Should I worry about small pleural effusions seen on MRI for myeloma diagnosis?

Small Pleural Effusions in Myeloma: Clinical Significance

Small pleural effusions incidentally detected on MRI in myeloma patients are generally not concerning and do not require immediate intervention unless symptomatic or accompanied by specific high-risk features. 1

Understanding the Context

Small pleural effusions in myeloma patients are typically not due to direct myelomatous pleural involvement, which is exceedingly rare:

• True myelomatous pleural effusion (MPE) occurs in less than 1% of all myeloma patients and represents an extremely poor prognostic sign with mean survival under 4 months 2, 3
• Most pleural effusions in myeloma patients (>99%) are caused by other conditions: congestive heart failure, renal failure, hypoalbuminemia, pulmonary embolism, or infection 3
• About 15% of patients with malignancy-related effusions have small effusions (<500 ml) that are relatively asymptomatic 1

When Small Effusions Are Likely Benign

Small, asymptomatic effusions discovered incidentally on MRI typically represent paramalignant rather than myelomatous effusions 1:

• Paramalignant effusions result from indirect effects: mediastinal lymph node involvement causing lymphatic obstruction, hypoalbuminemia from disease burden, or concurrent heart/kidney dysfunction 1
• CT scans in cancer patients frequently identify previously unrecognized small effusions that have no clinical significance 1
• MRI has limited utility for pleural disease evaluation compared to CT, though it can assess chest wall involvement 1

Red Flags Requiring Further Investigation

Pursue diagnostic thoracentesis if any of the following are present 1, 4:

• Symptomatic dyspnea that cannot be explained by other causes
• Progressive enlargement on serial imaging
• Unilateral effusion without clear alternative explanation
• Bilateral effusions with normal heart size on imaging
• Clinical deterioration or new symptoms

Diagnostic Approach When Indicated

If investigation is warranted based on symptoms or progression 4:

• Ultrasound-guided thoracentesis is the preferred initial approach, with 97% success rate even for small effusions 1, 4
• Send pleural fluid for: cell count with differential, protein, LDH, glucose, pH, cytology, and flow cytometry (critical for detecting myeloma cells, which have 85% sensitivity versus conventional cytology) 5, 6, 3
• Flow cytometry showing CD38+, CD138+, CD56+ with absent CD19, CD10, CD45 confirms myelomatous involvement 6
• Contrast-enhanced CT should be performed before complete drainage to visualize pleural abnormalities if malignant involvement is suspected 1

Practical Management Algorithm

For small, asymptomatic effusions detected on MRI 1, 4:

1. Watchful waiting with interval imaging is appropriate management
2. Review for alternative causes: check cardiac function, renal function, albumin levels
3. Reassess if symptoms develop or effusion enlarges significantly
4. Do not pursue thoracentesis for effusions too small to sample safely (<1 cm thickness on lateral decubitus view) 1

Key Clinical Pitfall

The major pitfall is assuming all effusions in myeloma patients are myelomatous 3:

• This leads to unnecessary invasive procedures for benign paramalignant effusions
• True MPE presents with yellow exudates, yellow pleural nodules, and requires flow cytometry for diagnosis 3
• MPE typically occurs in advanced, relapsed disease with IgA subtype predominance 6, 7

In summary: small pleural effusions on MRI in myeloma are usually incidental paramalignant findings that require observation rather than immediate intervention, unless accompanied by symptoms or concerning features.