Assessment of the motor and sensomotor system in vegetative patients

Gimranov R.F., Maltseva E.A.

Burdenko Research Institute of Neurosurgery Russian Academy of Medical Science

Introduction

Coma is a state, watched in a severe brain injury with permanent absence of consciousness, but without signs of brain death. A patient in this state cannot be awakened and does not open eyes even in response to strong stimuli. Sometimes there is restoration of a cycle of sleep and being awake; patients open their eyes from time to time, but they are unaware of the environment and do not respond to speech, addressed to them. In this case one can speak of a vegetative status or appalic syndrome. A term "vegetative status" (VS) seems to be more preferable. It was proposed by B. Jennet and F. Plum in 1972 [4]. According to their opinion, it makes doctors to pay more attention to a contrast between severely impaired psychic functions and preserved autonomous and vegetative functions. This state is characterized by loss of cognitive ability against a background of relative preservation of ability to be awake.

More often VS develops in acute cerebral lesions both of traumatic origin (car injury and road trauma, gunshot wounds, etc.) and non-traumatic genesis (global cerebral ischemia, caused by cardiac or respiratory arrest and different types of asphyxia; acute disorders of cerebral circulation; CNS infections; tumors; endo- and exogenous intoxications). This syndrome develops in 1-14% of patients with long-term traumatic coma and 12% of cases with coma of non-traumatic etiology [7]. It is conditioned by spread structural damage of the cortex, limbic structures and basal ganglia with relative preservation of the brain stem.

Recording motor evoked potentials (MEP) during transcranial magnetic stimulation (TMS) is a non-invasive method of assessment of the motor cortex and pyramidal pathways [1], As for recording of sensomotor evoked potentials (SSEP), it allows to evaluate the sensomotor cortex and pathways [2].

The main goal of our investigation was estimation of a state of the motor and sensomotor systems (the cortex and pathways) in vegetative patients.

Material and Methods

Study was carried out on 45 vegetative patients (mean age of 29.6±14.2). VS resulted from severe craniocerebral trauma in 34 of them; it was caused by different complications, typical of cerebral tumors, in 11 cases. All patients were treated in Burdenko Research Neurosurgical Institute during 1997-2201. Diagnosis was based on clinical findings and results of CT and MRI examination. A control group consisted of 24 healthy volunteers (mean age of 25.7±11.6).

MEP were recorded with the help of MES-10 magnetic stimulator (Cadwell) and Viking-IVP averaging device (Nicolet). The magnetic stimulator allowed to induce a pulse magnetic field (up to 2.0 T). Electrodes, located on muscles under examination, were connected with Viking-IVP device through a commutator. Use of a trigger mechanism made it possible to synchronize recording of MEP with a magnetic pulse start.

A coil was placed above the motor cortex area in the right or left hemisphere for its stimulation. Recording of MEP in stimulation of the spinal cord segment was carried out with a coil, placed above a spinous process of this segment. Difference in latency above these two points in TMS served the basis for determining a period of central motor conduction (PCMC).

When a stimulation threshold (motor threshold) for relaxed muscles was ascertained, induction of a magnetic field was increased by 15-20%. It was done for stimulation of a maximum number of nervous elements and, thus, receiving responses with the highest possible amplitude. A value of motor threshold was calculated as percent from maximum induction of the magnetic stimulator. Recording cup electrodes were placed above the right and left abductor pollicis brevis; impedance of electrodes was not less than 5 kOhm. An amplifier frequency band and an analysis epoch were equal to 30-3000Hz and 150ms respectively. MEP were recorded several times; responses with the smallest latent period were selected for analysis. Then its amplitude and duration were determined. A diagram of MEP recording and location of electrodes is given in fig. 1.

As for SSEP recording, a frequency band was equal to 30-2000 Hz; 500 avera-ging procedures (two series) were carried out; right and left median nerves were stimulated in turn by single pulses with a frequency of 5.1-7.3/s, duration of 0.1 ms and a current of 15 mA; an epoch of 50 ms was analyzed. Recording electrodes were placed on Erb's point, at the level of the 7th cervical vertebra, on the scalp 2 cm backward from C3 and C4 (10-20% according to the international scheme) and a reference electrode was applied to a forehead. Impedance was not less than 5 kOhm. Viking-IVP device (Nicolet) was used for recording and analysis of SSEP. An amplitude and latency of P23 component and a period of central conduction (PCC), i.e. difference of latency between N13 and N19 components, were analyzed.

Obtained data were subject to statistical processing (Statistica program, Student's t-criterion). If p was less than 0.05, it was considered to be a reliable index.

Results

Amplitude-temporal indices of MEP in TMS, watched in the control group, are presented in Table 1.

Table 1

Mean Statistical Amplitude-Temporal Values of MEP (from m.abductor pollicis previs) in TMS in Healthy Individuals

Stimulation of the right hemisphere Stimulation of the left hemisphere
MEP amplitude 2.2±0.8 mV 2.2±0.7 mV
MEP latency 22.7±2.5 ms 22.5±2.7 ms
PCMC 7.9±2.1 ms 7.8±2.2 ms

MEP in stimulation of the cortex and C7 area of the cervical spine of health volunteers, as well as a period of central motor conduction, are given in fig.2.

TMS of the right or left hemisphere in vegetative patients demonstrated changes of amplitude-temporal indices, a form and configuration of MEP from muscles of a hand. The most typical changes were as follows:

  1. Complete absence of MEP in TMS (fig.3.1.), indicative of a severe lesion of the motor cortex or pyramidal tract.
  2. Amplitude reduction, latency increase and a polyphase form of MEP (fig.3.2.), were signs of a lesion of the brain subcortical structures.
  3. Amplitude reduction and increase of latency of MEP in TMS of both right and left hemispheres (fig.3.3.), reflecting a lesion at the brain stem level.

MEP in TMS of the right and/or left hemispheres of vegetative patients were recorded in 39 cases. We failed to record MEP in TMS of both right and left hemispheres in 6 patients. A mean statistical value of induction of a magnetic field, causing MEP in TSM of the right and left hemispheres of vegetative patients made respectively 91.2% and 90.4% from the maximum induction of the magnetic stimulator.

An effect of the pyramidal tract lesion on temporal indices of MEP in TMS (latency, a conduction period) was more marked than that of the cortex lesion.

Vegetative patients had considerable reduction of mean statistical values of response amplitudes and increase of latency and PCMC (table 2) in comparison with analogous indices of healthy individuals. These changes were statistically significant.

Table 2

Mean Statistical Amplitude-Temporal Values of MEP, MEP in TMS in Vegetative Patients

Stimulation on the right hemisphere Stimulation on the left hemisphere
MEP amplitude 0.2±0.1 mV 0.3±0.2 mV
MEP latency 29.5±2.8 ms 28.7±4.1 ms
PCMC 11.4±2.3 ms 10.9±3.5 ms

Amplitude-temporal parameters of SSEP and a period of central conduction were estimated in the control group (table 3). Cortical components of SSEP of vegetative patients were recorded from the right and/or left hemispheres in 33 cases. We failed to record responses either from the right or left hemisphere in 12 out of 45 cases. There were certain changes in amplitude-temporal indices of SSEP. They manifested themselves in marked reduction of a mean statistical value of P23 amplitude both from the right and left hemispheres. The analysis of temporal parameters of SSEP was indicative of increase of mean statistical values of P23 cortical components, as well as of PCC.

Table 3

Mean—Amplitude-Temporal Values of SSEP P23 Component Component in Healthy Individuals

Stimulation of the right hemisphere Stimulation of the left hemisphere
23amplitude 22.8±1.7 ms 22.7±1.5 ms
23amplitude 2.6±1.8 mV 2.5±1.7 mV
PCC (N13-N19) 5.9±0.6 ms 5.9±2.1 ms

Comparison of mean statistical amplitude-temporal values of SSEP in healthy individuals and vegetative patients revealed statistically reliable difference. It lied in amplitude reduction, increase of P23 latency and PCC.

Changes of temporal indices of SSEP were reflection of changes at the level from Goll-Burdach's nuclei up to the cortex, as peripheral conduction of an impulse remained the same. Mean statistical amplitude-temporal values of SSEP in vegetative patients are given in Table 4.

Table 4

Mean Statistical Amplitude-Temporal Values of SSEP in Vegetative Patients

Stimulation of the right hemisphere Stimulation of the left hemisphere
23amplitude 27.6±1.7 ms 26.5±1.5 ms
23amplitude 0.6±1.8 mV 0.5±1.7 mV
PCC (N13-N19) 7.8±1.6 ms 7.7±2.1 ms

Discussion

It is known, that complex recording of MEP in TMS and SSEP makes it possible to assess a state of the motor and sensomotor systems [9, 11]. Thus, our study, carried out in vegetative patients, has demonstrated more subtle neurophysiologic mechanisms of generation, processing and conduction of nervous impulses.

Possibility of estimating functional changes in the motor cortex, based on analysis of motor thresholds values [6, 10] and MEP in TSM, was proved some time ago [3, 8]. Increase of a motor threshold and decrease of MEP amplitude in TMS, watched in our study, reflects first of all changes in the motor cortex. These changes can have both an organic and functional character.

Changes of amplitude of SSEP P23 component is more indicative of dysfunc-tion of the sensomotor cortex. Increase of P23 latency and PCC reflects changes in conduction tracts [2]. Vegetative patients have marked changes both of amplitude and temporal values of SSEP, being a sign of the sensomotor system lesion.

Presence of changes in amplitude-temporal values of MEP in TMS and SSEP not only in an affected hemisphere and conduction tracts, but also in a hemisphere without organic lesions is indicative of a functional character of these changes, whose probable cause is both metabolism and apaptosis processes [5].

It is necessary to continue dynamic study of the motor and sensomotor systems in vegetative patients for deeper understanding of a character of changes, which take place in them.

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