Unilateral renal size discrepancy (URSD) refers to a difference in size between a person’s two kidneys. This finding is typically made using medical imaging, such as an ultrasound, which measures the length of the kidneys along their longest axis. In adults, a size difference of 1.5 centimeters or more usually requires further investigation. A significant discrepancy suggests the smaller kidney may be affected by an acquired disease or a congenital issue.
Problems Stemming from Reduced Blood Supply
Insufficient blood flow, or ischemia, is a major cause of kidney shrinkage, starving the organ of necessary oxygen and nutrients. The most common vascular problem is Renal Artery Stenosis (RAS), where the renal artery narrows, often due to atherosclerosis. This narrowing reduces the pressure and volume of blood entering the kidney, triggering a series of events that ultimately lead to tissue atrophy. The lack of adequate perfusion causes the filtering units (nephrons) to shrink and be destroyed, resulting in a smaller organ. RAS often causes high blood pressure that requires prompt diagnosis. A less common, acute cause is renal infarction, which occurs when a blood clot suddenly blocks the main renal artery or a major branch. This sudden blockage causes rapid tissue death, resulting in a scarred and smaller segment of the kidney.
Causes Related to Kidney Formation and Chronic Disease
In some instances, the smaller size is present from birth due to developmental issues in the womb. Renal hypoplasia is a congenital condition where the kidney never developed to its full size, resulting in fewer functional units. Another developmental issue is renal dysplasia, where the tissue is abnormally organized and non-functional, leading to a small, fixed-size organ.
Other causes of a small kidney are acquired over a person’s lifetime through chronic damage and scarring. Chronic pyelonephritis is a severe, long-term infection causing repeated inflammation and tissue destruction. This condition is often linked to Vesicoureteral Reflux (VUR), where urine flows backward from the bladder into the kidney, causing recurrent infections. The resulting damage is characterized by a scarred, shrunken kidney with significant loss of functional tissue.
Understanding Obstruction and Pressure Damage
A mechanical blockage in the urinary tract can cause one kidney to shrink by creating excessive internal pressure. Obstruction sources include kidney stones, ureter strictures (narrowing), or external tumors compressing the drainage system. When urine flow is impeded, it backs up, causing the renal pelvis and calyces to swell, a condition known as hydronephrosis.
If the obstruction is not relieved, the prolonged high pressure within the collecting system compresses the delicate filtering tissue, the renal parenchyma. This constant pressure hinders blood flow and causes slow, progressive destruction of the nephrons, resulting in pressure atrophy. Over time, the kidney becomes irreversibly damaged, leading to a smaller, shrunken appearance.
When the Larger Kidney Determines the Difference
Sometimes the size difference results from the unusual growth of the larger kidney, not a problem with the smaller one. This phenomenon is called compensatory hypertrophy, where the healthy kidney grows significantly to take on the workload of a damaged or absent kidney. If one kidney is congenitally small or has lost function due to disease, the remaining functional kidney increases in size to maintain the body’s total filtering capacity.
In this scenario, the smaller kidney may be the one that is actually the normal size, while the larger kidney is simply overcompensating to preserve overall renal function. Determining the exact cause requires specialized tests beyond simple imaging, such as renal scintigraphy, which uses a radioactive tracer to measure the percentage of function contributed by each kidney. This functional assessment helps clinicians determine if the discrepancy is due to a diseased, shrinking kidney or a healthy, compensating one.