Functional (psychogenic) movement disorders (FMDs) are part of the spectrum of functional neurologic disorders. They are common, can be disabling, but are treatable in many patients, even to the extent of complete recovery. However, doctors commonly report that they find such patients difficult to manage and that they are amongst their least favorite patients to see and treat (1).

TERMINOLOGY AND DEFINITION: A DIFFICULT TRANSITION

Historically and still currently, the term “psychogenic” has been most commonly used to describe such patients. This term has referred to disorders characterized by physical symptoms—which in the case of FMD are specifically abnormal movements (gait disorders, tremor, dystonia, etc.)—which are incongruous with movement disorders known to be due to organic disease and which are instead attributed to underlying psychological difficulty. In psychiatric terminology, such patients would be classified as suffering from “conversion disorder”—that is experiencing (real and involuntary) physical symptoms generated by the “conversion” of psychological stress/trauma into physical symptoms via an undefined process.

As has been argued elsewhere (2), this terminology has a number of difficulties. First, the presence of psychological stress or trauma is not a diagnostic requirement for the diagnosis of FMD. Indeed, rates of adverse life events, either recent or in the distant past, are not very different between patients with FMD and those with organic movement disorders (3,4). Patients with organic movement disorders commonly have psychiatric disturbance as a clinical feature. Even the latest edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), has downgraded the requirement for a psychological stressor to be identified from a feature that must be present to one that might be present. In DSM-5, the disorder is relabeled as “functional neurologic symptom disorder.” Second, the term “psychogenic” shifts focus away from the positive clinical features that are used to make the diagnosis, most of which rely on demonstrating that normal movement is possible when the patient is distracted or movement is accessed in a different way (including with the use of suggestion). Third, the use of the term “psychogenic” implies that the only treatment that is going to be of help to the patient is one that relies on discovering and treating the underlying psychological stress/trauma. However, evidence suggests (see treatment below) that specific physical therapy approaches can have significant and lasting benefits in this group of patients. Fourth, regardless of whether the physician using the term “psychogenic” is meaning that the patient has a genuine involuntary movement disorder, public (and therefore patient) perceptions of the prefix “psycho-” are generally that this applies to problems that are trivial, “made up,” or reflect mental frailty (5,6).

We therefore prefer to define this disorder according to a clinical appearance as a movement disorder that is significantly altered by distraction or nonphysiologic maneuvers (including dramatic placebo response) and that is clinically incongruent with movement disorders known to be caused by neurologic diseases (7). We would prefer the term “functional” as a broader label, while acknowledging that this term also has a number of difficulties (2). These include the historical use of the term “functional” to apply to any disorder where no apparent structural lesion exists, for example, Tourette’s syndrome, primary dystonia, the replacement of the false dichotomy of psychological vs. physical with another false dichotomy of structural vs. functional, the long history from a clinical and research perspective of the use of the term “psychogenic movement disorder,” and the possibility that the use of the term “functional” allows the avoidance of discussion by doctor and patient of psychological triggering and maintaining factors which are often relevant and require psychological approaches to treatment.

There is in the end no right answer currently to this terminology debate. Our suggestion is that regardless of the term used, the complex nature of these disorders needs to be acknowledged and an individual approach taken to diagnostic explanation and treatment planning. This approach should assume neither that explanation and treatment can only be considered from a psychological dimension nor that a purely “biologic” approach is likely to be a helpful mode of management for patients.

PHENOMENOLOGY

Below, we consider the main phenotypes of FMD. In Table 43.1, we have summarized some of the main positive features on history, examination, and tests that can help distinguish FMD from organic movement disorders. We have included some examination features that are seen in patients with functional weakness, such as Hoover’s sign. We have done this because patients with FMD commonly have other functional symptoms and signs, particularly weakness of the limbs.

FUNCTIONAL TREMOR

Functional tremor is typically variable in frequency, direction, and amplitude; it is usually present at rest, on posture, and during action. It typically varies with distraction in both distribution and frequency (8) and can be precipitated and exacerbated by suggestion. Onset is most often sudden, and as with other FMD, patients may report periods of remission and sudden worsening.

Diagnostically, the most useful maneuver is the “entrainment test.” Here, the patient is asked to tap with a limb (typically the hand) at a frequency set by the examiner which is different from the frequency of the tremor (9–11). Entrainment strictly speaking applies to the situation where the frequency of the tremor shifts to exactly match the frequency of the tapping. This is actually quite a rare finding: much more typically the tremor will pause briefly and shift in frequency for short bursts. If tremor recordings are being done, this will be reflected as a shift in tremor frequency or increased variability of the tremor frequency. Another important clinical sign during this test is “poor task performance”: here the patient is unable to perform the simple tapping task set by the examiner. The explanation is that the performance of the functional tremor draws so much attentional resources that the patient is unable to perform the simple tapping task as well. This is an example of a “dual task” effect. It is useful to vary the speed of the tapping that the patient is required to do to make the task more demanding (9–11). Another useful clinical sign in upper-limb functional tremor is “pause with ballistic movement” (12). Here, the patient is asked to reach out with one hand as quickly as they can to a target (for example, the examiner’s finger) when instructed to. A short (1–2 second) pause in the functional tremor of the other hand is typically seen.

Functional tremor of the legs is usually generated using clonus mechanisms with the patient sitting with foot plantar-flexed with the forefoot in contact with the floor. An internal rotation of the knee and the hip is often associated. A useful maneuver in clinical practice is to ask the patient to sit with flat feet, pressing the heels against the floor, which often stops the tremor. The entrainment test can also be performed in the legs with the patient asked to perform foot taps in time to a frequency set by the examiner. Functional head tremor tends not to entrain well with typical flexion/extension tapping movements of the fingers: pronation/supination movements of the wrists and movements of the protruded tongue back and forth are more useful. Functional tremor of the palate can also occur: this typically entrains with flexion/extension movements of the fingers (13–15).

Although a confident clinical diagnosis of functional tremor is often possible, in some cases, a “laboratory-supported” level of certainty might be useful, particularly for research, but also for patients where clinical diagnosis is difficult (for example, patients with overlay of functional tremor on organic tremor). A combination of electrophysiologic tests has been suggested to distinguish functional and organic tremor with acceptable sensitivity and specificity (16). Simple tremor analysis (i.e., frequency and amplitude) is generally not very helpful diagnostically; however, the combination of electromyography and accelerometry can be useful, showing, for example, a tonic discharge of antagonist muscles approximately 300 ms before the onset of tremor bursts, (8) an increase of tremor amplitude in response to weighting the limb (8), entrainment or an increase in variability and change of tremor frequency while tapping with the contralateral hand (17–20), and transient arrest of tremor during a ballistic movement of the other hand (12). The combination of these tests as a diagnostic tool shows promise (16), but awaits confirmation in a larger cohort of patients.

FUNCTIONAL DYSTONIA

Distinction between organic and functional dystonias is a clinical challenge, since these two entities share similar characteristics with a wide and overlapping spectrum of presentation (21,22). Historic sensitivities also exist as most types of dystonia were at one point labeled as psychogenic. There is still an unfortunate lack of positive clinical signs to differentiate functional from organic dystonia. This leaves clinicians with a diagnostic approach based on recognition of certain patterns which are commonly seen in functional dystonia and with an important reliance on familiarity with patterns of organic dystonia which are varied and may be “bizarre” when first encountered. The three common patterns of functional dystonia are “fixed” dystonia, functional facial dystonia, and functional paroxysmal dystonia.

Fixed dystonia is most often seen after a minor injury to a limb, or less commonly following operation to a limb or casting following an injury (23). Most patients are female and in their 20s to 30s. Joint hypermobility syndrome (Ehlers Danlos type III) appears to be a risk factor for development of fixed dystonia (24). Typically, a fixed abnormal posture of the limb develops suddenly after the injury, often accompanied by pain. The commonest postures are fixed flexion of the fingers and wrist sparing the thumb and forefinger and planter flexion and inversion of the foot. Some patients develop fixed abnormal postures of the neck following neck injury (25

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