Renal colic: pain mechanism and presentation
Renal colic is severe, wave-like pain from the ureter (tube from kidney to bladder) in spasm above a sudden blockage, typically spreading from loin (side of the back) to groin.
Remember
Renal colic is classically wave-like loin-to-groin pain from stretch and spasm of the ureter above a sudden blockage, in a patient who is often restless and unable to lie still.
Memory aid
colic writhes, peritonitis lies still
Restlessness points towards renal colic, while lying still points towards peritonitis.
Renal colic is the pain of the ureter (the tube that takes urine from the kidney to the bladder) that has been suddenly blocked. A kidney stone is the usual cause, but a blood clot or a sloughed papilla (dead tissue shed from the kidney) can do the same.
Urine keeps arriving from the kidney, so pressure builds up behind the blockage and the renal pelvis (the funnel-shaped chamber where the kidney collects urine) and the ureter stretch. The ureter responds with extra peristalsis and smooth muscle spasm, helped by prostaglandins (local chemical messengers released as the wall is stretched). The contractions come and go, so the pain is colicky, meaning it comes in waves.
The pain is severe and usually unilateral (one-sided). Classically it starts in the loin and spreads towards the groin. Blood in the urine can come with it, and the blood may be non-visible, meaning it is found only on urine testing.
Patients are often restless and unable to lie still. That helps to tell colic from peritonitis (inflammation of the abdominal lining), where movement worsens the pain and people tend to lie still.
Speed of onset matters: partial or slowly developing blockage often causes little pain. Fever or shivering with an obstructed kidney suggests infection behind the blockage, called pyonephrosis (pus collecting in the blocked kidney), an emergency needing urgent drainage.
Key facts
- Pattern
- severe, colicky pain that is usually on one side and classically runs from the loin to the groin.
- Mechanism
- pressure rises behind a sudden blockage, and the ureter stretches and goes into spasm, with prostaglandins released as the wall is stretched.
- Usual cause
- a kidney stone is by far the most common cause, though blood clots and sloughed papillae may also cause it.
- Behaviour
- patients are often restless and unable to lie still, unlike people with peritonitis, who tend to lie still.
- Blood in the urine
- it is common and may be non-visible, found only on testing.
- Speed of onset
- partial or slowly developing blockage often causes little pain.
- Red flag
- fever or shivering with an obstructed kidney suggests infection behind the blockage (pyonephrosis), an emergency needing urgent drainage.
Exam traps
- Reduced blood supply to the kidney is a recognised cause of acute loin pain, but it does not explain pain in waves from the loin to the groin: that points to a blocked ureter that contracts.
- A stone low in the ureter, near the bladder, can irritate the bladder wall muscle and cause lower urinary symptoms and straining, but that does not explain the loin-to-groin waves of colic, which come from stretch and spasm of the ureter above the blockage.
- Colic does not need infection. Fever suggests an infection that can sit alongside a stone, but the infection is not what produces the waves of pain.
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