PCS, FND & the limits of Metaphor

This is one of the most challenging and evolving areas I work in. Its one where our models often lag behind our clinical observations.

When someone sustains a mild traumatic brain injury (mTBI) or concussion, it’s common to experience symptoms such as:
• Poor concentration
• Light and noise sensitivity
• Nausea and dizziness
• Headaches

Most people recover fully within a few weeks. But around 15–20% don’t. Their symptoms persist for months—or even years—and are often described under the umbrella of Post-Concussion Syndrome (PCS).

What’s happening in these cases?

One increasingly cited explanation is functional overlay: the initial symptoms, caused by structural or metabolic changes, resolve—but a second layer of functional symptoms emerges and maintains the person’s distress and impairment.

This is strikingly similar to what we see in Functional Neurological Disorder (FND).
In fact, a 2025 Brain Sciences review argues that PCS and FND may lie on a continuum, with overlapping risk mechanisms and similar treatment implications.

But here’s the frustrating bit:
We still don’t fully understand what’s driving these persistent symptoms—whether in PCS or FND.

It’s likely linked to brain networks, altered functional connectivity, and subconscious processes.
And for those living with these conditions, it can be profoundly distressing—especially when they’re told there’s no structural damage to explain their symptoms.

So we fall back on metaphors to explain them:
• Hardware vs Software: the brain’s structure is intact, but the program isn’t running properly
• Nocebo effects: expectations of harm generate real symptoms
• Protective overdrive: the brain shuts things down to reduce perceived threat

Each of these captures something—but none quite hits the mark.

🌀 The reality is more complex.

As neuropsychologists, we’re often tasked with making sense of symptoms that sit in this grey area—between organic injury and functional change, between neurology and psychiatry, between science and metaphor.

Post‑Concussion Syndrome and Functional Neurological Disorder: Diagnostic Interfaces, Risk Mechanisms, and the Functional Overlay Model, Brain Sci. 2025, 15(7), 755.