Introduction
Renal hyperfiltration is a condition in which the filtration rate in the filtration elements (glomeruli) of the kidneys is increased. It can occur in various conditions, including kidney diseases. It also indicates the early phase of kidney disease in the presence of various conditions like diabetes and obesity. Renal hyperfiltration can be physiological or pathological. Hyperfiltration is an independent contributor to various conditions, including cardiovascular diseases.
What Is Renal Hyperfiltration?
The filtration process is the major function of the kidneys. Kidneys filter blood and produce urine through filtration. The glomeruli in the kidneys are the filtration elements responsible for the filtration process. Renal hyperfiltration refers to a condition in which the glomerular filtration rate is elevated. The normal glomerular filtration rate is 90 to 120 ml (milliliters) per minute per 1.73 meter square. An increase in glomerular filtration rate above this range indicates the presence of hyperfiltration. Various clinical conditions can cause it, including kidney diseases.
What Are the Causes of Renal Hyperfiltration?
Renal hyperfiltration is associated with various diseases. It includes the following:
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Polycystic kidney disease.
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Diabetes mellitus.
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Secondary focal segmental glomerulosclerosis due to a reduction in the renal mass.
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Sickle cell anemia.
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Obesity.
How Does Renal Hyperfiltration Occur?
Various clinical conditions cause renal hyperfiltration, and the pathological mechanism depends on the underlying disease. Renal hyperfiltration is usually associated with early diabetes mellitus. Hyperfiltration is attributed to tubular factors and hemodynamic factors of the glomeruli. The hemodynamic abnormalities are caused by afferent arteriolar vasodilatation, efferent vasoconstriction, and suppression of tubuloglomerular feedback. The concomitant action of various pathogenic factors like hyperinsulinemia (high levels of insulin in the blood), increased body mass index, activation of the sympathetic nervous system, hyperleptinemia (high levels of leptin), inflammatory cytokines, increased oxidative stress, etc., cause renal hyperfiltration.
What Are the Types of Hyperfiltration?
Hyperfiltration is of two types whole-kidney hyperfiltration or single-nephron hyperfiltration.
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Whole-Kidney Hyperfiltration - Itrefers to a two-standard increase in the glomerular filtration rate. The glomerular filtration rate varies between 130 and 140 ml per minute per 1.73 meter square in individuals with two functioning kidneys.
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Single-Nephron Hyperfiltration - It is seen in diabetic kidney disease. Due to the irreversible damage in the glomeruli, the remnant nephrons undergo structural and functional changes like hypertrophy to maintain renal filtration. It can also be due to metabolic and hormonal stimuli in diabetes and or obesity that enhance filtration in single nephrons while maintaining the glomerular filtration rate between 130 to 140 ml per minute per 1.73-meter square.
The glomerular filtration rate is the sum of the filtration rate of all single-nephrons. An increase in single-nephron filtration rate in the presence of a normal number of functioning nephrons results in single glomerular hyperfiltration. This is called absolute hyperfiltration in the kidneys. Absolute hyperfiltration can be seen in healthy individuals after a high protein intake, during pregnancy, or in individuals with diabetes, obesity, or autosomal-dominant polycystic kidney disease. Relative hyperfiltration is due to a congenital reduction in the number of nephrons or due to an acquired reduction in nephron mass as a result of surgery or kidney disease.
What Are the Sign and Symptoms of Renal Hyperfiltration?
Renal hyperfiltration causes the following symptoms:
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Pink urine.
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Foamy urine.
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High blood pressure.
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Swelling (edema) of face, hands, ankles, and feet.
What Are the Effects of Hyperfiltration on Kidneys?
Renal hypertension is the major cause of diabetic nephropathy. Renal hyperfiltration in diabetes and obesity indicates renal dysfunction. An increase in glomerular capillary pressure causes hyperfiltration. Increased glomerular capillary pressure increases the tensile stress in the capillary wall structures. Additionally, the increased flow of ultrafiltrate into Bowman’s space increases the shear stress on podocytes (specialized cells in the glomerulus). As a result of all these mechanical stresses, glomerular basement membrane length increases, and podocyte hypertrophy occurs. The podocyte injury progresses to segmental sclerosis. Mechanical stress results in tubulointerstitial inflammation, hypoxia, and fibrosis. These effects contribute to the development of glomerular disease.
Why Is Renal Hyperfiltration a Concern?
Renal hyperfiltration is an important precursor for chronic kidney disease and an independent risk factor for cardiovascular diseases. Hyperfiltration increases intraglomerular hypertension resulting in albuminuria. Chronic albuminuria results in end-stage renal failure. Renal hyperfiltration causes proteinuria. Proteins are essential for moving the extra fluid from the body into the bloodstream. Protein loss results in fluid build-up in the body resulting in swelling of hands, feet, face, abdomen, and ankles. Proteinuria is associated with the progression of kidney disease and increases the risk of cardiovascular events. If left untreated, hyperfiltration can cause a gradual loss of renal function. Renal hyperfiltration, persistent for a long period of time, can result in renal failure. Hence, it is essential to manage hyperfiltration.
How Is Renal Hyperfiltration Diagnosed?
Renal hyperfiltration can be diagnosed by performing tests like the glomerular filtration rate and blood and urine tests.
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Glomerular Filtration Rate Test - Glomerular filtration rate test is performed to determine how well the kidneys function. It estimates the amount of blood that passes through the glomeruli each minute. The normal glomerular filtration rate is 90 to 120 ml per minute per 1.73-meter square. It is considered hyperfiltration if the value exceeds 140 ml per minute per 1.73-meter square.
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Blood Test - A blood test is performed to determine the protein levels, creatinine, and urea nitrogen levels.
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Urine Test - A urine test is performed to diagnose the presence of protein, red blood cells, and white blood cells in urine.
How Is Renal Hyperfiltration Treated?
The treatment for renal hyperfiltration depends on identifying the pathogenic mechanisms related to hyperfiltration. Non-specific hypertensive agents and drugs that reduce the activity of the renin-angiotensin-aldosterone system are given to reduce glomerular pressure. Recent research has shown that hyperfiltration can be reduced by decreasing the proximal reabsorption of glucose and sodium by SGLT2 (sodium-glucose cotransporter-2) inhibition. It acts as a renoprotective agent.
Conclusion
Renal hyperfiltration can be a physiological or pathological elevation of the glomerular filtration rate in the kidneys. Hyperfiltration is a marker for chronic kidney disease and is associated with the risk of cardiovascular diseases. It is essential to attenuate hyperfiltration to prevent the progression of kidney diseases and the risk of cardiovascular events. Early diagnosis and proper treatment can help in managing hyperfiltration.
