Determinants of functional disability in Huntington's disease: role of cognitive and motor dysfunction

Abstract

The clinical syndrome of Huntington's disease (HD) is notable for a triad of motor, cognitive, and emotional features. All HD patients eventually become occupationally disabled; however, the factors that render HD patients unable to maintain employment have not been extensively studied. This review begins by discussing the clinical triad of HD, highlighting the distinction in the motor disorder between involuntary movements, such as chorea, and voluntary movement impairment, with the latter contributing more to functional disability. Cognitive disorder clearly contributes to disability, though the relative contribution compared to motor is difficult to unravel, especially because many of the tests used to asses "cognition" have a strong motor component. The role of emotional changes in disability needs more study. The literature on contributions to functional disability, driving impairment, and nursing home placement is reviewed. Relevant experience is presented from the long-standing JHU HD observational study on motor versus cognitive onset, as well as on cognitive and motor features at the time when individuals discontinued working. Finally, we briefly review government policies in several countries on disability determination. We interpret the data from our own studies and from the literature to indicate that there is usually a close relationship between cognitive and motor dysfunction, and that it is critical to take both into consideration in determining disability. © 2014 International Parkinson and Movement Disorder Society.

Keywords: CAG repeat; Chorea; driving; motor impairment.

Figure 1

CAG repeat length vs motor and cognitive onset. Motor onset is on average 4.1 years earlier than cognitive onset, with little variation over this range of repeat lengths. “Power” refers to trendlines for cognitive and motor onset respectively.

Figure 2

Schematic diagram of basal ganglia circuits, emphasizing the close relationship among motor, cognitive and emotional circuits. Cortical region A indicates motor areas such as supplemental motor cortex, B indicates cortical areas especially involved with cognitive function such as dorsolateral prefrontal or parietal association cortex, and C represents cortical areas especially involved with emotion such as orbito-frontal, temporal or cingulate cortex. The circuits are parallel, but projections from these regions interdigitate, and recurrent projections enable the motor, cognitive and emotional information to be integrated before the efferent streams return back to the cortex. However emotional information may remain relatively distinct, via the ventral striatum. Adapted from Alexander et al 1986.