Somatosensory evoked potential as related to pathological mechanisms of chronic spinal cord compression induced myelopathy

Objective To evaluate the altered somatosensory evoked potential (SEP) as related to pathological mechanisms of chronic spinal cord compression induced myelopathy. Methods A total of 20 rats were implanted with water-absorbing polymer sheet in the cervical spinal canal (C5-C6 level), which would expand outside the dura mater gradually to produce chronic compression to the spinal cord. Before and at 6 months after surgery, SEP, Micro-CT, HE-staining histology and FLB-staining histochemistry of spinal cord under chronic compression were measured. Results In all of the 20 rat models, typical morphology of chronic compression was seen posterolateral to the spinal cord, as reflected by distorted gray matter and white matter of spinal cord demonstrated at Micro-CT. The abnormal SEP responses were categorized into 5 types:Ⅰ (n=6), Ⅱ a (n=5), Ⅱ b (n=4), Ⅲ (n=5), and Ⅳ (n=0). Compared with normal group, altered SEP responses were associated with significantly less FLB-stained myelin of posterior column ( 106±27 vs 121±8;P=0.036, ＜ 0.05), higher contrast medium density in posterior dorsal colunn (95±5 vs 87±6; P=0.041 )and fewer neurons within posterior horn [ (21 ± 8)/mm2 vs (29 ± 6)/mm2, P＞0.05] by Micro- CT.Histopathologically, type Ⅰ SEP was associated with central canal enlargement of the spinal cord; type Ⅱ a with intra-gray matter bleeding, phlebectasia and constriction of central canal; type Ⅱ b with distortion of gray and white matters, and increase of blood vessels; and type Ⅲ with loss of white-gray matter structure,fewer neurons and even cavernous degeneration of the matrix. Conclusions Different SEP responses to chronic compression-induced myelopathy may reflecte the severity of damage to posterior tract and neurons in gray matter of the spinal cord. As a prognostic indicator, SEP is shown to be correlated well with certain pathological characteristics. Key words: Myelopathy; Spinal cord compression; Model,animal; Somatosensory-evoked potential; Pathology