Operative findings in cases of trigeminal neuralgia without vascular compression: proposal of a different mechanism.

Trigeminal neuralgia is known to be caused by vascular compression at the trigeminal root entry zone (REZ) and microvascular decompression provides good outcome in most of cases. However, in some cases, no vascular compression was observed at the REZ. Over the last 2(1/2) years, the first author operated on 53 cases of trigeminal neuralgia with microvascular decompression and encountered nine cases where no offending vessels were noted at or near the REZ. They were divided into two groups: five cases involving an initial operation and four cases involving a second operation. In the former, arachnoid thickening, angulation or torsion of the root axis were common findings. Dissection of thick arachnoid around the root along the whole length reversed the root to be straight and flaccid. Complete pain relief was noted in four of five cases. In one case of atypical pain, constant facial pain remained. In the latter four cases, where the first operations were done more than 4 years before, thick granulation was noted around REZ without new offending vessels in two cases. In the remaining two cases, where no offending vessels were noted in the first operation, thick adhesion of a distal portion of the root with dura on the pyramidal bone was noted. Meticulous dissection of t he whole length of the root was done and complete pain relief was obtained. Delayed but complete pain relief in these nine cases was noted. Based on operative findings, arachnoid thickening or granulomatous adhesion between the root and surrounding structures can cause an abnormal course of the trigeminal nerve root, which causes root angulation and/or torsion. They can also cause pulsatile movement of the trigeminal nerve root. This tethering effect can promote abnormal root stretching force, especially at REZ, which might promote hyperexitability of the nerve. This speculative mechanism suggests that it is important to make the root free along the entire length, especially at its distal portion in cases with no offending vessels.