What are the rare lining cells observed in the cytospin smear of a child's ankle joint fluid aspirate?

Identification of Rare Lining Cells in Synovial Fluid Aspirate

The rare lining cells observed in the cytospin smear are synovial lining cells (synoviocytes), which are fragments of the synovial membrane that have been shed into the joint fluid during the inflammatory process.

Characteristics of Synovial Lining Cells

Synovial lining cells (synoviocytes) are specialized cells that form the inner lining of synovial joints. When examining joint fluid, these cells can be identified in cytospin preparations, though they are often rare findings. These cells have several key characteristics:

• They appear as fragments of tissue in the synovial fluid rather than as individual cells 1
• They maintain the histological appearance of synovial lining even when detached 1
• They are typically not visible in standard Wright-stained cell smears but can be detected in ethanol-fixed, hematoxylin/eosin-stained cytospins 1
• They consist of two main cell types:
  • Type A cells (macrophage-like synoviocytes) - express CD68 marker
  • Type B cells (fibroblast-like synoviocytes) 2

Diagnostic Significance

The presence of synovial lining fragments in this 2-year-old's ankle joint fluid has important diagnostic implications:

1. Inflammatory Process: The detection of synovial lining fragments along with the high nucleated cell count (52,000/μL) with 95% neutrophils strongly suggests an inflammatory or infectious process in the joint 1.

2. Septic Arthritis Likelihood: The synovial fluid findings (yellow and turbid appearance, high cell count with neutrophil predominance) are consistent with septic arthritis. A synovial fluid white blood cell count of 50,000 cells/mm³ or higher is the traditional threshold for diagnosing septic arthritis 3.

3. Differential Diagnosis: While the synovial fluid cell count exceeds 50,000/μL, suggesting septic arthritis, it's important to note that some infectious causes like brucellosis can present with lower cell counts 4. However, in this clinical context of a 2-year-old with refusal to walk, pain, and joint effusion, bacterial septic arthritis is the most likely diagnosis.

Technical Considerations

The detection of synovial lining fragments is enhanced by specific laboratory techniques:

• Cytospin preparations are superior to standard smears for detecting these cells 1, 5
• Processing with hyaluronidase can improve cell morphology and differential counts, particularly in non-inflammatory synovial fluids 5
• These fragments can be further characterized by immunohistochemistry, with CD68 staining helping to identify the macrophage-like synoviocytes 1

Management Implications

The identification of synovial lining cells, combined with the clinical presentation and synovial fluid analysis, has important management implications:

• Urgent Treatment: Septic arthritis requires prompt surgical drainage and antibiotic therapy to prevent joint destruction 3
• Surgical Options: Arthroscopic debridement and lavage for early infections, or open arthrotomy for severe infections 3
• Antibiotic Therapy: Empiric antibiotics should target Staphylococcus aureus initially, then be adjusted based on culture results 3

Pitfalls to Avoid

• Do not dismiss the presence of synovial lining cells as insignificant; they can provide valuable diagnostic information about the joint pathology 1
• Remember that the absence of organisms on Gram stain does not exclude infection 3
• Consider that cytospin preparations may reveal cell populations that are underdetected in routine smears 5

In this 2-year-old child with ankle pain, edema, and refusal to walk, the synovial fluid findings including synovial lining cells, high cell count with neutrophil predominance, and turbid appearance strongly suggest septic arthritis requiring urgent orthopedic consultation and treatment.