Homonymous hemianopsia is a common visual impairment resulting from damage to the brain, not the eye itself. The condition causes a loss of vision in the same half of the visual field in both eyes, meaning a person loses their entire left or right visual space. It is a frequent consequence of neurological events, particularly stroke. This visual field cut occurs because the brain centers responsible for processing vision have been damaged, disrupting the communication pathway from the eyes.
How Vision Is Affected
The term “homonymous” means “the same side,” referring to the loss of vision in the corresponding half of the visual field in each eye. For example, a person with left homonymous hemianopsia cannot see anything to their left side, regardless of which eye is open. This creates a large, unseen area that respects the vertical midline of the visual world.
This visual field loss significantly impacts daily life, causing difficulties in navigation and mobility. Individuals often bump into objects or walk into people on the side of their vision loss because they are unaware of the obstruction. Reading becomes a major challenge, especially when the right visual field is lost, as the person struggles to see the next words on a line. The condition can also lead to an increased risk of falls and discomfort in crowded environments.
The Neurological Pathway and Damage Location
The visual system is structured so that information from one half of the visual world is processed by the opposite side of the brain. Visual information from both eyes travels along the optic nerves until they reach the optic chiasm. Here, fibers from the nasal (inner) half of each retina cross over to the opposite side of the brain, while fibers from the temporal (outer) half continue to the same side.
After this crossing, the visual information is bundled in the optic tract, carrying a complete representation of the opposite visual field (e.g., the left optic tract carries information for the right visual field). Damage occurring at any point after the optic chiasm—along the optic tract, the optic radiations, or the visual cortex—results in a homonymous visual field defect. The most frequent site of injury is the occipital lobe, the posterior region of the brain dedicated to processing visual input.
Primary Causes of Vision Loss
Homonymous hemianopsia is a direct result of damage to the retrochiasmal visual pathway in the brain. The most common cause in adults is a stroke, which can be either ischemic (caused by a blockage) or hemorrhagic (caused by bleeding). Strokes account for a large majority of cases, with an estimated eight to ten percent of all stroke survivors experiencing this condition permanently.
The sudden loss of blood flow or an intracranial hemorrhage destroys brain tissue, leading to the functional impairment of the visual centers. Other significant causes include traumatic brain injury, where force damages the visual pathways, and brain tumors or lesions. These masses can compress or invade the optic radiations or the visual cortex, disrupting the transmission of visual signals.
Strategies for Adapting to Vision Loss
Since the brain damage causing homonymous hemianopsia is often permanent, rehabilitation focuses on compensatory strategies to maximize the use of remaining vision. One primary technique is visual scanning training, a behavioral strategy that teaches the person to make rapid, deliberate eye movements into the blind field. This trained sweeping action helps them actively search for objects and information in the unseen space, effectively “filling in the blanks” of their environment.
Specialized optical devices, such as prismatic glasses, are another adaptation used in some cases. These prisms are designed to shift images from the blind field into the area of spared vision, expanding the functional visual field. Vision rehabilitation therapists and low vision specialists work with individuals to implement these behavioral and optical solutions. The goal is to improve independence and safety during activities like walking, driving, and reading.