Visual assessment of train-of-four and double burst-induced fade at submaximal stimulating currents

The influence of current intensity on visual assessment of fade in response to train-of-four (TOF) and two modes of double-burst stimulation (DBS) was determined to assess the utility of low-current neurostimulation. Each of 150 sets of assessments (in 51 patients) included a mechanographic TOF at 60 mA followed by visual assessments of TOF, DBS3,3 (two minitetanic bursts of three stimuli each), and DBS3,2 (a burst of three followed by a burst of two impulses) at 20, 30, 50, and 60 mA in random order. For the range of mechanographic TOF ratios between 0.41 and 0.70, visual assessment of TOF fade failed to identify fade in 33%, 36%, 44%, and 58% of cases at 20, 30, 50, and 60 mA, respectively. Corresponding false-negative rates for DBS3,3 were 11%, 17%, 36%, and 33%, and for DBS3,2 they were 6%, 6%, 17%, and 28%. Within each method, P less than 0.05 (by Mantel-Haenszel analysis) for a linear trend of increasing accuracy as current decreased. For the range between 0.41 and 0.70, quantitative assessment overestimated the actual ratio at all currents for TOF, at 30, 50, and 60 mA for DBS3,3, and at 50 and 60 mA for DBS3,2 (P less than 0.05 by Wilcoxon signed rank test). At each current tested, DBS was more sensitive in detecting fade visually than TOF. The accuracy of visual fade detection was not influenced significantly by level of observer training. In conclusion, visual assessment of fade by novice and expert observers is improved by testing at low currents.