Functional Disorders Presenting to the Stroke Service

Introduction

About one-quarter of patients admitted to specialist stroke units will be discharged with a diagnosis other than stroke; these patients are commonly referred to as stroke mimics . Functional neurological disorder (FND) is one of the commonest causes of stroke mimic, representing with a reported range of 7–45% . FND can be broadly defined as a condition characterized by neurological symptoms that lack internal consistency, are genuine, but not explained by a defined disease process. Also called conversion disorder, or referred to as psychogenic, FND is common in neurological practice making up about 16% of new referrals to neurology outpatient clinics .

Within specialist stroke services, FND or functional stroke mimic (FSM) represents a reported range of 1.4–8.4% of all admissions and 0.5–5% of all patients who receive intravenous thrombolysis for presumed stroke (23–47% of stroke mimics receiving thrombolysis) . Large discrepancies in the numbers exist in the literature and are related to the difficulties of case ascertainment. Many studies are likely to underestimate the incidence of FND as these patients may be represented in other diagnostic categories such as migraine, hemiplegic migraine, seizure, vertigo, or may be discharged without a clear diagnosis.

Etiology

The etiology of FND is not fully understood and there is debate over the relative contribution of psychopathology. Disagreement here is represented in the different preferences of terminology. The terms conversion disorder and psychogenic presume an etiology based in psychopathology. Critics of these terms point out the absence of clear or substantial psychological factors in many patients; problems explaining symptom mechanism in purely psychological terms and the limitations to research and treatment imposed by a monothematic etiological model . Attributing all FND to psychological causes is a bit like blaming all strokes on smoking. A full discussion of etiological theories is beyond the scope of this brief introduction to FND. In summary, evidence from clinical and laboratory studies support the concept of a biopsychosocial etiological model. Functional neuroimaging studies, while limited by low numbers and mixed results have shown hypoactivity in areas associated with action selection and abnormal connections between limbic structures and motor areas . Clinical and laboratory studies of symptoms demonstrate the central importance of abnormally focused attention directed toward the body . When this attention is directed away from the body, symptoms resolve to a greater or lesser extent. Beliefs and expectations about symptoms and an abnormal sense of agency have also been described as part of the mechanism driving FND. A biopsychosocial etiological model that is generally well accepted considers symptoms in terms of a heterogeneous mixture of predisposing, precipitating, and perpetuating factors . We encourage clinicians to consider mechanism of the symptoms. For example, a panic attack (causing unilateral sensory symptoms), acute dissociation, migraine, physical injury, asymmetric pain, and fatigue are all physiological experiences that can form the basis for FND .

Diagnosis

Differentiating symptoms due to FND from stroke can be particularly challenging in the emergency department, when the patient may be anxious and confused, and there is pressure to make an early diagnosis within the window of opportunity to receive intravenous thrombolysis. There may be clues such as a gradual onset, a dissociative (nonepileptic) seizure or a history of multiple previous functional symptoms. Psychological problems such as anxiety, depression, and recent stress are more common in functional disorder but not universal and should not be used to make a diagnosis . The key to a positive diagnosis is positive identification of internal inconsistency during the physical examination or incongruity with recognized neurological disease .

A number of clinical signs have been described to identify FND, with high specificity and sensitivity, Hoover’s sign for functional lower limb weakness may be the most useful ( Fig. 121.1 ). This describes weakness of hip extension that returns to normal power with contralateral hip flexion against resistance. Pain on assessment or sensory inattention/neglect may lead to false positives on some tests. Parietal stroke is especially prone to lead to an apparent “functional” presentation. Nonetheless this is a sign that appears to perform reasonably well in an acute stroke setting. For example it had a sensitivity of 63% and a specificity of 100% in one study of 337 patients with suspected stroke .

Functional facial symptoms are common, usually due to muscle overactivity, typically orbicularis oculi, orbicularis oris, and platysma muscles with a depressed eyebrow or pulled down mouth. This can give the appearance of combined upper and lower facial weakness and is one of the few stroke mimics that may pass a FAST (Facial drooping, Arm weakness, Speech difficulties, and Time) test, but when combined with unilateral functional limb weakness, should be a red flag for a functional disorder ( Fig. 121.2 ). Examples of incongruity with recognized clinical disease include midline splitting of sensory disturbance, global pattern of limb weakness, and a tubular visual field ( Table 121.1 ) . In general, it is easier to base the diagnosis on motor symptoms, as they are more amenable to objective examination, whereas sensory symptoms are subjective experiences. Where sensory symptoms dominate the history, a thorough physical examination often reveals new or initially unobserved motor symptoms that can support the diagnosis .