Introduction
Magnetic motor stimulation is useful in the evaluation of a wide spectrum of nervous
system disorders including multiple sclerosis, spinal cord lesions, motor neuron diseases,
stroke, cervical spondylosis, intraoperative monitoring, epilepsy, pelvic floor disorders,
movement disorders and some investigative conditions such as brain mapping studies
[[1],[2],[3],[4]].
Technical advances in this method occurred during the 1980s and this method has gained
approval for clinical applications involving diagnostic and prognostic issues [[5],[6]]. Different techniques using magnetic stimulation and normal values for each technique
have not yet been studied to the same extent as conventional electrodiagnostic techniques.
Cortical magnetic stimulation has remarkable advantages over electrical cortical stimulation.
It is more convenient for the user, patients tolerate it much better, less time is
required for magnetic stimulation and no special preparation is needed for this study.1,2 Specificity of the site for magnetic stimulation is not as critical as it is for
electrical stimulation [[1],[7]].
One of the challenging topics in electrodiagnostic medicine is the diagnosis of proximal
brachial plexus entrapment syndromes such as neurogenic thoracic outlet syndrome,
especially in the early stages, when there is no significant axonal degeneration.
At this stage there is only demyelination and/or a focal conduction block involving
a short segment of plexus that can’t be evaluated by routine peripheral nerve conduction
studies and has no needle EMG findings. In this setting, use of Central motor conduction
time (CMCT) can be a potentially useful technique to confirm the clinical diagnosis.
Central motor conduction time (CMCT) is obtained when the peripheral conduction time
(PCT) is subtracted from the absolute latency of cortex to target muscle conduction
time. PCT is obtained by different methods including; F-wave latency, magnetic or
electrical nerve root stimulation and stimulation of the brachial plexus [[1],[8],[9]]. CMCT coefficients of variation for these techniques are; 15% for cervical magnetic
stimulation, 13% for F-wave latency and 11% for cervical needle electrical stimulation
[[10]]. Facilitation and intensity of stimulation can affect all the indices of motor
evoked responses including; amplitude, area and latency[[1],[9]]. but the effects of these variants on latency of motor evoked response are far
less than on area and amplitude. So the latency of motor evoked response is the most
reliable index and is more frequently used for investigative purposes [[1],[8]].
Methods
This study was performed in the electrodiagnostic medicine clinic of Shohada Tajrish
Medical Center Tehran, Iran, between May 2006 and December 2006. Overall 112 upper
limbs (66 persons) were tested, with 66 limbs belonging to healthy females and 46
belonging to healthy male volunteers. They had no history of convulsive disorders.
Their neurologic clinical evaluation was normal and they had no signs of neuromuscular
disorders. The medical ethics committee of Shahid Beheshti Medical University, Physical
Medicine and Rehabilitation Branch approved our study. After explanation of the procedure,
the volunteers signed an informed consent that was written in their native language
(Persian). They were also asked if they had cardiac pacemakers, implanted metallic
devices or intracranial metallic clips from neurosurgical operations. Cases having
one or more of these criteria were excluded from the study. If the limb temperature
was below 32°C their limbs were warmed up. All the volunteers who have undergone nerve
conduction studies on upper and lower limbs and cases suspected of having neuropathies
were excluded. After giving thorough explanations about the process of study the volunteers
were deliberately included in the study. To obtain the absolute latencies of median
and ulnar nerves, the magnetic coil stimulator was placed on the motor cortex 7 cm
lateral to Cz (a line connecting both tragi together) ([Figure 1]) in the transverse plane and the best response was obtained from thenar and hypothenar
muscles by elevating the intensity of stimulation. To obtain peripheral conduction
time (PCT,) we used a second stimulation on the brachial plexus in the supraclavicular
fossa by placing the magnetic coil stimulator in a plane that was parallel to the
body surface ([Figure 2]). The recording was done on the same muscles as for cortical stimulation. The central
motor conduction time (CMCT) was calculated by subtracting PCT from the absolute latency
of the above mentioned nerves.
Figure 1
Magnetic stimulation of cortical area.
Figure 2
Magnetic stimulation of brachial plexus.
Adjustment of coil stimulator angle on the scalp and ipsilateral slight contraction
of the target muscle, as the facilitation maneuvers, were used to improve the quality
of response. The stimulator machine used in this study was Mag-stim 200 set on 90–100%
of its maximal output (1.5 Tesla) for cortical stimulation and 70–80% of its maximal
output for brachial plexus stimulation. The coil used was circular in shape with an
internal diameter of 7.5 cm and its central point was used to stimulate the above
mentioned targets. The recording instrument was a four channel “Toennis Neuroscreen
Plus” set on: time division 5 ms, sensitivity 500–1000 μv/div. Recording electrodes
were conventional bar electrodes.
Results
Data obtained in this study was analyzed by SPSS-9 software. The mean age of males
was 44.7 years (range: 24–65 yrs) and that of females was 42.0 yrs (range: 18–67 yrs).
The mean for the absolute latency (cortex to muscle) of the median nerve with recording
from the thenar muscles was 21.4 (SD = 1.7) ms. This value was 21.9 (SD = 1.4) ms
in males and 21.0 (SD = 1.7) ms in females.
The mean for the absolute latency of the ulnar nerve with recording from the hypothenar
muscles was 21.3 (SD = 1.6) ms. This value was 21.9 (SD = 1.5) ms in males and 20.9
(SD = 1.7) ms in females. The mean for the central motor conduction time (CMCT) of
the median nerve with recording from the thenar muscles was 9.6 (SD = 1.9) ms. This
value was 9.6 (SD = 2.0) ms in males and 9.6 (SD = 1.8) ms in females. The mean for
the central motor conduction time (CMCT) of the ulnar nerve with recording from the
hypothanar muscles was 9.4 (SD = 1.8) ms. This value was 9.2 (SD = 1.9) ms in males
and 9.7 (SD = 1.7) ms in females ([Table 1]).
Table 1
Absolute latency and central motor conduction time (CMCT) of median and ulnar nerves
in 112 upper limbs of normal volunteers
|
Recording site
|
All patients mean(SD)
|
Males mean(SD)
|
Females mean(SD)
|
|
Absolute latency(ms)
|
Thenar
|
21.4 (1.7)
|
21.9 (1.4)
|
21.0 (1.7)
|
|
Hypothenar
|
21.3 (1.6)
|
21.9 (1.5)
|
20.9 (1.7)
|
|
CMCT (ms)
|
Thenar
|
9.6 (1.9)
|
9.6 (2.0)
|
9.6 (1.8)
|
|
Hypothenar
|
9.4 (1.8)
|
9.0 (1.9)
|
9.7 (1.7)
|
Discussion
The number of cases entered in this study is remarkably larger than those used in
similar studies. Zwarts in his study with a sample size of 36 obtained these results:
latency of cortex to APB muscle = 20.6 ms (SD = 1.2) and CMCT recorded from APB =
7.4 ms (SD = 0.9) [[11]].
In Eisen’s study with a sample size of 90, he obtained these normal values: absolute
latency from cortex to thenar muscles = 20.4 ± 1.5 (16.8 – 23.8) and CMCT with thenar
recording = 6.7 ± 1.2 (4.9 – 8.8) [[12]]. We made use of magnetic stimulation for cortical and peripheral stimulation. Our
results show that there is no meaningful difference between the two genders. CMCT
obtained by this method are more prolonged than values obtained when near nerve stimulation
is used for PCT [[8],[11],[12]]. The reasons for this finding are: (1) PCT was obtained by brachial plexus stimulation
and, (2) this was done by magnetic stimulation. These together make the PCT somewhat
shorter and consequently CMCT is calculated to be longer. Some peripheral nervous
system injuries such as nerve root lesions and proximal brachial plexopathies e.g.
TOS, can be potentially evaluated by this method of CMCT calculation. Finally it seems
that the technique for calculating CMCT as we explained in this manuscript has advantages
over conventional electrodiagnostic methods, including; non-invasiveness, and convenience,
taking less time from the physician., Since this method measures the proximal part
of the lower brachial plexus and related ventral primary rami, it may help diagnose
early stages of entrapment syndromes with mainly demyelinating and/or conduction block
type of involvement. It also has its own disadvantages such as lack of specificity
of stimulation site that makes its uses limited to central nervous system and long
segment peripheral nervous system disorders,
