A kidney stone is a hard, crystalline mass that forms within the kidney when high concentrations of minerals and salts in the urine crystallize. While stones can remain silently in the kidney without symptoms, passing one begins when the stone detaches and attempts to exit the urinary tract. The journey is a chronological sequence defined by the stone’s location and the resulting physical reaction to its movement and obstruction. This process involves distinct stages, each characterized by a shift in discomfort and physical symptoms.
The Initial Descent: Symptom Onset
The initial stage of passing a kidney stone is defined by its movement from the kidney into the ureter, the tube connecting the kidney to the bladder. The stone itself does not contain nerve endings, meaning its formation within the kidney is typically painless. Pain suddenly begins when the stone dislodges and causes a blockage or begins to stretch the delicate collecting system of the kidney.
This sudden and severe pain is clinically known as renal colic, and it is the first clear indication that a stone has begun its descent. The pain is usually localized in the flank, the side and back area just below the ribs. It often radiates downward toward the abdomen as the stone enters the ureter.
The characteristic feature of renal colic is its waxing and waning intensity, often described as coming in waves. These spasms are caused by the ureter’s muscular walls contracting vigorously to push the stone downward and clear the obstruction. This intense, intermittent pain results directly from the pressure buildup and distention of the renal capsule and the ureteral wall.
The Ureteral Journey and Obstruction
Once the stone is lodged within the ureter, the stage shifts to prolonged obstruction. The ureter’s diameter is small, and any stone greater than a few millimeters can become stuck, blocking urine flow from the kidney. This blockage causes the back-up of urine, known as hydronephrosis, which increases pressure and prolongs the intense, spasmodic pain.
During this phase, the patient often experiences secondary symptoms due to the proximity of the ureter’s nerve supply to the gastrointestinal tract. Nausea and vomiting are common, a reflex reaction to the severe pain and the shared neural pathways. Hematuria, or blood in the urine, frequently occurs as the stone scrapes the inner lining of the ureter during its movement.
As the stone progresses down the ureter toward the bladder, the location of the pain often shifts inferolaterally, moving from the flank to the lower abdomen and groin. The sustained presence of the stone also raises the risk of complications, particularly a urinary tract infection; a fever, chills, and cloudy urine signal a potentially serious complication requiring immediate medical attention. This active journey is often the longest phase, lasting from a few days up to several weeks for stones that can pass spontaneously.
Entry into the Bladder and Final Expulsion
The transition from the ureter to the bladder marks a distinct and welcome change in the patient’s experience. Once the stone successfully passes through the narrowest point of the ureter and drops into the bladder, the intense, wave-like flank pain of renal colic typically subsides immediately. The relief is caused by the sudden cessation of the ureteral obstruction and the subsequent drop in pressure within the kidney.
A new set of symptoms related to the stone’s presence in the bladder replaces the colic. The foreign object irritates the bladder lining, leading to symptoms that mimic a bladder infection. Patients often report urinary urgency and increased frequency, as the bladder attempts to expel the stone.
The final act of expulsion occurs when the stone leaves the body through the urethra during urination. This passage can sometimes cause a sharp, brief sensation of discomfort or a burning feeling, known as dysuria. Patients are often instructed to strain their urine during this phase to capture the stone, which is crucial for chemical analysis to determine its composition and guide future prevention strategies.
Variables and Intervention Points
The duration of the entire passage process is highly dependent on several physiological and physical variables. The most significant factor determining the likelihood and speed of natural passage is the stone’s size; stones less than 4 millimeters have a high chance of passing spontaneously, typically within a few days to a few weeks. The stone’s location also matters, as those located closer to the bladder in the lower ureter have a greater spontaneous passage rate than those high up near the kidney.
A failure to pass the stone naturally, or a “trial of passage,” is typically defined by a stone being too large (often greater than 10 millimeters), persistent and uncontrolled pain, or signs of a complete obstruction that threatens kidney function. An associated, persistent infection that cannot be cleared is also a clear indication for medical intervention.
When natural passage is deemed unsuccessful or unsafe, a urologist must intervene to remove or break up the stone. Common procedures include ureteroscopy, where a small scope is passed through the urethra and bladder to retrieve or fragment the stone, and extracorporeal shock wave lithotripsy (ESWL), which uses targeted shock waves from outside the body to break the stone into smaller pieces that can be passed more easily. These interventions are made to prevent lasting damage to the kidney.