Effects of subthalamic deep brain stimulation on fixational eye movements in Parkinson's disease

The reduction of LEDD leads to visual dysfunction in patients with PD after STN-DBS: a randomized clinical trial

BACKGROUND

Medication adjustment after deep brain stimulation (DBS) of the subthalamic nucleus (STN) in patients with Parkinson's disease (PD) may influence visual function. However, no clinical trials have been designed specifically to investigate this effect.

OBJECTIVES

To compare the effects of levodopa-equivalent daily dose (LEDD) reduction and non-reduction on visual function in patients with Parkinson's disease (PD) following STN-DBS.

METHODS

This was a multi-center, prospective, randomized, double-blinded clinical trial. A total of 208 patients with Parkinson's disease were referred for DBS between June 2019 and July 2021 and analyzed between June 2023 and July 2023. STN-DBS was performed, and the LEDD was reduced in one study arm but not in the other. The primary outcome measure was visual impairment in Parkinson's disease questionnaire (VIPD-Q) with or without LEDD reduction 12 months postoperatively, and the secondary outcomes included retinal nerve fiber layer (RNFL) thickness, vessel density, eye-tracking system results, contrast sensitivity and visual field.

RESULTS

During the short-term follow-up, DBS implantation and stimulation did not significantly affect visual function (VIPD-Q, baseline vs. 1 month, 9.269±8.385 vs. 8.938±7.666, Mann-Whitney U tests; P=0.6746). In the long-term follow-up, the reduction group demonstrated a significant decline in visual function, RNFL thickness, and vessel density after STN-DBS compared with the control group without STN-DBS (P<0.001).

CONCLUSIONS

Visual dysfunction, particularly a thinner RNFL and lower vessel density, is related to LEDD reduction after STN-DBS. Prolonged administration of dopamine-mimetic drugs prevents visual symptoms. Thus, physicians should consider LEDD adjustment when patients report visual dysfunction before surgery or severe visual symptoms after STN-DBS.

Assessing inter-ocular fixational eye movements throughout the lifespan

This study aims to assess fixational eye movements (FEMs) obtained under binocular and monocular viewing in normal individuals across different age groups. We recruited 68 healthy participants divided into Group 1 (children, 3-9 years, n = 20), Group 2 (adolescents, 10-19 years, n = 26), and Group 3 (adults, 20-73 years, n = 22). FEMs were collected using a high-resolution video-based tracker under 3 viewing conditions: binocular viewing (BV), monocular viewing right eye (MV_RE), and monocular viewing left eye (MV_LE). We quantified fixation stability, the frequency, amplitude, and disconjugacy of fixational saccades, and inter-saccadic drift velocity in BV, MV_RE, and MV_LE. We also computed inter-ocular fixation stability under binocular viewing and monocular viewing in the 3 groups. Fixation instability (FI) and fixational saccade amplitudes were higher in Group 1 than in Group 3 whereas inter-saccadic drifts were increased in Group 3. Vergence stability was greater in binocular viewing than in monocular viewing likely due to binocular summation in all groups. However, the fixational saccade amplitude and drift velocity of the right and left eye did not significantly differ across different viewing conditions within each group. Interestingly, the inter-ocular fixation stability ratio and vergence stability showed no significant differences between the groups. In conclusion, FEMs differ across age groups but inter-ocular FEMs are immune to the effects of age and can be a valuable parameter while evaluating FEM abnormalities in diseases like amblyopia.

Naturalistic Eye Movement Tasks in Parkinson's Disease: A Systematic Review

Background

Eye tracking assessments in the laboratory have previously highlighted clear differences in eye movements between Parkinson's disease (PD) and healthy aging. However, laboratory-based eye movement tasks are artificial and limit the ecological validity of observed results. Eye movement tasks utilizing more naturalistic scenarios may provide more accurate insight into cognitive function but research in this area is limited.

Objective

This systematic review aims to ascertain what naturalistic tasks have revealed about oculomotor deficits in PD and what this information may help us understand about the underlying sensorimotor and cognitive processes.

Methods

Adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement, a literature search of PsycInfo, Medline, Scopus, and Web of Science was conducted using predetermined search terms. Articles including both individuals with PD and healthy older adults completing eye tracking tasks involving naturalistic eye movements (e.g., reading, video-watching, unrestricted visual search) or naturalistic stimuli were included.

Results

After screening, 30 studies were identified as matching the inclusion criteria. Results revealed consistent findings across tasks, including longer fixation durations and smaller saccadic amplitudes in PD compared to healthy aging. However, inconsistencies in the literature and a lack of standardization in tasks limit interpretation of these results.

Conclusions

Naturalistic eye movement tasks highlight some consistent differences in eye movements between people with PD and healthy aging. However, future research should expand the current literature in this area and strive towards standardization of naturalistic tasks that can preferably be conducted remotely.

Eye movements in Parkinson's disease: from neurophysiological mechanisms to diagnostic tools

Movement disorders such as Parkinson's disease (PD) impact oculomotor function - the ability to move the eyes accurately and purposefully to serve a multitude of sensory, cognitive, and secondary motor tasks. Decades of neurophysiological research in monkeys and behavioral studies in humans have characterized the neural basis of healthy oculomotor control. This review links eye movement abnormalities in persons living with PD to the underlying neurophysiological mechanisms and pathways. Building on this foundation, we highlight recent progress in using eye movements to gauge symptom severity, assess treatment effects, and serve as potential precision biomarkers. We conclude that whereas eye movements provide insights into PD mechanisms, based on current evidence they appear to lack sufficient sensitivity and specificity to serve as a standalone diagnostic tool. Their full potential may be realized when combined with other disease indicators in big datasets.

Monitoring Eye Movement in Patients with Parkinson's Disease: What Can It Tell Us?

Parkinson's disease (PD) affects approximately 10 million individuals worldwide. Visual impairments are a common feature of PD. Patients report difficulties with visual scanning, impaired depth perception and spatial navigation, and blurry and double vision. Examination of PD patients reveals abnormal fixational saccades, strabismus, impaired convergence, and abnormal visually-guided saccades. This review aims to describe objective features of abnormal eye movements in PD and to discuss the structures and pathways through which these abnormalities may manifest.

Amblyopia and fixation eye movements

Amblyopia is a neurodevelopmental disorder caused by abnormal visual experience in early life that affects 3-5% of the population. Amblyopia results in a host of monocular and binocular visual afferent function deficits including reduced visual acuity, contrast sensitivity, depth perception, interocular suppression, and efferent function abnormalities such as unstable and inaccurate fixation. Conventional treatments such as patching therapy and newer dichoptic treatments are not always successful as 30-40% of patients experience recurrence/regression of amblyopia. There are numerous review articles focused on visual afferent function deficits and treatment modalities and outcomes in amblyopia. Recently, the advent of high spatial and temporal resolution eye trackers has spurred studies on fixation eye movements (FEMs) in healthy controls and neurologic and ophthalmic disorders. In this focused review, we will summarize studies evaluating FEM abnormalities in amblyopia. We will first describe the common devices and techniques used to quantify fixation abnormalities, and then highlight the importance of systematically evaluating the eye movements under different viewing conditions and describe the parameters crucial in assessing FEM abnormalities in amblyopia. We will summarize the evidence suggesting that FEM abnormalities are not limited to the amblyopic eye only but also affects the fellow eye and that FEM abnormalities can serve as biomarkers to predict the impact of amblyopia on visual functions. Beyond diagnosis, we will discuss the treatment and prognostic implications of the evaluation of FEM abnormalities in clinical practice.