Waveform Window #21: Anomaly during Pedicle Screw Stimulation

Empirical data support the stimulation of pedicle screws intraoperatively to reduce the risk of neural damage to exiting nerve roots post-operatively. Lenke and colleagues (1995) published a range of threshold intensities to indicate whether a screw is directly making dural contact ( < 4 rnA), medially breached without dural contact (4 rnA to 8 rnA), or completely within pedicle(> 8 rnA). However, false negatives occur with screw stimulation which include: fluid in the wound (which would cause current shunting, requiring higher stimulus output), something other than the screw head being stimulated (for example, stimulus to wound tissue or screw crown would impede the current, making the threshold value of a distal electromyogram (EMG) response greater than it should be), or the patient is pharmacologically paralyzed (different levels of paralytic agents would cause higher thresholds needed to obtain a response). These false negatives tend to manifest as high impedance threshold values (>50 rnA). We present a case where eight pedicle screws were stimulated via a monopolar intraoperative stimulation probe. The reference was placed in the wound rostral to stimulation, not far from the stimulus. A rep rate of 2. 79 Hz and duration of .2 msec was utilized. Sub-maximal stimulation was utilized to obtain the response. This is the least amount of current needed to elicit a compound muscle action potential (CMAP). Note in the figures presented the morphology and amplitude differences from each CMAP acquired, as screw stimulation elicits variable responses at every level tested. A time base of 5 msec/div and a sensitivity of 100 11 V /div were used. Each screw had a threshold value greater than 50 rnA with the exception of the left S 1 screw which elicited a response at 12 rnA. After deducing there were no indications for a false negative to occur, we decided to remove the screw and manually probe the hole.