Changes of magnetic stimulation motor evoked potential and the corresponding characteristics of the extent of chronic nerve root compression

Hongtao LIU; Dazhi YANG; Wanxin ZHEN; Duo WANG; Liang XU

ABSTRACT

BACKGROUND:

The assessment of chronic nerve root injury is mainly depend on indirect evidences of imaging results.

OBJECTIVE:

This study is designed to find out the relationship between the degrees of the pathological changes of nerve roots and motor evoked potential (MEP) changes due to chronic compression.

DESIGN:

A self-controlled trial with animals as subjects.

SETTING:

Spine Surgery Department of the Second Clinical Hospital of Jinan University.MATERIALS The trial was completed in the Spinal Surgery Department of the Second Clinical Hospital of Jinan University from October 2001 to May 2003. The subjects were 30 healthy cats of either gender weighing 3 to 5 kg.

INTERVENTIONS:

The compression model was established by wedging a Ⅴ-shape bone flap into the right intervertebral foramens between C7, C8 and T1 vertebrae. The left foramens served as controls. The cats underwent CT examination on the 2nd, 4th, 8th and 12th postoperative week to show the foramen changes. Six randomly selected cats underwent magnetic stimulation MEP each time on the 2nd, 4th, 8th, 12th and 24th week. The injuries to the compressed nerve roots were assessed according to Mackinnon Standard and they were compared with the MEP wave.MAIN OUTCOME

MEASURES:

Whether the superficial magnetic stimulated MEP monitoring result was in correspondence with the pathological changes of the chronically compressed nerve roots.

RESULTS:

The MEP did not show any abnormal changes for Grade Ⅰ injury prolonged to 9.6- 10. 2 ms. There was almost no evoke potential at all stimulation points for grade Ⅴ nerve root injury.

CONCLUSION:

That the injured nerve root presents dysesthesia and normal MEP results suggests grade Ⅰ injury. Prolonged latency including prolonged F wave suggests grade Ⅱ injury. And additional amplitude decrease together with widened and unclassifiable waves implies the injury be grade Ⅲ. The prominent features of grade Ⅳ nerve injury is much more decreased amplitudes and more prolonged latency. For grade Ⅴ injury there is always disappearance of whole MEP or some potential components. The superficial magnetic stimulated MEP monitoring results are in correspondence with the pathological changes of the nerve roots due to chronic compression, which means MEP monitoring results could serve as the quantitative indication to pathological changes of nerve injury.