Quantitative Measurements for Trigeminal Nerve Tethering Secondary to Cystic Degeneration of Vestibular Schwannomas: Clinical Implications

Trigeminal neuralgia-like symptoms are very rare (2.2–5.7%) but can be encountered in patients with vestibular schwannomas (VS) compressing the trigeminal nerve (TN). Recently, TN tethering has been reported as a cause of trigeminal neuralgia-like symptoms mostly after stereotactic radiation therapy. The objective of this study is to perform multiple quantitative measurements, using widely available tools and identify variables that can evaluate potential TN tethering causing trigeminal neuralgia-like symptoms. An illustrative case is demonstrated for clarity.

A multicenter and multiprovider database of two tertiary academic medical centers was queried for patients with large VSs associated with TN symptoms. Eight patients with large VSs and, or, associated with TN compression were included. Patients had a mean age of 57 and a mean tumor volume of 4,988 mm³. To assess tethering of the VS onto the TN, the following quantitative measurements were used ([Fig. 1]).

The VS and the ipsilateral TN were segmented in 3D slicer for all patients MRIs. Volumetric tumor measurements were made on these 3D-images. To quantify the tumor to TN interface, mean and maximum curvature measurements ([Fig. 1A]) were calculated by placing multiple fiducials at the midportion of the TN in 3D slicer. To better characterize the anatomical distortion of the TN, angle measurements of the TN were obtained in visage-imaging ([Fig. 1B, C]). Under the assumption that chronic injury to the TN would lead to fatty atrophy of the ipsilateral muscles of mastication, and therefore elevated T1 signal intensity, we measured the signal intensity of the bilateral medial pterygoid muscles on T1-weighted images ([Fig. 1D]). Lastly, to evaluate for direct axonal damage of the TN due to tumor compression or tethering, bilateral T2-weighted signals were measured on T2 space images ([Fig. 1E]).

Fig. 1

For the illustrative case the volume increased from 309 to 2,103 mm³ followed by cystic involution to 377 mm³ ([Fig. 2]). However, the contrary occurs with the TN curvature and angle, where the curvature increased from 0.146 to 0.399 mm and the angle increased from 16.3° to 31.1° as the tumor involutes. T1 signals of the pterygoid muscle did not significantly differ in all cases. Additionally, a greater T2-weighted MRI signal was measured in the TN on the ipsilateral side of our illustrative case.

Fig. 2

Our illustrative case represented a 52-year-old diagnosed with a VS. During active surveillance, mild trigeminal neuralgia-like symptoms developed, with corresponding growth of a cystic part of the VS and impingement of the TN. The patient elected continued observation, resulting in worsening of trigeminal symptoms, however, corresponding MRIs were showing tumor involution. Quantitative measurements outlined below highly suggest TN tethering onto the VS during tumor involution.

All quantitative measurements were easily and accurately quantifiable for the illustrative case as well as the control cases. If VS symptoms do not correspond to tumor growth over the course of the disease, the various imaging modalities that we have mentioned in this paper could be of additional value to explore the course of the disease and symptom explanation and possibly aid the surgeon on treatment recommendations.

Publikationsverlauf

Artikel online veröffentlicht:
05. Februar 2024