Instant and Early Efficacy of Gamma Knife Treatment on Trigeminal Neuralgia

The current approach is to place a radiosurgical 4 mm isocenter onto the trigeminal nerve as it is traversing the prepontine cistern. This treatment plan enables the precise targeting of trigeminal nerve, while minimizing the risk of damage to surrounding structures, such as the brainstem and temporal lobe. Likewise, Kondziolka et al. reported that the dorsal root entry zone is more radiosensitive than the more distal segments of the trigeminal nerve because of the more radiosensitive oligodendrocytes ensheathing the nerve proximally.[,] However, recent series demonstrated a comparable pain relief when the more distal portion of the nerve such as the pars triangularis (i.e., the transdural segment contained in the Meckel cave) was targeted. Current approach favors the pars triangularis since it is relatively distal to pons but still covers the main nerve trunk before branching. Similarly, we also chose the pars triangularis as the optimum isocenter location with the 20%-23% isodose line tangential to pons with no resultant paresthesia or hypesthesia. On the contrary, Marshall et al. recently reported that patients receiving 80 Gy with a proximal isocenter (50% isodose line entering brainstem) experienced a greater durability of pain relief than those with a distal isocenter on the pars triangularis (20% isodose line tangential to the brainstem).[]

The efficacy of GKS in TN is generally evaluated by several parameters, including pain relief, being medication free, and an improvement in the quality of life. Régis et al. reported that 70% of patients were able to come off medications after GKS.[] Marshall et al. reported a 86% response to pain within 3 months following GKS.[] Additionally, Marshall et al. reported a median durability of 4.9 years for patients with classical Type-1 trigeminal neuralgia after GKS.[] Lucas et al. claimed that an early successful response and cutting off the medications were the significant factors predictive of a longer pain relief after GKS.[] In addition, Kondziolka et al. published that the post-radiosurgical paresthesia was a major factor predicting the treatment success, while Maesawa et al. reported that the prior MVD surgery and radiofrequency ablation appeared to decrease the treatment efficacy.[,] Notably, Brisman et al. claimed that the visualization of contact between a blood vessel and the trigeminal nerve predicts a better response after GKS.[] Additionally, Balamucki et al. concluded that the dosage of radiation delivered by GKS was not a significant factor with regard to the degree of pain relief.[] In accordance with the current data in the literature, we were able to manage the pain relief by applying average doses of radiation-65 Gy mean. This approach also minimized the risk of radiosurgical toxicity on the sensory fibers in TN. Kondziolka et al. reported that the mechanism of pain relief is focal axonal degeneration of the trigeminal nerve that the pain fibers are more prone to axonal degeneration than the sensory fibers.[]

Lastly, Sheehan and Régis et al. reported that the effect of GKS on TN starts between 1 to 180 days in large cohorts.[,] However, the time range is so great, and it needs to be optimized by further studies so as to offer GKS as a more reliable and quick-acting alternative in the treatment of GKS in selected patients over MVD and percutaneous procedures.