Anatomy Associated with Ataxia
As mentioned early, the most common problem in damage to the cerebellum is limb ataxia as well as ataxic gait. How does the cerebellum get damaged? It can get damaged by stroke, disease, tumor, and any number of things. It results in this high amplitude tremor that accompanies the movement. Ipsilateral ataxia occurs when we have damage to only one cerebellar hemisphere. Remember that our cerebellum is divided into two hemispheres, right and left hemisphere much like our cerebral cortex is. However, the difference is if we have damage to the right hemisphere we expect to see problems on the left in our cerebral cortex. Contralateral expression is therefore seen in the cortex versus in the cerebellum, you expect to see ipsilateral expression.
So why is it ipsilateral? Pathways from the cerebellum to the lateral motor systems and then to the periphery are what is called double-crossed. So if we think about the reason why fibers from our right cerebral cortex innervate things on the left is that there's one crossing the decussation on the pyramids at the brainstem level. Well in the fibers that go from the cerebellum to the lateral motor systems and then to the periphery, they're double-crossed. They cross twice. Which means that they end up innervating the same side. The first crossing occurs at the cerebellar output pathway's exit in the decussation of the superior cerebellar peduncle. The second crossing that occurs as the corticospinal and rubrospinal tracts descend to the spinal cord. So you'll see those ipsilateral signs when you have a patient with cerebellar damage.
Ataxia can result from damage (in addition to the actual cerebellum) to any of the pathways providing cerebellar input or output. This may include the dorsal and ventral spinal cerebellar pathways, the pontine nuclei, the three cerebellar peduncles. Damage in any one of these places, which is essentially in the brainstem, could cause the same sort of ataxia that we see when there's actual damage to the cerebellum.
What does this mean when we have a patient that has some type of brainstem lesion? It can impact the cerebellum. Think about the close proximity of your brainstem to your cerebellum. The cerebellum sits on top of the brainstem. So because of that close proximity and because of the damage to these connections, you could have a lesion that's technically inside the brainstem, but if it affects one of these three areas here or three categories here, then you get damage to the cerebellum and you get ataxia as a result of that. You also can see that damage to structures that receive cerebellar input such as the thalamus can cause ataxia. So some of our patients that have a stroke or some other disorder that impacts the thalamus can result in ataxia. As well as demyelinating diseases in the cerebellum or related pathways can result in ataxia. Oftentimes people with MS will experience ataxia because of the demyelination of fibers and pathways that are related to the cerebellum.
Most people oftentimes think of cerebellar ataxia when discussing types of ataxia, but there are actually quite a few different ataxia classifications. Cerebellar ataxia is damage to the cerebellum and or the connections to the cerebellum like we just talked about. This results in upper limb incoordination, disturbance of posture and walking, dysmetria (overshooting or undershooting like we discussed before), dysdiadochokinesia, nystagmus or dysarthria. Dysdiadochokinesia which means that inability to perform rapid alternating movements. To test for it, we have the patient take their right hand and place it on their thigh in pronation, and then we have them alternate back and forth between supination and pronation. We have them tap their toes or we have them alternate tapping their right and left foot as well to test for dysdiadochokinesia. Nystagmus is those rapid eye movements that we see sometimes involved in vestibular issues. Dysarthria is a motor problem with speech. Oftentimes in cerebellar ataxia, the symptoms that we see are worse at faster speeds. One of the tricks that we teach our patients is to slow down to try to overcome that. Faster speeds tend to sort of enhance the ataxia that they have.
Sensory ataxia is ataxia following disruption of proprioceptive input from the periphery. Damage may occur to either the afferent component of the peripheral nerves, the dorsal nerve roots entering the spinal cord, the dorsal column of the spinal cord, the medial lemnisci of the brainstem, or some of the sensory receiving organs of the thalamus and the parietal cortex. Ataxia of voluntary limb movement or gait can occur depending on the actual lesion location of the sensory disruption.
Ataxia Differential Diagnosis
Sometimes it may be difficult to differentiate whether or not somebody has cerebellar damage ataxia or sensory loss issue. One of the key things that we can do here is to look at what happens when we take away vision or when vision is present. For example, if you have the person close their eyes, you'll see that with sensory ataxia, they have a marked worsening of their ataxia when their eyes are closed. If it's cerebellar ataxia, you really won't see much change at all when you have them close their eyes. Now, I will say that whether it's sensory ataxia or cerebellar ataxia, both of those may cause an increased reliance on visual feedback during movement. If there's something going on where I can't coordinate my movement well, obviously a key strategy for me would be to use vision to help me more. That is a common strategy that patients with any kind of ataxia will use, but when you take vision away completely, if it's sensory ataxia they're gonna have a marked worsening of their symptoms.
Vestibular ataxia results from damage to the vestibular cochlear nerve, cranial nerve eight, or some of its central connections. The central connections for the vestibular cochlear nerve are the brainstem and the vestibular nuclei. With vestibular ataxia, cerebellar damage occurs that also impacts the vestibular cochlear nerve or its connections. Therefore, not only are you getting ataxia from the cerebellar damage but on top of that, you have vertigo, nausea, loss of balance and nystagmus, which is a bad combination of symptoms. Again, because of the cerebellum's close proximity to the brainstem, if you remember back to your brainstem anatomy, our vital functions and a lot of our cranial nerves are there. So this is a cranial nerve that's often involved when you have cerebellar damage and can further complicate the ataxia.
Episodic ataxia is an autosomal dominant disorder. It's a hereditary disorder with sporadic bouts of ataxia with or without myokymia. Myokymia is defined as continuous muscle movement/tremors of various muscle groups. There are at least seven subtypes, but essentially this is, as it says, a sporadic kind of ataxia, sometimes provoked by stress, startle heavy exertion. It can first appear very early on in infancy. Ataxia is the most common symptoms of this particular type of hereditary disorder, and it's caused by this misfiring of Purkinje cells in the cerebellum. There are several different classifications such as EA1 which is improper regulation of these cells and EA2 which is the malfunction of these cells. They are all based on exactly what's going on wrong with these cells.