FND Might Be Nothing More Than a CNS Reflex

07 Jul 2025 Christoff Truter Spousal caregiver

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What do I mean when I hypothesize that Functional Neurological Disorder (FND) might be nothing more than a central nervous system reflex?

Let me explain by drawing a parallel with referred pain.

Referred pain occurs when an issue in one part of the body causes pain somewhere else. For example, a heart attack can cause pain in the jaw, left arm or shoulder, even though the origin of the problem lies in the heart, not those other areas. Similarly, irritation in the spinal cord can manifest far from its source. Sciatica, for instance, causes pain down the leg even though the root cause lies in the lower back.

If we propose that something similar happens in the central nervous system, where signals are misrouted, amplified or misinterpreted, then FND could be understood not as a brain disorder in itself, but as a knee-jerk reaction of the CNS in response to some other trigger.

In this light, FND is an expression rather than the origin of the problem. Instead of being comorbid with other disorders, it is simply a reaction to other disorders, much like a car's engine light indicating an underlying issue with the engine rather than a fault with the light itself.

The implication is significant: doctors may be looking for answers in the wrong place. It might not always be a disorder originating in the brain at all, but rather a reaction to something external, such as a systemic, immune, inflammatory, or structural trigger that the central nervous system is merely responding to. This could explain the persistent failure to identify an organic cause, a topic I will explore in much greater depth in future posts (e.g. how organs like the liver influences the CNS, vestibular system involvement, neurovascular etc).

There are some historical parallels here as well, back in FND's wild days, illnesses like endometriosis were effectively diagnosed as FND, or more specifically, as hysteria, conversion disorder, or other so-called “functional” or “psychogenic” conditions, because its symptoms were real, disabling, and invisible to tests of the time. Endometriosis and the History of Hysteria" (article in The Journal of Medical Humanities, 2010)Lesley A. Sharp, "Hysteria and Women’s Health: A Historical Perspective," in The Lancet Psychiatry, 2018

The confusion is understandable, given that endometriosis can produce symptoms that mimic neurological conditions, particularly when nerve involvement leads to muscle weakness, numbness, or tingling. For instance, a case study reported a woman with endometriosis who experienced foot drop and cyclic sensory disturbances. In some cases, endometriosis can affect the central nervous system, including menstrual-linked headaches, seizures, visual disturbances, motor and sensory deficits.

This diagnosis is historically riddled with real underlying causes, such as Multiple Sclerosis The emergence of multiple sclerosis, 1870-1950: a puzzle of historical epidemiology Functional neurological disorder and multiple sclerosis: a systematic review of misdiagnosis and clinical overlap Systematic review of misdiagnosis of conversion symptoms and "hysteria" , Parkinson’s disease In Men, It's Parkinson's. In Women, It's Hysteria. From Hysteria to Parkinson's: An Introduction , epilepsy Epilepsy, hysteria, and "possession". A historical essay , hypothyroidism & hyperthyroidism Does Sex Bias Play a Role for Dissatisfied Patients With Hypothyroidism? , lupus For Some Women With Serious Physical Ailments, Mental Illness Has Become a Scapegoat Diagnosis [source-reference], Lyme disease [source-reference] Neuropsychiatric Lyme Disease: Infection, Not Mental Illness Mistaken Identity: Many Diagnoses are Frequently Misattributed to Lyme Disease , fibromyalgia Functional neurological disorder and multiple sclerosis: a systematic review of misdiagnosis and clinical overlap (incl fibromyalgia) , and chronic fatigue syndrome Conversion disorder Ending the Somatization Myth in ME/CFS (or "Who's the Deluded One Now"?) An Exploration of How Functional Neurological Disorder Is Discussed on X (Twitter): Mixed Methods Study Using Social Network and Content Analysis , to name just a few.

If we get distracted by what the CNS is doing, give that distraction a name and an address, and stop searching for what might be triggering that reflex response, we risk overlooking the real source of the problem, just as doctors once (often still) did when they focused only on referred pain without investigating its true cause.