral roles of connective tissues [61].Understanding the fascial system provides a more precise comprehension of anatomy, which is essential for diagnosing and treating musculoskeletal issues.
In our case, the fascia structures formed the basis for the AFR hypothesis.We mentioned similar cases involving various muscles and adjacent joints in the upper and lower limbs.Te muscle-joint-capsule fascial relationship should be studied specifcally for each joint.For instance, in the upper limb, the brachial fascia originates from the intermuscular septum, which provides insertion points for some arm and forearm muscles before merging with the elbow capsule [62].

Conclusions and Recommendations
Aseptic joint efusion following pyomyositis of an adjacent muscle is described in several case reports.Te incidence is unknown.
We presented a case of pyomyositis of the quadriceps and knee efusion of the ipsilateral knee and suggested a hypothesis: Tere is a myofascial continuity between the muscle and the joint capsule.Tis continuity creates a track between the muscle and the joint space.Infammatory mediators due to myositis may trigger adjacent fascial structures along this track, leading to an infammation reaction in the adjacent joint.
Early diagnosis is challenging, as the initial presentation of AFR arthritis mimics septic arthritis.Te authors suggest that, given the nonspecifc diagnostic tools currently available, AFR arthritis should be managed as septic arthritis until proven otherwise.Septic arthritis is an orthopedic emergency that requires urgent surgical lavage [63], and delay in treatment can result in joint destruction [64].
Future research is needed to identify rapid and more specifc fndings to diferentiate myositis with septic arthritis from myositis with AFR aseptic arthritis.
Once septic arthritis is ruled out, treatment of AFR should be focused on the main afected site.Infection of the main afected site should be addressed and treated with antibiotics.Given the suspicion that this process has an infammatory component, incorporating anti-infammatory therapy may ofer potential benefts.Additional therapy should include physiotherapy and analgesics.A further study should be conducted to prove this hypothesis.

Figure 1 :
Figure 1: Coronal view of a CT scan, demonstrating an increase in right thigh circumference.

Figure 2 :
Figure 2: Axial view of a CT scan, demonstrating an increase in right thigh circumference.

Table 1 :
Previously published cases of pyomyositis with aseptic arthritis.