Department of Neurological Neurosurgical and Behavioural Science, University of Siena, Siena 53100, Italy.
Brain. 2011 Mar;134(Pt 3):879-91. doi: 10.1093/brain/awr009.
The cerebellum is implicated in maintaining the saccadic subsystem efficient for vision by minimizing movement inaccuracy and by learning from endpoint errors. This ability is often disrupted in degenerative cerebellar diseases, as demonstrated by saccade kinetic abnormalities. The study of saccades in these patients may therefore provide insights into the neural substrate underlying saccadic motor control. We investigated the different extent of saccade dynamic abnormalities in spinocerebellar ataxia type 2 and late-onset cerebellar ataxias, genetically undefined and with prevalent cerebellar atrophy. Reflexive and voluntary saccades of different amplitude (10°-18°) were studied in seven patients with spinocerebellar ataxia 2, eight patients with late-onset cerebellar ataxia and 25 healthy controls. Quantitative analysis of saccade parameters and measures of saccade accuracy were performed. Detailed neurological, neurophysiological and magnetic resonance imaging assessment was obtained for each patient. Genetic and laboratory screening for spinocerebellar ataxias and other forms of late-onset cerebellar ataxias were also performed. A lower peak saccade velocity and longer duration was observed in patients with spinocerebellar ataxia 2 with respect to those with late-onset cerebellar ataxia and controls. Unlike subjects with spinocerebellar ataxia 2, patients with late-onset cerebellar ataxia showed main sequence relationships to similar saccades made by normal subjects. Saccades were significantly more inaccurate, namely hypometric, in late-onset cerebellar ataxia than in spinocerebellar ataxia 2 and inaccuracy increased with saccade amplitude. The percentage of hypometric primary saccades and of larger secondary corrective saccades were consistently higher in late-onset cerebellar ataxia than in spinocerebellar ataxia 2 and controls. No other significant differences were found between groups. Two different mechanisms were adopted to redirect the fovea as fast and/or accurately as possible to peripheral targets by the two groups of cerebellar patients. Patients with spinocerebellar ataxia 2 maintained accuracy using slow saccades with longer duration. This reflects prevalent degenerative processes affecting the pontine burst generator and leading to saccade velocity failure. On the other hand, patients with late-onset cerebellar ataxia reached the target with a number of fast inaccurate, mostly hypometric saccades. Different degrees of cerebellar oculomotor vermis involvement may account for differences in optimizing the trade-off between velocity and accuracy in the two groups. In addition, as suggested by spinocerebellar patients having slow saccades that are no longer ballistic, visual feedback might be continuously available during the movement execution to guide the eye to its target.
小脑被认为通过最小化运动的不准确性和从终点误差中学习，来维持视觉的扫视子系统的效率。这种能力在退行性小脑疾病中经常受到干扰，这可以从扫视运动异常中得到证明。因此，研究这些患者的扫视可以提供有关扫视运动控制的神经基础的见解。我们研究了不同程度的脊髓小脑共济失调 2 型和迟发性小脑共济失调患者的扫视动力学异常，这些患者的遗传不明确，且以明显的小脑萎缩为特征。在七名脊髓小脑共济失调 2 型患者、八名迟发性小脑共济失调患者和 25 名健康对照者中，研究了不同幅度（10°-18°）的反射性和自主性扫视。对扫视参数和扫视准确性进行了定量分析。对每位患者进行了详细的神经、神经生理学和磁共振成像评估。还对脊髓小脑共济失调和其他形式的迟发性小脑共济失调患者进行了遗传和实验室筛查。与迟发性小脑共济失调患者和对照组相比，脊髓小脑共济失调 2 型患者的扫视峰值速度较低，持续时间较长。与脊髓小脑共济失调 2 型患者不同，迟发性小脑共济失调患者的主序列关系与正常受试者进行的类似扫视相似。与脊髓小脑共济失调 2 型患者相比，迟发性小脑共济失调患者的扫视准确性显著降低，即扫视幅度较小，且扫视幅度越大，准确性越低。迟发性小脑共济失调患者的原发性扫视幅度较小和较大的继发性校正扫视的百分比始终高于脊髓小脑共济失调 2 型患者和对照组。两组之间没有发现其他显著差异。两组小脑患者采用不同的机制来尽可能快速和/或准确地将注视点重定向到外周目标。脊髓小脑共济失调 2 型患者通过持续时间较长的缓慢扫视来保持准确性。这反映了普遍的退行性过程影响桥脑爆发发生器，导致扫视速度失败。另一方面，迟发性小脑共济失调患者通过多次快速不准确、主要是幅度较小的扫视来达到目标。不同程度的小脑眼球运动小脑蚓部受累可能解释了两组之间在速度和准确性之间的权衡优化的差异。此外，正如脊髓小脑共济失调患者的缓慢扫视不再是弹道的那样，在运动执行过程中可能会不断提供视觉反馈，以引导眼睛到达目标。
