Low-Grade Glioneuronal Tumor Adjacent to the Fifth Cranial Nerve Root With KIAA1549::BRAF Fusion Presenting With Trigeminal Neuralgia.

Low-grade glioneuronal tumors of the cranial nerves are rare, with only a few case reports describing their association with trigeminal neuralgia and no prior reports including genetic analysis. We present a case of a low-grade glioneuronal tumor located adjacent to the trigeminal nerve root in a 70-year-old man who had been experiencing severe left-sided trigeminal neuralgia for several months. He had initially presented with cranial polyneuropathy, for which oral steroid therapy was initiated. Magnetic resonance imaging of the head revealed the left superior cerebellar artery running near the left trigeminal nerve; however, no mass lesions were detected. During microvascular decompression, an 8-mm white tumor was identified on the trigeminal nerve and subsequently removed. Histopathological examination of the surgical specimen revealed round tumor cells with slightly eosinophilic or vacuolated cytoplasm, arranged in nests or rosettes. Foci of calcification, hemorrhage, and hemosiderin deposition were present, but eosinophilic granular bodies, Rosenthal fibers, necrosis, pleomorphism, or mitosis was absent. The tumor cells were immunopositive for synaptophysin and anti-Neuronal Nuclei, focally positive for glial fibrillary acidic protein and S-100 protein, and immunonegative for Olig2 and epithelial membrane antigen, with a Ki-67 labeling index of < 1%. Molecular analyses confirmed the fusion of KIAA1549 exon 16 with BRAF exon 9, with no mutations detected in IDH1/2, H3F3A, BRAF (V600), or FGFR1. These findings provide novel molecular and embryological insights into low-grade glioneuronal tumors of the trigeminal nerve, which may aid in their classification and understanding of the development of the tumor and neuralgia.