Why Kidney Stones Cause Flank-to-Groin Pain: The Ureteric Pathway Explained
From the Kidney to the Flank
Kidney stones produce one of the most characteristic and dramatic patterns of pain in clinical medicine: a sharp, colicky ache that begins in the flank and radiates downward toward the groin. This predictable pain pattern is not random. It reflects the precise anatomical course of the ureter, the arrangement of its sensory innervation, and the locations where stones most commonly obstruct the urinary tract.
The kidneys lie high in the posterior abdominal wall, embedded in perirenal fat and enclosed by the fibrous renal capsule. When a stone obstructs the renal pelvis or upper ureter, pressure increases in the collecting system. This distension activates stretch-sensitive nociceptors within the renal capsule and surrounding tissues of the kidney coverings. The result is flank pain, typically located at the costovertebral angle, corresponding to the lower thoracic dermatomes.
This region correlates with the segmental sensory supply to the kidney, carried predominantly by sympathetic fibers from T10–L1. These visceral afferents travel through the renal plexus, which is part of the autonomic networks described under the abdominal autonomic plexuses.
The Ureter and Its Constriction Points
As the stone descends, the pain migrates along the anatomical path of the ureter. The ureter begins at the renal pelvis, courses vertically on the psoas major muscle along the posterior abdominal wall, and then enters the pelvis to reach the urinary bladder. This route is closely related to structures of the posterior abdominal wall discussed in associated structures of the posterior abdominal wall.
Three classical ureteric constriction sites are especially prone to stone impaction, as highlighted in the section on anatomical constrictions of the ureter:
- The ureteropelvic junction (UPJ), where the renal pelvis narrows into the ureter
- The point where the ureter crosses the pelvic brim over the common iliac vessels
- The ureterovesical junction (UVJ), where the ureter pierces the bladder wall
Obstruction at each of these levels produces pain localized to dermatomal segments receiving afferents from that region.
Dermatomes and the Flank-to-Groin Radiation
The visceral sensory fibers from the ureter travel alongside sympathetic fibers from T11–L2 before entering the spinal cord. Because these spinal levels also supply the skin of the lower abdomen, inguinal region, and anterior thigh, the brain often misinterprets the origin of the pain. This phenomenon of referred pain is discussed further under clinical correlation of dermatomes and nerve plexuses.
Clinically, this produces the classic pattern of migrating pain:
- Flank pain when the stone is in the renal pelvis or proximal ureter
- Lower abdominal pain as it descends to the mid-ureter
- Groin or genital pain as it approaches the distal ureter and UVJ
Occasionally, pain may radiate to the scrotum or labia due to shared innervation via the genitofemoral and ilioinguinal nerves, which also arise from L1–L2.
Why the Pain Comes in Waves
Ureteric pain is typically colicky rather than constant. The ureter is a muscular tube with intrinsic peristaltic activity. When a stone obstructs the lumen, each peristaltic wave increases intraluminal pressure, stretching the smooth muscle and activating visceral nociceptors. As a result, patients experience episodic, severe waves of pain that correspond to peristaltic contractions attempting to force the stone distally.
Clinical Significance
The distinctive flank-to-groin pain pattern of renal colic directly mirrors the anatomical course and innervation of the ureter. Pain that remains high in the flank suggests obstruction near the renal pelvis or UPJ. Pain that migrates downward over hours indicates stone progression along the ureter, while groin-dominant pain suggests distal ureter or UVJ involvement.
Recognising this anatomical pattern, combined with supportive findings such as hematuria and imaging, allows clinicians to differentiate ureteric colic from other causes of acute abdomen. Ultimately, kidney stone pain is a precise clinical expression of the underlying anatomy of the kidney, ureter, and their shared segmental innervation.