Causes and Symptoms
Causes of FND
Contrary to popular belief, anyone can experience the sudden onset of FND. Age, gender, mental health, level of education, or other demographic factors do not seem to have a direct bearing on the onset of FND.
FND doesn’t have one single cause. It is generally thought to occur when the brain’s coordination of movement, sensation, or responses becomes disrupted, often after exposure to certain risk factors. These can include stress or trauma (physical or psychological), medical triggers such as infection or surgery, and nervous-system dysregulation from prolonged overload, poor sleep, or chronic pain and fear/threat sensitivity.
People often also have co-occurring conditions like anxiety, depression, PTSD, migraine, or chronic pain, which can contribute to symptom intensity and persistence, even though they aren’t necessarily the only reason FND develops.
FND is usually explained as a problem with how the brain’s networks for movement, sensation, and prediction are functioning, rather than structural damage that you can see on standard tests.
Mechanisms commonly described by clinicians/researchers include:
Miscommunication in motor/sensory control loops: the brain’s “command” signals and the brain’s expectations don’t line up, so movements/sensations can come out differently.
Abnormal attention/threat processing: when the brain treats sensations as especially threatening or “important,” it can amplify abnormal patterns and make symptoms more likely.
Prediction/protective error learning: the nervous system may learn a protective response (like bracing, avoidance, or altered movement) that becomes reinforced over time.
Disrupted coordination and timing: changes in how networks synchronize can produce symptoms that vary, fluctuate, or are influenced by distraction, focus, or context.
Co-occurring nervous-system factors: stress, trauma, dysautonomia, pain, and fatigue can shift baseline regulation and make these patterns easier to trigger.
Symptoms of FND
Functional neurological disorder (FND) is when you have real nervous-system symptoms (like weakness, movement problems, tremor, seizures/blackouts, or speech/swallowing difficulties), but brain/nerve tests don’t show a structural injury or disease that would explain them. The symptoms are thought to result from how the brain and nervous system are functioning—often influenced by stress, trauma, illness, or learned nervous-system patterns—so the symptoms can vary in intensity, fluctuate over time, and sometimes change with attention, distraction, or nervous system activation.
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Functional Movement Disorders can be divided into tremors, twitches and jerks, gait problems (difficulty walking), functional weakness. These functional symptoms are not caused by damage or disease of the nervous system.
Dr. Moenter talks about involuntary and voluntary movements, and how to relate to and potentially ease involuntary symptoms in your body.
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For some individuals with FND, symptoms are more pronounced in the realm of dorsal vagal collapse (not enough activation). When the nervous system is in a state of parasympathetic-dominant hypo-activation, the body shuts down. This shut-down can be experienced as partial or full paralysis or weakness.
Other symptoms that can accompany the dorsal vagal collapse can be the inability to think, slurred speech, drop attacks and blackouts, spaciness and extreme fatigue, loss of bladder/bowel function, collapse, numbness, depersonalization, or derealization.
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Functional seizures, also called dissociative seizures or non-epileptic seizures (NES), are often part of FND. Functional seizures can be viewed as an expression of a sympathetically over-activated nervous system. Dr. Moenter views functional seizures as a “reset mechanism” preventing the prolonged overactivation of the nervous system which could potentially lead to more severe psycho-physiological damages.
A typical feature of functional seizures is involuntary movement that resembles epileptic seizures. Those movements can be micro-movements (twitches, spasms, subtle shaking etc.) or macro-movements (visible large movements of the limps, head, upper or lower body) often with loss of consciousness. Some individuals have described still having self-awareness but not being able to react to the environment (responding to people asking questions, or protecting oneself from physical harm) while also not being able to self-regulate their nervous system enough to stop the seizure activity.
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Functional speech problems can show up in different ways and may fluctuate.
Stuttering can develop in someone who has never stuttered before (an acquired stammer), and speech may become hesitant with gaps or pauses during words, or with a “telegrammatic” style where some shorter words (like “and” or “the”) seem to be left out.
Foreign Accent Syndrome is a rare change in the “melody” and rhythm of speech that can make someone sound as if they’ve developed a new accent; in some cases this has been described alongside functional neurological symptoms, and the way it presents can become more consistent over time.
Slurred speech (dysarthria) may come and go and may sound worse when you’re tired. Clinicians may consider neurological causes such as myasthenia gravis when symptoms worsen with fatigue, but slurred speech can also occur as a functional neurological symptom.
Swallowing problems may include a persistent sensation of something stuck in the upper throat even when you aren’t actively swallowing, often called globus (globus pharyngis); this is commonly assessed by an ENT to rule out a structural cause, and acid reflux can sometimes contribute even if it isn’t the whole explanation.
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Many individuals with FND experience sensory sensitivities or sensory symptoms. Sensory sensitivities can impact all senses (touch, smell, sound, taste, vision, and interoception). Lights can feel too bright, touch uncomfortable or even painful, sounds intrusive, smells overpowering, taste intolerable and the ability to sense inner body sensations (interoception) is often lacking.
The central nervous system is designed to filter out redundant and unnecessary stimuli and organize sensorimotor information into an integrated experience. This process is called the “gating mechanism,” and it may be dysregulated in many clients with FND who describe a variety of sensory challenges that manifest as over-sensitivity or under-sensitivity.
Over-sensitivity, or “sensory overload,” implies a heightened sensitivity to touch, sounds, sights, textures, smells, and other sensory stimuli. Under-sensitivity can manifest as a lack of responsiveness to factors such as temperature, touch, or noise. It is possible that these symptoms are caused by sensorimotor gating impairments that prevent individuals from integrating information from internal and external sources as normal.
Dr. Moenter explains how sensory processing works and how you can regulate and increase your ability to be with sensory stimuli. Learning about the gating mechanism of the brain, your boundary style, and concrete boundary tools might increase your ability to process sensory stimuli and feel less negatively impacted by sensory stimuli.
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