Benign Enhancing Foramen Magnum Lesions

DISCUSSION

This is the first pathology-proved case of benign enhancing foramen magnum lesions (BEFML). We found that these lesions are fibrotic arachnoid nodules adherent to the dorsal aspect of the SAN. It is important to demonstrate that these pathology-proved lesions are benign and do not need surgical resection.

McGuinness et al¹ were the first to describe BEFML by their MR imaging appearance. They noted that the enhancing lesions were hyperintense on 3D T2-weighted FLAIR, round or ovoid in morphology, and posterior to the intradural vertebral artery. The lesions were thought to represent a venous varix or a ganglia/pseudoganglia of the C1 nerve or spinal roots of the SAN. The occult appearance on 2D T2-weighted imaging helps differentiate these lesions from the typical appearance of other lesions including meningioma, schwannoma, aneurysm, and metastasis.² A minority of meningiomas, schwannomas, and metastatic diseases can appear T2 hyperintense but not necessarily occult on T2-weighted imaging. While cystic lesions such as arachnoid, neurenteric, or synovial cysts could be occult on T2, they would not enhance.³ Our patient presented and underwent surgery before the published work by McGuinness et al.

Rosskopf et al⁴ published a case report on the intraoperative appearance of BEFML without pathologic confirmation and thought that these lesions represented a venous structure. Antonucci et al⁵ presented a single case of a patient with intracranial melanoma metastases in addition to a small enhancing left foramen magnum lesion that remained stable for 18 months, suggesting a benign process.

The largest study of these foramen magnum lesions was performed by Kogue et al,⁶ who reviewed 3D T2-weighted FLAIR imaging of 3717 patients and found what they termed high-signal lesions (HSL) posterior to the intradural vertebral artery in 127 patients (3.4%). They noted that all lesions were in contact with the SAN. Their lesions had a mean maximum diameter of 3.8 mm, with an upper range of 11.5 mm. As was the case with our patient, they found that 8.7% of patients had foramen magnum lesions that increased in size with time. On follow-up imaging of our patient 1 year after resection, there were no signs of recurrence or progression. A subsequent study by Kogue et al⁷ using a 3D balanced fast-field echo sequence found that all lesions contacted the SAN.

The SAN is classically known as providing motor fibers to the sternocleidomastoid and trapezius muscles. However, other studies have suggested that the SAN may transmit sensory/nociceptive signals as well.^8,9 Anatomically, the SAN has 2 origins: one that is derived from the upper 5 or 6 cervical spinal cord rootlets and the other arising from the brainstem. Some propose that the SAN starts with mixed motor and sensory components; however, the sensory neurons migrate to the spinal nerve. This process is often incomplete; therefore, neuron cell bodies can be found scattered throughout the nerve.¹⁰

Fahmy¹¹ suggested that ganglion cells along the SAN migrate distally with age, given the greater abundance and conspicuity of ganglion cells in a 3-month-old child compared with an adult and greater in the distal SAN relative to proximal. In a postmortem study, Tubbs et al¹² found that 14.8% of the specimens had focal enlargements contacting the dorsal aspect of the SAN at the foramen magnum. These enlargements were described as ectopic glial rests or heterotopias within the leptomeninges of the SAN and did not contain ganglion or neuronal cells.

Further work is needed to elucidate the true process that leads to these benign nodules associated with the SAN. Radiologists should be familiar with the typical appearance and benign nature of these lesions so that other entities do not have to be invoked in a differential diagnosis. This is the first work to show pathologic confirmation of what has been previously termed BEFML or HSL.