Visual symptoms in Parkinson's disease
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.253603
Source of Support: None, Conflict of Interest: None
'Why do people see things that do not exist'
The various visual disorders seen in Parkinson's disease (PD) are impaired visual acuity, impaired 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. The common symptoms revealed by PD patients are difficulty in reading and double vision, illusions, feelings of presence and passage, and complex visual hallucinations (CVH). We, however, tend to group presence, passage, and illusions as 'hallucinations' in PD patients. Their pathophysiology appears to be different, as similar experiences are reported in the general populations as well, particularly in the 'sleep-wake' transition phase or in brainstem disorders and narcolepsy.
Visual acuity (VA), contrast sensitivity (CS), color perception, and motion perception are all impaired in PD, with VA and CS identified as risk factors for CVH.
The prodromal features of PD may include autonomic system involvement affecting pupil reactivity, abnormal colour vision, abnormal stereopsis with postural instability, defects in smooth pursuit eye movements, and insufficiencies in visuo-motor adaptation. PD dementia is associated with worsening of many ocular and visual problems involving eye movements, visuo-spatial function, and visual hallucinations.
Visual misperception and hallucinations are seen in more than 50% of PD patients in advanced stages of the disease. The common symptoms reported by PD patients are vivid dreams, visual misperceptions and hallucinations, which may or may not lead to frank psychosis and dementia. Visual misperceptions are the images misinterpreted in presence of the stimulus, e.g., any specks on the wall are seen as ants or insects. Visual hallucinations are the visual images or perceptions in the absence of the stimuli.
The occurrence of visual misperceptions and visual hallucinations is probably due to inability of the neural circuits to integrate information. Alteration in the attention and sleep-wake cycle may be the cause of these visual phenomenon. There have been postulations that these may be rapid eye movement (REM) sleep dream intrusions in the awake state. However, the polysomnography recordings failed to demonstrate such intrusions in PD patients.
PD patients with a poor vision report more visual misperceptions than with normal vision. The possibility of loss of dopamine in the retinal cells with combination of dopaminergic overstimulation of these sensitized cells may contribute to the poor visual discrimination and subsequent visual misperception and visual hallucinations.
Patients with eye disease report recurrent CVH, with an incidence from less than 1% to above 10%, depending on the selection and exclusion criteria. Patients with diffuse Lewy body disease (DLBD) with particularly high rates of CVH have visual hallucinations as one of the possible diagnostic criteria. The acquired eye disease, occipital stroke, and sensory deprivation, with reduced visual input, however, have low rates of CVH ranging from 3– 18%. Patients with disturbed consciousness, as seen in narcolepsy and delirium, and patients with dementia with Lewy bodies, PD with dementia, and vascular dementia, and schizophrenia have high rates of CVH, ranging from 30–59%.
In contrast, simple hallucinations are relatively frequent in sensory deprivation and eye disease, but rare in dementia, delirium, and PD. This double dissociation between simple and complex hallucinations suggests two things to us. First, that each type of hallucination has a single primary cause, and second, that these causes are separable within this complex system of perception of vision.
Although CVHs are linked to cognitive decline in PD, the association between cognition and other visual phenomena, such as illusions, presence, and passage, has not been specifically addressed.
In the study by Archibald et al., on PD patients, diplopia was seen in 38% of the total patients with PD and PD dementia. The PD duration, the Epworth Sleepiness Scale (ESS) scores, abnormal ocular alignment, and hypometric saccades were independent factors predictive of diplopia. The CVH group consisted of 38% of the total PD/PD dementia group. Three independent factors that were predictive of CVH were the diagnosis of dementia, the Beck Depression Inventory-II (BDI-2) score, and 'best at presentation' visual acuity (BAPVA). The diagnosis of dementia was the strongest individual predictor. Illusions were seen in 37%, presence in 40%, and passage in 54% patients. Independent factors for illusion were the ESS and the Unified Parkinson's Disease Rating Scale (UPDRS) III scores. The present state had the REM (rapid eye movement) sleep behavior disorder (RBD) and ESS scores as independent predictors. None of these variables contributed to a model predictive of sensations of passage.
Results from neuropathological, structural, and functional imaging studies indicate that the occipitotemporal, occipitoparietal, limbic, and frontoparietal dysfunctions are likely to be vital in the pathophysiology of CVH.
In contrast, measures of daytime somnolence and the presence of RBD contributed to models predictive of illusions and presence, suggesting that these visual experiences may be influenced by brainstem regions involved in sleep regulation and arousal.
In the current study, PD patients as compared to controls, had a higher total visual misperception [VM] score and misperceptions of bistable percepts (VMB) compared to healthy controls. However, misperceptions for monostable percepts (VMS) and missed images were similar in cases and controls. PD patients who were taking a longer time to complete the trail making test parts A and B (TMT A and B) had increased frequency of VMs. An inverse correlation was also observed between the mini-mental state examination (MMSE) and VMs, and between the frontal assessment battery (FAB) scores and visual misperceptions. One unit decrease in the FAB and MMSE score increased VM score by 1.226 and 1.304, respectively. In the control group, TMT-A was a significant predictor for VMs.
There was also a correlation of VMs with disease duration, treatment duration, UPDRS-III, and Hoehn and Yahr stage in PD patients. Levodopa equivalent dose (LEDD) and visual misperceptions did not show any significant correlation.
The subgroup with “impaired visual perception” had a longer disease and treatment duration, higher mean UPDRS-III scores, and advanced modified Hoehn and Yahr stages compared to the patient subgroup with normal visual perception. No significant difference for LEDD was observed between the two subgroups. There was no significant difference in the MMSE scores of the two subsets of patients (P = 0.199).
However, the two subsets differed significantly with respect to TMT-A, TMT-B, TMT-B − A, and FAB scores. There was a significant difference in the identification of bistable percepts (VMB) between the two subsets. The mean VMB scores were 7.73 (±1.16) and 5.25 (±1.04) for those with impaired visual perception (VMES >6.75) and normal visual perception (visual misperception error score [VMES] ≤6.75), respectively (P value <0.0005). VMS and missed images were not different for the two subsets of patients. The risk of rise in VMs increased significantly with an increased disease duration, treatment duration, and scores on UPDRS, H and Y scale, TMTAs, TMTBs, TMT-B − A, and FAB.
The questions that arise include the following: Are these visual symptoms characteristic of the prodromal phase of PD?; and, is PD dementia associated with specific visual changes? Oculomotor and visual dysfunction in PD can involve the 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 rapid eye movement (REM) sleep behaviour disorder. PD dementia is associated with the exacerbation of impaired eye movements, visuo-spatial function, and visual hallucinations.
PD patients can exhibit significant non-motor symptoms including apathy, depression, sleep disturbances, cognitive impairment, dementia, and autonomic, gastrointestinal, and sensory dysfunction. Sensory symptoms may include visual dysfunction, eye movement abnormalities, loss of smell, auditory problems, pain and 'restless legs' syndrome. They have the inability to judge distances or the shape of an object. Visual deficits in PD may affect motor disability. They are also risk factors for developing hallucinations and can have a significant influence on the general quality of life.
Colour vision dysfunction in PD may be disease specific. PD patients frequently exhibit higher error scores than controls after age adjustment using the Farnsworth-Munsell 100-hue test. Frequency of errors increase with the severity of motor symptoms. Defective colour vision may also be an early sign of dopamine dysfunction in PD. However, the pathophysiology of colour discrimination deficits in PD is likely to be complex. Colour discrimination deficits in PD worsen with cognitive decline. In addition, the green light improved gait and reduced freezing of gait better than red or no light.
Abnormal saccadic and smooth pursuit eye movements are seen in about 75% of PD patients. Saccadic eye movements may be hypometric 'under reaching of task', while smooth pursuit movements interrupted by small additional saccades reveal saccadic pursuits that are necessary before reaching the final eye position and the patients also exhibited an increase in errors while remembering the target sequence.
Impairment of depth perception in PD is associated with impaired VA and colour perception (stereopsis) and is due to the involvement of extrastriate cortical areas, neural pathways involving the thalamus and the posterior parietal lobes, which are also areas affected in PD dementia.
Perceptual abnormalities in PD may result from dopamine dysfunction in the retina or can be attributable to deficiencies in attention due to dysfunction of the striato-frontal system. Hence, mild PD is associated with little perceptual abnormality, moderate PD with attention deficits, and more severe PD with both types of abnormality. This suggests that retinal influence on perception occur later in the disease. In addition, the presence of visuo-perceptual impairment could be an indication of developing PD dementia.
Visual hallucinations are seen in PD patients especially when treated with L-dopa, dopamine agonists and with PD dementia. In this study, hallucinations were seen in 40% patients, being visual in 22% and auditory in 10% of patients. Minor hallucinations were associated with higher depression scores. Hallucinations were associated with severe cognitive defects, daytime somnolence, a longer duration of disease, poor vision, and reduced activity in the primary visual cortex.
Many of the oculo-visual features present in early and middle stage PD will become more severe if the patient develops PD dementia. Deficits in colour vision and changes in pupillary function are often seen in PD dementia. Prominent visual hallucinations are significantly more frequent in PD dementia than in PD alone. Severe eye movement problems are more likely to be present in PD dementia and become more extensive with declining cognitive function. Defects in visuospatial orientation are likely to be greater in PD dementia, especially when associated with greater cortical atrophy.
Separating PD from dementia with Lewy bodies DLB was thought to be important because patients with visual hallucinations may be treated with antipsychotic drugs, which may exacerbate symptoms in DLB. In addition, DLB patients were thought to exhibit fewer tremors, more asymmetry of motor symptoms, more falls, and respond less well to dopamine treatment than those with PD. DLB and PD patients, however, exhibit similarities on a variety of saccadic eye movement tasks. Deficits in orientation, 'trail-making', and reading the names of colours ('Stroop test') suggest DLB rather than PD. Visual perception tasks (visual discrimination, space-motion and object-form recognition), however, are usually equally impaired in DLB and PD, especially in patients with visual hallucinations.
In conclusion, PD affects vision and eye movements in the form of impaired visual acuity, colour discrimination, pupillary reactivity, eye movements, movement perception, and visual processing speeds. Visuo-spatial orientation, facial recognition difficulties, and CVH may also be present. The possible prodromal features of PD include autonomic system dysfunction (which could affect pupil reactivity), abnormal colour vision and stereopsis associated with postural instability; and abnormal smooth pursuit eye movement function. PD dementia is associated with the escalation of many of the visual difficulties present in PD but those involving eye movements, visuo-spatial function and CVH appear most specific.