Visual field loss is a common sign of stroke and brain injury with about 30% of patients suffering vision disorders. The question is, why do hemianopia and quadrantanopia involve such distinctive patterns of vision loss?
The answer lies in the brain. When a stroke or brain injury patient experiences homonymous visual field loss (i.e. visual field loss in the same area on both eyes), it is a sign of brain damage, not damage to the eyes. Visual field loss occurs when the optic pathways to the brain are damaged during a stroke or brain injury. Because of how the brain’s visual system is wired, vision loss after a brain injury often occurs in the same visual field areas of both eyes at the same time, as a result one eye cannot compensate for the deficit in the other eye.
Patients suffering from these visual field deficits may run into objects, trip or fall, knock things over, lose their place when reading, or be surprised by people or objects that seem to appear suddenly out of nowhere. They may become afraid of venturing out in public, often because they easily get lost in crowded areas.
In addition, some visual field deficit patients experience visual neglect; that is, they may be unaware that they cannot see to one side. They may orient their body to compensate, bump into objects on the affected side, or miss parts of words when reading.
It is also common for those affected by hemianopia or quadrantanopia to believe that they have vision loss in only one eye.