An in depth understanding of the anatomy of the lateral neck is essential to avoid injury to the IJV during SAN dissection. A literature review of the anatomic relationship of the SAN to the IJV is presented in Table 1.
As shown Krause, Soo, and Kierner, conducted cadaveric studies [7, 8, 9]. Soo and Kierner reported an equal frequency of the lateral (56%) and medial (44%) position of the nerve compared to the IJV while Krause reported an incidence of 72.5% of the nerve crossing lateral to the IJV. Krause also observed one case of SAN fenestrating through the IJV. Saman’s cadaveric study supported Krause’s data, reporting a 79.8% of SAN coursing lateral to the IJV, 19% of SAN medial to the IJV and 1.2% of SAN fenestrating through the IJV [10].
In vivo studies reported a higher frequencies of the nerve crossing lateral to the IJV. Levy, Hinsley and Taylor’s results showed the SAN lateral to the IJV in 99.25%, 96.6%, 95.7% and the SAN medial to the IJV in 0.75%, 2.6% and 2.8% [11, 12, 13]. Taylor and Hinsley also reported < 1% of cases of SAN fenestrating to the IJV; moreover Taylor described a new anatomical variant of the SAN in which the nerve divides and travels both medial and lateral to the IJV.
The higher incidence of the nerve crossing lateral to the IJV in in vivo studies compared to cadaveric is explained by Hinsley and Taylor as follows. The internal jugular vein of cadavers can collapse determining a higher incidence of the medial position of the nerve in relation to the IJV. Moreover, in vivo studies which are conducted on oncological patients, focus on the relation between the nerve and IJV at the level of the posterior belly of the digastric muscle while cadaveric studies don’t always keep the same reference points.
In 2009, Lee published an in vivo study in which he found a surprising 57.4% incidence of SAN medial to the IJV. However, these results have not been confirmed by other studies [14].
In his in vivo study, Lee pointed out that the variation of the course of the SAN correlates with a variation in the number of lymph node of the level IIa and IIb. Therefore, a lateral course of the SAN increases the level IIb area and subsequently the number of lymph nodes.
Regarding the rarer anatomic variants of the SAN, Table 1 shows an incidence of the SAN fenestrating the IJV ranging from 0.8 to 2.2% [15, 16, 17]. Taylor also describes a new anatomical variant of the spinal nerve splitting around the IJV. However, there are no other reports of this variant of the SAN and no iconographic material results available.
So what tools can a surgeon use to uncover a rare anatomical variant of the SAN? In 2012 Hashimoto published a case report where they were able to identify the fenestration of the internal jugular vein with a contrast enhanced CT. However, Ozturk warns us that even though the fenestration of the IJV is strongly associated with the SAN travelling through the fenestration this is not to be taken for granted as it can also travel medial or lateral to the fenestration
In both cases presented in this article, the surgeon noticed the fenestration of the IJV during the pre-operative imaging which was not indicated in the radiologist report. As the radiologist priority concerns the extension of the tumour and lymph nodal spread, it falls to the surgeon to enquire about potential anatomical variants of the IJV during the scan.
The aetiology of internal jugular vein (IJV) fenestrations and duplications remains elusive in the field of literature; however, several theories have been proposed. To better grasp these theories, it is essential to distinguish between the two anatomical variants of the internal jugular vein. Fenestration of the IJV is the condition in which the branches of the vein reunite into a single vessel proximal to the subclavian vein. Duplication, also known as bifurcation, refers to branches of the vein that remain separate throughout their entire course and drain separately into the subclavian vein.
Various authors have put forth neural, vascular, bony, and muscular theories to elucidate the cause of these variants. The neural theory suggests that the fenestration of the IJV is the result of an obstructed growth by the spinal accessory nerve during development. This theory is supported by the cases in which the SAN travels through the fenestration of the IJV. However, Contrera reported cases of IJV fenestration without the involvement of the spinal accessory nerve, challenging this theory.
The vascular theory attributes fenestration or duplication to inadequate condensation of the embryonic capillary plexus. As for the bony theory, the cause of IJV fenestration is linked with an obstruction at the level of the jugular foramen. Finally, Bachoo documented a case of IJV duplication surrounding the posterior belly of the omohyoid muscle, suggesting a muscular etiology for IJV duplication.