Kidney stones are solid, pebble-like formations that develop in the kidneys. They form from chemicals in the urine that crystallize and stick together. These stones vary greatly in size, from as small as a grain of sand to, rarely, the size of a golf ball.
Approximately 1 in 10 people will experience a kidney stone, with their prevalence increasing over recent decades. While some small stones may pass unnoticed, larger stones can cause considerable pain as they move through the urinary tract. A stone’s ability to pass naturally is influenced by several factors.
Factors Influencing Stone Passage
A kidney stone’s journey through the urinary tract is affected by its characteristics and the individual’s anatomy. The shape of the stone plays a role; smooth stones may pass more easily than those with jagged edges, which can cause more irritation and potentially get stuck. The stone’s chemical composition can also influence its texture and hardness, impacting its ability to pass.
The location of the stone within the urinary tract significantly affects its passability. Stones closer to the bladder, especially in the distal ureter, have a higher chance of spontaneous passage than those higher up in the ureter or within the kidney. Individual anatomical differences, such as the width of the ureter, can also influence whether a stone can navigate this pathway. Other factors like hydration status and the presence of any blockages or strictures in the urinary tract can further determine the likelihood of natural stone passage.
Understanding Stone Size and Passability
Stone size is a primary determinant of whether it can pass spontaneously. Stones 4 millimeters (mm) or smaller generally have a high chance of passing on their own, with spontaneous passage rates around 80-90%. These smaller stones typically pass within one to two weeks.
Stones between 5 mm and 10 mm in size present a more varied outcome; a 5 mm stone has about a 50-68% chance of passing, while a 6 mm stone has a roughly 33% chance. Stones in this range may take several weeks to pass and often cause more significant pain, sometimes requiring medical assistance.
Stones larger than 10 mm are unlikely to pass naturally and almost always require medical intervention. While rare, some anecdotal reports suggest natural passage of stones larger than 10 mm, though these are considered exceptional cases. The ureters, which are tubes between 6 to 8 mm in diameter, are the primary pathway for stones, making passage of stones larger than this range particularly challenging.
When Medical Intervention is Necessary
When a kidney stone is too large to pass naturally, causes severe symptoms, or leads to complications like infection or kidney damage, medical intervention is necessary. Several procedures can remove or break down kidney stones.
Extracorporeal Shock Wave Lithotripsy (ESWL) is a non-invasive procedure that uses high-energy shock waves to break stones into smaller fragments that can be passed in the urine. ESWL is effective for stones less than 2 centimeters (20 mm).
Ureteroscopy is another common procedure where a small, flexible telescope is inserted through the urethra and bladder, up into the ureter to locate the stone. Small stones can be removed with a basket device; larger ones are often broken with a laser for removal. This procedure is suitable for stones in various locations within the ureter and kidney.
For very large or complex stones, Percutaneous Nephrolithotomy (PCNL) may be recommended. PCNL involves a small incision in the back to directly access the kidney and remove the stone, often by breaking it into pieces. PCNL is effective for stones larger than 2 cm.