In the evolving landscape of retinal imaging, Optical Coherence Tomography Angiography (OCTA) has revolutionized how we visualize the retinal vasculature without the need for invasive dyes. However, as with any advanced technology, interpreting its signals requires a nuanced understanding of what is “true flow” versus “pseudo-flow.” One such phenomenon gaining clinical attention is Suspended Scattering Particles in Motion (SSPiM).
What is SSPiM?
SSPiM is a finding on OCTA where hyperreflective particulate material trapped within intraretinal fluid (IRF) spaces—such as those seen in Diabetic Macular Edema (DME)—exhibits Brownian motion. Because OCTA algorithms are designed to detect movement (decorrelation signals), they often misinterpret this motion as blood flow, creating a “pseudo-flow” signal within non-vascular cystoid spaces.
Far from being a mere imaging artifact, SSPiM is a window into the pathology of the blood-retinal barrier (BRB).
The Pathological Link: BRB Breakdown
Recent research indicates that SSPiM occurs when there is a significant breakdown of the inner BRB. When the barrier is severely compromised, larger molecules and serum proteins leak into the retinal interstitium. These particles create a high-turbidity environment within intraretinal cysts, which is reflected in two ways on imaging:
- Hyperreflective Foci (HRF): A higher number of these dots on structural OCT is strongly associated with the presence of SSPiM.
- Optical Density Ratio (ODR): Cysts with SSPiM have a significantly higher mean ODR (greater turbidity) compared to those without.
SSPiM as a Predictor of Treatment Response
The most critical clinical takeaway is the relationship between SSPiM and how a patient responds to intravitreal injections (Anti-VEGF or Steroids). Studies have shown that the presence of SSPiM is a biomarker for a poor structural response to treatment.
Key findings include:
- Reduced Cyst Shrinkage: In patients with DME, intraretinal cysts with SSPiM decreased in size by only about 30% after treatment, whereas those without SSPiM decreased by 69%.
- Lower Rates of Resolution: Cysts showing SSPiM are significantly less likely to achieve a “complete response” (full resolution) and are more likely to show “no response” to therapy.
- Anatomical Location Matters: SSPiM located in the Inner Nuclear Layer (INL) is more strongly related to poor treatment response than SSPiM in the Outer Nuclear Layer (ONL). This may be because ONL edema can sometimes be caused by disturbances in dehydrating functions rather than direct vascular leakage.
Clinical Perspective
When evaluating a patient with DME, identifying SSPiM on OCTA can help set realistic expectations for treatment outcomes. While both steroids and anti-VEGF agents are less effective in these “SSPiM-positive” cysts, the presence of this biomarker signals a more recalcitrant form of edema driven by severe inflammatory and vascular barrier dysfunction.
As we move toward more personalized medicine in ophthalmology, SSPiM serves as a valuable structural indicator that can be assessed rapidly in a clinical setting to guide long-term management strategies.
References
- Manthalkar AR, Sharma V, Vignesh TP. When fluid mimics flow: suspended scattering particles in motion on OCTA. Eye. Published online June 29, 2026. doi:10.1038/s41433-026-04691-y.
- Ahn J, Han S, Ahn SM, Kim SW, Oh J. Clinical implications of Suspended Scattering particles in Motion observed by optical coherence tomography Angiography. Scientific Reports. 2020;10:1-11. doi:10.1038/s41598-019-55606-9.
