MR Imaging of the Superior Cervical Ganglion and Inferior Ganglion of the Vagus Nerve: Structures That Can Mimic Pathologic Retropharyngeal Lymph Nodes

Discussion

This is the first MR imaging analysis focused on identifying SCGs and IGVNs, to our knowledge. The SCG drew little attention in the past, and several previous reports mistakenly described it as retropharyngeal lymph node metastasis or tumor.^18,19 The results of the current study indicate that MR imaging can be useful in identifying these structures and would potentially facilitate safer and more accurate planning of surgical and interventional procedures such as SCG blocks for facial pain.

RPLNs are usually less evident in elderly populations than in children.²⁰ Because we assessed the MR imaging of elderly patients in cohort 1, most of whom presented with oral or pharyngeal cancers, the detection of SCGs was comparatively straightforward because of less evident RPLNs. The enhancement and DWI characteristics were most useful for differentiation in the analysis. The SCG has a large number of capillary vessels around the ganglion cells^8,21 and lacks a blood-nerve barrier.²² These features might cause avid enhancement of the SCGs. Although metastatic lymph nodes can be strongly enhanced due to angiogenesis by cancer cells,²³ signals were often heterogeneous. Homogeneous enhancement was considered one of the characteristics of SCGs. The signal difference on DWI was likely due to differences in the ADC values because the signal intensity was visually about the same with conventional T2WI. ADC values can reflect histologic characteristics such as cell density. The SCG is constructed of ganglion cells, nerve fibers, vascular structures, and collagen fibers.^8,21 By contrast, lymph nodes show low ADC values due to lymphocyte accumulation.

Intraganglionic hypointense spots on T2WI might be consistent with the 2 previous reports that concluded that they represent venules.^8,9 Although this finding can be a characteristic feature of SCGs, metastatic lymph nodes were often heterogeneous in signal and might mimic this finding. In contrast, slits or spot-shaped fat tissue had not been previously described and were thought to be much more specific. The figure of T2WI presented by Loke et al⁸ appeared to have FS-T2WI, presumably due to signal intensity changes caused by the low body temperature of the corpus. Our assumption is that the fat tissue in the SCGs may represent spaces among the neural branches. SCGs have many branches: superior, lateral, medial, and anterior. The superior branches enter the cranial cavities along with the ICA. The gray rami of the lateral branches communicate with the upper 4 cervical spinal nerves, as well as with some of the cranial nerves such as the vagus and hypoglossal nerves. The medial branches are laryngopharyngeal and cardiac. The anterior branches are rami and those on the common and external carotid arteries. These branches may entrap fat tissue near the SCGs and mimic intraganglionic fat. Caution should be taken not to mistake fat tissue for lymph node hilum.

Cadaver studies have reported that the length and width of the SCGs are about 10–30 and 5–8 mm and are located posterior to the ICA.^{1⇓⇓⇓⇓⇓–7} The height of the SCGs varies in these reports from the level of C2–C3, just C2, just C4, and so forth.^{1⇓⇓⇓⇓⇓–7} Although our results were mostly consistent with these cadaveric studies, there were some discrepancies. Our results showed that SCGs were located not posterior to but anteromedial or medial to the ICA in almost all cases. In addition, SCGs were widely distributed from C1 to C5 in our study. Cadaveric studies potentially present a risk of artifactual changes in location due to postmortem procedures and postmortem changes. Only MR imaging can reveal precise shapes and locations of the SCGs in vivo.

MR imaging allowed detection of bilateral SCGs in all cases. SCGs have elongated, cylindric, and fusiform shapes, and the longus capitis muscle and ICA were landmarks to detect them. The RPLNs were also located along the longus capitis muscle and had an elongated appearance along the body axis; therefore, the size and positional relationships were the most important factors in assessing them correctly. While many SCGs were, in general, longer than the RPLNs in the craniocaudal direction, some of the RPLNs were larger than the SCGs. In such cases, focusing on the relative positions will be of pivotal importance. SCGs are usually located caudal to the RPLNs. IGVNs also have a fusiform shape; however, once again, the location was the most important key to differentiating IGVNs and SCGs. The RPLNs were located at the anteromedial side of the SCGs, and IGVNs were located lateral or posterolateral to the SCGs. As a result, the RPLN, SCG, the ICA, and IGVN formed a line on an axial plane from anteromedial to posterolateral (Fig 5D). In addition, IGVNs might not cause confusion using routine neck MR imaging. Although we attempted to detect IGVNs, retrospectively, in cases of routine neck MR imaging, IGVNs were difficult to detect with confidence, presumably because the volume of the IGVNs was small and the signals due to vascular structures compromised their detection.

Probability mapping showed that the positional relationship had a large degree of variation. If the sites with maximum probability were visually checked, the SCGs could not be detected in more than half of the cases. The SCGs are one of the targets for ganglionic local opioid injection for migraine, trigeminal neuralgia, postherpetic neuralgia, facial pain, complex regional pain syndrome, pain-associated depression, and vasospasms following subarachnoid hemorrhage.^24⇓–26 Blinded or fluoroscopically guided approaches have been described.^24,27,28 These blinded techniques were expected to have a high risk of inaccurate localization of the targets. A potential risk of vertebral artery puncture and epi-/subdural injection has also been demonstrated.²⁶ Sonography-guided identification of the SCGs could be a potentially reliable technique, though this has only been validated in cadavers.²⁵ For the sonography study, the CCAB and transverse processes of the spinal bones were used as landmarks. The probability maps created do not favor one landmark over the other because the probability densities of SCGs showed no evident difference between the 2 maps. We believe that MR imaging–guided intervention might be one of the options for failed pain control under a blinded injection. Although an SCG block under MR imaging still requires controlled studies with blinded injection,^26,29 MR imaging may have a potential role in identifying the precise location of SCGs.

Limitations of this study include a lack of direct clinicopathologic correlation of the identified ganglia by either an operation or postmortem examination. Because one of our purposes was to characterize normal SCGs, invasive procedures for confirmation were not acceptable; however, our results were consistent with those of anatomic reports.^{1⇓⇓⇓⇓⇓–7} Also, as many participants as possible were examined to reveal the variations of morphology and location, minimizing the effects of misdetection for the ganglia and lymph nodes. Second, 2 different MR imaging instruments were used for this study because we assigned the specific protocols on separate instruments. The SCG signal was evaluated by thick sections of routine neck MR imaging. Evaluations using thin section thickness, such as neurography, might have been better to avoid partial volume effects and reveal detailed intraganglionic structures. Third, the lymph nodes that we classified into normal lymph nodes might have contained metastases, though there were significant differences in ADC values between normal and metastatic lymph node groups. Finally, we selected only adult cases. Our data might not be relevant to the pediatric population. In fact, the relative size of the SCGs and RPLNs may be different between adults and children because reactive retropharyngeal lymph nodes are more commonly seen in pediatric populations.