First, imagine a vertical line coming down and splitting your vision exactly in half. Then imagine your vision on one side of that line being entirely missing, or black. You would be seeing only ½ of the world. This is what many patients who have had strokes affecting their visual system experience.
This type of vision loss is called “homonymous hemianopia”. It refers to an absence of vision towards one side of the visual world in each eye. The damage that caused this problem is in the brain and not in the eyes.
It is difficult to explain the sensation of a homonymous hemianopia. Affected people often “feel like” the problem is in the right eye when they have a right homonymous hemianopia, but checking each eye by itself shows that the right side of each eye is not seeing.
People often bump into things on the side of the visual loss. Actions such as crossing the street may be dangerous. Driving may be particularly problematic where there is an oncoming car. Objects on a desk or table may not be seen. Sometimes, even the food on one side of a plate is left uneaten.
Reading has its own special set of difficulties. People with a left homonymous hemianopia have difficulty finding the correct line when the finish one line and try to move their eyes back to the start of the next line. Since we read from left to right, people with a right homonymous hemianopia have difficulty.
Visual acuity, that is the ability to see the 20/20 characters on an eye chart, is not affected by a homonymous hemianopia. Only the awareness of the world on that side is lost.
Homonymous hemianopia may be caused by any disorder that affects the brain including tumors, inflammation, and trauma, but most commonly is due to stroke. Imaging of the brain by MRI is the diagnostic test most commonly used to diagnose the location and cause of the brain injury.
Recovery of a homonymous hemianopia depends on the underlying cause and the severity of the injury. There is often poor recovery if the cause is a stroke. Any recovery that occurs is likely to begin soon after the stroke and reach its maximum in the first 6 months although some improvement may occur after that. If the underlying cause is not a stroke, the potential for improvement is determined by the ability of treatment to reverse that underlying condition.
Attempts to improve visual function in people with homonymous hemianopia may be directed at one of two areas, reading and dealing with the environment. If you or a loved one have a homonymous hemianopia, be sure to see a neuro-ophthalmologist and discuss strategies to manage the visual loss. Your doctor can also give you recommendations for safe driving.
Keep in mind that though there is no proven treatment yet for homonymous hemianopia, research studies are underway. Make sure to ask about any new developments, as there is always hope for improving the blind side.