Left renal vein obstruction, also known as Nutcracker phenomenon (NCP), occurs due to the external compression of the left renal vein (LRV) in between the angle formed by the origin of the superior mesenteric artery (SMA) from the aorta. The word "Nutcracker" was first used by Chait et al. in 1971, but in 1972, the Belgian physician De Schepper named it after his observation of the compression of LRV between the aorta and SMA which behave as 2 levers of the Nutcracker.
LRV compression is a developmental anomaly. It occurs when the average angle between the SMA and the aorta of approximately 90 degrees reduces to less than 35 degrees. This leads to the NCP. The term Nutcracker syndrome (NCS) is interchangeably used with NCP when clinical symptoms accompany the anatomical findings. The LRV courses in front of the aorta before draining into the inferior vena cava. It has an extended course of 6 to 7 cm compared to 3 to 4 cm of the right renal vein. There have been several arterial and venous anomalies associated with NCS. Venous anomalies include LRV bifurcation with branches coursing in front and back of the aorta. The LRV compresses between the aorta and vertebral body in cases where it takes a retro-aortic route leading to posterior NCS. Arterial anomalies include the low or lateral origin of the SMA from the aorta leading to the most common type of compression between the SMA and aorta which is known as anterior Nutcracker syndrome. Anterior NCS can also co-exist with compression of the duodenum as part of the Wilkie syndrome (SMA syndrome). Excessive fibrous tissue along the origin of SMA which compresses the left renal vein. Les frequently, LRV can be externally compressed by pancreatic neoplasm, para-aortic lymphadenopathy, retroperitoneal tumor, and abdominal aortic aneurysm.
The exact prevalence of Nutcracker syndrome is unknown. Although it is more common in women especially in the second and third decade of life as suggested by the reported cases, it can occur from first to the seventh decade of life. A low body mass index (BMI) has shown a positive association with the NCS, due to the rapid increase in body height and vertebral bodies maturation during puberty leading to narrowing of the angle between SMA and Aorta. It is not a hereditary phenomenon, but coincidental cases in siblings have been reported.
Clinical manifestations differ widely among patients with NCS, but the most common complaint is hematuria with or without flank pain. Presentation in children is usually asymptomatic hematuria. In the female, it often presents with pelvic congestion syndrome with associated symptoms of dysmenorrhea, dyspareunia, dysuria, lower abdominal pain, post-coital pain, varices (gluteal/thigh) and emotional disturbance. Other presentations include orthostatic proteinuria and varicocele in males.
Hematuria occurs in the background of rupture of thin-walled varices into the collecting system secondary to venous congestion and hypertension. Hematuria can be macroscopic or microscopic which can lead to resultant anemia. Pain is part of the pelvic congestion syndrome and can be aggravated by sitting, walking or riding in a vehicle. Sometimes, Left flank pain could be due to the passage of blood clots in the ureter leading to ureteric colic. Orthostatic proteinuria occurs more in puberty. Increased LRV pressure along with subclinical immune injury is thought to be the mechanism behind it. Left-sided varicoceles secondary to the Left renal vein hypertension occur in males.
The path to diagnose NCS is often a difficult one, and most of the times it comes under the diagnosis of exclusion. The workup is usually related to the presentation of typical Hematuria which includes urinalysis, urine culture, cystoscopy, ureterorenoscopy, and even renal biopsy. The vascular anomaly can then be ruled out by doing ultrasound imaging and color Doppler scanning which has a sensitivity of 78% and specificity of about 100%. The anteroposterior diameter and peak velocities of LRV should be measured at 2 points in the transverse plane. First, at the level of renal hilum and second, when it crosses the aorta and SMA. When the peak velocity and AP diameter at these 2 points exceed by factor 5, the diagnosis should be suspected of NCS. Then further workup should be done by CT angiography or MRA to define the anatomical relation of LRV to aorta and SMA. With MRA being superior to CTA due to the fact of absence of radiation and having multi-planer imaging. Anatomical abnormalities that could be found include an abnormal origin of SMA, an abnormal high course of LRV, dorsolateral torsion of the L kidney, abnormally low or lateral origin of the SMA.
The gold standard then to establish the final diagnosis is by doing the cine-video angiography and retrograde phlebography along with reno-caval pressure gradient measurement. The normal mean pressure gradients range from 0 to 1 mm Hg. To establish the diagnosis of NCS, the pressure gradient between the left renal vein and IVC should be greater than 3 mm Hg. The advantage of cine-video angiography is not only it allows visualization of LRV compression at the level of crossover between aorta and SMA, but it also shows reflux of collaterals into the adrenal and gonadal veins with stasis of contrast in the renal vein.
Since NCS is uncommon, management over the recent decades has been a subject of debate, and it varies a lot depending on the symptoms and age of the patient. Young patients who are in their adolescence with short periods of mild hematuria and insignificant pain can be observed without any treatment for at least 2 years as with physical development the increase in fat and fibrous tissue relieve the LRV obstruction. In patients with orthostatic proteinuria ACE inhibitors have a role in improving it. On the other hand, patients who are symptomatic with flank pain and hematuria can be tried for different surgical options ranging from intravascular or extravascular stenting, intrapelvic chemical cauterization and open surgical procedures such as nephrectomy to renal autotransplant.
Pastershank did the first surgical procedure in 1974. It involved renal vein release via cutting through the fibrous tunnel between aorta and SMA. The most reported procedure was LRV transposition which included separation of the LRV at its convergence with the inferior vena cava and reanastomosis of the excised LRV at a lower level of IVC incision aside from the SMA. Transposition of SMA has the similar type of principal as LRV transposition has that is the division of SMA from its origin of aorta and reanastomosis at a lower level away from LRV.but it has a risk of intestinal ischemia which explains its avoidance nowadays. Renal autotransplant which has wide surgical exploration is more invasive than LRV transposition.
Intravascular and extravascular stenting have been applied recently. Extravascular stenting utilizing a ring polytetrafluoroethylene graft is used by open surgery after mobilization of the duodenum the PTFE ring is applied around the LRV. During Intravascular stenting the endograft is being delivered to the narrow portion of the LRV via the femoral vein. Some of the complications of stenting include stent migration, restenosis, and embolization.
NCS remains underdiagnosed, and a proper understanding of NCS or NCP is essential to keep it in mind when seeing a patient with clinical symptoms and signs of hematuria, pelvic pain, pelvic varicosities, and varicoceles. Lack of knowledge of the natural history of this condition leads to improper diagnostic criteria and treatment. Patients who are in their adolescence with a benign course require a more conservative approach as they will outgrow their symptoms with physical development. Those with severe symptoms require vascular intervention or surgery to treat the condition.