The kidneys are two bean-shaped organs that remove waste from your blood, balance body fluids, form urine, and regulate other important functions of the body. Each kidney has a vein that carries blood filtered by the kidney into the circulatory system. These are called the renal veins.

In nutcracker syndrome, the renal vein coming from the left kidney becomes compressed and blood can’t flow normally through it. Instead, blood can flow backwards into other veins and cause them to swell, which causes symptoms such as blood in your urine.

There are two types of nutcracker syndrome: anterior and posterior. In anterior nutcracker syndrome, the left renal vein is compressed between two abdominal arteries. This is the most common type of nutcracker syndrome.

In posterior nutcracker syndrome, the left renal vein is compressed between an artery and the spine. Usually, the artery is the abdominal aorta, which is largest artery in the abdomen.

Nutcracker syndrome got its name because the compression of the renal vein is like a nutcracker cracking a nut.

Some people with nutcracker syndrome don’t have any symptoms, while others will have severe symptoms. Children are less likely to have symptoms. Common symptoms include:

  • blood in your urine (most common symptom)
  • pelvic pain, usually from varicose veins in the lower abdomen
  • pain in your side or abdomen
  • protein in your urine, which can be determined by a doctor and used to diagnose nutcracker syndrome
  • pain during intercourse for women
  • enlarged veins in testicles for men
  • lightheadedness while standing, but not while sitting (orthostatic intolerance)

The specific causes of nutcracker syndrome are unknown. The condition is more common in females in their teens and 20s, but can affect anyone of any age. People who have a low body mass index (BMI) are more likely to develop nutcracker syndrome.

Some conditions that may increase the chance of developing nutcracker syndrome include:

  • pancreatic tumors
  • tumors in the tissue that lines the abdominal wall
  • a severe lower spine curve
  • nephroptosis (“floating kidney”), when the kidney drops into the pelvis when you stand up
  • an aneurysm in the abdominal aorta

In children, rapid growth during puberty can cause nutcracker syndrome. As body proportions change, the renal vein can become compressed. Children are less likely to have symptoms than people of other ages.

Nutcracker syndrome is not genetic.

First, your doctor will perform a physical exam. Next, they’ll take a medical history and ask about your symptoms to help them narrow down a possible diagnosis.

If they suspect nutcracker syndrome, your doctor will take blood and urine samples to look for blood, protein, and bacteria. This will help them narrow down your diagnosis even further.

Next, your doctor will do a Doppler ultrasound of your kidney area to see how the blood is flowing through your veins. That will allow them to see if the veins are not allowing blood to flow normally.

If necessary to confirm a diagnosis, your doctor will do a CT scan or MRI to look more closely at the kidney, and to see exactly where the vein is compressed. They might also do a kidney biopsy to help rule out other conditions that can cause similar symptoms.

In many cases, especially if your symptoms are mild, your doctor will recommend observation of your nutcracker syndrome. This is because it can sometimes go away on its own.

In children under 18, studies show that nutcracker syndrome will resolve itself approximately 75 percent of the time.

If your doctor does recommend observation, they’ll do regular urine tests to track your condition’s progression.

If your symptoms are more severe or don’t improve after a period (usually 18 to 24 months) of observation, you might need either a stent or surgery to relieve nutcracker syndrome.

A stent is a small mesh tube that holds the compressed vein open and allows blood to flow normally. Your doctor will insert it by cutting a small slit in your leg and using a catheter to move the stent into the vein. Stent placement requires an overnight hospital stay and full recovery can take two to three months.

If you have more severe symptoms, such as severe pain or a lot of blood in your urine, your doctor might recommend surgery to relieve pressure on the vein. One option for surgery is to move the vein and reattach it, so it’s no longer in an area where it would be compressed.

Another option is bypass surgery, in which a vein taken from somewhere else in your body is attached to replace the compressed vein. Blood then flows out of the left kidney through the new vein instead of the compressed one. Recovery from surgery generally takes around three months.

Nutcracker syndrome is hard for doctors to diagnose, but once it’s diagnosed, the outlook is good for the condition to be cured. In many cases, nutcracker syndrome with mild symptoms will resolve itself within two years. In people with more severe symptoms, inserting a stent and surgery to move or bypass the affected vein have good results for short- and long-term relief.