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The paper by Perrine et al (this issue, pp 150-4) is one in a series of recent reports of cognitive outcome after unilateral pallidotomy for patients with Parkinson’s disease.1-4 These studies have taken advantage of sophisticated and selective surgical techniques (and outcome measures) unavailable to earlier, poorly controlled, studies in the pre-levodopa era. More reliable targeting of globus pallidus interna lesions is likely to have reduced cognitive complications, although potentially, the margin for error remains small and there has been at least one report of hemisphere specific postoperative cognitive changes.4 Nevertheless, despite a considerable degree of individual variability (postoperative gains and deficits), experienced centres have not generally found clinically significant cognitive sequelae (in terms of statistically significant changes in group mean scores) associated with good neurological outcomes (at 3 to 12 month follow ups). Similar findings have been reported after bilateral pallidotomy, although postoperative deterioration in verbal fluency, speech-motor apparatus, and possibly “executive” functions, remains a concern.3
This individual variability in outcome has yet to be explained and it may require larger sample sizes for any consistent pattern of group cognitive changes to emerge. However, refinements are also needed in the methodology of plotting the volume, contour, and location of pallidal lesions relative to individual neuroanatomy, before any exploration of associations between lesions and selective postoperative cognitive change can proceed. Such studies might advance mapping the functional architecture of the globus pallidus.Until more is known about the role, if any, of the globus pallidus interna in cognition (or the reliability of surgical targeting), psychometric test batteries should arguably be as comprehensive as is clinically practicable (rather than selective in terms of cognitive domains known to be impaired in Parkinson’s disease), if they are to represent an adequate audit.
Careful consideration needs to be given to the design of a neuropsychological audit, and the interpretation of any postoperative changes. For example, when tests are repeatedly administered, performance may improve due to practice and/or familiarity effects. With a comprehensive battery of tests, many comparisons will need to be made from baseline to follow up(s), and a careful balance therefore struck between the clinical significance of making type I or type II errors in statistical analyses. The recruitment of appropriately matched control groups or employing tests with parallel forms partially addresses the problem of test-retest effects, but it is not known if these are different for control and pallidotomy groups and postoperative scores may also be indirectly influenced by other factors. For example, a reduction in dyskinesias will improve dexterity and may also have a secondary, beneficial effect on attention; further, the effects of any postoperative changes in parkinsonian medications are largely unknown.
Despite these difficulties, adequate neuropsychological audit of ablative neurosurgery (or the implantation of stimulators) in consecutive series is important for several reasons, not the least of which is to enable meaningful comparison of the safety of different procedures—for example, surgery guided by microelectrorecording as opposed to (various methods of) image guidance with macrostimulation. Clinicians may also find baseline psychometric measures of cognition helpful in interpreting any subjective reports of postoperative change; “effort after meaning” can be a problem after any neurosurgical procedure. In addition, baseline assessment is an important screening device. Dementia is widely accepted as a contraindication to pallidotomy, but as many patients present with advanced disease and complex symptomatology, brief screening tests such the mini mental state examination are often too crude to adequately identify suitable surgical candidates. Retrospective evaluation of baseline assessments (in the light of subsequent outcome) may ultimately help establish empirically, more selective cognitive contraindications.
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