Kidney stones are hard, crystalline masses that form within the kidney when high concentrations of certain minerals and salts are present in the urine. A 3-millimeter (mm) stone is generally considered small, but its journey from the kidney to the bladder involves passing through the ureter, a muscular tube only a few millimeters wide. Understanding the likelihood of passage and potential points of impaction is important for anyone managing this common condition.
Passage Rate Expectations for a 3mm Stone
A 3mm kidney stone has a high probability of passing spontaneously without the need for surgical intervention. Medical data indicates that stones 3mm or smaller have an estimated spontaneous passage rate nearing 98%. While this success rate is high, the process can still be uncomfortable and take time.
The timeframe for passage varies widely among individuals. The average time for a stone under 4mm to pass is around 31 days. However, most stones that pass naturally do so within a range of about six to 29 days.
Factors such as the stone’s shape and the patient’s hydration level influence the speed and ease of its journey. Smooth stones are less likely to catch on the ureter wall than those with jagged edges. Maintaining a high fluid intake, often recommended at two to three liters of water daily, increases urine flow to help flush the stone along the tract.
Narrow Points in the Urinary Tract
The ureter contains naturally occurring constrictions where even a 3mm stone can become temporarily lodged. The first common point of impaction is the ureteropelvic junction (UPJ), where the kidney pelvis narrows to connect with the ureter.
The second area of potential obstruction is where the ureter crosses over the iliac blood vessels near the pelvic brim. Here, the ureter makes a sharp bend, creating another natural choke point. The most common location for any stone to become stuck is the ureterovesical junction (UVJ), where the ureter enters the wall of the bladder.
The UVJ is often considered the narrowest segment of the entire urinary tract, making it the final and most frequent barrier for a stone. If the 3mm stone successfully navigates this last junction, it enters the bladder. From the bladder, it generally passes out of the body during urination without further issue.
Signs of Complete Obstruction
When a 3mm stone becomes lodged in the ureter, it creates a blockage preventing urine from draining to the bladder. This obstruction causes renal colic, typically described as sudden, severe, and fluctuating pain in the flank or back. The intense pain results from the buildup of urine pressure behind the stone, which stretches the lining and capsule of the kidney.
The location of the pain often shifts as the stone moves down the ureter, starting in the back and radiating toward the groin or abdomen. Nausea and vomiting are common symptoms accompanying the severe pain. The physical irritation caused by the stone scraping the ureter wall can lead to hematuria, which is the presence of blood in the urine.
A complete obstruction can become an emergency if signs of infection develop. A fever accompanied by chills, especially with severe pain, indicates a complication requiring immediate medical attention. If a stone fails to move for an extended period, it can cause hydronephrosis, which is the swelling of the kidney due to sustained pressure and urine backup.
Treatment Options When Small Stones Lodge
For a small 3mm stone causing manageable symptoms, the initial treatment is often conservative management. This approach involves pain control, usually with non-steroidal anti-inflammatory drugs (NSAIDs), and maintaining high fluid intake. Persistent pain or signs of infection are the primary signals that a conservative approach is failing.
A common medical intervention is the use of alpha-blockers, such as tamsulosin, prescribed as medical expulsive therapy (MET). These medications relax the smooth muscles in the ureter, particularly around the narrow UVJ, facilitating stone passage. Alpha-blockers can increase the stone passage rate by nearly 30% and are considered for stones likely to pass naturally.
If the stone remains lodged after several weeks, causes uncontrolled pain, or leads to kidney damage, a urologist may recommend a minimally invasive procedure. Ureteroscopy is the most common intervention for small, persistent ureteral stones. This procedure involves passing a thin, flexible scope up to the stone, where it can be removed directly or broken into smaller fragments using a laser.