When Stroke Affects the Occipital Lobe
The occipital lobe of the brain is located at the back of the head and is named for the occipital bone that covers it (Latin ob, behind, and caput, the head). It is the smallest of the four lobes of the cerebral cortex, and it’s pretty much a one trick pony. But that one trick — vision — is an important one.
When we look at something, that visual stimuli are sent along the optic nerves, which run the length of the brain all the way to the occipital lobe, which processes that visual information for us. “The occipital lobe has two hemispheres, and just like the other lobes in the brain, each control the opposite side of the body,” said neurologist Elisabeth Marsh, associate professor of neurology at the Johns Hopkins School of Medicine. For both of our eyes, the left side of the occipital lobe is responsible for what we see in the right side of our field of vision — and the right side of the occipital lobe, for the left side of our field of vision.
In other words, the left hemisphere doesn’t control the right eye, it processes visual stimuli for the right side of each eye. That fact leads to quite a vexing stroke deficit, and the most common visual deficit — a field cut (homonymous hemianopsia) where a person sees only half the world. Unfortunately, a field cut is often misinterpreted as a problem with the eye. “When people come to my clinic and say, ‘I can’t see out of my right eye,’ my first job as a neurologist is to confirm that’s truly the case,” Marsh said. “‘Is it really your eye you can’t see out of? What happens when you close the eye you think is the problem?’ If their vision becomes normal, then it really is the eye that’s the problem. But more commonly, people close the eye and realize they still can’t see on one side of space. The same deficit affecting both eyes is a brain, or more specifically, an occipital lobe problem.”
It's Not Just the Eyes
Dr. Elisabeth Marsh
We often limit vision to our eyes, but vision is really an additive process. Impulses from the retina travel along the optic tracts to the occipital lobe where they are processed. “There are discrete areas within the occipital lobe that are responsible for different parts of your vision,” Marsh said. For instance, motion is detected in a separate place in the occipital lobe than pattern recognition. Some parts of the occipital lobe identify movement, or color, or even the different parts of an object that make up the whole.”
But after the occipital lobe processes vision, the rest of the brain is important for actually perceiving and figuring out what that means. “After vision gets processed in the occipital lobe, the movement and patterns have to be perceived, which is done in the parietal lobe,” she said. “Then how one responds to the stimulus, how they feel about what they’re seeing, is determined in the temporal lobe. In other words, the entire brain is important for processing that visual stimuli, but it starts with the occipital lobes.”
Marsh pointed out that recognizing the difference between a field cut and neglect, a common stroke deficit involving inattention, can be difficult: “If someone is not seeing that side of space, sometimes it can be hard to tell whether their attention there is good or not. But neglect tends to be more of a temporoparietal [controlled by the temporal and parietal lobes] function in general. These two syndromes can be difficult to differentiate in patients,” she said.
Most of the occipital lobe’s blood flow is from the artery in the back of the brain (posterior cerebral artery or PCA). However, the middle cerebral artery (MCA) supplies a small portion of the occipital lobe. “That part is the center of the vision called the macula, which is good because if you have a sizable PCA stroke, sometimes the macula and the central vision can be spared,” Marsh said.
Marsh says that strokes in the PCA are less common than those in the MCA. The PCA has more twists and turns, so clots tend to travel the easier path of the MCA. “But certainly, PCA strokes are not infrequent,” she said.
“Two types of disease result in PCA strokes. One is cardiac disease, more specifically, atrial fibrillation (AFib). When a patient is in AFib, blood can pool in the heart and form clots that then travel to the brain and lodge in the big blood vessels like the PCA. The other major stroke type leading to occipital lobe strokes is large vessel disease, where lipid plaques form due to vascular risk factors like hypertension [high blood pressure], hyperlipidemia, and smoking, that can result in a stroke of the occipital lobe.”
There is also a difference between ischemic strokes and hemorrhagic strokes. “Ischemic strokes tend to obey vascular borders. The neurons fed by the PCA are the ones affected when a clot blocks blood flow through the vessel. With a hemorrhagic stroke, there’s rupture of the vessel so these borders are disrupted, and in addition there can be swelling that expands the territories involved. Patients may be weak or have aphasia because of all the edema.”
How Some Visual Field Cuts May Appear To Stroke Survivors
With field cuts, the survivor typically can’t see what’s there, but occipital lobe strokes can produce the opposite effect — seeing something that is not there. That can be for two reasons. “First, it can be because you have cells that are injured and malfunction, resulting in seeing lights or zigzags,” Marsh said. “But in addition, the brain is really smart and knows it’s supposed to be seeing something. It doesn’t like it when it’s not getting the input it expects, so it starts to make things up to fill in the gaps. This can actually happen with occipital lobe strokes or any cause of visual impairment, such as macular degeneration. Most commonly, people will detect movement and then perceive that those movements are performed by people. Hallucinations can be quite complex.”
These kind of vivid hallucinations, called the Charles- Bonnet syndrome, apparently are rare, but Marsh says it is hard to know precisely. “People who experience them may think they are crazy, so they don’t talk about it unless prompted,” she said. “Whenever anybody has a visual problem, I always ask if they’re seeing things that they know aren’t there? If you bring it up carefully, you can often get them to open up and then reassure them.”
The thalamus and brainstem can also be affected with an occipital lobe stroke. The vessel that supplies the PCA, which supplies both the occipital lobe and the thalamus, is the basilar artery, which also feeds the brainstem. It’s not uncommon for a clot traveling up the basilar to break up and affect multiple locations.
A stroke that affects both the thalamus and the occipital lobe can produce a second deficit — confusion. “It’s not uncommon for people to not only present with a visual problem but also with confusion when they have a PCA stroke,” Marsh said. “It’s not that they are unable speak, they’re not aphasic. But they’re often confused, and families will say that they’re ‘not right,’ not behaving the way they normally do.”
Compensating For Deficits
Therapy and recovery from occipital lobe stroke can be difficult. Occipital lobe strokes don’t impact the ability to move, nor are they likely to produce aphasia. This leads many people to think the survivor got off lucky, with “minor deficits.”
“But when you talk to people who have had an occipital lobe stroke and can’t see out of half of their visual field, it’s a very big deal,” Marsh said. “Not being able to see on one side of space means you can’t drive, which takes away independence. Depending on what you do, a field cut can also affect one’s ability to work. For instance, I have a patient who’s a welder and can’t go back to operating heavy machinery because it would be unsafe. Strokes resulting in vision problems can be devastating.”
Whereas rehabilitation can often help survivors with weakness or movement problems recover some of their previous function, survivors with visual deficits are far less likely to recover any of their vision. “Unfortunately, once those neurons are lost, there’s no getting them back, and the opposite hemisphere can’t take over the function” she said. “However, occupational therapy can teach a patient how to compensate for their deficit and become quite functional.”
The occipital lobe doesn’t have an operational role in emotions or executive function, but Marsh pointed out, “Having a stroke itself changes the brain chemistry and makes post-stroke depression far more likely.”
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