Self-paced saccades and saccades to oddball targets in Parkinson's disease

Dopamine, Alpha-Synuclein, and Mitochondrial Dysfunctions in Parkinsonian Eyes

Parkinson’s disease (PD) is characterized by motor dysfunctions including bradykinesia, tremor at rest and motor instability. These symptoms are associated with the progressive degeneration of dopaminergic neurons originating in the substantia nigra pars compacta and projecting to the corpus striatum, and by accumulation of cytoplasmic inclusions mainly consisting of aggregated alpha-synuclein, called Lewy bodies. PD is a complex, multifactorial disorder and its pathogenesis involves multiple pathways and mechanisms such as α-synuclein proteostasis, mitochondrial function, oxidative stress, calcium homeostasis, axonal transport, and neuroinflammation. Motor symptoms manifest when there is already an extensive dopamine denervation. There is therefore an urgent need for early biomarkers to apply disease-modifying therapeutic strategies. Visual defects and retinal abnormalities, including decreased visual acuity, abnormal spatial contrast sensitivity, color vision defects, or deficits in more complex visual tasks are present in the majority of PD patients. They are being considered for early diagnosis together with retinal imaging techniques are being considered as non-invasive biomarkers for PD. Dopaminergic cells can be found in the retina in a subpopulation of amacrine cells; however, the molecular mechanisms leading to visual deficits observed in PD patients are still largely unknown. This review provides a comprehensive analysis of the retinal abnormalities observed in PD patients and animal models and of the molecular mechanisms underlying neurodegeneration in parkinsonian eyes. We will review the role of α-synuclein aggregates in the retina pathology and/or in the onset of visual symptoms in PD suggesting that α-synuclein aggregates are harmful for the retina as well as for the brain. Moreover, we will summarize experimental evidence suggesting that the optic nerve pathology observed in PD resembles that seen in mitochondrial optic neuropathies highlighting the possible involvement of mitochondrial abnormalities in the development of PD visual defects. We finally propose that the eye may be considered as a complementary experimental model to identify possible novel disease’ pathways or to test novel therapeutic approaches for PD.

Eye Movement Evaluation in Multiple Sclerosis and Parkinson's Disease Using a Standardized Oculomotor and Neuro-Ophthalmic Disorder Assessment (SONDA)

Evaluating the state of the oculomotor system of a patient is one of the fundamental tests done in neuro-ophthalmology. However, up to date, very few quantitative standardized tests of eye movements' quality exist, limiting this assessment to confrontational tests reliant on subjective interpretation. Furthermore, quantitative tests relying on eye movement properties, such as pursuit gain and saccade dynamics are often insufficient to capture the complexity of the underlying disorders and are often (too) long and tiring. In this study, we present SONDA (Standardized Oculomotor and Neurological Disorder Assessment): this test is based on analyzing eye tracking recorded during a short and intuitive continuous tracking task. We tested patients affected by Multiple Sclerosis (MS) and Parkinson's Disease (PD) and find that: (1) the saccadic dynamics of the main sequence alone are not sufficient to separate patients from healthy controls; (2) the combination of spatio-temporal and statistical properties of saccades and saccadic dynamics enables an identification of oculomotor abnormalities in both MS and PD patients. We conclude that SONDA constitutes a powerful screening tool that allows an in-depth evaluation of (deviant) oculomotor behavior in a few minutes of non-invasive testing.

Visual Dysfunction in Parkinson's Disease

This chapter describes the visual problems likely to be encountered in Parkinson's disease (PD) and whether such signs are useful in differentiating the parkinsonian syndromes. Visual dysfunction in PD may involve visual acuity, contrast sensitivity, color discrimination, pupil reactivity, saccadic and pursuit eye movements, motion perception, visual fields, and visual processing speeds. In addition, disturbance of visuospatial orientation, facial recognition problems, rapid eye movement (REM) sleep behavior disorder, and chronic visual hallucinations may be present. Problems affecting pupil reactivity, stereopsis, pursuit eye movement, and visuomotor adaptation, when accompanied by REM sleep behavior disorder, could be early features of PD. Dementia associated with PD is associated with enhanced eye movement problems, visuospatial deficits, and visual hallucinations. Visual dysfunction may be a useful diagnostic feature in differentiating PD from other parkinsonian symptoms, visual hallucinations, visuospatial dysfunction, and variation in saccadic eye movement problems being particularly useful discriminating features.

Oculo-Visual Dysfunction in Parkinson's Disease

This review describes the oculo-visual problems likely to be encountered in Parkinson's disease (PD) with special reference to three questions: (1) are there visual symptoms characteristic of the prodromal phase of PD, (2) is PD dementia associated with specific visual changes, and (3) can visual symptoms help in the differential diagnosis of the parkinsonian syndromes, viz. PD, progressive supranuclear palsy (PSP), dementia with Lewy bodies (DLB), multiple system atrophy (MSA), and corticobasal degeneration (CBD)? Oculo-visual dysfunction in PD can involve visual acuity, dynamic contrast sensitivity, colour discrimination, pupil reactivity, eye movement, motion perception, and visual processing speeds. In addition, disturbance of visuo-spatial orientation, facial recognition problems, and chronic visual hallucinations may be present. Prodromal features of PD may include autonomic system dysfunction potentially affecting pupil reactivity, abnormal colour vision, abnormal stereopsis associated with postural instability, defects in smooth pursuit eye movements, and deficits in visuo-motor adaptation, especially when accompanied by idiopathic rapid eye movement (REM) sleep behaviour disorder. PD dementia is associated with the exacerbation of many oculo-visual problems but those involving eye movements, visuo-spatial function, and visual hallucinations are most characteristic. Useful diagnostic features in differentiating the parkinsonian symptoms are the presence of visual hallucinations, visuo-spatial problems, and variation in saccadic eye movement dysfunction.

Ocular motor abnormalities in neurodegenerative disorders

Eye movements are a source of valuable information to both clinicians and scientists as abnormalities of them frequently act as clues to the localization of a disease process. Classically, they are divided into two main types: those that hold the gaze, keeping images steady on the retina (vestibulo-ocular and optokinetic reflexes) and those that shift gaze and redirect the line of sight to a new object of interest (saccades, vergence, and smooth pursuit). Here we will review some of the major ocular motor abnormalities present in neurodegenerative disorders.

Saccade reprogramming in Friedreich ataxia reveals impairments in the cognitive control of saccadic eye movement

Although cerebellar dysfunction has known effects on motor function in Friedreich ataxia (FRDA), it remains unclear the extent to which the reprogramming of eye movements (saccades) and inhibition of well-learned automatic responses are similarly compromised in affected individuals. Here we examined saccade reprogramming to assess the ability of people with FRDA to respond toward unexpected changes in either the amplitude or direction of an "oddball" target. Thirteen individuals with genetically confirmed FRDA and 12 age-matched controls participated in the study. The saccade reprogramming paradigm was used to examine the effect of an unpredictable "oddball" target on saccade latencies and accuracy when compared to a well-learned sequence of reciprocating movements. Horizontal eye movements were recorded using a scleral search coil eye tracking technique. The results showed a proportionally greater increase in latencies for reprogrammed saccades toward an oddball-direction target in the FRDA group when compared to controls. The FRDA group were also less accurate in primary saccade gain (i.e. ratio of saccade amplitude to target amplitude) when reprogramming saccades toward an unexpected change in direction. No significant group differences were found on any of the oddball-amplitude targets. Significant correlations were revealed between latency and disease severity as measured by the Friedreich Ataxia Rating Scale. These findings provide further support to the view that cognitive changes in FRDA may arise from disruption of cerebellar connections to cortical structures.

Ischaemic stroke: the ocular motor system as a sensitive marker for motor and cognitive recovery

OBJECTIVE

To evaluate the sensitivity of measuring cognitive processing in the ocular motor system as a marker for recovery of deficit in post stroke patients.

METHODS

15 patients (mean age 60.6 years, mean National Institutes of Health Stroke Scale (NIHSS) score 2.25) and 10 age matched control subjects (mean age 63.3 years) participated in the study. We included mildly affected acute stroke patients without a visual field defect or gaze palsy. Patients were examined at onset and at 1 month and 3 months post stroke by testing ocular motor function, NIHSS, modified Rankin Scale (mRS) and standard cognitive function assessments.

RESULTS

Significant differences were found in measures of ocular motor function between groups at stroke onset as well as between the first test and follow-up in patients. At 3 months, function had not returned to normal baseline. Ocular motor function was more sensitive in identifying cognitive dysfunction and improvement compared with NIHSS or mRS.

CONCLUSIONS

Standard neurological assessments of stroke patients are weighted significantly towards motor and sensory function, underestimating cognitive deficits. Ocular motor assessment demonstrates cognitive effects of even mild stroke and may provide improved quantifiable measurements of cognitive recovery post stroke. We demonstrated abnormality in patients just after onset, extending beyond 3 months, when there was apparent full recovery of motor and sensory function, implying more widespread disruption of cognitive mechanisms, consistent with the subjective complaints received from patients. This may provide insight into cognitive rehabilitation strategies leading to improved functional outcomes.

Movement orientation switching with the eyes and lower limb in Parkinson disease

Difficulty switching between motor programs is a proposed cause of motor blocks in Parkinson disease (PD). Switching from one movement to another has been studied in the upper extremity and during postural control tasks, but not yet in the eyes and lower limb in PD. The purpose of this study was to compare movement orientation switching ability between people with PD and age-matched controls (CON) and to determine if switching ability is correlated between the eyes and lower limb. Twenty-six persons with PD and 19 age-matched controls participated. Movement orientation switching was studied in a seated position with the head fixed in a chinrest. In response to a randomly generated tone, participants switched from a continuous back-and-forth movement in either the horizontal or vertical orientation to the opposite orientation as quickly as possible. Lower limb movements were performed with the great toe pointing back and forth between targets positioned on a 45° angled floor platform. Eye movements were back and forth between the same targets. Eye and lower limb switch time was reduced in PD (p<0.01), but after normalizing switch time to movement velocity, no differences existed between PD and CON. Eye and lower limb switch times were correlated in PD (r=0.513, p<0.01) but not in CON. In PD, switch time and movement velocity of the lower limb, but not the eyes, correlated with bradykinesia and postural instability/gait. Our results suggest that individuals with PD experience movement-switching deficits with both the eyes and lower limb, perhaps driven by overall bradykinesia.

Visual symptoms in Parkinson's disease

Parkinson's disease (PD) is a common disorder of middle-aged and elderly people in which degeneration of the extrapyramidal motor system causes significant movement problems. In some patients, however, there are additional disturbances in sensory systems including loss of the sense of smell and auditory and/or visual problems. This paper is a general overview of the visual problems likely to be encountered in PD. Changes in vision in PD may result from alterations in visual acuity, contrast sensitivity, colour discrimination, pupil reactivity, eye movements, motion perception, visual field sensitivity, and visual processing speeds. Slower visual processing speeds can also lead to a decline in visual perception especially for rapidly changing visual stimuli. In addition, there may be disturbances of visuospatial orientation, facial recognition problems, and chronic visual hallucinations. Some of the treatments used in PD may also have adverse ocular reactions. The pattern electroretinogram (PERG) is useful in evaluating retinal dopamine mechanisms and in monitoring dopamine therapies in PD. If visual problems are present, they can have an important effect on the quality of life of the patient, which can be improved by accurate diagnosis and where possible, correction of such defects.

The search for biomarkers in Parkinson's disease: a critical review

Parkinson's disease (PD) is a progressive neurodegenerative disorder that primarily presents with features of bradykinesia, rigidity and tremor, and has, as part of its core pathology, the degeneration of dopaminergic neurons in the substantia nigra pars compacta. There is a great need for the development of a reliable diagnostic tool to improve promptness of diagnosis, definition of disease subtypes, and to monitor disease progression and demonstrate treatment efficacy in the case of disease modifying therapies. Current biomarkers range from objective clinical tools, to neuroimaging, to 'wet' markers involving blood and cerebrospinal fluid. To date, all candidate biomarkers for PD have failed to be developed into a clinically useful tool. Ideally, a combination of sensitive markers will be needed, not only to predict the onset of PD, but also to help in subtype classification and to follow progression. Here, we critically review various PD biomarker studies.