When Stroke Affects the Cerebellum
The word cerebellum is derived from the Latin for “little brain.” The cerebellum has been referred to as the brain’s quality assurance center. “One of the things it does is make things better,” said Dr. Jeremy D. Schmahmann, professor of neurology at Harvard Medical School and director of the ataxia unit at Massachusetts General Hospital. “It is involved in the coordination of voluntary motor movement, balance and equilibrium, and muscle tone — but it also plays a similar role in emotion and cognition.” All of this occurs outside of our conscious awareness.
The cerebellum is located at the back of the head, underneath the occipital lobe of the cerebrum — which Schmahmann sometimes refers to as “the brain upstairs.” Situated behind the brainstem, the cerebellum has two halves, or hemispheres. They are separated by the vermis, or midline. The cerebellum is divided into three lobes—the anterior lobe, the posterior lobe and the flocculonodular lobe.
When a stroke injures the cerebellum, the survivor’s movement may become slow and uncoordinated; they may stagger when walking, often accompanied by tremors in the trunk of the body. Other symptoms may include slow, unsteady and jerky movement of the arms or legs, slowed and slurred speech, and rapid, small movements of the eyes (nystagmus).
The cortex (outer layer) of the cerebellum actually has three layers:
- The top, or molecular, layer contains important cables that link the different brain cells in the cerebellum with each other.
- “The middle one is called the Purkinje cell layer, and this contains the very large and elegant neurons that are characteristic of the cerebellum,” Schmahmann said.
- The bottom, or granule, cell layer is densely packed with literally billions of tiny granule cells, which are important for bringing neuro-messages into the cerebellum. Because of these granule cells, the cerebellum contains about 80 percent of the brain cells that we have. Amazingly, though, it only makes up about 10 percent of the brain’s total weight.
There are three main syndromes that may occur, depending on which of the three lobes are affected by the stroke. Schmahmann summarizes the syndromes this way: “There’s a motor cerebellum in the anterior lobe, a balance cerebellum in the flocculonodular lobe, and a cognitive cerebellum in the posterior lobe. The midline of the cerebellum seems to be important for emotional processing.”
A stroke in the anterior lobe is likely to produce motor deficits. “Those motor deficits are problems with coordination of the arm or hand dexterity, and with leg movements,” Schmahmann said. “It’s not weakness. It’s not inability to use the limb. It’s difficulty controlling it. That has importance for typing, writing, brushing your teeth—whatever you like to do. It may also affect how you place your leg as you walk. If it affects both sides of the anterior lobe, it can affect gait in a marked way, making a person walk as if they had drunk too much alcohol. It can also affect speech and eye movements. There are particular patterns of eye movements that are characteristic of cerebellar motor control problems.”
A stroke in the flocculonodular lobe of the cerebellum produces vertigo. “In this syndrome, the world feels like it is spinning around you, as if someone got you off a merry-go-round,” Schmahmann said. “Patients can have terrible nausea and vomiting at the time the stroke happens, often with headache.”
A stroke in the posterior lobe of the cerebellum produces what is known as cerebellar cognitive affective syndrome. The major characteristics of this are problems with executive function, language processing, visual spatial function and emotional regulation. “The executive functions I am referring to are things like organizing, multitasking, developing strategies, long-range planning, mental flexibility, being able to think on your feet and adapt to situations,” Schmahmann said. “You can also have problems with ability to see the big picture of a visual-spatial diagram.
“Unlike strokes in the brain upstairs, which produce aphasia, cerebellar strokes produce difficulty with constructing complex sentences. Survivors with this syndrome have trouble finding words, and they may have trouble with what we call ‘metalinguistics,’ the ability to recognize implicit meanings, or if someone is making an inference to something else. It’s the way we use language for social discourse.”
Dr. Jeremy D. Schmahmann
A lack of metalinguistic ability may hamper what is called social cognition and create difficulty with understanding how one engages in a socially appropriate way.
Schmahmann said the “affective” component in cerebellar cognitive affective syndrome occurs when the stroke involves the midline of the cerebellum. “You can have problems with regulation of mood. Is the way you’re feeling appropriate to the environment?” Survivors may involuntarily laugh or cry in inappropriate situations, a condition known as pseudobulbar affect.
A cerebellar stroke can affect working memory, which is unlike the memory challenges that occur with Alzheimer’s disease or damage to the hippocampus. “A person can have difficulty sorting things out in memory,” Schmahmann said. “You may have trouble recalling information. Sometimes, jogging the memory can be helpful, which is important, because if you can develop a strategy to remember things, that will improve function. People with cerebellar troubles can have difficulty with what’s called ‘unstructured recall’ when they’re trying to remember. But, if they can find a strategy or a trick to make sure they can remember things, the information is usually there. They just need to find a way to go get it.”
Knowing one’s own position
Schmahmann tells us that the spinal cord relays info to the cerebellum about where a person’s limbs are in space (proprioception). The cerebellum isn’t providing the sensations a limb is experiencing, the sensory cortex of the brain does that.
“The cerebellum is concerned with where the limbs are, how they are situated in space, how the limbs are related to the world around you and your own position. But all this happens without you being aware of it.” Schmahmann said. He mentions that patients, for example, having trouble backing their car into a driveway, don’t talk about having trouble controlling their movements, but that they have difficulty judging distances. “So, they report their problem not in motor terms but as awareness of space.”
Like the cerebrum, the cerebellum has two hemispheres. But unlike the cerebral hemispheres, each of which controls the opposite side of the body, the cerebellar hemispheres control coordination on the same side of the body, Schmahmann explains. For example, injury to the right side of the cerebellum’s anterior lobe affects motor functions in the right arm, hand or leg, and the same goes for left.
“The right side of the posterior lobe is involved in language processing, while spatial processing is more on the left side,” Schmahmann said, “In the cerebral cortex, language is on the left side, usually, and spatial information on the right.”
“The reason for this is that the cerebral connections to the cerebellum are crossed; so, the left cerebral cortex, which is important for language, talks to the right cerebellum. Therefore, the right cerebellum is important for language. And the right cerebral hemisphere talks to the left cerebellum, which makes the left cerebellum important in spatial processing.”
Schmahmann tells us that executive function seems to be processed on both sides of the cerebellum and emotional affect seems to be processed in the midline and the areas just next to it, for the most part.
Plasticity and improvement
As for plasticity (the brain’s ability to adjust), Schmahmann says there’s no definitive evidence on whether the right side can make up for a loss on the left side. “We don’t know whether that happens in the cerebellum or the brain upstairs,” Schmahmann said. “What we do know is that there is a lot of opportunity for recovery of function, and the recovery has to do both with the reorganization of brain itself and with working on rehabilitation techniques to learn new ways to make up for, as much as possible, the damage that was done from the injury to the cerebellum.”
Sometimes, medications are used to improve deficits, but he emphasized the importance of teaching the brain new methods. Rehab is a prime tool for recovery. “In patients who engage in rehabilitation therapy it can make a substantial difference,” Schmahmann said. “We think the cerebellum makes things better; it refines both our movements and our mood. If you bring these things to conscious awareness and pay attention to them with rehab, there’s a good opportunity, especially after a stroke, to recover from many of the deficits that occur after a cerebellar injury.”
Common blood supply
Because the blood vessels that supply the cerebellum also supply other areas, a blockage to the cerebellum may also affect the brainstem. Schmahmann says this can cause weakness or paralysis because of the loss of the function of the motor system in the brainstem, or problems with speech or moving the face and eyes. “Further, the blood vessels that supply the brainstem and cerebellum continue up into the cerebrum and supply the thalamus, the occipital lobe and the memory system in the hippocampus,” Schmahmann said. “Damage to the cerebellum does not cause loss of vision or severe memory loss, but the same major blood vessels that provide blood supply to the cerebellum (the basilar and vertebral arteries) also supply the brain regions responsible for these other functions.”
Hallmarks of cerebellar stroke — location, location, location
As with strokes in the cerebrum, the deficits from cerebellar stroke depend on where the injury occurs. “For instance, a stroke in the anterior lobe will produce difficulty with the coordination of arms and legs and clarity of speech. Gait is impaired. People can stumble and fall,” Schmahmann said. “Somebody with a stroke in the posterior lobe may have difficulties with nausea, vomiting and vertigo early on and then be left with some cognitive issues.”
A large stroke in the posterior lobe can cause the cerebellum to become swollen. Schmahmann tells us that this swelling can be life-threatening because it can compress the brainstem and that can lead to death.
Depression can also occur, especially in strokes that involve the midline of the cerebellum, which influences mood regulation. Depression is generally improved with appropriate treatment. Treatment can include medication as well as talk therapy, “The nice thing about knowing that is, you can treat it with medications and support the patient and family with counseling and care, which makes a big impact on the patient’s quality of life,” Schmahmann said.
Schmahmann points out that three things affect the prognosis for cerebellar stroke:
- The size and location of the stroke.
- The need to determine a cause so that the risk of another stroke can be lessened or eliminated, especially if an embolism caused the stroke.
- Getting plenty of appropriate rehab.
As much as we know about the cerebellum, there are relatively recent things we’ve learned and more to discover. Schmahmann sums up, “The cerebellum is part of the neural circuits engaged in a diverse array of human functions, whether they’re motor, intellectual, emotional or psychological.
“We’ve learned that large parts the cerebellum are linked to the non-motor areas of the brain. We have a clear sense of the cerebellar motor syndrome, which has been identified now for a couple hundred years, including gait imbalance (ataxia), impaired control of arm and leg movements (dysmetria), and dysarthria (slurred speech).
“What has been identified recently is the cerebellar cognitive affective syndrome in people who have cerebellar injury. The importance of this going forward is the big question: How much is there still to be discovered about the role of the cerebellum in the other psychiatric diseases that we’re still trying to figure out? Because so much of the cerebellum is tied to the areas of the brain concerned with higher-order function, the prediction is that in the psychiatric diseases in which there seems to be some cerebellar pathology, the role of cerebellum will become increasingly important and this is relevant for understanding disease and also for treatment options.”