Motor pathway analysis in HAM/TSP using magnetic stimulation and F-waves

Median Nerve Somatosensory Evoked Potential in HTLV-I Associated Myelopathy

INTRODUCTION

HTLV-I Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP) is a progressive Myelopathy that mainly involves the corticospinal tract. Despite pronounced involvement of the lower limbs, patients also have abnormalities in their upper limbs. So, we studied somatosensory-evoked potentials (SSEPs) of the median nerve in HAM/TSP patients to determine the extent of the involvement of the pathway of the central nervous system, especially the cervical spinal cord.

METHODS

In this cross sectional study, 48 patients with HAM/TSP who were referred to Qaem Hospital in Mashhad from October 2010 to October 2011 were evaluated for various indices, including SSEPs of the median nerve for N9, N11, N13, and N20 waveforms and also N11-13 and N13-20 Inter Peak Latency (IPL), severity of disease (based on Osama criteria), disease duration (less or more than 2 years), age, and gender. SPSS software was used for data analysis. The t-test was used for quantitative data, and the chi-squared test was used for the qualitative variables.

RESULTS

Thirty-four patients (70.2%) were females. The mean age was 45.6 ± 14.2 years. About SSEPs indices of the median nerve, N9 and N11 were normal in all patients, but N13 (50%), N20 (16.7%), IPL11-13 (58.3%), and IPL13-20 (22.9%) were abnormal. No significant relationships were found between age, gender, disease duration, and SSEPs indices (p > 0.05), but IPL11-13 and IPL13-20 had significant relationships with disease disability (p = 0.017 and p = 0.01, respectively).

CONCLUSION

Despite the lack of obvious complaints of upper limbs, SSEPs indices of the median nerve from the cervical spinal cord to the cortex were abnormal, which indicated extension of the lesion from the thoracic spinal cord up to the cervical spinal cord and thalamocortical pathways. Also, abnormalities in the cervical spinal cord had a direct correlation with the severity of disability in patients with HAM/TSP.

The motor evoked potential in aids and HAM/TSP State of the evidence

OBJECTIVE: We aimed to better understand the involvement of the corticospinal tract, assessed by non-invasive transcranial stimulation, in order to determine the actual involvement of the motor system in patients with HAM/TSP and AIDS. METHOD: An exhaustive MEDLINE search for the period of 1985 to 2008 for all articles cross-referenced for "HTLV-I, HTLV-II, HTLV-III and HIV, HIV1, HIV2, evoked potential, motor evoked potential, high voltage electrical stimulation, transcranial magnetic stimulation, magnetic stimulation, corticomotor physiology, motor pathways, acquired immunodeficiency syndrome, AIDS, SIDA, tropical spastic paraparesis, HTLV-I-associated myelopathy, HAM, TSP, and HAM/TSP" were selected and analysed. RESULTS: Eighteen papers published in English, Spanish, Portuguese, French and Japanese were identified. Only the central motor conduction time has been analyzed in seropositive patients to human retroviruses. The investigations done on HAM/TSP support the involvement of the pyramidal tract mainly at lower levels, following a centripetal pattern; in AIDS, such an involvement seems to be more prominent at brain levels following a centrifugal pattern. CONCLUSION: The central motor conduction time abnormalities and involvement differences of the corticospinal tract of patients with AIDS and HAM/TSP dissected here would allow to re-orient early neurorehabilitation measures in these retroviruses-associated neurodegenerative disorders. Besides this, more sophisticated and sensitive non-invasive corticospinal stimulation measures that detect early changes in thalamocortical-basal ganglia circuitry will be needed in both clinically established as well as asymptomatic patients at times when the fastest corticospinal fibers remain uninvolved.

Magnetic stimulation of the central and peripheral nervous systems

Since 1985, when the technique of transcranial magnetic stimulation (TMS) was first developed, a wide range of applications in healthy and diseased subjects has been described. Comprehension of the physiological basis of motor control and cortical function has been improved. Modifications of the basic technique of measuring central motor conduction time (CMCT) have included measurement of the cortical silent period, paired stimulation in a conditioning test paradigm, repetitive transcranial magnetic stimulation (rTMS), and peristimulus time histograms (PSTH). These methods allow dissection of central motor excitatory versus inhibitory interplay on the cortical motor neuron and its presynaptic connections at the spinal cord, and have proven to be powerful investigational techniques. TMS can be used to assess upper and lower motor neuron dysfunction, monitor the effects of many pharmacological agents, predict stroke outcome, document the plasticity of the motor system, and assess its maturation and the effects of aging, as well as perform intraoperative monitoring. The recent use of rTMS in the treatment of depression and movement disorders is novel, and opens the way for other potential therapeutic applications.

Clinical and physiological significance of abnormally prolonged central motor conduction time in HAM/TSP

We measured the central motor conduction time (CMCT), central sensory conduction time (CSCT), F wave and mean F wave/M wave amplitude ratio in patients with HTLV-I associated myelopathy/tropical spastic paraparesis (HAM/TSP) and controls. CMCTs in upper (U) and lower (L) limbs were significantly prolonged in HAM/TSP. L-CSCT was significantly prolonged in HAM/TSP, but U-CSCT in HAM/TSP and controls were not significantly different. CMCT and CSCT were significantly correlated in HAM/TSP. U-CMCT, but not L-CMCT, correlated with the clinical severity of HAM/TSP. Although F wave conduction velocity and its occurrence were normal in HAM/TSP, U- and L-mean F wave/M wave amplitude ratio tended to be higher in HAM/TSP, and the L-mean F wave/M wave amplitude ratio was significantly correlated with the L- and thoracic CMCT. These findings demonstrate that the prolongation of CMCT sensitively reflects the extension of the lesions and the disinhibition to the anterior horn cells in HAM/TSP.

Motor and somatosensory evoked potential findings in HTLV-I associated myelopathy

The central motor and sensory conduction times were measured using the motor and somatosensory evoked potentials in 21 patients with HTLV-I associated myelopathy/tropical spastic paraparesis (HAM/TSP) to evaluate the functions of the corticospinal tract and posterior column. The control subjects were closely matched for gender, age and height with the patients in the central sensory conduction time (CSCT) and central motor conduction time (CMCT) studies. An abnormal upper limb (UL) CMCT was present in six of 34 limbs (17.6%) while the lower limb (LL) CMCT was abnormal in 31 of 42 limbs (73.8%). No patients had an abnormal UL CSCT while the LL CSCT was abnormal in 10 of 32 limbs (31.3%). The frequency of abnormal LL CMCT was significantly higher than that for an abnormal LL CSCT. The duration of the disease and the disability score did not significantly influence the evoked potential findings. These results indicate that major lesions of the spinal cord in HAM/TSP patients tend to be localized more in the descending corticospinal pathways than in the ascending posterior columns at the thoracic spinal cord level. This finding also closely agrees with the clinico-pathological findings of HAM/TSP.