IJV is the vein of greater size in the cervical region and is considered the main cerebral venous outflow route, particularly in the supine position. The cerebral venous flow goes mainly from the superficial and deep venous system to the transverse sinus (TS) which in its turn continues in the sigmoid sinus (SyS), which drains into the IJV. The IJV joins the subclavian vein (SV) to form the BCV. The confluence of the two brachiocephalic veins gives rise to the superior vena cava (SVC), which drains the cerebral venous blood in the right atrium.

The IJV therefore begins at the level of the jugular foramen, after the junction of the inferior petrosal sinus (IPS) with the SyS, as a direct continuation of the latter. At this level, there is a slight expansion, called jugular bulb or gulf, which cannot be explored by ultrasound, and which continues relying on the anterior surface of the transverse process of the atlas [6]. The interaction between these two structures may obstruct the jugular venous outflow in some cases evidenced by neurosurgery. However, not even this segment can be investigated by ultrasound, while the next segment, at the level of epistropheus or C2, becomes explorable. In this segment, the relationships of contiguity of the IJV with the carotid axis are easily recognizable. The most common course of IJV is slightly anterior and lateral compared to the internal carotid artery (ICA), with a distance of about 1 mm from it in 69.5 % of examined cases. Variations in the anatomy of the IJV and its correlation to CCA are frequent [7]. The subsequent course of the IJV maintains this relationship with CCA, passing under the anterior edge of the sternocleidomastoid muscle to join the SV and converge into the BCV [8].

In a study performed with computed tomography angiography (CTA) aimed to evaluate the IJV anatomy and its relationship to the carotid artery, out of 176 IJV examined both from the right and from the left side compared to the CCA, 85.2 % of IJV were in a lateral position, 12.5 % in the anterior position, 1.1 % in the medial position, and 1.1 % in the posterior position [7]. Furthermore, in an ultrasonic study designed to evaluate the degree of overlap between the IJV and CCA in the position of the head suitable for IJV cannulation, in over 1,000 veins, the IJV covers even if only partially the CCA in 54 % of patients, predisposing them to the accidental puncture of the carotid artery [9]. The variability may be even greater if you consider the effect of the rotation of the head; in fact, the overlap between IJV and CCA increases passing from the neutral position of the head to a rightward rotation (23.3 vs. 39.2 %) and to a leftward one (35.3 vs. 52.8 %), so that the incidence of lateral positioning of the IJV compared to the CCA is significantly reduced with the rotation of the head (40 vs. 21 % at right, 26.5 vs. 10.5 % at left). The right side appears, however, associated with a minor overlap, whatever the position of the head [10].

An important and well-studied element of the IJV is its valve apparatus, located near the confluence with the SV, and present in 86–93 % of the veins in autopsy case studies [11].

From the ultrasound point of view, it is therefore possible to divide the IJV into three different segments:

Morphological anatomical abnormalities of IJV are considered rare; for example, duplication would have a ratio of 4/1000 unilateral cervical dissections [12].

The high-resolution ultrasound examination revealed the presence of a valve in one or both veins in 87 % of cases in the series of Lepori et al. [13] and in 72 % in the series of Macchi and Catini [14]; the series of Darge et al. [15] in a population of children and young adults has found instead a prevalence of 96 %. In autoptic studies on adults, the 86–88 % of the examined subjects had a valve [16, 17]. Gender differences in the prevalence of the valvular apparatus of the IJV are not reported, and in the subjects with unilateral valve, it is more commonly located to the right side.