In human anatomy, the loin refers to an area extending vertically from just below the waist, or more specifically from the bottom of the ribcage, to just below the pelvis. The term is somewhat inexact, and is most often used to describe the sides of the body in the area below the ribs, sometimes also called the flanks, and the parts of the lower back on either side of the spine. Sometimes the term also includes the genitals, the groin, the hips, and the lower abdomen. Loin is used to describe this part of the anatomy in humans, primates, and many species of four-legged mammals, including cattle and horses. The word loins is sometimes used as a euphemism for both the male and female reproductive organs, and the original Latin word lumbus, from which loin originated, referred both to the general loin area and the genitals.
Loin is not a term commonly used by doctors or in medical science. In the field of medicine, the term lumbar region is more commonly used to describe a similar, though not identical, area of the body. This part of the body includes the major psoas muscle, a large muscle that is used for example when one is sitting up or bending down and is also involved in hip movement. The lumbar vertebrae, numbered as L1-L5, are also located in the loin area. Back pain is often located in this part of the spine because it carries most of the body's weight.
Even though the word loin is not commonly used as an anatomical term in medicine, the word is included in the names of some disorders, for example loin pain hematuria syndrome. The two main symptoms of this syndrome are pain in the loins and blood in the urine. It is not fully understood what causes the pain associated with this syndrome, but it is thought to be related to the kidneys.
Pain in the loins is commonly caused by problems related to the kidneys, such as kidney stones and renal colic, or by problems related to the ureter, meaning the tubes that carry urine from the kidneys to the bladder. The reason for this is that the nerves from the ureter enter the spinal cord in the lumbar region. Other causes of pain in the loins are injuries to the muscles or vertebrae in the lumbar region, herpes zoster, and radiculitis, a type of nerve pain that often affects the sciatic nerve.
Common Causes of Loin Pain
Discussed above were a few causes and conditions concerning loin pain, but this section will dive deeper into the many common causes of loin discomfort and pain.
Kidney Stones: Kidney stones are probably the most common culprit of loin and abdominal pain. Kidney stones are a small build-up of calcium, minerals, and salt in the urinary tract. They can be extremely painful. They can typically pass through urination but sometimes require surgery. When you need surgery to remove a stone, it’s called Renal Colic.
Kidney Infection: A kidney infection occurs when a UTI travels backward to the kidneys, causing the infection to spread, and can be highly painful in your lower torso area.
Kidney Abscess: A kidney abscess is the formation of a pocket of pus inside the kidney tissue. It often occurs after a UTI or kidney infection.
Dehydration: Dehydration can also lead to loin pain, as your organs, especially the kidney and liver, are not receiving enough water to function. If you experience loin pain, ensure you hydrate well and often.
Tietze’s Syndrome: This syndrome is an inflammatory disorder that occurs in the torso. While it typically affects the upper rib cage and shoulders, it can also travel to the lower ribs and cause pain in the loin and pelvic area.
Arthritis: Pelvic arthritis can cause chronic pain in the loin. People usually feel this pain most severely on the sides of their loin around their hips, but it can also cause back pain.
Spinal Fracture: A spinal fracture, anywhere along the spine, not just toward the bottom, can lead to loin pain. A fracture often causes a ripple of pain and discomfort throughout the back and can easily affect the abdominal and loin area.
Disc Disease: Disc disease is a degenerative disease where you lose cushioning between your vertebrae, causing pain with any movement.
Pinched Nerve in the Back: A pinched nerve can happen when you apply too much pressure to a nerve around bones, cartilage, muscles, or tendons. If near the pelvic region, it will cause loin pain and discomfort.
Less Common Causes of Kidney Pain
The conditions and causes discussed below are less common than the ones mentioned in the previous section. However, loin pain can still be due to the medical issues below, and it's helpful to be aware of them.
Loin Pain Hematuria Syndrome: The syndrome’s definition details that it includes periods of severe intermittent, persistent unilateral, or bilateral loin pain. This pain also comes with either microscopic or gross hematuria.
Pneumonia: Pneumonia, and infection of the lungs, can sometimes cause loin pain. This pain is a rare symptom of pneumonia, leading many to misdiagnose the cause of the loin pain.
Pancreatitis: Pancreatitis occurs when enzymes or digestive juices accidentally attack the pancreas, leading to redness and inflammation.
Appendicitis: Appendicitis is when the appendix ruptures, located in the loin area.
Inflammatory Bowel Disease: Bowel diseases, such as Crohn’s Disease, can sometimes cause loin pain.
Shingles: Shingles are a form of the chickenpox virus, so anyone who ever had chickenpox can develop shingles later in life. It’s commonly associated with painful rashes but can also cause loin pain.
Renal Infarct: This occurs when a blood clot blocks the blood supply to the kidney, resulting in pain in the kidney area.
Abdominal Aortic Aneurysm: This condition is rare but life-threatening. It occurs when a bulge forms in the main artery supplying blood to your belly, pelvis, and legs.
Diagnosing Loin Pain
The problem with loin and flank pain is that it can be difficult to pinpoint the cause and treat the pain. As mentioned, some viruses and infections can cause loin pain unexpectedly, like shingles and pneumonia.
Therefore, diagnosing the cause of loin pain can take a long time and typically requires X-rays, such as an abdominal CT scan or ultrasounds. Doctors also frequently run blood tests to check the health of the kidneys.
Urine tests are also a helpful method for finding the problem, either via a urinalysis or a urine culture.
And lastly, they may perform a cystoscopy. This method is a minor procedure where the doctor inserts a thin tube up the urethra and into the bladder to check for signs of kidney stones, inflammation, or infection.