The Retromandibular Vein and Its Relation With Facial Nerve Branches: Practical Implications of an Anatomic Variation

Significance Statement

The retromandibular vein is an important imaging landmark for locating parotid neoplasms in its superficial or deep lobe. Although undeniable, its utility in identifying the facial nerve trunk and its branches during parotidectomy may be inconsistent due to the unpredictable anatomic variations visualized only at surgery. This article describes one such unusual relationship noted during a superficial parotidectomy where the retromandibular vein was immediately lateral to the cervicofacial branch, and descended medial to the zygomaticotemporal division of the facial nerve trunk. The implications and caveats of such anatomic variations are discussed.

A 29-year-old woman presented with a discrete, firm, nontender, and mobile swelling in her left parotid gland. Magnetic resonance (MR) imaging revealed a predominantly cystic lesion of ~3.5 cm diameter limited in the superficial lobe; the retromandibular vein (RMV) could be seen abutting the medial aspect of the lesion (Figure 1). The fine needle aspirate was a dark fluid rich in mucin-containing cells and mucinophages, and was inconclusive. After the routine investigations, the patient was put up for superficial parotidectomy under general anesthesia.

Figure 1.

Magnetic resonance imaging of the face (contrast-enhanced and T2-weighted; A and B, respectively; axial sections) revealed a predominantly cystic lesion within the superficial lobe of the left parotid gland. The retromandibular vein (arrows) could be seen passing through the parotid substance dividing it into the larger superficial lobe and a smaller deeper lobe.

The standard steps in incision and soft tissue dissection for superficial parotidectomy were carried out. The facial nerve trunk and its branches were delineated following the known surgical landmarks. However, during this procedure, considerable bleeding was encountered in spite of meticulous dissection. The primary divisions of the extratemporal facial nerve trunk were difficult to find initially, and attempts to delineate the cervicofacial branch led to brisk bleeding. For once, the possibility of iatrogenic injury to the cervicofacial branch, or inadequate removal of the superficial lobe through an approach into the wrong surgical plane was considered. The surgical field was explored and hemostasis was finally secured. At this point, it was noted that the main trunk of the RMV passed superficial (lateral) to the cervicofacial division of the facial nerve trunk (Figure 2). In fact, it ran between the cervicofacial and zygomaticotemporal branches, immediately superficial to the former. The anatomic variation was noted, and the rest of the surgery was completed uneventfully.

Figure 2.

The facial nerve trunk and its branches were traced during superficial parotidectomy. The retromandibular vein (arrowhead in A; separated by a mosquito forceps in B) could be seen running immediately lateral to the cervicofacial division (yellow arrow); its distal end emerged from within the substance of the deep lobe (asterisk), medial to the zygomaticotemporal division (blue arrow). Black arrow indicates greater auricular nerve. M, masseter; SCM, sternocleidomastoid; T, tragal pointer.

Post-surgery, the patient recuperated well; the mild loss of deviation of the left angle of mouth improved to normalcy within a month.

Searching for the extratemporal facial nerve trunk is one of the key steps during parotid surgery and its identification is essentially landmark-based. The RMV is one such landmark, apart from the routine ones like the tympanomastoid suture, posterior belly of digastric muscle, tragal pointer, and stylomastoid artery. ¹ RMV is formed by the superficial temporal vein and the maxillary vein and runs through the substance of the parotid tissue. Since the extratemporal facial nerve is not well-visualized on conventional imaging techniques (computed tomography/MR) and because ultrasonography does not provide reliable information about the deep lobe of parotid, the RMV is often used by the radiologists and surgeons as a landmark that divides the gland into superficial and deep lobes, as an on-table guide for identifying the facial nerve trunk and its divisions, and to delineate a lesion with regard to the facial nerve branches and the superficial/deep lobes of parotid.^2-5

At surgery, the RMV is generally found medial to the facial nerve and its branches (83%-100%; average: 88.17%).^5,6 However, this relation is not constant, and there are documented variations. In a robust study on cadavers, Touré and Vacher described 6 such variations; the RMV was found to be lateral in 28% cases, either with the main facial trunk or with its subsequent divisions. ⁴ Of these, however, the RMV descended lateral to the cervicofacial division in only 7.6% (type 4). ⁴ A pioneering study on fetuses with healthy faces also revealed the RMV remaining lateral to the facial nerve trunk and its branches in ~11.48% of hemi-faces, although in only 4.9% it was lateral to the cervicofacial branch. ⁷

Thus, in spite of such documented anatomic variations, the position of RMV lateral to the extratemporal facial nerve and its major branches is neither anticipated prior to surgery nor expected on table.

There are conflicting and varied observations regarding the frequency of RMV being lateral to the cervicofacial and zygomaticotemporal divisions of the facial nerve. That RMV is more prone to have variations with the cervicofacial branch can be explained from an embryologic perspective. During development, as the parotid primordium grows caudally, the RMV and the cervicofacial branch come in proximity in a deeper plane within the primordium compared to the zygomaticotemporal branch.^4,8 As in the present patient, the RMV was found to descend medial to the zygomaticotemporal division but, taking a variant course, turned lateral to the cervicofacial division.

It becomes imperative for the surgeons to anticipate and look for such variations in the course of the RMV and facial nerve branches. Otherwise, during a parotidectomy, the stepwise search and dissection, especially of the cervicofacial branch, may lead to inadvertent injury to the RMV and result in brisk bleeding. Staining of the surgical field with blood, and distraction due to attempts at securing the bleeding may further increase the chances of injury to the facial nerve branches which can be as close as 5 mm from the RMV. ⁶ During imaging, the RMV is often used as a landmark for localizing an intraparotid lesion with regard to the parotid lobes; the same is corroborated at surgery through dissection of the facial nerve trunk and pes anserinus. With variations in the position of the RMV with the facial nerve (cervicofacial) branches, the surgeon may enter into a wrong plane of dissection resulting in complications (bleeding, facial nerve injury). The situation may get further complicated with larger volume tumors/space occupying lesions that may splay or displace the anatomic structures and their landmarks. ¹ Therefore, proper knowledge of the relative anatomic disposition of the facial nerve branches and their landmarks (including the RMV), and anticipating the variations on table, are essential skills for a good surgical team that can obviate untoward complications to a large extent.