Unlocking the Freeze: How Stress Impairs Movement in Functional Disorders
"New research reveals the hidden link between stress, body movement, and functional movement disorders, offering insights into potential treatments and therapies."
Functional neurological disorders (FND), often misunderstood and characterized by symptoms like paralysis, tremors, or dystonia, have long puzzled medical professionals. These conditions, where neurological symptoms occur without visible lesions in the nervous system, affect a significant portion of the population, with up to 18% of neurological patients presenting such 'unexplained' symptoms. Understanding the intricate relationship between the brain, stress, and motor function is crucial for effective diagnosis and treatment.
Stress, a well-known trigger for various health issues, has been increasingly recognized as a key factor in FND. Psychological and physical stressors can initiate or exacerbate motor symptoms, highlighting the need to investigate how stress influences automatic motor behaviors. One such behavior is the 'freeze response,' a defensive mechanism characterized by decreased body movement and, sometimes, bradycardia (slowing of the heart rate) in response to perceived threats.
Recent studies employing advanced technologies like accelerometers and gyroscopes are providing new insights into postural behavior under stress. These tools allow researchers to analyze movement dynamics in detail, revealing subtle patterns that may be indicative of underlying neurological conditions. By examining how individuals with FND respond to stress-induced situations, scientists hope to unlock new therapeutic strategies and improve the lives of those affected.
Stress Processing and Movement
Patients with functional movement disorders (FMD) show abnormal stress processing. In one study, healthy controls reduced thorax movement during stress, consistent with a freeze response, whereas patients with FMD did not. The same study reported lower movement fractality in FMD patients than in controls, while a separate observational study compared circulating cortisol, the end-product of the hypothalamic-pituitary-adrenal axis, in patients and matched healthy controls.
Diagnosis and Treatment Challenges
The DSM-5 adopted the term Functional Neurological Symptom Disorder or Conversion Disorder, replacing psychogenic terminology and removing psychological stress as a prerequisite for diagnosis. FMD may present with limb weakness, gait disorders, tremor, dystonia, myoclonus, or parkinsonism, often without a single agreed treatment model. Current reviews describe FMD as common but often difficult to diagnose and manage, and psychological interventions research has found little consensus on treatment models.
From Hysteria to FMD
The field has moved from the historical terminology of “hysteria” toward the modern designation functional movement disorder. FMD remains described as occupying a border zone between neurology and psychiatry. More recent work emphasizes that FMD can follow diverse phenotype trajectories over time, while contemporary clinical guidance recommends a clear diagnosis based on positive examination findings, honest expectations, and discussion of potential recovery.
What is the 'Freeze Response' and Why Is It Important?
The 'freeze response' is an automatic defensive mechanism deeply rooted in our evolutionary history. When faced with a threatening situation, our bodies instinctively reduce movement to assess the danger and prepare for action, whether it be fight or flight. This response is characterized by a decrease in body sway, often accompanied by a slowing of the heart rate. In healthy individuals, the freeze response is a temporary and adaptive reaction to stress.
- Reduced Body Sway: Healthy controls show a decrease in body sway under stress, indicating a typical freeze response.
- Lack of Freeze Response: FMD patients do not exhibit this reduction, suggesting an impaired defensive mechanism.
- Lower Movement Complexity: FMD patients show a smoother, more regular movement pattern compared to controls, indicated by lower fractal dimension (HFD).
- Cortisol Correlation: In healthy controls, changes in body sway correlate with changes in cortisol levels, but this correlation is absent in FMD patients.
Evolving Mechanisms
A May 2025 review presents FMD through integrated clinical, neurobiological, diagnostic, and therapeutic dimensions. A separate November 2025 review describes FMD as involving involuntary motor symptoms incongruent with known neuroanatomy. It also reports a shift away from explanations based solely on psychological factors toward recognition of disrupted brain networks as part of the neurobiological basis of FMD.
Limits of Current Explanations
FMD does not appear to be explained adequately by a single framework. Psychological, neurological, and behavioral accounts may each capture part of the disorder, but available summaries indicate continuing uncertainty about mechanisms and treatment. This uncertainty supports cautious interpretation of stress-related explanations rather than treating stress as a universal or necessary cause.
A Multidimensional Comparison
FMD differs from disorders defined by a single structural neurological lesion because its motor symptoms are described as incongruent with known neuroanatomy. At the same time, contemporary accounts increasingly place FMD within neurobiological models involving disrupted brain networks. Comparisons are therefore most useful when they distinguish observable movement symptoms, diagnostic mechanisms, and proposed biological explanations rather than assuming one category explains every presentation.
Moving Forward: Implications and Future Directions
This research sheds light on the critical role of stress and the freeze response in functional movement disorders. Recognizing the impaired defensive mechanisms in FMD patients opens new avenues for developing targeted therapies aimed at restoring normal motor control and improving patients' quality of life. Future studies should focus on further elucidating the neural circuits involved and exploring potential interventions, such as stress management techniques and rehabilitation programs, to help individuals with FMD regain control over their bodies and their lives.
Early Diagnosis and Coordinated Care
Quantitative studies report reduced quality of life and poor well-being among people with FMD. Expert clinical opinion recommends an early positive diagnosis and multidisciplinary interventions. Patient-centered research also describes poor prognosis, substantial physical and psychological comorbidity, unclear pathogenesis, treatment difficulty, and frequent disagreement between healthcare providers and patients.
Tracking Changing Clinical Demand
A September 2025 study examined new FMD consultations from 2014 through 2024 at a tertiary movement disorders clinic. It analyzed consultation trends and the impact of the COVID-19 pandemic using linear and segmented regression. This approach points toward a future in which service demand and changes over time are evaluated systematically alongside clinical research.
Coordination and Access
FMD care may be complicated by uncertainty about diagnosis, differing explanations among clinicians, and limited agreement about treatment. These challenges can make coordinated care difficult and may affect how patients understand their symptoms. A broader response therefore requires communication across specialties as well as attention to both movement symptoms and psychological well-being.
Delayed Recognition and Lived Burden
FMD can produce symptoms that mimic neurological disorders, including Parkinson’s disease, and its diagnosis is often delayed or missed. A 2024 case report source describes stress, trauma, and emotional distress as factors explored in relation to symptom triggering, while also reporting that complete remission is generally poor. A separate 2026 study examined emotional correlates of motor symptoms using real-world patient-reported data from a mobile health tracking platform.