Nutcracker Syndrome

Prevalence, Demographics and Risk Factors

The exact prevalence of NCS is unknown primarily due to an absence of unified diagnostic criteria and variability of presenting symptom(s). Although not limited to a specific gender or age range, based on data from several studies it is thought that NCS is more prevalent in females and the young patients (second and third decade of life), with a secondary peak in middle age adults. ^, However, further studies have shown that NCS is equally prevalent among male and female patients, and that it can affect older patients, as well.

Radiologic studies demonstrated a 10.4% prevalence of NCP on evaluation of abdominal computed tomographic (CT) scans performed for other indications, with no difference in the prevalence of NCP by gender alone. Owing to causes elaborated upon later, a lower BMI has been shown to have a correlation with NCS. A large proportion of the nutcracker literature includes patients of Asian origin, although it should be noted that no true population-based studies have been performed to determine if there are any ethnic, racial and/or geographic risk factors for NCS. Finally, with the exception of a single case report in which NCS was found in two young siblings, data from the current body of literature shows no evidence of genetic hereditary link in the inheritance of this disorder.

Anatomy

Based on underlying anatomy there are two main types of NCS. The most common type, anterior NCS, refers to compression of the LRV between the abdominal aorta and the SMA, as mentioned above ( Fig. 165.1A ). In posterior NCS the LRV is in retro-aortic position and is compressed between the abdominal aorta and the vertebral column ( Fig. 165.1B ). There is also atypical NCS caused by very rare anatomic variants including the compression of the LRV and IVC as they pass between the aorta and the SMA in patients with left-sided IVC ( Fig. 165.1C ). A number of other anatomic variations pertinent to the anatomy and angulation of the SMA may also result in symptomatic compression of the LRV, although whether they can be truly considered as causes of the NCS is controversial. These include compression by pancreatic neoplasms, paraaortic lymphadenopathy, retroperitoneal tumors, an overarching testicular artery, fibrolymphatic tissue or “webs” between the SMA and aorta, as well as compression of larger veins by a gravid uterus. ^, The normal distance between the SMA and aorta has been noted to average 4 to 5 mm at the level of the LRV. LRV diameters average approximately 1.2 ± 0.2 cm (1.1 cm proximal segment; 1.2 cm distal segment) in cadaveric studies, with ultrasound studies showing slightly smaller diameters of LRV size (4–5 mm) in patients without NCS.

Anterior NCS can result in dilation and engorgement of the gonadal vein, as well as a preponderance of pelvic collaterals that can be visualized with cross-sectional or angiographic imaging. It is very important to note that a recent study has shown that gonadal vein diameter may not correlate with gonadal venous reflux in patients with pelvic venous congestion syndrome not related to renal vein compression. Dos Santos et al. demonstrated that many normally sized ovarian veins exhibit pathologic reflux, and some large ovarian veins are competent. The authors measured diameters of 34 ovarian veins (17 right, 17 left) using digital subtraction venography. The findings were compared with previously diagnosed reflux (by transvaginal DUS) in 16 out of these 34 veins (47.1%). Data showed that the mean diameters of the non-refluxing and refluxing veins were 7.2 mm (range 3–13 mm) and 8.5 mm (range 4–13 mm), respectively, with no statistically significant difference ( P = 0.204) among groups. Based on these findings the accuracy for diagnosis of ovarian venous reflux based on vein diameter is less than 56%, which approximates the probability of random guessing (50%). Whether or not these finding can be extrapolated to patients with NCS is not clear, nevertheless these findings do underscore the importance of clinical features, rather than radiologic findings alone, to make the diagnosis of NCS.

Anatomically, there are a variety of explanations as to the mechanism of SMA compression of the LRV, including a more acute angle between the aorta and SMA (<16 degrees; normal range: 35–40 degrees), which is often attributed to a paucity of retroperitoneal or mesenteric fat, wasting of the paraspinal muscles, as well as profound weight loss. ^, However, according to other studies that used magnetic resonance angiography (MRA) the sagittal plane angle between the SMA and the abdominal aorta needs to be less than 35 degrees for a definitive NCS diagnosis. Similar with these findings, Kim et al. demonstrated, by comparing CT results with LRV venography and measurement of the pressure gradients between the LRV and IVC, that sagittal plane angle of <39 degrees on CT had high sensitivity and specificity for detecting symptomatic NCS with respective rates of 92% and 89%.

The paucity of retroperitoneal fat correlates well with the observation that NCS is oftentimes seen in those with a particularly thin body habitus. Posterior ptosis of the left kidney resulting in abnormal angulation and subsequent stretching and traction of the LRV has also been implicated as a possible cause of LRV narrowing and subsequent hypertension. Additional anatomic variants resulting in anterior NCS include aberrant branching of the SMA and an abnormal, particularly cephalad course of the LRV. ^{, ,}

Although prior studies have demonstrated that posterior NCP may be similar in prevalence to anterior NCP, there are far fewer reports in the literature of the management of posterior NCS patients, although their symptomatology is noted to be no different from that of their anterior NCS counterparts. In truth, a retro-aortic LRV remains an uncommonly observed anatomic phenomenon, with a recent study showing a prevalence of 0.77% to 3.18%, with only a minority of cases being symptomatic (6.5%).

A combined anterior and posterior NCS has also been reported as a result of the LRV duplication (the circumaortic renal vein). ^, This is a rare anatomic variant of the LRV where the vein has both antero- and retro-aortic segment on its course to the IVC (with reported incidence of 0.3–5.7%). This anatomic variant can be prone to the compression of the anterior LRV segment between the aorta and SMA, while the posterior LRV segment is compressed by the aorta against the vertebral column resulting in symptoms secondary to the LRV hypertension and/or gonadal vein reflux. Lastly, right-sided NCS, although possible, is even more infrequently reported in the literature and oftentimes results from the condition of left-sided IVC and the unique combination of left-sided IVC, hemiazygos continuation, and persistent left SVC.