A 62-year-old woman presents with a fever, right knee pain and swelling for 24 hours. The patient says she was gardening two weeks ago and bumped her right knee on the cement. At that time, she experienced slight discomfort in the knee but was able to continue to garden. The patient states the right knee pain is not like her “typical” pain from rheumatoid arthritis (RA). She describes a sudden onset of non-radiating pain associated with fever and chills. She is unable to bear weight due to the right knee pain.
The patient’s past medical history is significant for RA, hypertension, mitral valve prolapse and depression. Her medications include: Zoloft 50 mg daily, HCTZ 50 mg daily, methotrexate 15 mg weekly, and folate 1 mg daily.

The physical examination reveals a very pleasant, slightly obese woman in acute distress. Vital signs include a temperature of 101ºF, a sitting blood pressure of 134/90, a heart rate of 78 with regular rhythm and respirations of 16 per minute. She weighs 186 lbs with clothes. She is 5’4” without shoes.


The physical examination of the right knee reveals tenderness, warmth and erythema. The patient has a moderate joint effusion, a positive bulge sign, decreased range of motion in all planes and no ligamentous laxity. A neurologic examination reveals 5/5 strength and intact sensation throughout both lower extremities. We noted 2+ knee and ankle jerk reflexes bilaterally although the examination of the right knee was limited due to tenderness.



Anteroposterior, lateral and oblique radiographs of the right knee reveal joint spacing narrowing and soft tissue swelling. Arthrocentesis yields purulent synovial fluid. Synovial fluid cultures and a Lyme titer are pending.

Which of the following conditions is the most likely diagnosis?

A. Lyme disease
B. Rheumatoid arthritis
C. Nongonococcal acute bacterial
arthritis
D. Meniscal tear

Pertinent Insights On The Differential Diagnosis
A. Lyme disease is not the correct answer. Lyme disease is the most common vector-borne infection in the United States. It is caused by the spirochete Borrelia burgdorferi, which is transmitted to humans by ixodid ticks in North America, Europe, Australia and Asia. Most infections occur in the spring and summer.

There are three stages of the disease: rash stage (early), neurological stage (disseminated infection) and arthritis stage (late disease). The rash stage occurs three to 30 days after the bite and is typified by erythema migrans, which is a large “bull’s eye” rash. This rash is noticeably swollen in the center and it expands radially from the site of the tick bite. The rash is frequently accompanied by flu-like symptoms, profound fatigue, fever, chills, headache and/or backache. However, keep in mind that as many as 25 percent of patients do not have the characteristic rash.

During the neurological stage, one may note neurologic complications and migratory musculoskeletal pain. Approximately 15 percent of patients may develop more severe complications including meningitis, inflamed nerve roots in the neck, Bell’s palsy, myopericarditis with atrial or ventricular arrhythmias, and heart block.
The arthritis stage is marked by chronic synovitis and joint pain similar to what clinicians might see with RA. These symptoms occur several months to two years after the rash. Recurrent attacks of arthritis last a few days to a few weeks and primarily affect the knees and other large joints.

The diagnosis of Lyme disease is based on clinical manifestations and laboratory findings, usually by the detection of specific antibodies to B. burgdorferi in serum. One may obtain positive cultures for B. burgdorferi early in the course of the disease. The early stage of the disease readily responds to doxycycline, amoxicillin or cefuroxime axetil. When it comes to treating the late stage of the disease, clinicians may utilize intravenous penicillin or ceftriaxone.

Dr. Feld is an Associate Professor within the Drexel Hahnemann Physician Assistant Program at Drexel University in Philadelphia.

B. Rheumatoid arthritis is not the correct answer. While this patient has the diagnosis of RA, the sudden onset of severe monoarticular arthritis is not typical of the disease. Rheumatoid arthritis is the most common form of inflammatory arthritis. It affects women more than men. The clinical presentation is an insidious onset of morning stiffness, fatigue and polyarthritis.

The joint involvement includes hands (PIPs and MCPs), wrists, MTPs, knees, elbows, shoulders, ankles and the neck. There is chronic swelling and thickening of metacarpophalangeal and proximal interphalangeal joints. The pannus ingrowth gradually denudes articular cartilage and leads to chondrocyte death. Additionally, the range of motion is limited and there may be ulnar deviation of the MCP joints. Hand deformities include swan-neck and boutonniere deformities.

There may also be systemic involvement affecting the lung, heart, skin, eye and the gastrointestinal and genitourinary systems. Laboratory tests may show an elevated sedimentation rate and C-reactive protein as well as a positive rheumatoid factor. Radiographic changes include soft tissue swelling, juxtaarticular demineralization, joint space narrowing and erosions. In terms of treatment, the goals are controlling synovitis and pain, maintaining joint function and preventing deformities.

C. Septic arthritis is the correct answer. When it comes to septic arthritis, it most commonly occurs in a hematogenous manner. The condition may also occur via trauma, inoculation in a patient with predisposing factors such as intravenous drug use, the presence of in-dwelling catheters and/or underlying immunocompromised states. Another predisposing factor is a preexisting arthritis such as rheumatoid arthritis, gout or osteoarthritis. When entering the joint space, bacteria initially deposit in the synovial membrane and produce an inflammatory reaction, usually with polymorphonuclear and synovial cells.

Staphylococcus aureus is the most common cause of nongonoccocal septic arthritis. Other common causes are group A and group B streptococci. Staphylococcus epidermidis is the usual organism involved in prosthetic joint arthritis. Patients usually present with a single swollen erythematous joint with pain upon active and passive range of motion. The knee is the joint most often involved. A classic presentation is a febrile patient with rigors, an increased leukocyte count and an elevated sedimentation rate.

Dr. Auth is a physician assistant and is the Director of the Drexel Hahnemann Physician Assistant Program at Drexel University in Philadelphia.

Blood cultures are positive in approximately 50 percent of patients. The leukocyte count of the synovial fluid exceeds 50,000/ul. The gram stain of the synovial fluid is positive in 75 percent of staphylococcal infections and 50 percent of gram-negative infections. A joint aspiration is required to establish a diagnosis. However, clinicians can make a definitive diagnosis by obtaining a positive synovial fluid culture.
Treatment includes antibiotic therapy. Clinicians should give antibiotics to all patients suspected of having septic arthritis even before culture results are available.

Immediate surgical drainage is reserved for septic arthritis of the hip. For other joints, one would opt for or make an appropriate referral for surgical drainage only if medical therapy fails over two to four days to improve the fever, the synovial fluid volume, white blood cell count and culture results. Be aware that polyarthritis may occur, especially in the presence of an immunocompromised state or chronic connective tissue disease, and these patients have a mortality of 30 percent.

D. Meniscal tear is not the correct answer. Meniscal tears of the knee result from trauma or degenerative changes. The meniscus fibrocartilage serves to increase knee stability, joint congruency and improve nutrition and lubrication of articular cartilage. These patients may complain of joint line pain, catching, popping or a locking sensation. There may be an effusion evident and compression testing may elicit pain. Magnetic resonance imaging is the diagnostic test of choice. Treatment includes activity modification, NSAIDs and rehabilitation. If conservative treatment fails, arthroscopic surgery is indicated.