Varying length of central myelin along the trigeminal nerve might contribute to trigeminal neuralgia.

Several studies have suggested that vascular compression of more distal portions of the trigeminal nerve (Vth cranial nerve: VN) may cause trigeminal neuralgia (TN). However, neurosurgeons performing microvascular decompression intraoperatively cannot identify which type of myelin is being compressed by blood vessels. The aim of this study was to clarify the histological anatomy of central and peripheral myelin in the human VN. Histological analyses were conducted using photomicrographs from 134 cisternal segments of the VN from the brains of 67 cadavers. The three dimensions of the VN were measured in these sections: distance from the point at which the lateral-most pontine VN merges with the boundary between central and peripheral myelin (line-a), distance along the medial aspect (line-b), and the length of the transitional zone (TZ), known as the Obersteiner-Redlich zone. Twenty-nine of 134 VNs were available for study. The length of central myelin ranged from 0.69 to 8.66 mm (mean, 3.56 mm; median, 3.10 mm) along the lateral aspect and from 0.36 to 5 mm (mean, 1.81 mm; median, 1.40 mm) along the medial aspect of the VN. The length of the TZ ranged from 0.31 to 3.37 mm (mean, 1.75 mm; median, 1.63 mm). We report here, for the first time, that some individuals had much longer spans of central myelin than those reported previously. Some cases of TN may thus be caused by vascular compression of VN peripheral myelin, especially in cases where central myelin is extended to an unprecedented degree. Clin. Anat. 32:541-545, 2019.