Hemianopia is the loss of vision in half of the visual field, originating from damage within the brain’s visual pathways rather than the eyes themselves. When this system is disrupted, a person can lose the ability to see in a specific portion of their visual field. This loss can affect one or both eyes, fundamentally altering how an individual perceives their environment.
Causes of Vision Loss
The most frequent cause of hemianopia is a stroke, which occurs when blood flow to a part of the brain is interrupted. This deprives brain cells of oxygen, leading to damage in the visual pathways. Strokes in the occipital lobe, the brain’s visual processing center, are a common source of this vision loss. Strokes account for a significant majority of adult hemianopia cases, with some studies showing this figure to be as high as 69.7%.
Traumatic brain injury (TBI) is another prominent cause. A forceful impact to the head from a fall, accident, or blow can directly damage the delicate tissues of the brain’s visual cortex or the nerve fibers leading to it. The location and severity of the injury determine the nature of the resulting vision loss.
Brain tumors can also lead to hemianopia by exerting pressure on or invading the structures of the visual pathway. As a tumor grows, it can compress the optic tract, optic radiations, or the occipital lobe itself, disrupting the transmission of nerve signals. Similarly, brain aneurysms, which are bulges in a blood vessel wall, can rupture or press on these same visual structures, causing vision loss.
Types of Hemianopia
The patterns of vision loss are classified based on which parts of the visual field are affected. The most common type is homonymous hemianopia, which involves the loss of the same half of the visual field in both eyes. For instance, a person with right homonymous hemianopia cannot see the right half of their visual world from either eye. This occurs because damage to the visual pathway on one side of the brain affects signals from the opposite visual field of both eyes.
Homonymous hemianopia can be complete, affecting the entire half of the visual field, or partial, where some vision remains. When the deficit is confined to one-quarter of the visual field, it is known as quadrantanopia. For example, a lesion in the brain’s temporal lobe might cause vision loss in only the upper left quadrant of both eyes.
A different classification is heteronymous hemianopia, where vision loss affects different sides of the visual field in each eye. One form is bitemporal hemianopia, the loss of vision in the outer halves of both visual fields, which results from damage to the optic chiasm. A rarer form, binasal hemianopia, involves the loss of the inner, or nasal, visual fields of each eye.
Diagnosis and Medical Evaluation
Diagnosing hemianopia involves mapping the vision loss and identifying its cause. The primary tool for charting the visual field defect is perimetry, or visual field testing. During this test, a patient looks at a central point while lights are flashed in their peripheral vision. By indicating which lights they see, a precise map of their visual field is created, confirming the pattern of hemianopia.
Once the visual field defect is confirmed, neuroimaging is used to locate the source of the problem. Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) scans create detailed pictures of the brain’s structure. These scans can reveal the location and extent of damage from a stroke, a brain tumor, or a traumatic injury. Correlating the pattern of vision loss with the lesion’s location on the scan allows doctors to make a definitive diagnosis.
Management and Adaptation Strategies
Although the vision loss from hemianopia is often permanent, individuals can learn techniques to compensate. A central approach is visual scanning therapy, which trains a person to systematically move their head and eyes toward the blind side. This practice helps make scanning a more automatic behavior, improving environmental awareness and reducing the risk of collisions. These exercises can be low-tech, like finding letters on a wall, or use high-tech programs to improve reaction time.
Another adaptation involves special glasses with prisms. Prism lenses work by bending light, shifting an image from the blind part of the visual field into the person’s remaining seeing field. For example, a prism on the outer edge of a lens can bring peripheral objects into view, alerting the wearer to hazards. This does not restore lost vision but expands the effective field of view, enhancing safety.
Modifying one’s environment and daily routines is also a practical strategy. This can include organizing living spaces to keep pathways clear and items within the field of vision. When reading, using a ruler or a colored line guide helps prevent skipping words. For navigating unfamiliar places, assistive technologies like GPS devices can provide verbal cues to supplement visual information.