Reduced Frequency of Ipsilateral Express Saccades in Cervical Dystonia: Probing the Nigro-Tectal Pathway.

Background: Cervical dystonia is a hyperkinetic movement disorder of unknown cause. Symptoms of cervical dystonia have been induced in animals in which the integrity of the nigro-tectal pathway is disrupted, resulting in reduced inhibition of the deep layers of the superior colliculus. This same pathway is believed to play a critical role in saccade generation, particularly visually guided, express saccades. It was hypothesized that individuals with cervical dystonia would present with a higher frequency of express saccades and more directional errors. Methods: Eight individuals with cervical dystonia and 11 age- and sex-matched control participants performed three saccadic paradigms: pro-saccade, gap, and anti-saccade (120 trials per task). Eye movements were recorded using electro-oculography. Results: Mean saccadic reaction times were slower in the cervical dystonia group (only statistically significant in the anti-saccade task, F(1, 35)  =  4.76, p  =  0.036); participants with cervical dystonia produced fewer directional errors (mean 14% vs. 22%) in the anti-saccade task; and had similar frequencies of express saccades in the gap task relative to our control population (chi-square  =  1.13, p  =  0.287). All cervical dystonia participants had lower frequencies of express saccades ipsilateral to their dystonic side (the side to which their head turns), (chi-square  =  3.57, p  =  0.059). Discussion: The finding of slower saccadic reaction times in cervical dystonia does not support the concept of reduced inhibition in the nigro-tectal pathway. Further research is required to confirm the observed relationship between the lateralization of lower frequencies of express saccades and direction of head rotation in cervical dystonia.