Correlation between midline gait function performance and verbal fluency in patients with Parkinson's disease.

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Verbal fluency impairments are among the earliest and most common cognitive deficits in Parkinson's disease (PD), yet more than one study has shown that as a group, verbal fluency performance is not always different between patients with PD and age-matched controls. Here we examined whether PD-related deficits in speech and language capabilities are detectable in patients who exhibit poor midline motor function, such as control of gait. We postulated that if phonemic verbal fluency (PVF) performance in PD is dissociable from other factors such as age in this patient subgroup, a low PVF will likely reflect a general cognitive-motor deficit attributable to Parkinsonian pathology. Thirty-one PD and thirteen controls were evaluated on PVF and the Metronome-Paced Square Step Test (MPSST), a complex sensorimotor task that challenges axial and midline function. The MPSST requires a patient to initiate and maintain a consecutive series of diagonal and midline cross-over steps during the test. We found that despite similar PVF performance between groups, the vast majority of controls but not patients completed the MPSST without any errors, and the odds of completing the MPSST without any errors was 15 times greater among control. More importantly, a significant amount of PVF variability was explained by MPSST performance in PD but not controls even after controlling for age and disease duration. Together, these results provide evidence that PVF performance in PD likely reflects a PD-specific process and suggests that the MPSST may be a more sensitive test of PD-specific pathology than PVF.