A stroke occurs when the blood supply to a part of the brain is interrupted, either by a blockage (ischemic stroke) or a burst blood vessel (hemorrhagic stroke). This interruption deprives brain cells of oxygen and nutrients, causing damage that can affect various functions, including vision. A stroke can definitively affect vision, though the visual problems do not arise from damage to the eyeballs themselves. Visual symptoms develop because the injury occurs in the brain regions responsible for processing and interpreting sight, which affects a significant number of stroke survivors.
The Anatomy Connecting the Brain and Vision
The eyes function primarily as receptors, gathering visual data and converting light into electrical signals. These signals travel along the optic nerves, which are extensions of the central nervous system, carrying information along the visual pathways deep into the brain.
The final destination for visual processing is the visual cortex, located in the occipital lobe at the back of the brain. The brainstem houses the control centers for the cranial nerves that dictate eye movement and alignment. A stroke impacting the blood flow to any of these processing or control centers results in a visual deficit.
Damage to the brainstem affects the nerves controlling eye movement, while injury to the occipital lobe impacts the brain’s ability to interpret the image. The severity and location of the stroke determine the specific type of visual impairment experienced. Because the visual pathways cross over, a stroke on one side of the brain typically affects the visual field on the opposite side.
Visual Field Loss After a Stroke
The most common visual impairment following a stroke is the loss of a portion of the visual field, known as a visual field defect. This condition is a perceptual issue where the brain fails to register images from a specific area. The most frequent form is homonymous hemianopia, characterized by the loss of the same half of the visual field in both eyes.
For example, a person with a right-sided stroke may lose the entire left side of their vision, meaning they cannot see anything to their left with either eye. This field loss often results from damage to the occipital lobe or the optic radiations, which are the nerve fibers that fan out from the thalamus to the visual cortex.
A less extensive form is quadrantanopia, where only a quarter of the visual field is missing. Damage to the optic radiations passing through the temporal lobe can cause a superior quadrantanopia, sometimes described as a “pie in the sky” defect. Conversely, injury to the optic radiations in the parietal lobe may result in an inferior quadrantanopia. These field losses significantly impair daily life, causing a person to bump into objects on the blind side or struggle to read a line of text.
Eye Alignment and Movement Deficits
A stroke can also disrupt the motor control of the eyes, leading to alignment and movement deficits. These problems often occur when a stroke affects the brainstem, the origin point for the cranial nerves responsible for eye movement (Cranial Nerves III, IV, and VI). Damage to these nerves disrupts the balance between the six muscles controlling each eye.
The resulting misalignment prevents the eyes from focusing on the same point simultaneously. This sensory conflict manifests as diplopia, or double vision. Double vision can be highly disorienting, affecting balance and depth perception.
Strokes can also cause gaze palsies, which are difficulties moving both eyes together in a specific direction, such as side-to-side or up-and-down. Patients may also experience problems with saccades (rapid eye movements used for scanning) or pursuits (smooth tracking of a moving object). These motor impairments make activities requiring coordinated eye use, like driving or following a conversation, difficult.
Rehabilitation and Visual Recovery Strategies
The visual system often shows a capacity for recovery due to neuroplasticity, the brain’s ability to reorganize and form new neural connections after injury. Visual recovery is generally most pronounced within the first few months following a stroke. Treatment is provided by vision rehabilitation specialists, such as neuro-optometrists.
Managing Visual Field Loss
One primary strategy for managing visual field loss is visual scanning training. This teaches the patient to make systematic head and eye movements into the area of field loss. This technique helps compensate for missing information and improves environmental awareness. Optical devices, such as prism lenses embedded in glasses, can also shift images from the blind field into the remaining field of view.
Treating Double Vision (Diplopia)
In cases of diplopia, prism lenses can be prescribed to merge the two images into a single picture. These prisms bend light before it reaches the eye, correcting the image misalignment. Vision therapy exercises are also used to retrain the eyes to work together, focusing on improving coordination and tracking skills.