Homonymous hemianopsia (HH) is a type of vision loss where an individual loses sight in the same half of the visual field in both eyes. The term “homonymous” means “the same side,” and “hemianopsia” refers to a loss of half the visual field. This condition is caused by an injury to the brain’s visual processing centers or the pathways leading to them, not by damage to the eyes themselves. The visual field loss can occur on either the right or the left side, depending on the location of the brain injury.
The Neurological Mechanism Behind Homonymous Hemianopsia
Homonymous hemianopsia is a direct result of how the brain is wired to process visual information. When a person looks forward, the entire visual field is split into a right half and a left half. Information from the entire left visual field is channeled exclusively to the right hemisphere of the brain, and input from the entire right visual field is directed to the left hemisphere.
This unique organization is achieved by a partial crossing of nerve fibers at the optic chiasm, which is located near the base of the brain. Fibers from the inner half of each retina cross over to the opposite side, while fibers from the outer half continue on the same side. Once past the optic chiasm, these bundled nerve fibers, called the optic tracts, carry all the information for one-half of the visual world.
The nerve signals travel through the thalamus to the occipital lobe, which serves as the primary visual cortex. An injury occurring anywhere along this visual pathway after the optic chiasm disrupts the flow of information to the visual cortex on that side of the brain.
If damage occurs in the right hemisphere—affecting the optic tract, optic radiations, or visual cortex—the entire left visual field in both eyes will be lost. This creates a mirror effect where the side of the brain injury is opposite to the side of the vision loss. The more posterior the injury is, such as in the occipital lobe, the more symmetric the visual field loss tends to be.
Primary Causes and Contributing Risk Factors
Homonymous hemianopsia is caused by an underlying neurological event that damages the visual pathway in the brain. The most frequent cause in adults is a cerebrovascular accident, or stroke. Strokes, whether ischemic (caused by a blockage) or hemorrhagic (caused by bleeding), disrupt the blood supply to the visual centers, leading to tissue damage.
Traumatic brain injury (TBI), such as from head trauma, is another major cause, often affecting the posterior regions where the visual cortex is located. Brain tumors, whether malignant or benign, can also lead to vision loss by physically pressing on visual pathway structures.
Other Causes
Transient ischemic attacks (TIAs), sometimes called mini-strokes, can temporarily cause HH and serve as a warning sign for stroke risk. Other causes include brain infections like encephalitis, inflammatory conditions such as multiple sclerosis, and complications from brain surgery. Temporary disruptions, such as migraine with aura or seizures, can also result in transient homonymous hemianopsia.
Recognizing the Visual Symptoms and Clinical Diagnosis
Patients often report specific difficulties reflecting the loss of half their visual world. Those with right-sided loss frequently bump into objects on their right, while those with left-sided loss struggle to see people approaching from the left. This reduced spatial awareness makes mobility difficult and can lead to feelings of clumsiness or discomfort in crowded environments.
Reading is challenging because the patient loses their place on the line of text, depending on the side of the field loss. Patients may also exhibit visual neglect, where they are unaware of objects or even people in the blind field, sometimes leaving food uneaten on one side of a plate. In sudden-onset cases, patients may experience visual hallucinations in the blind area, known as Charles Bonnet syndrome.
Diagnosis begins with a detailed patient history and a general eye exam to confirm the eyes are healthy. The definitive method for confirming the condition is formal visual field testing, or perimetry. This testing involves presenting lights or targets to various points in the visual field while the patient keeps their gaze fixed straight ahead, precisely mapping the extent of the vision loss.
Once the defect is confirmed, neuroimaging is essential to identify the underlying cause and location of the brain damage. A computed tomography (CT) scan or, more commonly, a magnetic resonance imaging (MRI) scan is used to look for evidence of stroke, tumor, or trauma along the visual pathway. The specific pattern of loss, such as macular sparing (sparing the center of vision), helps localize the injury.
Adaptive Strategies and Vision Rehabilitation
Since the damage causing homonymous hemianopsia is often permanent, management focuses on maximizing the use of remaining vision and adapting to the deficit. Visual rehabilitation programs teach compensatory strategies to navigate the environment safely. A cornerstone of this rehabilitation is visual scanning training.
This training teaches the patient to make rapid, purposeful eye and head movements, called saccades, into the blind field to search for objects. By repeatedly moving their gaze into the non-seeing area, patients learn to effectively expand their functional visual field for daily tasks. This practice significantly improves scene exploration and reduces search times.
Specialized optical devices, such as prism glasses, can also help expand the perceived visual field. These prisms are mounted in the peripheral part of the spectacle lens on the side of the field loss. They shift images from the blind field into the seeing field, helping alert the patient to obstacles or hazards. The goal of these methods is to improve independence by teaching the brain new ways to compensate for the lost visual input.