This communication is aimed at demonstrating how informatics can be used in a neurosurgical operation for relief of trigeminal pain. Because of its main property, excitability, the nervous tissue offers an ideal background for interaction between the information that can be gathered from it and modifications induced either functionally or anatomically. In the operation that we are to describe, information in the form of electrical signals is collected at various stages, and the entire surgical procedure is thus guided and monitored.

Background

Trigeminal neuralgia, a lightning pain in the face that usually affects the elderly, can successfully be treated with an operation called thermorhizotomy ¹. A long needle is inserted into one of the foramina of the skull base – the foramen ovale- until its tip reaches the sensory ganglion of the trigeminal nerve –the Gasserian ganglion- and then it is heated by means of a radiofrequency current. The heat damages some of the nerve fibres near the tip. For effective pain relief, the needle tip has to be properly positioned adjoint to the fibre bundles that convey sensation from the face painful areas. Also, it is essential that during the operation the surgeon may assess that the correct amount of damage has been performed. Too less damage to the fibres, and the pain will recur very quickly; too much damage, and a severe sensory loss of the face will affect the patient that may even be more annoying than the trigeminal neuralgia itself. Because the surgeon needs these two essential informations: position of the needle tip, and exent of damage, so far he/she has been compelled to ask for collaboration by the patient, who can just be anesthetized for short spells.

Our method, combining informatics and neurophysiology, will give the surgeon the possibility of very precise monitoring of the needle tip position and of the lesion exent, without asking for patient’s collaboration. This way the patient can be subject to uninterruped general anesthesia.

Recent progress in neurophysiology of the trigeminal nerve has provided us with the right toool for monitoring its function during the operation. The early trigeminal evoked potentials (TEPs) have been introduced 1985², and have been demostrated to be extremely reliable both for research and clinical applications. These are used to monitor the thermorhizotomy operation. The first use of this operation has been described 1996³.

At the present time we are using a dedicated computer programme for signal acquisition and analysis. This allows us an easier and quicker evaluation of modifications of the various relevant parameters.

Description of the method

The peripheral branches of the trigeminal nerve are stimulated with a small electric current. The ensuing response of the nervous system is both recorded at scalp level and from the tip of the needle. Form and amplitude of the response recorded from the tip tell the surgeon how near is the tip to the fibre bundles involved in the pain transmission.

After the tip has been placed in the best possible position, the lesion phase is carried out. The tip is heated several times at temperatures from 64 to 70° C, for 60s. After each time the effect of the lesion is monitored by means of the scalp and tip recorded response. The tip recorded response has the advantage of being larger and of allowing observation of the entire neurogram, with a broad spectrum of fibres. The scalp response is smaller, but in principle more reliable, as it is independent of possible movements of the needle tip. A 50% modification in amplitude of both responses will be proof of adequate lesion.

By means of the dedicated computer programme, we are able to compare the responses obtained after each lesion phase, and to precisely calculate amplitudes. As soon as the desired reduction in amplitude of the response is reached, the entire procedure is considered complete.

Results and comments

So far 52 patients have been operated with this procedure. 45 had complete relief of pain, whereas 7 had to be re-operated for incomplete relief. In the latter, the lack of success had been predicted by the neurophysiological data, as these were not as satisfactory as they should have been (either lack of evidence of good tip positioning or of sufficient lesion), no matter how the needle was moved in search of a better position. As the needle has to be passed across the foramen ovale, sometimes it may happen that anatomical restraints prevent it from getting the direction that would take its tip into the ideal position. And this is probably what happened in our 7 failed cases. None of the 52 operated patients developed anesthesia; only a light hypoestesia was noticeable in 38 cases. In conclusion, the monitored operation resulted in a high rate of success, due to the fact that positioning and extent of lesion were much more precisely assessed that it was possible before. Furthemore, the procedure was much smoother and completely painless, as no interruption of anesthesia was necessary.

Future developments

In future, we plan to perfect our informatic system, reaching several goals:

1-automatic interpretation of the responses so that the computer itself may take the decision of which is the best position of the needle tip.

2-automated sequence of stimulation. The three peripheral trigeminal branches will be stimulated by a computer guided interface, so as to obtain responses in a much faster way than it is now possible.

3-automated procedure for needle positioning. By implementing the first point, with the aid of a stereotactic frame and servomotors driven by computer, the best position of the needle tip may be reached very quickly and precisely. This will have also the advantage of providing emory for positions, in case more lesions have to be performed.

References

1-Sweet WH, Wepsic JG. Controlled thermocoagulation of trigeminal ganglion and results for differential destruction of pain fibres. Part 1: Trigeminal neuralgia. J Neurosurg 1974; 40: 143-156.

2-Leandri M, Parodi CI, Favale E. Early evoked potentials detected from the scalp of man following infraorbital nerve stimulation. Electroenceph clin Neurophysiol 1985; 62: 99-107.

3-Leandri M, Gottlieb A. Trigeminal evoked potential-monitored thermorhizotomy: a novel approach for relief of trigeminal pain. J Neurosurg , 1996; 84: 929-939.