A kidney stone is a hardened deposit of minerals and salts that forms inside the kidneys from chemicals in the urine. These stones vary greatly in size, from a grain of sand to much larger formations. An 8 millimeter (mm) stone is considered significant because it is large enough to frequently cause a painful obstruction. Although substantial, natural passage is sometimes attempted under close medical supervision before moving to other treatments.
Contextualizing the 8 mm Size
An 8 mm kidney stone is roughly the size of a standard pencil eraser or a large pea. The ureter, the urinary tract’s narrowest point connecting the kidney to the bladder, typically has an internal diameter of only 3 to 4 mm. This dimensional mismatch explains why an 8 mm stone can cause severe pain and blockages as it attempts to move through the ureter.
The stone’s size is the primary factor determining the likelihood of natural passage. Stones 5 mm or smaller have a relatively high chance of passing on their own, often cited around 68%. However, the probability drops sharply as size increases, placing an 8 mm stone in a gray area where intervention is frequently required. For stones between 5 mm and 10 mm, the overall spontaneous passage rate is significantly lower, approximately 47%.
Likelihood of Natural Passage and Limiting Factors
Whether an 8 mm stone can pass naturally depends on several clinical factors, though statistics for this size are not encouraging. Data suggests that for stones 6 mm, the spontaneous passage rate drops to about 33%. For stones 6.5 mm and larger, the rate can be as low as 9%. When a stone is 7 mm or 8 mm, the likelihood of a successful spontaneous passage is generally less than 50%.
The stone’s location within the urinary tract is a major factor influencing passage success. Stones that have reached the distal ureter (closest to the bladder) have a higher chance of passing. In contrast, stones lodged in the proximal ureter (closer to the kidney) are far less likely to pass naturally. The shape and composition of the stone also play a role, as smooth stones move more easily than jagged ones.
When natural passage is attempted, medical expulsive therapy (MET) is often used, involving medication like alpha-blockers. These medications relax the muscles in the ureter, helping to widen the passage and improve the stone’s movement. This conservative approach is typically monitored for a maximum of four to six weeks, as prolonged obstruction can lead to irreversible kidney damage. Hydration and pain management support the process, but the patient must be monitored closely for signs of infection or worsening blockage.
Standard Medical Interventions for 8 mm Stones
When an 8 mm stone is not progressing or is causing uncontrolled pain, intervention is usually recommended to prevent complications like infection or kidney damage. Two primary procedures, non-invasive or minimally invasive, are commonly used for stones in this size range. The choice between them often depends on the stone’s location and the patient’s anatomy.
Extracorporeal Shock Wave Lithotripsy (ESWL)
Extracorporeal Shock Wave Lithotripsy (ESWL) is a non-invasive treatment that uses high-energy sound waves to break the stone into tiny fragments. These fragments are then small enough to pass out of the body in the urine. ESWL is generally a first choice for 8 mm stones that are still located high up in the kidney.
Ureteroscopy (URS)
The alternative, Ureteroscopy (URS), is a minimally invasive procedure involving the insertion of a small, flexible scope through the urethra and bladder up into the ureter. Once located, a laser is used to fragment the stone, or a small basket is used to physically remove the fragments. URS is frequently the preferred method for 8 mm stones lodged in the ureter or when ESWL has failed. A temporary stent may be placed in the ureter after URS to ensure the tube remains open and promote healing.