A difference in size between the two kidneys is a finding that prompts medical attention, though some natural variation is normal. The kidneys are generally expected to be similar in length, typically within one to two centimeters of each other in adults. A significant difference, known as unilateral renal size discrepancy, often signals an underlying medical condition that has either prevented one kidney from developing fully or has caused a normally sized kidney to shrink over time. Identifying the cause is important because it directs the appropriate management to preserve kidney function and manage associated issues like high blood pressure.
Kidney Size Discrepancy Due to Developmental Issues
Developmental issues present from birth are one category of causes, resulting in a kidney that never fully formed and is often detected incidentally later in life. The primary congenital issues are renal hypoplasia and renal dysplasia, both of which affect the internal structure and size of the organ.
Renal hypoplasia refers to a kidney that is small but otherwise appears structurally normal, simply containing a reduced number of nephrons, the microscopic filtering units. Unilateral hypoplasia is relatively common, and the unaffected kidney often grows larger to compensate.
In contrast, renal dysplasia involves abnormal tissue growth and disorganization within the kidney, which also results in a smaller size. Dysplastic kidneys contain primitive ducts and disorganized tissue, sometimes including cysts. This abnormal development can lead to scarring and a non-functional kidney.
Acquired Causes Related to Restricted Blood Flow
A common acquired cause is restricted blood supply, which starves the organ of oxygen and nutrients. This condition is known as ischemic nephropathy, where the kidney shrinks over time due to chronic under-perfusion. The most frequent cause is renal artery stenosis, a narrowing of the main artery supplying blood to the kidney.
In older individuals, the narrowing is most often caused by atherosclerosis, the build-up of plaque within the artery. In younger patients, a condition called fibromuscular dysplasia may cause the narrowing. When the artery is significantly narrowed, the blood flow to the kidney decreases, leading to chronic ischemia.
The reduced blood flow triggers a physiological response, including the activation of the renin-angiotensin-aldosterone system, which attempts to raise blood pressure. However, the lack of sufficient perfusion causes structural changes in the kidney. Over time, this chronic ischemia leads to the atrophy of the tubular cells, inflammation, fibrosis, and ultimately, a decrease in the overall size of the kidney, which can compromise its function.
Acquired Causes Related to Blockage and Scarring
Another major group of acquired causes involves damage from issues related to urine flow and subsequent infection, leading to scarring and shrinkage. Chronic pyelonephritis, a persistent or recurrent kidney infection, is a frequent contributor to a unilateral small kidney.
A significant underlying factor in chronic pyelonephritis is vesicoureteral reflux (VUR), a condition where urine flows backward from the bladder up toward the kidney. VUR allows bacteria to reach the kidney, causing recurrent infections that lead to patchy scarring of the renal tissue, a process called reflux nephropathy. This scarring destroys the nephrons and results in a smaller, irregular kidney.
Chronic obstruction of the urinary tract, such as from severe kidney stones or a stricture, also contributes to kidney size discrepancy. Blockage causes a build-up of pressure within the kidney’s drainage system, which can lead to a condition called hydronephrosis. If this obstruction persists, the sustained back-pressure can damage the renal parenchyma, resulting in atrophy and subsequent kidney shrinkage.
Significance of Finding and Diagnostic Steps
A difference in kidney size is significant because it indicates a potential loss of functional tissue. Even if total kidney function remains adequate due to the compensating kidney, a small kidney may be a source of problems like difficult-to-control high blood pressure or recurrent urinary tract infections. For example, the scarring associated with reflux nephropathy is a known cause of hypertension in children and young adults.
Diagnosis typically begins with imaging, usually a renal ultrasound, which is a non-invasive tool used to measure kidney length and look for structural abnormalities. A size disparity of more than two centimeters between the two kidneys is considered abnormal and warrants further investigation. Ultrasound can also detect signs of obstruction, such as hydronephrosis, or features of chronic disease like small size.
To pinpoint the specific cause, specialized tests are often employed:
- If restricted blood flow is suspected, a renal arteriogram, magnetic resonance angiography (MRA), or a captopril-enhanced renal radionuclide scan can assess the blood flow and look for renal artery stenosis.
- For issues related to scarring from infection or reflux, a dimercaptosuccinic acid (DMSA) scan is used to visualize the extent and location of the renal scarring.
- Computed tomography (CT) or magnetic resonance imaging (MRI) scans provide highly detailed anatomical images.
- These detailed scans help differentiate between congenital issues, vascular problems, and damage from obstruction.