The nutcracker syndrome (NCS) results most commonly from the compression of the left renal vein (LRV) between the abdominal aorta (AA) and superior mesenteric artery (SMA), although other variants exist. The name derives from the fact that, in the sagittal plane and/or transverse plane, the SMA and AA (with some imagination) appear to be a nutcracker crushing a nut (the renal vein). There is a wide spectrum of clinical presentations and diagnostic criteria are not well defined, which frequently results in delayed or incorrect diagnosis. The first clinical report of Nutcracker phenomenon appeared in 1950. This condition is not to be confused with superior mesenteric artery syndrome, which is the compression of the third portion of the duodenum by the SMA and the AA.
Signs and symptoms The signs and symptoms of NCS are all derived from the outflow obstruction of the left renal vein. The compression causes renal vein hypertension, leading to hematuria (which can lead to anemia) and abdominal pain (classically left flank or pelvic pain). The abdominal pain may improve or worsen depending on positioning. Patients may also have orthostatic proteinuria, or the presence of protein in their urine depending on how they sit or stand. Since the left gonadal vein drains via the left renal vein, it can also result in left testicular pain in men or left lower quadrant pain in women, especially during intercourse and during menstruation. Occasionally, the gonadal vein swelling may lead to ovarian vein syndrome in women. Nausea and vomiting can result due to compression of the splanchnic veins. An unusual manifestation of NCS includes varicocele formation and varicose veins in the lower limbs. Another clinical study has shown that nutcracker syndrome is a frequent finding in varicocele-affected patients and possibly, nutcracker syndrome should be routinely excluded as a possible cause of varicocele and pelvic congestion. In women, the hypertension in the left gonadal vein can also cause increased pain during menses. Headaches can develop when collateral veins branch out from the kidney into the spinal plexus. Also, common findings that develop alongside NCS are POTS and pelvic congestion syndrome. Pelvic congestion occurs when blood flows into the pelvis from the kidney which enlarges pelvic veins resulting in internal varicose veins. This can result in pelvic pain and also GI tract irritation leading to bloating, abdominal fullness, constipation and/or diarrhea. Often times nutcracker syndrome occurs alongside other abdominal compressions such as May–Thurner syndrome, superior mesenteric artery syndrome, and median arcuate ligament syndrome.
Cause In normal anatomy, the LRV travels between the SMA and the AA. Occasionally, the LRV travels behind the AA and in front of the spinal column. NCS is divided based on how the LRV travels, with anterior NCS being entrapment by the SMA and AA and posterior NCS being compression by the AA and spinal column. NCS can also be due to other causes such as compression by pancreatic cancer, retroperitoneal tumors, and abdominal aortic aneurysms. Although other subtypes exist, these causes are more uncommon in comparison to entrapment by the SMA and the AA. Patients with NCS usually have a low BMI, as this can lead to a narrower gap between the SMA and the AA for the LRV.
Diagnosis Nutcracker syndrome is diagnosed through imaging such as doppler ultrasound (DUS), computed tomography (CT) with contrast, magnetic resonance imaging (MRI), and venography. The selection of the imaging modality is a step-wise process. DUS is the initial choice after clinical suspicion based on symptoms. However, often vascular compressions can be missed and CT with and without contrast is needed to visualize the vascular structures. MRI can be used if CT is not assessable. Venography with IVUS is gold standard for diagnosing.
Doppler Ultrasound Although its ability to detect renal vein compression is dependent on how a patient is positioned during imaging and technician knowledge and skill, DUS is recommended as an initial screening tool as it has a high sensitivity (69–90%) and specificity (89–100%). DUS measures the anteroposterior diameter, and a peak systolic velocity at least four times as fast as an uncompressed vein is indicative of NCS.
CT and MRI CT and MRI with contrast can be used afterward to confirm compression by the AA and SMA with comprehensive measurements of the abdominal vasculature. A "beak sign" can often be seen in CT scans due to the LRV compression. However, CT and MRI cannot demonstrate the flow within the compressed vein. These two modalities can be used to confirm other evidence for NCS such as back-up of blood flow into the ovarian veins.
Venography If further confirmation is necessary, venography is used as the gold standard test in diagnosing nutcracker syndrome. A renocaval pullback mean gradient of >3 mmHg is considered diagnostic. Although this method continues to be the gold standard, values in unaffected individuals may vary considerably, leading to some measurements in NCS patients to be similar to those in normal individuals. This may be partly due to compensatory mechanisms in the vasculature as a result of the increased blood pressure. The invasive nature of the procedure is another consideration in comparison to DUS and CT/MRI as imaging modalities.
Differential diagnosis Pelvic congestion syndrome Renal stones May–Thurner syndrome Genitourinary malignancy Loin pain hematuria syndrome
Treatment Treatment depends on the severity and symptoms. In addition to conservative measures, more invasive therapies include endovascular stenting, renal vein re-implantation, and gonadal vein embolization. The decision between conservative and surgical management is dependent on the severity of the symptoms. Conservative management is used if the patient is a child and the hematuria is mild. In contrast, more severe symptoms such as reduced renal function, flank pain, and anemia are managed with surgical interventions.
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