Objective : Ossification of the posterior longitudinal ligament (OPLL) often bring out spinal canal stenosis, and has been recognized as a common cause of compression myelopathy. Somatosensory evoked potential (SEP) is a noninvasive electrodiagnostic study to monitor somatosensory pathway including spinal cord integrity. There have been some reports about correlation between size, subtype of OPLL and clinical symptoms including weakness, sensory change and pain. But, there has been no trial about the correlation between OPLL characteristics and SEP. Therefore, the objective of this study is to find out the correlation between size, shape, subtype of OPLL and SEP in cervical OPLL patients, and demonstrate cut-off value of OPLL size to deteriorate spinal cord integrity.
Materials & Methods : We retrospectively studied 153 cervical OPLL patients who underwent SEP study between January 2012 and October 2014. OPLL anterior-posterior (AP) diameter, area, occupying ratio of diameter and area were measured most accurately on cervical axial CT. The involved longitudinal vertebral level of OPLL was measured through 1~7 level on sagittal image on CT. And OPLL was classified into subtypes according to longitudinal continuity and shape on sagittal and axial CT. SEPs were recorded by stimulation of median nerve at the wrist and the posterior tibial nerve at the ankle. Recordings were performed via surface electrodes from the scalp at C3 (right median nerve stimulation), C4 (left median nerve stimulation) and Cz (right and left tibial nerve stimulation) positions and from a reference electrode at FPZ according to the 10-20 international EEG system. We performed correlation analysis between the characteristics of OPLL and SEP and Receiver operating characteristic curve (ROC) was used to analyze the cut-off value of OPLL size for abnormal SEP.
Results : Correlation analysis revealed that tibial SEP latency showed significant positive correlation with OPLL AP diameter (p = 0.001), and OPLL AP diameter occupying ratio. (p < 0.05) OPLL area, OPLL area occupying ratio and involved longitudinal vertebral level also revealed significant positive correlation with tibial SEP.(p < 0.05) And, Space available for spinal cord (SAC) is negatively correlated with SEP prolongation (p < 0.05). The cut-off values of OPLL AP diameter, SAC, OPLL AP diameter occupying ratio, OPLL area, OPLL area occupying ratio, and involved longitudinal vertebra level that were predictive for SEP prolongation suggesting myelopathy were 4.91 mm, 6.02mm, 44.5%, 63.4 mm², 36.1%, 2 level.
Conclusions : This is the first study to investigate the correlation between OPLL characteristics and SEP. Our results reveal that tibial SEP latency is significantly correlated with OPLL size and suggest cut-off values of OPLL diameter and area for meaningful early prediction of myelopathy. And these results would be helpful to establish treatment plan including surgery timing.
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