Tag Archives: cerebellum

The hindbrain steps forward

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The cerebellum is gradually being understood as a contributor to cognitive and behavioral function in both in health and disease.  A new publication has teased out MRI changes in the cerebellum that differentiate PSP from other dementing disorders early in the disease.  This pattern could be developed into a diagnostic test and as a marker of disease progression and even as a guide to rehabilitation measures.

The cerebellum is classically thought of as a regulator of movement.  In its most simplistic essence, its job is to put a brake on voluntary movement instructions from the cerebrum.  The cerebellum is guided in this task by perception of the position and motion of the trunk, head and limbs, by the effect of gravity, all complemented by visual input.

More recently, the cerebellum has demonstrated a memory function when it comes to movement regulation (making “muscle memory” more than just a metaphorical expression), and damage to certain parts of the cerebellum can cause a behavior disinhibition and cognitive impulsivity similar to the frontal lobe damage seen in PSP. In that sense, the cerebellum still functions as a “brake,” but on behavior and cognition rather than just on movement.

Now, researchers from the University of California San Francisco have carefully analyzed routine MRI scans from people with dementia arising from a variety of neurodegenerative conditions including PSP.  They specifically quantified gray matter damage.  (Gray matter is brain tissue composed mostly of cell bodies — as opposed to white matter, which is mostly axons.  In the cerebellum, unlike the cerebrum, the gray matter is the deeper layer and the white matter is superficial.)

The figure below shows the principal results. Illustration from Chen Y, et al. Alzheimer’s & Dementia, 2023. The senior author is Dr. Katherine Rankin. Each MRI image has been reconstructed by computer from routine scans to show the cerebellum splayed out flat.  The randomly assigned colored areas represent a loss of gray matter relative to non-demented people of similar age (“Controls”).  Note that the pattern for PSP differs in obvious ways from the other diseases, though at present the differences are only between the averages for groups, not individual differences useful for diagnosis in routine care. 

Notes: The small type abbreviations are the sub-areas of the cerebellum.  AD=Alzheimer’s disease; CBD=corticobasal degeneration; LBD=Lewy body dementia; TDP=frontotemporal dementia with TDP-43 protein aggregation.  It comes in 3 types. “Pick’s” is a form of frontotemporal dementia.  LBD is combined with AD because at autopsy, the former is always accompanied by some of the latter.  This paper did not include Parkinson’s disease or multiple system atrophy, as those diseases rarely include dementia early in the course, the focus of the present study.

The authors conclude, “These findings suggest the potential for cerebellar neuroimaging as a non-invasive biomarker for differential diagnosis and monitoring.”  They hasten to add that to understand the reasons for these different patterns of cerebellar loss, future studies will have to image the areas of the cerebrum where brain cell activity has been lost and to correlate that with corresponding loss of activity in the cerebellum.  That’s called “functional neuroimaging” as opposed to the “structural neuroimaging” of the current study.   

These insights, aside from their qualitative and quantitative diagnostic value, could provide guidance for electrical or magnetic transcranial stimulation (i.e., delivered across the scalp and skull rather than by inserting hardware onto or into the brain) as symptomatic treatment for PSP and the other dementing disorders. 

In through the back door?

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One of my retirement activities is giving an occasional lecture to the neurology residents in my old department at Rutgers Robert Wood Johnson Medical School.  In March, my topic will be the anatomy of the cerebellum.  I’ve never lectured specifically on that before, so I’ve been updating my knowledge and preparing slides.  That led me to something very interesting. 

It turns out that one or more discrete lesions, typically small strokes, of the posterior half of the cerebellum (hence the anatomical connection) can produce behavioral and cognitive abnormalities.  Mind you, the cerebellum has classically been considered only a center of motor control.  But in 1997, Dr. Jeremy Schmahmann, a neurologist at Harvard, and his psychologist colleague Dr. Janet Sherman described what they called the “cerebellar cognitive affective syndrome.” 

This is a mouthful, and “Schmahmann-Sherman syndrome” would have been shorter but more of a tongue-twister.  So the world, as usual, decided to recognize the contribution of the man over that of the woman, and the condition is usually called “Schmahmann’s syndrome.”  In protest, I’ll call it CCAS. 

From: Stoodley CJ, MacMore JP, Makris N, Sherman JC, Schmahmann JD. Location of lesion determines motor vs. cognitive consequences in patients with cerebellar stroke. NeuroImage Clin. 2016;12:765–75.
The Roman numerals are areas of the cerebellum. X, y and z are the three axes of space and the numerical values are the distance of the image plane from a standard reference plane for that axis. Each color is arbitrarily chosen and represents one patient’s stroke(s).

It’s not clear why CCAS arises from disruption of only the back half of the cerebellum, as both halves have pretty much the same kind of circuitry, as far as we know.  The features of CCAS include loss of:

  • executive functions (difficulty with shifting tasks or multitasking, problem solving, planning, organizing, and sequencing)
  • some aspects of affect (disinhibition of speech or behavior, making inappropriate jokes, behaving childishly, inability to suppress laughter or crying, being obsessive or compulsive) 
  • some language functions (loss of fluency of speech, grammatical rule-breaking)
  • some visuospatial skills (inability to copy or understand pictures or to distinguish two objects presented at the same time)

Ring a bell (especially the first two)?  Sound something like PSP? 

We’ve known since its original 1964 description of PSP by Steele, Richardson and Olszewski that the cerebellum, specifically in an important part called the dentate nucleus (because its layers are in a saw-tooth pattern), is a site of tau deposits and cell damage.  But the balance problems and dizziness produced by dentate damage may be swamped by the symptoms from the degeneration in the cerebrum, specifically the basal ganglia.  So, the symptoms of PSP have never been considered to arise importantly from the cerebellum damage.

But now, thanks to Drs. Schmahmann and Sherman, we know that cerebellar problems can cause cognitive and behavioral problems, and that they look like those of PSP.  True, the well-known loss of function in the frontal lobes can explain those symptoms as well.  But here’s the rub:  Cerebellar symptoms in PSP might be amenable to treatment by transcranial magnetic stimulation (TMS).

TMS involves the painless, nearly harmless (as far as we know) application of magnetic fields to the scalp.  It’s an emerging field for a wide variety of neurological problems and has been FDA-approved for depression and migraine.  The European Union has approved it for several other conditions as well, including Alzheimer’s, Parkinson’s, autism, bipolar disorder, epilepsy, chronic pain and PTSD. Adverse effects are very rare, with the most common being fainting.  Some others are seizures, pain, confusion, hearing loss and hyperactivity.  Caution must be exercised in the presence of pacemakers or other implanted or worn devices.  The procedure is typically repeated once or twice a month for six to 12 months.  Unfortunately, Medicare and commercial insurance do not yet cover it, and the cost is several thousand dollars for a course.

In PSP, seven studies of TMS have been published, involving a total of only 47 patients.  Two of the seven were single cases reports.  Three of the seven, involving 32 patients, stimulated over the cerebellum. They all reported modest improvement in motor symptoms, and two studied reported speech improvements.  None found side effects, at least over short-term follow-up.  Unfortunately, none of the three studies evaluated cognition or behavior in detail.

A closely related technique is transcranial direct current stimulation. Its advantage over magnetic stimulation is that it can reach more deeply into the brain, but with more side effects. It has not been studied as well in PSP as TMS. My November 8, 2021 post was about one such study. I’ll return to TDCS in another post.

So we have work to do.  The frequency of the magnetic impulses, their strength, temporal pattern and precise location could make big differences in the outcome, so this will not be simple.  But if careful study shows that the benefit amounts to even a modest improvement in quality of life for those with PSP, and if Medicare eventually decides to pay for it, let’s get busy.