Misdirected horizontal saccades in pan-cerebellar atrophy

Static and Dynamic Ocular Motor Abnormalities as Potential Biomarkers in Spinocerebellar Ataxia Type 3

While dynamic ocular motor abnormalities (e.g., gaze-evoked nystagmus (GEN), low optokinetic nystagmus (OKN), pursuit and vestibulo-ocular reflex (VOR) gains, and dysmetric saccades) have been shown to be potential biomarkers in spinocerebellar ataxia type 3 (SCA3), the value of static abnormalities (e.g., convergent [esodeviation] and divergent strabismus [exodeviation]) is unknown. Moreover, studies on dynamic abnormalities in SCA3 usually do not take into account the existence of potential abduction-adduction asymmetries in patients with degenerative ataxia. Thirty-eight patients with genetically confirmed SCA3 (24 females; mean age ± SD, 49.8± 12.2 years) and 22 healthy controls (12 females, p = 0.589; mean age ± SD, 50.7± 12.5 years, p = 0.651) underwent clinical and video-oculographic assessment. A p value < 0.002 (between- and within-group analyses) and < 0.001 (correlation analysis) was considered significant. Patients showed larger esodeviation at distance (p < 0.001), became more esodeviated in lateral gaze (p < 0.001), and their near exodeviation correlated with scale for the assessment and rating of ataxia (SARA) score (p = 0.004). Pursuit, OKN, and VOR gains were lower in patients, both for their adducting and abducting components (p < 0.001). Saccades showed higher velocities (p < 0.001), abducting saccades showed lower amplitude (p < 0.001), and adducting saccades tended to show greater vertical bias (p = 0.018) in patients. Abducting saccades showed relatively lower velocity (p < 0.001) and lower amplitude (p = 0.015) than abducting saccades within patients. All dynamic ocular motor abnormalities except saccades correlated with SARA score, CAG repeat number, and/or disease duration (p < 0.001). Static and dynamic ocular motor abnormalities are potential biomarkers in SCA3. SCA3 studies using saccades should take into account the existence of potential abduction-adduction asymmetries.

Impaired Saccade Adaptation in Tremor-Dominant Cervical Dystonia-Evidence for Maladaptive Cerebellum

We examined the role of the cerebellum in patients with tremor-dominant cervical dystonia by measuring the adaptive capacity of rapid reflexive eye movements (saccades). We chose the saccade adaptation paradigm because, unlike other motor learning paradigms, the real-time modification of saccades cannot "wait" for the sensory (visual) feedback. Instead, saccades rely primarily on the internal reafference modulated by the cerebellum. The saccade adaptation happens over fast and slow timescales. The fast timescale has poor retention of learned response, while the slow timescale has strong retention. Cerebellar defects resulting in loss of function affect the fast timescale but the slow timescale of saccade adaptation is retained. In contrast, maladaptive cerebellar disorders feature the absence of both fast and slow timescales. We were able to measure both timescales using noninvasive oculography in 6 normal individuals. In contrast, both timescales were absent in 12 patients with tremor-dominant cervical dystonia. These findings are consistent with maladaptive cerebellar outflow as a putative pathophysiological basis for tremor-dominant cervical dystonia.

Saccades in progressive supranuclear palsy - maladapted, irregular, curved, and slow

BACKGROUND AND OBJECTIVES

Slowed and curved rapid eye movements, saccades, are the well-known features of progressive supranuclear palsy (PSP). We hypothesized that the saccades in PSP are not only slow and curved, but they are also irregular and have timing deficits.

METHODS

We tested this hypothesis in 12 patients with PSP by measuring vertical and horizontal visually guided saccades using a limbus tracker.

RESULTS

Both, horizontal and vertical saccades were slow and had irregular trajectory and velocity profiles, but deficits were much more robust in vertical saccades. The irregularity in the saccade velocity was due to premature interruptions that either completely stopped the eyes, or moved the eyes at much slower velocity along or in the opposite direction of the ongoing saccade. The direction of the eyes' trajectory was often changed after the interruption. We simulated a conductance based single-compartment model of the burst neurons embedded in local feedback circuit for saccade generation. This model mimicked anatomical and physiological realism, while allowing the liberty to selectively change the activation of individual burst neurons or the pause neurons. The PSP saccades were comparable to the simulations during reduced activity of the inhibitory and excitatory burst neurons.

CONCLUSION

PSP saccades are due to the paucity in burst generation at the excitatory and imprecise timing signal from the inhibitory burst neurons. Premature discharge of the inhibitory burst neuron further leads to breaks in the saccade trajectory, and maladaptive superior colliculus activity leading to aberrant saccades changing the intended trajectory of the ongoing saccade.