Neurobiology of FMD
The fact that functional movement disorders (FMD) are not strange, mysterious or particularly unexplained phenomena was emphasized during a workshop presentation at the EAN 2019 Congress.
Functional disorders are not strange, mysterious or particularly unexplained phenomena
An FMD is a condition where a person experiences abnormal function in a system that is capable of normal function when attention is diverted from the abnormal movements or from peripheral signals.1Understanding the neurobiological mechanisms of FMD allows us to explain “how things have gone wrong”. However, in order to understand how functional disorders occur, we first need to appreciate the process of normal functioning.
Understanding the neurobiological mechanisms of FMD allows us to explain “how things have gone wrong”.
The brain as a processor
The brain is an organ that mediates the relationship between “us”, our bodies, and the world. There are two types of data sources that guide action and perception in a dynamic fashion:
- Sensory data (feedback) – “bottom up”
- Predictions about (future) sensory data (feed-forward) – “top down”.
Our experience of our bodies and the world is a fluid, changeable phenomenon modulated by predictive systems that play a role in “ownership” over our bodies and movement. Functional disorders are a problem at the interface between you and the body itself at the level of predictions and attention to the body. In FMD, movement is not identified correctly as coming from the self, and so is experienced as “out of control”.2,3
In FMD, movement is not identified correctly as coming from the self, and so is experienced as “out of control”.
FMD commonly occur after a physical precipitating factor or event, e.g. after injury, infection, neurological disorder, pain, adverse drug reaction, or surgery.4-6 The new salient information about the body acts as a trigger of malfunction of the predictive systems.
Stressful life events and maltreatment may be considered as risk factors for FMD rather than causes.7 Neurobiological interest focuses on how these risk factors generate vulnerability in relevant systems.
Epidemiology of FMD
New cases of FMD arise in 4-12 people per 100,000 annually, and FMD have a prevalence of 50 per 100,000 in the community among whom 5% of cases are referred to a Neurology service.8 The age of onset of FMD is 35-50 years, and women are more likely to be affected (60-85% of cases).
FMD typically manifest as tremor of the limbs, dystonia, myoclonus, gait disorders and Parkinsonism. FMD are common, persistent and associated with disability, and may be diagnosed by a combination of clinical history, examination and electrophysiology.
FMD are common, persistent and associated with disability
Management of functional disorders
The management of functional disorders is multimodal and may include physiotherapy, neurology, psychiatry, and chronic pain services.9 Currently, obstacles to effective treatment include limited evidence from randomized controlled trials, an absence of guidelines, and lack of determinants of outcome for different approaches. Further elucidation of the neurobiology of functional disorders may lead to new treatments.