Abstract
Maintaining proper eye alignment is necessary to generate a cohesive visual image. This involves the coordination of complex neural networks, which can become impaired by various neurodegenerative diseases. When the vergence system is affected, this can result in strabismus and disorienting diplopia. While previous studies have detailed the effect of these disorders on other eye movements, such as saccades, relatively little is known about strabismus. Here, we focus on the prevalence, clinical characteristics, and treatment of strabismus and disorders of vergence in Parkinson’s disease, spinocerebellar ataxia, Huntington disease, and multiple system atrophy. We find that vergence abnormalities may be more common in these disorders than previously thought. In Parkinson’s disease, the evidence suggests that strabismus is related to convergence insufficiency; however, it is responsive to dopamine replacement therapy and can, therefore, fluctuate with medication “on” and “off” periods throughout the day. Diplopia is also established as a side effect of deep brain stimulation and is thought to be related to stimulation of the subthalamic nucleus and extraocular motor nucleus among other structures. In regards to the spinocerebellar ataxias, oculomotor symptoms are common in many subtypes, but diplopia is most common in SCA3 also known as Machado–Joseph disease. Ophthalmoplegia and vergence insufficiency have both been implicated in strabismus in these patients, but cannot fully explain the properties of the strabismus, suggesting the involvement of other structures as well. Strabismus has not been reported as a common finding in Huntington disease or atypical parkinsonian syndromes and more studies are needed to determine how these disorders affect binocular alignment.
Highlights
Proper alignment and coordination of the eyes is essential for accurately perceiving the visual environment
The presence of new-onset strabismus in an adult can range in severity from mild to debilitating and merits consideration of an underlying neurodegenerative disorder
Diplopia generally responds well to treatment in Parkinson’s, it fluctuates with dopamine dosage, which can present a challenge in management
Summary
Proper alignment and coordination of the eyes is essential for accurately perceiving the visual environment. Because the eyes are separated in space and receive different images, fine ocular motor control is required in order to reconcile this disparity and achieve a cohesive image This is done in part via sensory fusion, which is a cortical neurological process by which the cortex perceives the two retinal images as one. Understanding the neuroanatomical substrates involved in vergence aids in understanding how these pathways are affected by disease and how they interact with other ocular motor networks such as saccades. Many ocular motor effects have been well-documented and can even aid in the diagnosis of disorders that have cha racteristic eye movement abnormalities [46, 47] These deficits have been shown to have a significant negative impact on vision-related quality of life [48, 49]. Parkinson’s disease is a progressive neurological disorder characterized by loss of dopaminergic neurons in the substantia
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