Dr. Tóth Zoltán Foundation, MÁV Central Hospital Budapest, Department of Neurological Surgery
Examinations of evoked potentials do not rejoice great popularity among neurosurgeons because of their low specificity regarding the qualitative and quantitative properties of the underlying pathological process. As a result, there are very few studies dealing with pre- and postoperative EP-data analysis. Despite of this, EPs have important role in assessing functional features of the nervous system under altered conditions. In our study we try to present the diagnostic and prognostic value of EPs by patients operated due to mass lesions.
We have examined 69 randomly selected patients with intracranial supratentorial tumour by means of somatosensory and auditory evoked potentials with Dantec Counterpoint MK-2 equipment. SSEPs were performed by median and tibial nerve stimulation, averaging 200 stimuli, repeating each series for assessing wave-form reproducibility. Stimulus intensity was adjusted to the appearance of finger twitch. Active electrodes were placed on C2 and P3, P4 (medianus) or Pz (tibialis) standard EEG site with Fpz reference. Recording of evoked potentials was performed in the first 300 ms to analyse the early, specific (from 0 to 60-80 ms), as well the late, "spreading, associative" (from 60-80 to 300 ms) potentials. Significant alteration was defined as an at least 10 % difference in the latency and 30% difference in amplitude between the two sides. Diminished amplitude and latency prolongation was considered as a "lesion sign", while increased amplitude as a "synchronisation sign". AEPs were performed by 10 Hz rarefaction stimuli, 30 dB above hearing level, averaging 2000 stimuli, also repeating each series for assessing wave-form reproducibility. The first 20 ms and the 20-200 ms distance was evaluated independently. Mastoid active electrodes were referenced to a vertex electrode.
I., In preoperative evaluation of 69 patients with intracranial supratentorial tumours we have observed mainly two sort of EP-changes: "lesion signs" and "synchronisation signs".
1, Lesion signs are the "official", well-known type of alterations,
which are the prolongation and amplitude reduction of the primer potentials,
and the more or less pronounced disorganisation of the wave-complexes.
According to our observations these changes in general, and especially
the prolongation of the latencies are not too frequent, and nearly always
associated with severe or moderate neurological signs (hemiparesis, hemihypaesthesia),
which are indicative of the involvement of the specific sensory pathways.
The type of involvement can be tumour infiltration, vascular lesion or
severe distortion of these structures, and often can be seen by central
region gliomas. Lesion signs ipsilateral to the side of the tumour was
observed at 17 of 69 patients (24.6 %). We found amplitude reduction and
wave-form distortion of the N20-P25 medianus, and P37-N45 tibialis EP-complex
more frequently than absolute or interpeak latency prolongation.
2, Synchronisation signs, i.e. significant unilateral amplitude increase of one or more late waves, most often a high and wide negative or biphasic negative-positive potential at 90-120 ms by SSEP (Fig.1.), -according to our experiences- are generally ignored, but have two helpful characteristics. Firstly, it can be the only change in cases, when specific sensory pathways are not directly or severely involved, and thus primer potential-complexes are not distorted at all (see the tibial SSEP on Fig.1.). Secondly, pronounced increase of these late waves often accompany epileptic dysfunction, thus the presence of unilateral high amplitude late potentials can indicate that the brain is more "susceptible" to focal epileptic discharges. This sort of change perhaps reflects the exaggerated excitative state of the brain during the transcortical, associative spreading of potentials, or -alternatively- the altered electroconductive properties of the oedematous and compressed parenchyma. Significant synchronisation signs, which were not present in 20 normal control persons were observed at 32 of the 69 patients with tumour (46.3 %), while evaluating the focal epileptic patients alone, the incidence have risen to 79.4 % (27 of 34 pts). This "synchronisation-type" of EP-changes were observed predominantly when the parenchymal mass lesion was relatively far from the specific sensory pathways or, -even more-, when the tumour was extraparenchymal, for example a meningioma. As appears from the above mentioned, synchronisation signs of the late potentials were nearly twice as frequent than lesion signs of the primer complexes (46.3% vs. 24.6%). After tumour removal, by patients with improving neurological signs, reduction or cessation of synchronisation could be observed in one third (32.2 %) of the cases within the first 2-3 weeks, but this ratio was considerable time-dependent and much higher in later stages (no exact data). Indeed, sometimes lesion signs became predominant in the operated hemisphere.
II., In postoperative evaluation EPs can be used as prognostic guide by patients who were, or became severely hemiparetic or hemiplegic. Examining these patients with similar clinical-neurological status in the early postoperative period, we observed good later functional outcome in patients who showed only moderately changed, or at least relatively spared EPs, and, -reversely-, we experienced definitely poorer outcome in patients with absent or severely distorted EPs. Unfortunately (or rather fortunately) we can't support this observation by means of strict statistical evaluation because of the low number (7) of such patients. Examples are presented on Fig. 2.
Evoked potentials, -while clearly not competitive with the modern neuroradiological diagnostic procedures regarding localization and the pathoetiologic approach of the neurosurgical patient-provide a useful and non-substitutable tool in evaluating the functional aspects of the normal and abnormal nervous tissue. Functional systems and pathways of the CNS can be tested and monitored in the peri-, and intraoperative periods, so as to minimise the neural damage due to the intervention itself and the threat of an unnoticed complication. Furthermore, EP-examinations can give us a more or less reliable prognostic help in the early postoperative days by patients with severe neurological signs. Intraoperative use of EPs has appeared principally in spinal surgery, endarterectomy, aneurysm surgery, infratentorial procedures and in localization of central region by supratentorial central mass lesions. In our mini-paper we would have liked to call attention on some possibilities of neurosurgical using of evoked potentials and the need for the necessity of further refinement and extension of the methods.
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