Flank pain

Flank pain refers to discomfort or pain located in the lateral aspect of the trunk — the area between the lower rib cage and the upper border of the pelvis, on one or both sides of the body. It corresponds anatomically to the region of the kidneys, adrenal glands, and upper ureters, though it can also arise from musculoskeletal structures (muscles, vertebrae, and spinal nerves) in the same region.

Flank pain ranges in character from a dull, persistent ache to a severe, sudden, cramping pain that comes in waves and radiates toward the groin or inner thigh. The nature, timing, and associated symptoms of flank pain provide important clues to its underlying cause.

In the context of urological and oncological disease, flank pain is a significant symptom that can indicate obstruction of the urinary tract, renal parenchymal disease, adrenal gland pathology, or, less commonly, retroperitoneal malignancy.

Ureteral and renal pelvis obstruction (obstructive uropathy)

• Obstruction of the ureter — by a tumor (ureteral cancer or renal pelvis cancer), a calculus (kidney stone), or external compression — impairs the drainage of urine from the kidney, causing progressive distension of the renal collecting system (hydronephrosis) and the characteristic pain of ureteral colic. Ureteral colic is one of the most intense pains in clinical medicine: it is typically sudden in onset, extremely severe, crampy, and radiates from the flank toward the ipsilateral groin and inner thigh. A tumor-related obstruction tends to cause a more gradual, dull ache as the hydronephrosis develops slowly, in contrast to the abrupt severe pain of an acutely impacted stone.

Kidney stones (nephrolithiasis)

• Kidney stones are among the most common causes of acute, severe flank pain. As a stone migrates from the renal pelvis into the ureter, it causes ureteral obstruction and smooth muscle spasm, producing the characteristic colicky pain. Hematuria almost always accompanies stone-related flank pain.

Kidney infection (pyelonephritis)

• Bacterial infection of the kidney — typically ascending from the lower urinary tract — causes inflammation and swelling of the renal parenchyma. The resulting flank pain is typically constant rather than colicky, associated with fever, chills, and systemic signs of infection, and accompanied by lower urinary tract symptoms.

Renal pelvis and ureteral cancer

• Tumors arising in the renal collecting system or ureter can obstruct urine flow and produce flank pain from hydronephrosis, or cause a dull aching discomfort from local tumor growth. Flank pain in upper tract urothelial carcinoma is typically present alongside hematuria.

Adrenal tumors

• Large adrenal tumors — particularly adrenocortical carcinomas — can cause flank, back, or abdominal pain through local mass effect and pressure on adjacent structures. Pheochromocytoma may cause episodic pain associated with hypertensive crises.

Renal cell carcinoma

• Advanced kidney cancers can produce flank or back pain through distension of the renal capsule or invasion of adjacent structures. Flank pain as part of the classic triad (hematuria, flank pain, palpable abdominal mass) suggests locally advanced disease.

Musculoskeletal causes

• Muscle strain, vertebral disc pathology, spinal nerve compression, and rib fractures are common non-urological causes of flank pain and can closely mimic renal colic in location and severity. A careful history and physical examination — supplemented by imaging — help distinguish musculoskeletal from urological causes.

Other urological causes:

• Acute distension of the renal pelvis from any cause
• Polycystic kidney disease
• •Renal infarction (arterial occlusion)

See a physician promptly if:

• Flank pain is new, unexplained, or persistent for more than a few days
• Flank pain is accompanied by blood in the urine (hematuria)
• Flank pain is accompanied by fever, chills, nausea, or vomiting — these may indicate an infected obstructed kidney (pyonephrosis), which is a urological emergency requiring immediate drainage
• Flank pain is associated with a palpable abdominal or flank mass
• Flank pain occurs in a patient with a known or suspected history of urological malignancy

Seek emergency evaluation immediately if:

• Severe, unrelenting flank or abdominal pain is accompanied by high fever and rigors — this combination may indicate urosepsis from an obstructed, infected kidney and is a medical emergency
• Flank pain is associated with significant trauma to the abdomen, back, or flank
• Pain is accompanied by inability to urinate