Wednesday, June 6, 2012

Alzheimer's Disease: PET scan article to me by a patient. Interesting.

A patient of mine came in with her very concerned and caring husband yesterday. She was having subtle memory difficulties which did not affect her overall ability to function well, but raised their concern for her developing Alzheimer's dementia in the future (if not already), given that her mother developed dementia late in life.

This is not going to be an entry about the dozens of other causes of dementia besides the Alzheimer's type. This is not going to address the fuzzy inheretence pattern, nor the specifics of Mild Cognitive Impairment (MCI) versus definitive dementia. This is not going to address treatment. Those are topics that need to be addressed in other entries.

This is just to address the PET scan and the understandable desire by family to diagnose Alzheimer's dementia as early as possible.

What is a PET image? And how is this different than a CT scan of the brain or MRI?

--In short, a CT scan shoots low-level radiation at you; an MRI uses a powerful magnet to "pull" on water hydrogen/water molecules in your tissue (no radiation); and PET scan detects radiation from a radiated substance (safe) put in the bloodstream.
--The PET scan stands for Positronic Emission Tomography.
--It creates a 3D image.
--Essentially, sugar is made very slightly radioactive (generally safe), and injected.... Then this machine watches the brain use the sugar in its regular metabolic processes and sees where it lights up. The more sugar is used, the more metabolically active it is. Now, we have done enough PET scans of "normal people" of every age group to know what normal baseline metabolism is (how much sugar is used) in all the different parts of the brain. So the PET scan image of your brain is compared to these normal values, and if yours is less active in a certain area, then that can mean something.
--The advantage is that it can detect very subtle changes, and is great for things such as looking for sneaky cancers that won't show up on regular imaging, or looking for subtle changes in brain activity which can't be detected by MRI or CT.
--It's also great for detecting the difference between different types of dementia potentially, which is helpful for studies in the literature to help develop and tailor new treatments to specific dementias (such as Lewy Body Dementia versus Alzheimer's versus Frontotemporal Dementia, etc.).

Here is what a series of PET scan images might look like in an Alzheimer's patient.

So what this shows to the eye of your neurologist/neuroradiologist is hypometabolism (less uptake of sugar) in the posterior (back) parietal and temporal lobes (see below). This hypometabolism is compared to other people's brains of the same age without any concerns of thinking difficulty. This PET scan finding would be characteristic of Alzheimer's Disease and might not be seen on MRI or CT.

It costs about $3000 to $6000 depending on who owns the machine (A hospital-owned machine costs more). The cost is broken down into the Radiopharmaceutical fee ($ for the radioactive element used), the Technical fee (where most of the money goes... the actual profit for whoever owns it), and the Doctor's fee for actually reading it (By far the the least part actually).

So here is my opinion on it:
Unless you are in a study for an academic reason, it's a waste (for dementia). You know what's better than a PET scan for subtle changes? The human brains around the patient. We have a great capacity to recognize subtle changes in people, and if enough people notice a change that is concerning for dementia, or its oft-precurser Mild Cognitive Impairment (MCI), then it's time to start treatment or get quality psychological testing (part IQ test, part a bunch of other tests) which can be just as sensitive, tell us more about the areas of thinking difficulty, and is far less expensive. By the time the level of concern is high enough to seek out a PET scan for "possible" dementia, you either already have it, or you have something else that is mimicking it, and you're just wasting time trying to define it further.
In general, as a public, we are often too hung up on some definitive positive or negative tangible test, instead of the clean and just-as-accurate case which can be built by the history, family & friends' suspicion, the exam, and clinical accumen of your neurologist/primary care doctor. I tease out different forms of very early dementia and Mild Cognitive Impairment on a weekly basis without the need for a PET scan and without any significant delay of treatment.

Lastly, there's controversy about whether or not amyloid mentioned below is even specific to Alzheimer's alone... which means if you diagnose some other dementia early, then it may not be responsive to the same medications anyway. So you would have to go back to the history to see if their changes clinically fit Alzheimer's Disease or if it's some other dementia... which is where we started anyway.

So, what became of the pleasant woman who came in with her husband and this article below?
She had a severe B12 deficiency due to an absorption issue, not dementia at all. We figured it out based on history, looking over her other medications, an exam showing markedly decreased vibration sensation and toe proprioception (location of toes when your eyes are closed and I reposition them)... and a cheap B12/Methylmalonic Acid/Homocysteine blood test.
Her symptoms resolved with supplement.