Acute Monoarticular Arthritis (Approach)

 

 

Risk Factors for Septic Arthritis:

• Rheumatoid Arthritis or Osteoarthritis (RA >OA)
• Joint Prosthesis (orthopedics should tap these!)
• Low Socioeconomic status
• IV drug use (predilection for axial joints!)
• Alcoholism
• Diabetes
• Intraarticular Steroids
• Cutaneous ulcers.
• The history and physical exam typically reflect the acute presentation of  a painful, restricted joint.  The physical exam can help you clinch many of the diagnoses on your list if you look for other systemic manifestations that can be associated with these diagnoses (we went through a few "spotter" style cases which I've attached in a powerpoint below).
• Synovial Fluid analysis is the KEY to diagnosis.  The fluid analysis is tremendously helpful in steering your differential diagnosis.  Gram Stains identify an organism in 50% of septic arthritis and Culture increases this to 67%.
• Typical organisms include gram (+)'s like strep and staph. 
• Risk factors for MRSA:  Assisted living communities, recent hospitalizations, recent antibiotics, chronic infusion centers or wound care visits.
• Patient's with chronic immunosuppression, diabetes, and extremes of age can also get Gram(-) infections (Pseudomonas, E. Coli, etc) which would require broader empiric antibiotic coverage.
• Empiric coverage for septic arthritis should include vancomycin until sensitivies confirm that there's no MRSA.  If there are risk factors for gram (-) organisms, consider adding a FQ or 3rd generation cephalosporin.

 

 

Main categories in the DDx:
1. Traumatic – usually fairly easy to sort out by history, but also consider less obvious causes such as hemophilia, coagulopathies that can predispose to hemarthroses with minimal or no trauma.  On tap, expect to see bloody fluid.

• Fracture
• Stress Fracture
• Hemarthrosis
• Acute Charcot Foot

 

2. Infectious – main distinction is Gonococcal, vs Nongonococcal.  This distinction is important because gram stain and cultures are much less sensitive for Gonococcus vs typical bacterial pathogens.

• Bacterial (Staph Aureus, Streptococcal species, Neisseria Gonorrhea, Pseudomonas, Ecoli, Burrelia Burgdorferi). 
• Fungal/Mycobacterial (Cocci/histo/blasto, tuberculosis). 
• Viral (Parvovirus B19)

 

3. Crystal Deposition – Gout vs Pseudogout.  Keep in mind that clinically, they are virtually indistinguishable except by tap and crystal analysis.

• Gout
• Pseudogout (CPPD)

 

4. Inflammatory – includes seronegative spondyloarthropathies (Reactive arthritis, psoriatic, IBD-associated), and Rheumatoid Arthritis.  Caveat is that these typically present with polyarticular arthritis and more chronic, but can certainly present with acute monoarticular arthritis.

• Fracture
• Stress Fracture
• Hemarthrosis
• Acute Charcot Foot

 

In general, you should have a fairly low threshold for performing joint arthrocentesis and sending the fluid for analysis.   

 

Things to look for in the fluid:

1. Appearance – purulent, bloody, clear

 

2. Cell count:

• Normal - < 200 WBC
• Osteoarthritis – 200 – 2000 WBC
• Inflammatory (including Gout, Pseudogout) – 2000 – 100,000
• Infectious- > 50,000-100,000

** Important to note that inflammatory and infectious arthritis have considerable overlap in the WBC count – i.e. with a WBC of 50 – 100k, it can be either inflammatory or infectious **

 

3. Gram stain and culture – note that the sensitivity of gram stain in infectious arthritis is only 50-70% at best (and much lower for gonococcal arthritis).  Cultures are much more sensitive, although they take longer and still are insensitive for gonococcal arthritis.

 

4. Crystal Analysis – looking for negatively birefringent needle-shaped crystals for gout, and positively birefringent rhomboid shaped crystals for pseudogout.

** Remember that gout/pseudogout and infection can co-exist, so ALWAYS ALWAYS send for gram stain and culture, and have low threshold to treat with Abxs even if crystals are present, until you are sure there is no infection!! **
Along those lines, according to the American College of Rheumatology committee, synovial fluid analysis should be performed in the febrile patient with an acute flare of established arthritis (eg, rheumatoid arthritis, osteoarthritis) to rule out superimposed septic arthritis.  Repeated aspiration and synovial fluid analysis may be used to monitor the response of septic arthritis to treatment and may also be valuable for diagnosis of some cases of gout in which the initial aspirate does not have detectable crystals.

 

 

(Andy Copland MD, 6/10/11)

(Chanu Rhee MD, 1/27/11)