Impairment of smooth pursuit eye movements in patients with multiple system atrophy and idiopathic Parkinson's disease

Objectives: It is well known that patients with multiple system atrophy of cerebellar type (MSA-C) show impaired smooth pursuit eye movements (SPEM) due to their cerebellar pathology. To what extent SPEM are also affected in other MSA subtypes or in idiopathic Parkinson's disease (IPD) is less clear. In this study, we investigated if quantitative measurements of SPEM may help to discriminate between subtypes of MSA as well as between MSA and IPD.

Methods: Twenty-seven healthy controls, 17 patients clinically diagnosed as IPD and 11 patients diagnosed as MSA, in different stages of disease, tracked sinusoidal target movements of 0.125 and 0.375Hz in the horizontal (amplitude±20°) and vertical (±15°) plane. Eye movements were recorded by video-nystagmography (EyeLink with 250Hz sampling rate). Using an interactive program (in-house software based on MATLAB), SPEM of the subjects' responses were separated from all interspersed saccades and artefacts under careful visual inspection. SPEM velocity was fitted by sinusoids and referred to stimulus velocity, yielding SPEM gain.

Results: Nine out of the 11 patients with MSA showed reduced SPEM gain as compared to controls, although only 3 of them were clinically diagnosed as MSA-C. Of the 17 IPD patients, 5 had normal gains and 6 had slightly reduced values which possibly were due to an increased amount of extra-foveal tracking caused by square wave jerks. In the remaining 5 IPD patients, SPEM was markedly impaired. Closer inspection revealed two different tracking patterns: Three patients made large saccades during some half-cycles of the stimulus that anticipated the oncoming displacement of the target. We hold that the level of attention fluctuated in these patients leading them to partly “replace“ pursuit by (probably more automatic) saccades, although in principle they were still able to generate SPEM. The other 2 patients showed a pattern similar to the MSA-C patients. Indeed, one of them recently has been diagnosed as MSA-C.

Conclusions: In summary, careful investigation of SPEM may help to discriminate IPD from MSA especially in early cases, whereas the cerebellar involvement in MSA-P does not aid in the differentiation between MSA subtypes C and P. Conceivably MSA-P with lesions restricted to flocculus and/or vermis can be visualized by SPEM analysis, and can be contrasted to the changes observed in association with the more widespread cerebellar involvement in MSA-C.