A case of a bum knee

Case

A young adult patient presents with a few months of constant left knee pain and stiffness with gradually worsening swelling, but no preceding trauma or illness. The knee is particularly stiff in the mornings when he wakes up. He also endorses mild discomfort in his left shoulder and elbow as well as a rash along his gluteal crease. He is sexually active. There is a FHx of gout and diabetes. He denies fever, abdominal pain, nausea, vomiting, diarrhea, dysuria, or penile discharge. 

On exam of the knee, there is tenderness along the medial aspect, a small knee effusion is present, flexion is mildly decreased, and he ambulates with an antalgic gait. The joint is not warm or red. The remainder of his exam is unremarkable with the exception of a scaly erythematous plaque along the superior aspect of his gluteal cleft. 

Labs are notable for CRP 103, ESR 108, and +ANA 1:160; CBC, BMP, uric acid, rheumatoid factor (RF), and STI testing are unremarkable. XR of the knee is unremarkable apart from a small effusion. Arthrocentesis is attempted but unsuccessful.


DDx

The DDx for knee pain is broad, but there are several salient features about this case that help us narrow the possibilities. Specifically, persistent non-traumatic pain in the presence of an effusion is most suggestive of crystal arthropathy (e.g., gout), infectious arthritis, or rheumatological disease. 


Back to the case…

Over the next weeks, the patient develops a rash over his forehead. 

Diagnosis: Guttate psoriasis with psoriatic arthritis

  • Fun fact: Guttate comes from the Latin words for speckled or drop. The abbreviation “gtt”, commonly used when referring to an IV drip, has the same root.

Psoriatic arthritis (PsA)

PsA is an inflammatory MSK disease that affects around 20% of patients with psoriasis. 

In the majority of patients, skin disease precedes the onset of arthritis with a median time of 7-8 years; however, arthritis precedes skin disease in 15% and occurs simultaneously in another 15%. 

Clinical features suggestive of PsA:

  • Polyarthritis or asymmetric oligoarthritis, often affecting distal joints
  • Presence of nail lesions (pitting/onycholysis) or “hidden” psoriatic plaques (e.g., scalp, gluteal fold, umbilicus)
  • Dactylitis

Diagnostic criteria (Classification Criteria for Psoriatic Arthritis, aka CASPAR) requires 3+ points among the following:

  • Skin psoriasis
    • Present – 2 points
    • Previously present – 1
  • FHx of psoriasis (even if patient is not affected) – 1
  • Nail lesions – 1
  • Dactylitis (past or present) – 1
  • Negative RF – 1
  • Juxtaarticular bone formation on radiographs (distinct from osteophytes) – 1

Management of PsA: 

  • Mild disease: NSAIDs
  • Moderate to severe: DMARDs (e.g., MTX, leflonamide, apremilast)
  • Severe with erosive disease or functional limitation: TNF inhibitor (e.g., adalimubab, infliximab)

Blog post based on Med-Peds Forum talk by Ann Ding and Sam Masur, PGY4