March 20, 2023
Michael Merzenich, PhD

When I read the news that Bruce Willis had disclosed his diagnosis of frontotemporal dementia (FTD), I was reminded that all of us are at risk for spending our final epoch lost in a neurologic swamp. What is remarkable about the swamp that we call FTD is that it’s a somewhat rare and unusual type of dementia. We tend to characterize dementia as the erosion of memory, but FTD is more characterized by the loss of control over emotions and other cognitive functions. What’s especially tragic for performers like Willis is the loss of the verbal fluency required for delivering one’s lines.

Frontotemporal Dementia
To this casual observer, Bruce Willis was an almost invincible force, vigorous, vital, one of the “immortals.” Alas, with his FTD diagnosis, we know that even a die-hard like Willis, now only 67 years of age, may have to endure years of progressive decline. If the disease follows its typical path, that will probably include slowly disconnecting and progressively losing emotional judgment and control as well as losing a reasonable understanding of what or why any of it is happening. He may also experience a progressive deterioration of the control of bodily functions and general health.

Most people with dementia lose their neurocognitive abilities through a number of different pathways, all of which result in brain shrinkage, disconnection, evident neuropathology, neurobehavioral expressions of loss, and forms of befuddlement. Alzheimer’s disease leads the list as the most common form of dementia, but vascular dementias; dementia with Lewy bodies; “mixed” dementias; dementias associated with Parkinson’s, Huntington, or other diseases; dementia rising from alcoholic or other brain poisoning, HIV, Lyme disease, or a host of other brain infections; or from traumatic encephalopathy (chronic or more current) may present at any active neurology clinic. These are what you might think of as your “grandpa’s dementia” — the common types often associated with old age.

FTD is a particularly interesting variant for several reasons. First, it usually arises in relatively young individuals, with initial symptoms emerging in one’s 50s or 60s. In most cases, there is no genetic and, with rare exception, any other explanation of origin — except that old medical standby, bad luck.

Second, FTD has little initial impact on a patient’s broader memory and associated cognitive abilities. The patient will stumble to come up with that next word and ultimately slow down their speech as their brain struggles with verbal fluency; they will struggle with translating their feelings and emotions into fast and appropriate actions expressed in their mind and their physical body while their memory will appear intact.

In all other dementias, cognitive losses can be profound, whereas social and emotional control and voluble speech production are generally better sustained. Imagine the impact that these struggles in verbal fluency and in emotional calibration and response must have for an established actor. By all reports, Willis vigorously pursued the work that he loved right up until the time of his dementia diagnosis, even as his colleagues would almost certainly have seen that he was struggling. Sadly, a lack of that type of self-awareness is an expected consequence of FTD.

The Salience Network and von Economo Neurons
Third and most intriguing to a neuroscientific nerd like me is that patients with FTD experience an initial loss of a special population of cortical neurons located within the salience network in our brains, called the von Economo neurons. That salience network is designed to quickly read and evaluate our complex thoughts and emotions and via those Economo neurons, initiate appropriate neurologic and physical responses.

We share this special von Economo machinery with great apes, whales, elephants, and a handful of other especially social mammalian species.

When we see or hear or otherwise sense something that induces fear, alarm, or a potential reward, the salience network in our brain acts as a kind of gatekeeper. First, it assesses the emergent or changing situation, then it rapidly initiates an emotional and physical response. As I sit with a patient in obvious distress in my office, my salience network turns on an empathetic alarm. My brain and body immediately adjust to initiate appropriately sympathetic reactions. The von Economo neurons — those very neurons that have substantially died off in a brain with FTD — are the linchpins in this fast-response emotion and complex body signal-informed system.

Controlled emotional response is at the heart of our humanity. It’s a sad day when we lose it.

In other neurologic clinical conditions marked by the loss of specific brain cells, different forms of “disuse atrophy” are partly the cause. We don’t know whether that’s the case for FTD. Scientists have shown that specific forms of computerized brain exercises can sharply increase activity levels in the salience network which is linked to improvements in the regulatory control of the autonomic nervous system — one of the key response-mediating targets of the network’s von Economo neurons.

Interestingly, super-agers who sustain body and brain health into their 90s (and beyond) die with a full complement of von Economo neurons operating happily in a still-vigorous salience network.

This neuroscientist can foresee a day when we routinely assess the integrity of this important brain system and more reliably maintain its good health. Keeping those very special neurons alive would have probably allowed Willis to sustain himself on the soundstage and on the grander stage of life for a long time to come. Alas, like so many things in medicine, there is promise. But at this moment for this famous patient, our current medical science appears to be a day late, and a dollar short.