Parkinson’s disease and visual impairment

Table of contents

Parkinson’s is a slow-moving but progressive neurodegenerative disease that involves certain functions such as movement control and balance.

The disease belongs to a group of pathologies referred to as ‘Movement Disorders’; of these, it is the one with the highest incidence.

The cause of the disease is linked to the degeneration of dopaminergic neurons in the substantia nigra mesencephalic, resulting in a motor deficit of the nigro-striatal pathway.

James Parkinson is the 19th-century London physician who first reported most of the symptoms of Parkinson’s in his ‘Treatise on Agitating Paralysis’.

The disease is widespread throughout the world and affects all ethnic groups. The disease has a slight prevalence in the male sex and the average age at which the first symptoms appear is around 58-60 years.

Importantly, around 5% of patients may present with juvenile-onset, the age range being 21-40 years.

If before the age of 20, it is extremely rare for Parkinson’s to occur, for a group over the age of 60 this disease affects 1-2% of the population (3-5% when over the age of 85).

The symptoms of Parkinson’s

A resting tremor on one hand is often the first symptom of what is an extremely insidious onset of the disease. This tremor is usually slow and coarse, with a maximum point at rest, but decreases during movement.

Some characteristics are:

• emotions and fatigue can amplify the tremor;
• the limbs may be asymmetrically involved;
• both the tongue and the jaw can be involved in this type of tremor, which can, however, diminish as the disease progresses;
• bradykinesia (slow movements) is a typical condition of Parkinson’s disease; as the disease progresses, there is an increasing decrease in the amplitude of movements (akinesia);
• the rigidity associated with hypokinesia can induce muscle fatigue and a feeling of pain in the muscles;
• hypomimia (reduced facial expressiveness);
• sialorrhoea and dysarthria, sometimes stuttering, are other conditions that may arise;
• the patient may develop tremulous and small handwriting (micrographia) in an attempt to control rigidity.

Postural instability occurs later. Patients have difficulty initiating movements, staggering and taking small steps. Posture with hunched shoulders is associated with the destination, which may precede freezing of walking in later stages.

Non-motor symptoms

Non-motor symptoms include:

• dementia, a condition that in the advanced stages of Parkinson’s develops in about a third of patients
• sleep disorders
• depression.

Here is a video published by Azienda ULSS2 – Marca trevigiana that illustrates the non-motor symptoms.

Neurological symptoms

Neurological symptoms develop due to synucleinopathy occurring in other areas of the central nervous system, and consist of:

• orthostatic hypotension, related to cardiac sympathetic denervation;
• dysphagia, related to impaired oesophageal motility;
• constipation, associated with intestinal dysmotility;
• bladder overactivity;
• anosmia.

In some patients, these symptoms, in addition to progressing over time, may act as an anticipator of motor symptoms.

The Visual System

Patients with Parkinson’s disease have a high probability of developing visual deficits.

Decreased visual abilities can be triggered by a decrease in dopaminergic innervation in the retina.

Among the various problems that can occur are:

• visual-spatial deficits
• altered color sensitivity;
• decreased oculomotor control of the extrinsic eye muscles.

Parkinson’s disease and visual disturbances: there is often an underestimation of ophthalmological symptoms

Alterations in visual function are caused by an excess of alpha-synuclein which results in the death of dopaminergic cells in the retina.

Through the administration of levodopa (a dopamine-producing drug that is prescribed to correct motor deficits in patients with Parkinson’s disease), the visual and electrophysiological defects can be recovered.

The retina is thus identified as a tool for early diagnosis of Parkinson’s and other neurodegenerative diseases.

According to the study Seeing ophthalmologic problems in Parkinson’s disease – Neurology² these problems are more common among people with Parkinson’s:

• problems adapting to rapid changes in light;
• hallucinations;
• problems with depth perception;
• visual and ocular problems (e.g. dry eyes).

Parkinson’s and vision disorders: diplopia

Diplopia (or double vision) is one of the symptoms reported in the literature. This particular defect is triggered by a dysfunction of the oculomotor pathways in the brain stem, frontal lobes, and basal ganglia.

Some of these problems may result from a decrease in dopamine or cortical involvement within the neurological degeneration process caused by Parkinson’s disease.

A study published in Frontiers in Neurology ¹ mentions other eye disorders related to Parkinson’s disease are:

• hypsometric saccadic movements
• slow pursuit
• convergence insufficiency.

In these cases, patients with Parkinson’s disease need to request an ophthalmic and orthoptic assessment, aimed at analyzing and, if necessary, rehabilitating symptoms that affect the normal performance of everyday activities.

Visual disturbances in Parkinson’s disease: the saccadic system deficit

Saccadic movements are voluntary, rapid movements of the eyes whose task is to move points in the fovea of the environment we are observing.

The execution of saccades is associated with the activity of 3 main areas:

• superior colliculus;
• frontal ocular fields;
• oculomotor nuclei of the brainstem.

At the level of the brainstem nuclei two main classes of neurons are fundamental for saccadic movement:

• Pause cells: these are activated when the eyes are stationary, i.e. during fixation (WHEN system).
• Burst cells: they are activated before and during the saccade (WHERE system).

The centers of the brain stem are mainly the paramedian pontine reticular substance and the mesencephalic reticular substance.

These two centers ‘construct’ the typical saccadic motor packet to be sent to the ocular motor neurons based on end-position specifications from other nerve centers. In the centers of the superior colliculus, however, some important visuomotor transformations occur.

Selective lesions to these neural structures impair the ability to generate rapid eye movements to varying degrees.

In Parkinson’s disease, the saccades are therefore hypermetric and associated with discontinuous and fragmented pursuit movements in a series of saccades.

Saccades account play a role for about 10% of the time spent reading; therefore, targeted saccadic, reading speed, and visuospatial component rehabilitation can help.

Therapy and Management of Parkinson’s Disease

To date, there is no cure for Parkinson’s disease; however, several treatments can act in an attempt to control its symptoms. The goal is to restore dopamine levels to allow better brain function.

Several specialists are involved in the management of Parkinson’s patients such as neurologists, physiatrists, neuropsychologists, physiotherapists, speech therapists, and nurses.

Medications that are usually administered include L-dopa, dopamine agonists, anticholinergics, amantadines, selegiline and COMT inhibitors.

Surgery may be considered for patients in whom the symptoms of the disease cannot be adequately controlled with drug treatment.

The most effective intervention available today is Deep Brain Stimulation (DBS), which consists of the insertion of a lead associated with a neurostimulator placed at the level of the subthalamic nucleus.

Bibliography

1. Vergence and Strabismus in Neurodegenerative Disorders — PubMed (nih.gov) (Sarah L. Kang, Aasef G. Shaikh, Fatema F. Ghasia) in Front. Neurol., 16 May 2018
2. Seeing ophthalmologic problems in Parkinson disease | Neurology (Carlijn D.J.M. Borm et al.) in “Neurology”, n. 94, 11 Marzo 2020.
3. Parkinson: non trascurate i problemi alla vista — IAPB Italia Onlus: Agenzia Internazionale per la Prevenzione della Cecità