• STEREOTAXIC SURGERY
• NEUROSURGERY
• MOVEMENT DISORDERS

Dear editor

We read with interest the novel systematic review and meta-analysis of the relationship between targeting accuracy and motor outcome in subthalamic nucleus (STN) deep brain stimulation (DBS) presented by Kremer et al. 1 Efficacy of DBS is purported to be dependent on accurate placement of electrodes in target structures and, as the authors highlight, a tolerance of 2 mm from a desired target has gained informal traction in clinical practice. The authors’ rigorous approach reveals several shortcomings to reviewing peer-reviewed published data to test the validity of the ‘2 mm rule’. As pointed out in the paper, the studies included in their meta-analysis demonstrate generally accurate electrode placement and reveal publication bias towards series with greater accuracy. Some of the included studies even cited electrode misplacement as an exclusion criterion from their series, thus further reducing the range of accuracies available for meta-analysis. One might expect outliers in targeting accuracy to inform us most about how electrode placement affects clinical outcome. Furthermore, despite over 160 000 patients having been implanted with DBS systems worldwide, mostly for Parkinson’s disease, individual patient data were available for only 206. Although the available data do not refute the ‘2 mm rule’, these issues raise the question of whether meta-analysis of published data is an appropriate method for examining …