Background/Purpose:  Staphylococcus aureus is the most common cause of adult septic arthritis and the incidence of methicillin-resistant Staphylococcus aureus (MRSA) infections continues to rise. There is a relative lack of information on clinical presentations and outcomes in MRSA septic arthritis and how it differs from methicillin-sensitive Staphylococcus aureus (MSSA) septic arthritis. Our aim was to evaluate the differences in clinical features and outcomes between patients with MRSA and MSSA septic arthritis.

Methods: This is a retrospective chart review study performed at a tertiary level referral hospital. We queried the electronic database for patients with a discharge diagnosis of pyogenic arthritis between Jan 1st 2000 and Dec 31^st2013. We only included native joint septic arthritis. We collected data on patient demographics, clinical information, and patient outcomes. Statistical analysis was performed using SPSS staistics 20.0. The institutional review board at Texas Tech University approved this study.

Results: We identified 274 patients with native joint septic arthritis. Staphyloccocus aureus caused 122 cases. MRSA caused septic arthritis in 45 patients; MSSA caused septic arthritis in 77 patients. Patient characteristics, clinical features and outcomes of MRSA septic arthritis patients and MSSA septic arthritis patients are summarized in Table 1. MRSA and MSSA septic arthritis occurs predominantly in males. There were no differences between the two groups in mean age. MRSA septic arthritis patients had more comorbidities than MSSA septic arthritis patients. There were no statistically significant differences in the initial clinical presentation between both groups (fever, leukocytosis, presence of bacteremia, polyarticular involvement, mean leucocyte count on synovial analysis). Only 77.8% of MRSA patients were treated initially for MRSA. The MRSA septic arthritis patients had a higher mean number of joint surgeries and longer hospital lengths of stay. They also had worse patient and joint outcome and higher rates of development of osteomyelitis adjacent to the joint.  

Conclusion: We recorded a significant number of MRSA septic arthritis cases from the year 2000 to 2013. MRSA septic arthritis affects patients with multiple comorbidities and is associated with worse outcomes. Only 77.8% of MRSA septic arthritis patients were covered empirically for MRSA on initial presentation. Since MRSA is an emerging clinical entity in septic arthritis, rheumatologists have to consider this pathogen in patients with septic arthritis and more research is needed to improve patient outcomes.

Table 1

	MSSA	MRSA	P
Male	61/77 (79.2%)	31/45 (68.9%)	0.201
Age, mean (SD*)	46.68 (22.6)	44.78 (21.0)	0.647
Number of comobidities, mean (SD*)	1.2 (2.3)	2.2 (2.6)	0.082
One or less than 1 co morbidity	39/77 (50.6%)	14/45 (31.3%)	0.036
Pre-existing joint disease	7/77 (9.1%)	5/45 (11.1%)	0.758
Fever (More than 100F)	21/77 (56.8)	16/45 (43.2)	0.347
Mean WBC count on presentation(/μL)	13399 (6980)	13615 (4147)	0.853
Mean ESR (mm/hr)	79.7 (32.3)	83.6 (35.0)	0.543
Mean Joint WBC on presentation (mm3)	95461 (69768)	104784 (82679)	0.508
Polyarticular involvement	5/77 (6.5%)	4/45 (8.9%)	0.724
Bacteremia	25/77 (32.5%)	13/45 (28.9%)	0.680
Appropriate empiric coverage for MRSA	41/77 (53.2%)	34/45 (77.8%)	0.007
Mean number of weeks of antibiotics treatment (SD)	5.4 (1.8)	5.0 (1.3)	0.172
Mean number of joint surgeries (SD)	2.0 (1.3)	2.6 (1.7)	0.035
Mean number of admission days (SD)	10.2 (7.2)	16.2 (9.0)	0.001
Mortality	3/77 (3.9%)	4/45 (8.9%)	0.421
Poor patient outcome	7/77 (9.1%)	14/45 (31.1%)	0.002
Poor joint outcome	15/77 (19.5%)	15/45 (33.3%)	0.086
Development of osteomyelitis	8/77 (10.4%)	13/45 (28.9%)	0.009

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/comparison-of-outcomes-in-septic-arthritis-caused-by-mrsa-and-mssa/