Cortical blindness (CB) is a profound form of vision loss that originates within the brain, not the eyes. Most people associate the inability to see with damage to the physical structures of the eye or the optic nerve. CB challenges this understanding, demonstrating that the eyes can be perfectly healthy while the ability to perceive the world visually is completely absent. This neurological distinction is key to understanding this unique form of sightlessness.
Defining Cortical Blindness
CB is a total or partial loss of vision resulting from damage to the brain’s visual processing centers, primarily located in the occipital lobe. The condition is specifically linked to injury in the primary visual cortex (V1), which is the initial receiving area for visual information. In CB, the eyes and the entire anterior visual pathway, including the retina and optic nerves, are typically intact and functioning normally. The eyes capture light and send signals toward the back of the brain, but the cortex cannot interpret these signals into conscious sight.
The distinction between CB and ocular blindness is fundamental. Ocular blindness is caused by damage to the eye or optic nerve, leading to a failure in signal transmission. Conversely, CB is a failure in signal processing, compromising the brain’s ability to create a visual image. This neurological basis allows a patient’s eyes to appear normal during an examination, despite their inability to see.
For complete cortical blindness to occur, both sides of the occipital lobe must sustain injury. The damage in CB is localized to the final destination where conscious visual perception is created. When the V1 area is functionally destroyed, the person loses their subjective experience of seeing, even though the initial parts of the visual system remain active.
Common Causes and Risk Factors
CB is caused by any event that leads to bilateral damage or a severe lack of oxygen and blood flow to the visual cortex. The most frequent cause in adults is a stroke, specifically one affecting the posterior cerebral arteries (PCA). Since these arteries supply blood directly to the occipital lobes, their occlusion leads to ischemic damage and sudden vision loss.
Cerebral hypoxia, a lack of oxygen reaching the brain tissue, is another significant cause of damage. This occurs following events like cardiac arrest, severe respiratory failure, or carbon monoxide poisoning. The visual cortex is highly sensitive to oxygen deprivation, making it vulnerable during periods of systemic shock. Severe head trauma can also cause direct mechanical damage to the occipital lobes, leading to CB.
Risk factors for CB mirror those for underlying neurological events. Conditions that increase the likelihood of stroke, such as hypertension, diabetes, and heart disease, also increase the risk. Less common causes include severe eclampsia, infections like meningitis, and severe metabolic derangement. In children, congenital abnormalities of the occipital lobe or perinatal ischemic stroke are the most likely causes.
How Cortical Blindness Presents
The presentation of cortical blindness highlights its origin in the brain. A defining sign is the preservation of the pupillary light reflex. When light is shined into the eye, the pupil constricts normally because the reflex pathway bypasses the damaged visual cortex. This confirms the eyes are responding to light even if the brain cannot consciously perceive it.
Patients experience a total or near-total loss of conscious vision, manifesting as complete binocular blindness or large blind spots (scotomas). One paradoxical phenomenon associated with CB is blindsight. Blindsight occurs when a person reports no conscious awareness of seeing anything, yet can still accurately process certain visual information unconsciously.
Individuals with blindsight may correctly guess the location or orientation of an object, or navigate an obstacle course, without consciously “seeing” it. This ability relies on intact visual pathways that travel to other parts of the brain, bypassing the damaged V1 area. The brain retains a primitive, non-conscious way of detecting visual stimuli.
An equally unusual presentation is Anton-Babinski Syndrome, a rare condition where the patient denies being blind (visual anosognosia). The individual may adamantly claim to see, even while bumping into objects. They often confabulate, creating false descriptions to fill in the missing sensory input. This denial is a neurological symptom, thought to be a disconnection between the visual cortex and the brain areas responsible for self-awareness.
Diagnosis and Potential for Recovery
Diagnosis of cortical blindness begins with a comprehensive eye examination to rule out ocular causes of vision loss. The presence of normal eye structures, healthy optic nerves, and an intact pupillary light reflex strongly suggests a central origin for the blindness. This initial assessment establishes the neurological nature of the problem, shifting the focus to the brain.
Neuroimaging is the definitive diagnostic step. Magnetic Resonance Imaging (MRI) is the preferred method to visualize brain tissue, clearly showing areas of damage like ischemic infarcts or hemorrhage in the occipital lobes. While a Computed Tomography (CT) scan is often used initially in an emergency setting, MRI offers greater detail of soft tissue damage.
Specialized tests, such as Visual Evoked Potentials (VEP), assess the function of the visual pathway. The VEP measures the electrical activity generated in the brain in response to visual stimuli. In CB, the eyes transmit the signal correctly, but the VEP shows an absent or abnormal electrical response in the visual cortex, confirming the lack of cortical processing.
The potential for recovery is highly variable, depending on the underlying cause and the severity of the damage. If CB is caused by a temporary condition, such as swelling or reversible metabolic issues, vision may fully return. However, if the damage is due to a major stroke with extensive tissue death, the prognosis for full recovery is generally poor.
Recovery, if it occurs, is most likely within the first few months, with some improvement possible up to a year or more. This recovery is driven by neuroplasticity, the brain’s ability to reorganize and form new connections. Rehabilitation focuses on maximizing any residual vision or function through visual stimulation training and the use of compensatory strategies.