Loin (flank) pain

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This can present suddenly as severe pain in the flank reaching a peak within minutes or hours (acute loin pain). 

Alternatively, it may have a slower course of onset (chronic loin pain), developing over weeks or months. 

Loin pain is frequently presumed to be urological in origin on the simplistic basis that the kidneys are located in the loins. However, other organs are located in this region, pathology within which may be the source of the pain, and pain arising from extra-abdominal organs may radiate to the loins (referred pain). So, when faced with a patient with loin pain think laterally the list of differential diagnoses is long!

The speed of onset of loin pain gives some, though not an absolute, indication of the cause of urological loin pain. Acute loin pain is more likely to be due to something obstructing the ureter, such as a stone. Loin pain of more chronic onset suggests disease within the kidney or renal pelvis.

Acute loin pain
The most common cause of sudden onset of severe pain in the flank is the passage of a stone formed in the kidney, down through the ureter. Ureteric stone pain characteristically starts very suddenly (within minutes), is colicky in nature (waves of increasing severity are followed by a reduction in severity, though seldom going away completely), and it radiates to the groin as the stone passes into the lower ureter. The pain may change in location, from flank to groin, but its location does not provide a good indication of the position of the stone, except where the patient has pain or discomfort in the penis and a strong desire to void, which suggests that the stone has moved into the intramural part of the ureter (the segment within the bladder). The patient cannot get comfortable. They often roll around in agony.

50% of patients with these classic symptoms of ureteric colic do not have a stone confirmed on subsequent imaging studies, nor do they physically ever pass a stone. They have some other cause for their pain. A ureteric stone is only very rarely life-threatening, but many of these differential diagnoses may be life-threatening. Acute loin pain is less likely to be due to a ureteric stone in women and in patients at the extremes of age. It tends to be a disease of men (and to a lesser extent women) between the ages of ~20 to 60 years, though it can occur in younger and older individuals.

Acute loin pain non-stone, urological causes
  • Clot or tumour colic: a clot may form from a bleeding source within the kidney (e.g. renal cell cancer or transitional cell cancer of the renal pelvis). Similarly, a ureteric TCC may cause ureteric obstruction and acute loin pain. Loin pain and haematuria are often assumed to be due to a stone, but it is important to approach investigation of such patients from the perspective of haematuria (i.e. look to exclude cancer).
  • Pelviureteric junction obstruction (PUJO), also known as ureteropelvic junction obstruction (UPJO): may present acutely with flank pain severe enough to mimic a ureteric stone. A CT scan will demonstrate hydronephrosis, with a normal calibre ureter below the PUJ and no stone. MAG3 renography confirms the diagnosis.
  • Infection: acute pyelonephritis, pyonephrosis, emphysematous pyelonephritis, xanthogranulomatous pyelonephritis. These patients have a high fever (38°C), whereas ureteric stone patients do not (unless there is infection behind the obstructing stone) and are often systemically very unwell. Imaging studies may or may not show a stone, and there will be radiological evidence of infection within the kidney and peri-renal tissues (oedema).
Acute loin pain non-urological causes
  • Vascular
    • Leaking abdominal aortic aneurysm
  • Medical
    • Pneumonia
    • Myocardial infarction
    • Malaria presenting as bilateral loin pain and dark haematuria black water fever
  • Gynaecological and obstetric
    • Ovarian pathology (e.g. twisted ovarian cyst)
    • Ectopic pregnancy
  • Gastrointestinal
    • Acute appendicitis
    • Inflammatory bowel disease (Crohn's, ulcerative colitis)
    • Diverticulitis
    • Burst peptic ulcer
    • Bowel obstruction
  • Testicular torsion
  • Spinal cord disease
    • Prolapsed intervertebral disc
Distinguishing urological from non-urological loin pain

History and examination are clearly important. Patients with ureteric colic often move around the bed in agony. Those with peritonitis lie still. Palpate the abdomen for signs of peritonitis (abdominal tenderness and/or guarding) and examine for abdominal masses (pulsatile and expansile = leaking AAA). Examine the patient's back, chest, and testicles. In women, do a pregnancy test.

Chronic loin pain urological causes
  • Renal or ureteric cancer
    • Renal cell carcinoma
    • Transitional cell carcinoma of the renal pelvis or ureter
  • Renal stones
    • Staghorn calculi
    • Non-staghorn calculi  
  • Renal infection
    • TB
  • PUJO
  • Testicular pathology (referred pain)
    • Testicular neoplasms
  • Ureteric pathology
    • Ureteric reflux
    • Ureteric stone (may drop into the ureter causing severe pain which then subsides to a lower level of chronic pain)
Chronic loin pain non-urological causes
  • Gastrointestinal
    • Bowel neoplasms
    • Liver disease
  • Spinal disease
    • Prolapsed intervertebral disc
    • Degenerative disease
    • Spinal metastases

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