Do spinal meningiomas penetrate the pial layer? Correlation between magnetic resonance imaging, microsurgical findings and intracranial tumor interfaces.

OBJECTIVE: To study the relationships between spinal dura-arachnoid and tumor-cord interfaces in spinal meningiomas and to investigate whether a disruption of the pial layer and penetration of the tumor in the spinal cord occurs. METHODS: Fifteen patients with histologically proven meningiomas underwent magnetic resonance imaging (MRI) preoperatively. All patients underwent microsurgery. The histological characteristics of the tumors were compared with MRI and microsurgical findings. RESULTS: At surgery, the peritumoral hypointense rim revealed by MRI in 10 of 15 patients corresponded to a well-defined cerebrospinal fluid-containing space confined between the outer arachnoidal layer and the inner leptomeningeal layer. In those patients in whom the hypointense peritumoral rim was absent, the inner layer was either difficult to identify or clearly absent, and the blood vessels were extremely adherent to the tumor, requiring a more cautious dissection. Penetration of the tumors through disruption of the pial surface was not documented. CONCLUSION: Previous anatomic and electron microscopy studies demonstrated, in human spinal meninges, the presence of an intermediate layer attached to the inner aspect of the arachnoid, extending laterally over the dorsal surface of the spinal cord and arborizing over the nerve roots and blood vessels. The intermediate layer is not present in human cerebral leptomeninges. The presence/absence of this layer might explain the hypointense rim detected by MRI and might also explain why no penetration and no peritumoral edema is observed in spinal meningiomas as compared with intracranial meningiomas.