Homonymous hemianopsia is a specific form of vision loss resulting from damage to the brain, not the eyes themselves. This condition impairs the visual field, the entire area a person can see without moving their eyes. It results in a loss of vision on the same side of the visual world in both eyes. Although the eye structures remain healthy, the brain cannot process the visual information sent to it.
Defining the Visual Field Cut
The term “hemianopsia” translates to “half blindness,” referring to the loss of vision in half of the complete visual field. The descriptor “homonymous” indicates that the lost half is the same side for both eyes. For example, an individual might lose the entire left side of their vision in both the right and left eyes. This creates a blind spot that runs down the middle of the person’s visual world, respecting the vertical midline.
This unique pattern of vision loss means that when a person looks straight ahead, they are blind to everything on one side, but their central sharpness of vision (visual acuity) remains unaffected. The loss is not a blurry spot but a complete absence of sight in that half of the field. A person with a right homonymous hemianopsia, for instance, cannot see anything to their right side, regardless of which eye they are using.
This condition is different from being blind in one eye, as the loss is a field defect that affects both eyes symmetrically. Because the brain’s visual pathways are crossed, damage to the right side of the brain leads to a loss of the left visual field, and vice versa. Patients often report bumping into objects or having difficulty finding the end of a line when reading.
Primary Causes of the Condition
The root cause of homonymous hemianopsia is damage to the visual pathway after the optic chiasm, where nerve fibers from each eye cross. This pathway extends through the optic tracts, the optic radiations, and finally to the visual processing center in the occipital lobe. Any injury to these structures interrupts the flow of visual signals, causing the condition.
Cerebrovascular events, commonly known as strokes, are the most frequent cause in adults, accounting for between 42% and nearly 90% of cases. This includes both ischemic strokes, where blood flow is blocked, and hemorrhagic strokes, which involve bleeding in the brain. Stroke damage most often affects the occipital lobe or the optic radiations.
Other significant causes include traumatic brain injury (TBI), brain tumors, or lesions. The location of the brain damage determines the specific nature of the field cut; for example, occipital lobe damage often results in a more symmetrical or “congruous” field loss between the two eyes. Less common causes involve brain surgery, inflammatory conditions, or rare neurological diseases.
Diagnosis and Identification
The diagnosis of this visual field defect begins with a thorough neurological and eye examination. Initial screening often involves a confrontation visual field test, where the examiner compares the patient’s peripheral vision to their own by presenting targets in the four quadrants of the visual field. This quick test identifies the characteristic loss that respects the vertical midline.
A definitive diagnosis requires formal visual field testing, known as perimetry, which precisely maps the extent and location of the vision loss. Automated perimetry is used to document the pattern of the defect and track changes over time. This measurement confirms whether the field loss is complete or partial, such as a quadrantanopia, which affects only a quarter of the field.
Since the cause is neurological, neuroimaging is always indicated to locate the exact site of the brain injury. Magnetic Resonance Imaging (MRI) is the preferred method because it provides detailed images of soft tissues. This allows medical professionals to pinpoint damage in the optic tract, optic radiations, or the occipital cortex. The imaging confirms the underlying pathology, which is necessary for managing the patient’s overall health.
Strategies for Daily Adaptation
Living with a permanent visual field loss requires individuals to learn new ways of interacting with their environment to ensure safety and independence. The primary strategy is compensatory training, which focuses on teaching the brain and eyes to work differently. This involves visual scanning techniques, where the patient learns to consciously make large, repeated head and eye movements (saccades) into the blind field to bring objects into their seeing area.
Reading can be particularly challenging, as the patient may lose their place at the beginning or end of a line of text. Specialized reading rehabilitation involves techniques like using a visual anchor, such as a ruler or finger, and training predictable scanning sweeps to ensure the entire line is captured. This deliberate practice helps structure the visual search and improves reading speed.
Specialized optical devices can also assist in adaptation by physically shifting a portion of the blind field into the seeing field. Prism lenses, such as Peli prisms, are fitted into spectacles to bend incoming light. These prisms are placed on the side of the field loss and can expand the patient’s awareness by up to 20 degrees, improving mobility and reducing collisions.
Vision Restoration Therapy (VRT)
While most intervention focuses on adaptation, some patients explore vision restoration therapy (VRT). VRT uses computer-based exercises to stimulate the border between the seeing and blind regions. Although VRT aims to reduce the size of the blind spot, its effectiveness is debated and considered controversial compared to the proven benefits of compensatory scanning and optical aids.
Mobility and safety are also addressed through environmental modifications, such as decluttering paths and using strategic lighting. Patients must also understand local driving laws, which often restrict driving due to the increased hazard of the field loss.