Shrunken Pore Syndrome (SPS)
By Radhika Kumar, M.S., Ph.D.
Glomerular filtration rate (GFR) estimation is one of the ways to measure kidney function. Low molecular weight proteins (5-30 kDa) are usually filtered out into the urine under normal conditions. The standard way to estimate GFR (eGFR) is a simple blood test that measures serum creatinine levels (a waste product that comes from the normal wear and tear on muscles). When the kidney function is compromised in acute or chronic situations, GFR usually declines. Shrunken pore syndrome (SPS) is a renal disorder which is characterized by decreased renal clearance of low molecular weight proteins or peptides. However, creatinine levels remain normal, making it not a viable marker for measurement. In this case, Cystatin C (a protein that slows down the breakdown of other protein cells) serves as a superior marker in the prediction of SPS. The reason being, Cystatin C helps in identifying the disruptions in the filtration process in the pore model of glomerular filtration. The pathophysiology of this syndrome involves a decrease in the diameter of a fraction of the pores of the glomerular membrane impairing the filtration of big molecules and to a lesser extent water or creatinine. Also, the amount of Cystatin C varies much less with the muscle mass and no tubular section compared to creatinine. SPS is diagnosed when the eGFR cystatin C/eGFR creatinine-ratio <0.60 (<60-70%). The mortality of this condition is associated with cardiovascular risk. The incidence of SPS is mostly in the elderly population but highly variable owing to its poor prognosis. The prevalence of SPS is between 0.2 and 36%. Treatment options may involve monoclonal antibodies to reduce the levels of lethal signaling proteins. The limited information about SPS necessitates the procurement of better diagnostic measures and treatment modalities.
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