Given the complexity of the human anatomy, flank pain can originate from various organ systems. A comprehensive differential diagnosis is crucial for accurate patient management. Here, we explore some of the common and less common causes:

Urinary Tract Causes (Non-Stone Related)

• Pyelonephritis (Kidney Infection): A bacterial infection of the kidney, often ascending from the bladder. Symptoms include flank pain (usually dull and constant), fever, chills, nausea, vomiting, and painful urination. It requires prompt antibiotic treatment.
• Urinary Tract Infection (UTI): While typically causing lower abdominal pain and dysuria, a severe or upper UTI can sometimes present with flank discomfort.
• Hydronephrosis: Swelling of the kidney due to a blockage in the urinary tract, which can be caused by stones, tumours, strictures, or external compression. This can lead to dull, aching flank pain.
• Renal Tumours: Kidney cancers can sometimes present with flank pain, often dull and persistent, along with other symptoms like haematuria, weight loss, or a palpable mass.
• Renal Infarction: A rare condition where blood supply to part of the kidney is interrupted, leading to tissue death and acute, severe flank pain.

Musculoskeletal Causes

• Muscle Strain or Spasm: Injury or overuse of the back muscles in the flank region can cause localised pain, often exacerbated by movement or specific postures. This is a very common cause and often overlooked.
• Rib Fractures or Costochondritis: Trauma to the ribs or inflammation of the cartilage connecting the ribs to the breastbone can cause sharp, localised pain in the flank or chest wall.
• Spinal Issues: Conditions like herniated discs, spinal stenosis, or arthritis in the lumbar spine can cause referred pain to the flank region, often accompanied by back pain, numbness, or weakness in the legs.

Gastrointestinal Causes

• Appendicitis: While typically causing right lower abdominal pain, a retrocaecal appendix (located behind the caecum) can sometimes present with right flank pain. It is often accompanied by nausea, vomiting, and fever.
• Diverticulitis: Inflammation of diverticula (small pouches) in the colon, usually in the left lower abdomen, but can sometimes cause left flank pain.
• Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease can cause abdominal pain that may radiate to the flank.
• Constipation: Severe constipation can lead to generalised abdominal discomfort, which may include flank pain.
• Pancreatitis: Inflammation of the pancreas, located behind the stomach, can cause severe upper abdominal pain that often radiates to the back or flank.
• Gallstones/Cholecystitis: Gallbladder issues typically cause right upper abdominal pain, but this can sometimes radiate to the right flank or shoulder blade.

Gynaecological/Reproductive Causes

• Ectopic Pregnancy: A life-threatening condition where a fertilised egg implants outside the uterus. It can cause severe unilateral abdominal or flank pain, often with vaginal bleeding, in women of childbearing age.
• Ovarian Cysts/Torsion: Ruptured ovarian cysts or ovarian torsion (twisting of the ovary) can cause sudden, severe, unilateral lower abdominal or flank pain.
• Endometriosis: A condition where uterine tissue grows outside the uterus, causing chronic pelvic pain that can sometimes radiate to the flank.
• Pelvic Inflammatory Disease (PID): An infection of the female reproductive organs that can cause lower abdominal and flank pain, fever, and vaginal discharge.

Vascular Causes

• Abdominal Aortic Aneurysm (AAA): A bulging in the wall of the aorta. A ruptured or dissecting AAA is a medical emergency causing sudden, severe abdominal, back, or flank pain, often with signs of shock. This is a critical red flag.
• Renal Artery Stenosis/Thrombosis: Narrowing or blockage of the renal artery can cause flank pain, often associated with high blood pressure.

Other Causes

• Shingles (Herpes Zoster): A viral infection that causes a painful rash, typically on one side of the body, following a nerve pathway. Pain can precede the rash and be mistaken for other causes of flank pain.
• Referred Pain: Pain originating from other areas of the body, such as the lungs (e.g., pleurisy, pneumonia), can sometimes be perceived in the flank region.