BY G. DIERSSEN and E. MARG
Localisation is a major problem of stereotaxic procedures and anything that can increase our certainty of being in a certain structure if of vital importance. The X-ray studies as basis of the localisation have been and will continue to be invaluable but their limitations are well known. X-ray location is an indirect method depending on average values despite individual differences from patient to patient. These values become progressively less accurate as the distance from the used landmarks increases.
Stimulation can and does give information on localisation, its value, however, depends in part on the particular structure stimulated and in part on the reaction to stimulation of the particular brain we are acting upon.
Recording, specially with microelectrodes, has been shown to provide further information on localisation of the structures. This method appears to be worthwhile but it requires, to be effective, very special technical and instrumental conditions.
There are a number of physical measurements of the brain that can be used to provide further information on the location of our electrodes. Some of them are to be tried such as thermal gradients, mechanical resistance, pH, etc. Others have been tested experimentally (Robinson, 1962), but not generally in patients, such as the electrical impedance.
This is a report of the practicability of using impedance measurements to aid in the localisation of the structures during stcreotactic surgery.
Method
The electrode was an insulated tungsten needle 20 cm long with a 1/2 mm diameter. It was originally made as miroelectrode and adapted for impedance measurements by removing the insulation within 1/4 mm of the tip. This electrode showed less than 200 ohm impedance in saline solution. The return circuit was through the stereotactic apparatus screwed into the skull.
The impedance meter has been designed and manufactured by Biotronic Inc. incorporating the specifications suggested by Robinson and Tompkins (maximum current 1 microampere, range 1 to 8 kohm, 50 Kc). It is portable and operating on flashlight batteries. The impedance is read directly from a metre. An automatic recording device is being developed, in the meantime all values were read and graphs were drawn manually. The graphs obtained have been compared with the diagrams of the electrode paths obtained independently by analysing the X-ray plates using as landmarks the Ca-Cp line as proposed by Talairach and al.
Results
This method has been tested in the course of 50 introductions of the electrodes. Initial difficulties have derived from the selection of the electrode which in our experience should be monopolar and with a very small tip. A larger contact surface of the electrode is not advisable since it reduces the precision without giving any advantage.
Bipolar electrodes have the advantage of avoiding the return circuit but they too have much less precision. The readings had to be taken during penetration of the brain by the electrode because the impedance might remain low during withdrawal, probably due to the penetration of small amounts of fluid or blood into the opened path. In all our insertions the variations of the readings where compatible with the X-ray location. The boundaries between white and gray matter could be established very clearly by the variations of the impedance (Fig. 1).
The degree of myelinisation of the various structures conditions in our opinion the difference in the readings from DM, and anterior nucleus of the thalamus or nucleus caudatus to those obtained from structures such as VL, VPL or pallidum. Within the white matter, the corpus callosum and the capsule gave higher values of impedance than the subcortical white matter. As it was to be expected and has been shown by Fry et al. the penetration in the ventricle was shown by an immediate fall of the impedance or, if the ventricle was filled with air, by a rise of the values.
Occasionally a rhythmic pulse in chronic variation of the impedance could by detected. This oscillation appeared to be very localised and in our feeling might be due to the proximity of a blood vessel.
Discussion
We think that the experimental results of Robinson and Tompkins can be applied to human stereotactic surgery.
The readings obtained have, of course, only relative value, since the conditions of the return circuit have a great influence on them. But the differences due to the return circuit do not affect the validity of the method since the variations and not the absolute values of impedance are evaluated.
These variations used as complement to the X-ray studies give us a more precise idea of the location of our electrode. They might also provide a warning of danger of vascular disruption and information of the extent of any haemorrhage.
Impedance is just one of a number of physical measurements which could be used to localise structures of the brain more precisely.
Only the integrated use of a number of biological, physical and morphological data will provide us with a more complete certainty and precision in the location of our instruments.
References
Robinson, B.W.: Localization of intracerebral electrodes. Exp. Neurol. 6: 201-223 (1962).
Robinson, B.W. and Tompkins, H.E.: Impedance methods for localizin brain structures. An extension of the methods. Arch. Neurol., Chicago 10: 563-574 (1964).
Talairach, J.; David, M.; Tournaux, P.; Corredor, H. et Kausin, T.: Atlas d'anatomie stereotaxique (Masson, Paris 1957).
Author's address: Dr. G. Dierssen and Dr. E. Marg, Clinica de Nuestra Senora de la Concecion, Av. Reyes Catlolicos 2, Madrid-3 (Spain)