Glenohumeral Joint Sepsis Caused by Streptococcus mitis: A Case Report
Septic arthritis of the shoulder girdle remains relatively uncommon, with Staphylococcus aureus and β-hemolytic streptococci the typical offending organisms in adult patients. Rare cases of Streptococcus viridans, an oral bacterium with low virulence, have been reported in the setting of septic arthritis, mostly involving the knee joint or the sternoclavicular joint. In this article, we report a case of Streptococcus mitis infection of the glenohumeral joint that likely resulted from hematogenous spread after oral trauma in a patient with poor underlying dentition. Prompt diagnosis followed by arthroscopic irrigation and débridement of the glenohumeral joint resulted in a satisfactory clinical outcome.
Septic arthritis predominantly involves the weight-bearing joints of the hip and knee, which account for nearly 60% of cases.1 In contrast, the shoulder joint is involved in 10% to 15% of cases, though this number may be higher among intravenous (IV) drug users.2 The most common causative organisms are the Staphylococcus species, followed closely by β-hemolytic streptococci, with these 2 groups accounting for more than 90% of all cases.3 The Streptococcus viridans group belongs to normal oral flora residing predominantly on the surface of teeth. Although well known for its ability to colonize heart valves and frequently cause bacterial endocarditis, this group has rarely been associated with septic arthritis. Furthermore, Streptococcus mitis, a subgroup of S viridans, has been implicated even less commonly.
In this article, we report a case of glenohumeral joint septic arthritis caused by S mitis. To our knowledge, such a case has not been previously reported in the English literature. Given the low virulence of this orally based bacterium, treating physicians must maintain clinical suspicion for the organism in the setting of persistent joint effusion and pain in association with periodontal disease or trauma. The patient provided written informed consent for print and electronic publication of this case report.
Case Report
A right-hand-dominant 54-year-old man presented to Dr. Gruson with complaints of persistent right shoulder pain associated with worsening range of motion (ROM). Three weeks earlier, the patient reported being assaulted and noted progressive swelling about the right shoulder. He denied fevers, chills, or prior shoulder problems. Although his past medical history was remarkable for hepatitis C and diabetes, he was not taking any diabetic medications at that time. A review of systems was remarkable for poor dental hygiene, and the patient was missing several teeth, which he said had been knocked out during the assault. Physical examination revealed diffuse tenderness about the right shoulder and severe pain with all passive movement. The shoulder was pseudoparalyzed. There were no subcutaneous collections, wounds, or ecchymosis about the shoulder. Mild calor was noted on the right shoulder relative to the left. Radiographs of the right shoulder showed no acute osseous abnormalities.
Magnetic resonance imaging (MRI), which was urgently obtained to assess the integrity of the rotator cuff and the location of the effusion, showed a large subacromial and glenohumeral joint effusion as well as diffuse muscular edema (Figures 1A-1C). 
In light of the elevated infection findings of the laboratory tests and the positive culture, urgent arthroscopic irrigation and débridement of the right shoulder were indicated. Given the organism identified, transesophageal echocardiography was performed; there were no valvular vegetations. Creation of the posterior glenohumeral portal resulted in egress of turbid fluid, which was sent for culture. The subacromial space and the glenohumeral joint were thoroughly lavaged and the copious hemorrhagic synovitis débrided (Figures 2A, 2B). 
The 8-week course of antibiotics normalized the patient’s ESR to 13 mm/h. Follow-up MRI showed improvement in the soft-tissue edema. Clinically, the patient reported minimal shoulder pain. He was undergoing physical therapy to regain strength and ROM.
Discussion
Staphylococcus aureus is the leading causative organism of septic arthritis, accounting for more than 60% of all cases.4 Conversely, the Streptococcus viridans group is rarely implicated in septic arthritis, accounting for <1% of cases.4S viridans is part of the commensal oral flora and has low virulence. This heterogeneous group is subdivided into S mitis, S salivarius, S anginosus, S mutans, and S bovis. The S mitis group is further subdivided into S sanguinis (formerly known as S sanguis) and S mitis. Infection by an organism of the S viridans group usually occurs on a previously injured focus, and the organism is a causative agent of bacterial endocarditis.5 Reported cases of septic arthritis caused by S viridans have predominantly involved the knee joint—with severe osteoarthritis, poor dental hygiene, and prior IV drug use identified as risk factors.5-7The shoulder joint is seldom involved in septic arthritis; estimated incidence is under 8%.8 Although overall incidence may rise in an increasingly elderly patient population, incidence of shoulder infection remains low.2,9
