What is Cortical Blindness?
Cortical blindness, or CB, is a condition where a person loses their sight not because of an eye problem, but because of damage to a specific part of their brain, specifically the area responsible for sight in the occipital lobes. Even though their vision is lost, their eyes will still react normally to light. This condition is a subset of a broader category known as cerebral blindness, which is vision loss resulting from any damage to the back parts of the brain’s visual pathways.
The existence of CB has been acknowledged since the times of the Romans. For instance, the philosopher and politician Seneca wrote about a slave who, although blind, kept arguing about the room being dark. French writer Michel de Montaigne, in the 16th century, narrated an instance of a person who, despite clear signs of blindness, refused to believe they were blind. In the late 19th century, Austrian neuropsychiatrist Gabriel Anton reported cases of patients with damage to both parts of the sight-related area in their brain who were completely blind, but didn’t realize their loss of sight, leading to them making up stories to explain what they couldn’t see. This condition was later named ‘anosognosia’ by French neurologist Joseph François Babinski.
What Causes Cortical Blindness?
Cortical blindness, a condition where your brain can’t process sight despite having healthy eyes, can occur in both children and adults due to a variety of reasons.
In children, it’s usually caused by:
* Damage to the part of the brain that controls vision (the occipital lobe) due to a head injury
* Birth defects in the occipital lobe
* Lack of blood flow to the brain around the time of birth
In adults, this condition can emerge due to problems with the primary visual cortex of the occipital lobes. These problems can surface from a number of conditions, such as:
* A stroke, which interrupts blood flow in the brain
* A clot in the heart that travels to the brain (cardiac embolism)
* Trauma to the head that damages the brain
* Seizures starting in the occipital lobe (occipital lobe epilepsy)
* Low sodium in the blood (hyponatremia)
* Low blood sugar, severe enough to harm the brain (severe hypoglycemia)
* A rare brain disorder called Creutzfeldt-Jakob disease
* Infections, including HIV
* A condition called eclampsia, associated with high blood pressure during pregnancy
* A rare genetic disorder known as MELAS, which stands for mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes
There can also be temporary cases of cortical blindness due to:
* An infection of the heart’s inner lining, known as infective endocarditis
* Changes in the brain caused by high blood pressure, referred to as hypertensive encephalopathy
* An illness called Posterior Reversible Encephalopathy Syndrome (PRES), characterized by headaches, seizures, and visual problems.
Risk Factors and Frequency for Cortical Blindness
Exact statistics are not readily available, but research suggests that a significant number of stroke patients, between 20% to 57%, also experience Condition B (CB).
Signs and Symptoms of Cortical Blindness
Patients suffering from cortical blindness, also known as CB, may have symptoms like loss of vision, blurred vision, or issue seeing in certain areas. They might also have difficulty remembering things, hallucinations, difficulty recognizing faces, and trouble with their eye movements. To diagnose this condition, it is crucial to get a detailed medical history from the patient. Be sure to ask about any factors that may have led to CB, like birth complications, substance abuse, high blood pressure, diabetes, palpitations, and fever.
A full physical examination, including an analysis of the eyes and nerves, is also necessary. In the general physical examination, it is important to take note of the pulse, blood pressure, and body temperature. The examination of the eye will also include testing the eye’s reflex to light, which is usually intact in patients suffering from CB. Eye movement also remains normal. There should not be any changes in pupil size with light exposure (known as RAPD) in cortical blindness. Additionally, the heart should also be examined to rule out the possibility of stroke due to an embolus or clot from the heart.
A specialist eye doctor might find visual field defects, but the rest of the eye examination is usually unremarkable. The signs depend on the location of the brain’s lesion, and they might vary from patient to patient. Some specific signs include:
- Occlusion of the posterior cerebral artery results in hallucinations.
- Left-sided large PCA stroke can cause trouble recognizing objects (visual agnosia), while right-sided stroke can lead to trouble recognizing faces (prosopagnosia).
- In parietal lobe disease, your eye will follow a line smoothly on one side, but on the other side, the eye jumps from one line to the other (saccade).
- Testing optokinetic nystagmus (OKN) — eye movement provoked by moving objects — can help inform whether the lesion is in the parietal or occipital lobe of the brain. OKN is different on both sides in parietal lobe lesion (probably due to a tumor), while OKN is the same on both sides in occipital lobe lesion (usually due to stroke).
CB could be complete or incomplete. In incomplete CB, there might be defects in the vision of one side of both eyes. The vision loss varies, from mild (scotoma) to severe (hemianopsia). In most cases, the patient’s central vision is unaffected. The term “blindsight” is used when a person can perceive fast flicker movements in the blind field, due to the heterogeneous damage to V1. There are some peculiarities that can be seen in cortical blindness: Anton syndrome, Riddoch phenomenon, and formed visual hallucinations.
Anton syndrome happens when the patient can’t see but they deny being blind. In Riddoch phenomenon, the patient can only perceive moving objects in the blind field, not the stationary ones. Benson syndrome is a different form of Alzheimer’s disease and it affects the brain’s vision centers. A condition called posterior reversible encephalopathy syndrome (PRES) causes acute onset headache, seizures, consciousness changes, and visual disturbance. Lastly, Balint syndrome is characterized by failure to perceive the visual field as a whole, difficulty with eye movements, and inability to reach towards an object under visual guidance.
Testing for Cortical Blindness
To evaluate a patient with Cortical Blindness (CB), several tests should be completed. These include a complete blood count with an ESR (erythrocyte sedimentation rate) test, a metabolic profile, an electrocardiogram, visual and brain imaging tests. Initially, a CT scan (computer tomography) of the brain is usually done due to its easy availability. However, it’s important to note that CT scans can sometimes miss early or small strokes. An MRI (Magnetic Resonance Imaging) of the brain is better at diagnosing stroke, but it may not be readily available everywhere.
Visual tests, referred to as Humphrey visual field (HVF) tests, will show certain types of vision loss. For example, if a lesion is located in the temporal lobe (temporal optic radiation), it causes a homonymous quadrantanopia of the opposite upper side, also known as ‘pie in the sky’ vision loss. This means, if there is damage to the right temporal lobe, there will be a loss in the left upper visual field of both eyes. Similarly, a lesion of the anterior parietal lobe (anterior parietal radiations) leads to homonymous quadrantanopia (vision loss) of the opposite lower side, resembling ‘pie in the floor’. As an example, right parietal lobe damage will cause a loss in the left lower visual field in both eyes.
If the main optic radiation which is deep inside the parietal lobe, external to the trigone and occipital horn of the lateral ventricle, gets damaged, it results in complete vision loss on the opposite side. Lesions at the front part of the visual cortex typically caused by PCA stroke, result in similarly positioned vision loss, but with an exclusion zone, a.k.a ‘macular sparing’. This is because the part of the cortex responsible for the macular vision (the tip of the occipital cortex) and which is supplied by the MCA is usually spared in a PCA stroke. Lesions at the very tip of the occipital cortex, usually due to trauma, can similarly cause homonymous hemianopia (vision loss), including the half of macular vision on the opposite side.
While cortical blindness typically involves bilateral visual field defects, unilateral defects can occur when the anterior part of the calcarine cortex, responsible for the extreme temporal visual field of the opposite eye, is affected. This can cause a temporal crescent-like visual defect in the opposite eye alone.
Treatment Options for Cortical Blindness
Treatment for vision loss, often due to a stroke, usually focuses on visual training and rehabilitation. Three common methods include restitution therapy, compensation therapy, and substitution therapy.
Restitution therapy aims to restore deficits in the visual field, which is the total area that can be seen when the eye is focused straight ahead. This therapy is similar to perimetry, a test that measures how well you can see objects in your peripheral vision. In restitution therapy, the patient is asked to identify multiple spots of light on a black screen, some of which fall in the blind area and some in the normal vision area.
Compensation therapy helps make up for visual loss by encouraging the use of rapid, abrupt eye movements, known as saccadic eye movements. This method allows patients to capture visual signals that would typically miss their blind spot, instead directing these signals to parts of the visual field they can see.
Alternatively, substitution therapy uses devices like prisms to redirect visual signals from the blind side of the vision field to the normal side. This allows patients to capture and process visual information that they would normally miss due to their visual impairment.
What else can Cortical Blindness be?
When trying to diagnose cortical blindness, which is a total or partial loss of vision in a normal-appearing eye caused by damage to the brain’s occipital cortex, doctors could consider ruling out the following conditions:
- Hemineglect: Ignoring one side of the space around you.
- Prosopagnosia: Trouble recognizing familiar faces.
- Simultagnosia: Difficulty seeing more than one object at a time.
- Malingering: Pretending to have a vision loss to achieve some benefit.
These conditions can present similar symptoms but are caused by different problems in the brain, so it’s critical for the doctor to thoroughly examine the patient and conduct necessary tests to ensure an accurate diagnosis.
What to expect with Cortical Blindness
The outlook for a patient largely depends on how severely the visual cortex, the part of the brain that processes visual information, is damaged. Widespread damage to both sides of the occipital region of the brain, which houses the visual cortex, has a poorer outlook compared to temporary disruptions in blood flow or “transient ischemic attacks.”
With intensive training and tasks, some patients may regain certain aspects of their vision that match the visual field that wasn’t damaged. However, it’s important to note that full recovery for all visual abilities is usually not possible if the V1 area of the visual cortex, a primary area responsible for processing visual information, is damaged.
Possible Complications When Diagnosed with Cortical Blindness
Cortical blindness can greatly affect a person’s everyday life and cause distress, not only for the individual afflicted but also for their family members. This condition can lead to challenges in society and increased risks such as accidents or broken bones.
Preventing Cortical Blindness
Patients with CB often struggle with their regular activities, adding to the extra time they need for treatment. They might not even see satisfactory results, all of which add a lot of mental strain. This strain can lead to depression and other mental health issues. That’s why it’s critical to include patient education and rehabilitation as a main part of the treatment plan.
It should never be overlook, instead, well trained counselors should be involved for proper guidance. Regular sessions with patients and their families can help alleviate these issues and significantly improve their day-to-day life. This supportive approach not only benefits the patients but also their caregivers, aiding in an overall better quality of life.
