Role of NIVL in lower limb venous diseases. Though the anatomical anomaly of NIVL is widely prevalent, it was considered a rare cause of symptomatic CVD, identifiable only in 1–5 % of patients with lower limb symptoms [4]. This was because conventional venography fails to detect many of these lesions. Using intravascular ultrasound (IVUS) as the imaging modality, Raju and Neglen have reported that the incidence of NIVL is much higher in patients with symptomatic CVD [5]. Two basic issues were addressed by these workers in relation to NIVL: Why a silent lesion should become symptomatic? Further, is NIVL an incidental finding in CVD patients?

It is suggested that NIVL is a so called permissive lesion. Permissive lesions are pathologies that may remain silent until additional insult or pathology is superimposed. The triggering factors in NIVL could be trauma, cellulitis, distal thrombosis, decreasing mobility, pedal edema, etc. Very often the cause remains obscure. It was observed that isolated iliac segment stenting in symptomatic patients with NIVL detected on IVUS had produced marked clinical and hemodynamic improvement without correction of the coexisting superficial and deep vein reflux [5]. Stasis changes improved, and leg ulcers remained healed up to 2.5 years of follow-up after stenting. These findings confirm that NIVL can cause symptomatic venous disorder and is not an incidental finding in CVD patients.

The commonest type of NIVL is a compression of left common iliac vein by the right common iliac artery (Fig. 22.1). The further progression of this compression has been evaluated by helical CT. Based on the findings of helical CT, a system of classification has been evolved [7].

Kim et al. [8] reported a sequential progression in the evolution of clinical Iliac vein compression:

Although compression of the left common iliac vein by the right common iliac artery is the most common finding, several other patterns are also identified [7]:

May and Thurner postulated that the chronic pulsations of the overriding right iliac artery led to the development of a “spur” in the vein wall and that this spur would result in partial venous obstruction. Chronic trauma to the inner side of the vein wall due to adjacent arterial pulsations leads to the accumulation of elastin and collagen, contributing to spur formation. In addition to the chronic arterial pulsations, mechanic compression of the iliac vein by the thick-walled overriding iliac artery leads to extensive local intimal proliferation, impaired venous return, and venous thrombosis [3].

Patients with NIVL may remain totally asymptomatic. This is because NIVL is a slowly progressive permissive lesion. An additional pathology has to be superimposed for clinical manifestations to develop.

Symptomatic patients can present in two ways:

The disease is common in young women on the left side. But it can develop in both genders, at any age and on the right side too. In elderly patients, atherosclerotic changes in the overriding arterial wall can exert extra pressure on the vein. Ulcers are reported to occur in NIVL without venous reflux but are more common in the presence of reflux [5]. It is rare in nullipara and is commonly seen in multiparous women. In a subset of multiparous women, May-Thurner syndrome can present with features of pelvic congestion syndrome [9]. Many patients are asymptomatic especially when in stage I. With the development of spurs, patients become symptomatic and develop lower limb edema. The compromised blood flow often causes collateral blood vessels to form. These are most often horizontal transpelvic collaterals, connecting both internal iliac veins, thus creating outflow through the right common iliac vein. Sometimes vertical collaterals are formed, most often paralumbar, which can cause neurological symptoms, like tingling, numbness, etc.