Nutcracker syndrome happens when the vein that drains the left kidney is squeezed between two large arteries, and this causes symptoms. Blood backs up in the kidney vein, which can lead to blood in the urine, pain in the left side, and, in women, congested pelvic veins. It is considered rare, and it is easy to overdiagnose, because the same squeeze is often seen on scans of people who feel perfectly well.
What is nutcracker syndrome?
The left renal vein carries blood from the left kidney across the body to the inferior vena cava, the main vein that returns blood to the heart. On the way, it usually passes through a narrow gap between two arteries:
- The aorta, the main artery, which runs down behind it
- The superior mesenteric artery (SMA), which branches forward off the aorta to supply the gut
If the angle between these two arteries is narrow, the vein can be pinched. Doctors also call this left renal vein entrapment or renal vein compression.
Why is it called nutcracker syndrome?
The aorta and the SMA form a V shape, like the two arms of a nutcracker. The left renal vein sits between them like a nut being cracked.
Anterior and posterior nutcracker
- Anterior nutcracker is the usual type. The vein is squeezed between the SMA in front and the aorta behind.
- Posterior nutcracker is less common. Here the left renal vein runs behind the aorta and is squeezed between the aorta and the spine.
Phenomenon or syndrome?
This distinction matters a great deal.
- Nutcracker phenomenon (or nutcracker anatomy) means a scan shows the vein is narrowed, but there are no symptoms.
- Nutcracker syndrome means the narrowing is causing symptoms.
Some narrowing of the left renal vein is fairly common, especially in slim people, and it is often reported on scans done for other reasons. On its own, it does not need treatment.
Symptoms
When the vein is squeezed, pressure builds up in the kidney vein. Blood then looks for other routes out, such as the left ovarian vein in women or the left testicular vein in men. This explains the range of symptoms:
- Blood in the urine (haematuria). This is the most common sign. It may be visible, making urine pink, red or cola-coloured, or found only on a urine test.
- Left flank pain. An ache in the left side or back, below the ribs. It may be worse when standing, walking or after exercise.
- Protein in the urine, especially when standing. This is mainly seen in younger people.
- Pelvic congestion in women. Blood escapes down the left ovarian vein, causing a dull pelvic ache, pain with sex and sometimes vulval varicose veins. See pelvic congestion syndrome.
- A left-sided varicocele in men, a cluster of swollen veins around the testicle. See varicocele.
- Tiredness or dizziness on standing, reported by some people.
For more detail, see our guide to nutcracker syndrome symptoms.
What causes it?
It comes down to the shape and position of the blood vessels. Things that may narrow the gap include:
- A naturally narrow angle between the aorta and the SMA
- Very little fat around the vessels, which normally acts as a cushion. This is why it is more common in slim people and after weight loss.
- A rapid growth spurt in the teenage years
- An unusual course of the vein, as in posterior nutcracker
- Increased curvature of the lower spine
Sometimes no clear reason is found. A different condition, SMA syndrome, involves the same artery squeezing the bowel rather than the vein. See how nutcracker and SMA syndrome differ.
Nutcracker syndrome and pelvic congestion
In women, nutcracker syndrome is one of the less common causes of pelvic congestion syndrome. When the left renal vein is squeezed, the left ovarian vein can act as an overflow pipe. Blood flows down it into the pelvis instead of up towards the heart, and the pelvic veins swell.
This matters for treatment. If the pelvic veins are closed off without considering the narrowed kidney vein, pressure in the kidney vein could rise further. Specialists therefore look at both before deciding what to do. Many women with ovarian vein reflux do not have nutcracker syndrome, so it is not something everyone needs to worry about. For more on the other causes, see what causes pelvic congestion syndrome.
Who gets it, and how rare is it?
Nobody knows exactly how common it is. Diagnostic criteria are not agreed, and mild cases may never be investigated. It is considered rare. It is most often diagnosed in teenagers and adults up to middle age, and seems slightly more common in women.
Is nutcracker syndrome dangerous?
It is very rarely life-threatening. Many people have mild symptoms that settle or stay stable. Possible complications include:
- Anaemia from repeated blood loss in the urine
- A clot in the left renal vein
- Kidney damage from long-term high pressure, which is uncommon
- Severe pain that affects daily life
How it is diagnosed, and the risk of overdiagnosis
Diagnosis usually involves urine and blood tests, then imaging. A Doppler ultrasound can measure how fast blood flows through the narrowed vein. CT or MR scans show the angle between the arteries and how squashed the vein is. A venogram can measure the pressure difference across the narrowing.
Doctors also need to rule out more common causes first. Blood in the urine can come from infection, kidney stones, kidney disease or, especially in older adults, cancers of the kidney or bladder. These must be excluded before blaming the renal vein.
Because the anatomy is common in healthy people, a scan finding alone is not enough. The diagnosis should rest on symptoms that fit, a clear narrowing with signs of backed-up pressure, and no better explanation. See our guide to CT and MRI scans of the pelvic veins.
Treatment in brief
Treatment depends on how severe the symptoms are:
- Watching and waiting. Children and teenagers often improve as they grow and gain weight, so they are usually monitored, sometimes for up to two years. Adults with mild symptoms are often observed for some months.
- Procedures for persistent or severe symptoms. Options include moving the kidney vein to a new position (transposition), placing a stent, closing a refluxing ovarian vein for pelvic symptoms, and, rarely, moving the kidney itself.
Each option has risks, and long-term results are limited. Stenting in particular is debated. Read more in our guide to nutcracker syndrome treatment. A Singapore vascular surgeon’s overview of pelvic vein treatment types mentions renal vein stenting as one option when the kidney vein is involved.
When to see a doctor
See a doctor if you notice blood in your urine, even once, if you have ongoing pain in your left side or back, or if you have a dull pelvic ache or a swollen vein around the testicle. Blood in the urine always needs checking, because it has many possible causes.
Get urgent medical help if you pass large amounts of blood or clots in your urine, cannot pass urine, have severe flank pain with fever or vomiting, or feel faint. In Singapore, call 995 in an emergency.
Frequently asked questions
Why is it called nutcracker syndrome?
The two arteries that squeeze the left kidney vein, the aorta behind and the superior mesenteric artery in front, form a V shape that looks like the jaws of a nutcracker. The vein is caught between them like a nut.
What is posterior nutcracker syndrome?
In some people the left kidney vein runs behind the aorta instead of in front of it. It can then be squeezed between the aorta and the spine. This is called posterior nutcracker and is less common than the usual (anterior) type.
How rare is nutcracker syndrome?
Nobody knows exactly. Some narrowing of the vein is seen fairly often on scans of people without symptoms, but true nutcracker syndrome with symptoms is considered rare. There are no agreed diagnostic criteria, which makes it hard to count.
Is nutcracker syndrome dangerous? Can it kill you?
It is very rarely life-threatening. Most people have mild or moderate symptoms. Possible complications include anaemia from ongoing blood loss in the urine, a clot in the kidney vein, and, rarely, kidney damage. Severe pain can seriously affect quality of life.
What causes nutcracker syndrome?
It is caused by the shape and position of the blood vessels. A narrow angle between the aorta and the superior mesenteric artery, little fat around the vessels, a growth spurt, weight loss or an unusual course of the vein can all contribute. Sometimes no clear reason is found.
Sources and further reading
This article is general information, not personal medical advice. Please speak to a doctor about your own symptoms. How we create our content.