INTRODUCTION AND DEFINITIONS
Patients with medically unexplained symptoms (MUS) are commonly seen in all medical practices, especially in neurology and psychiatry. MUS comprise up to one-third of new referrals to an ambulatory neurology clinic.1 Many of these patients suffer from a functional neurological symptom disorder (FNSD), a type of somatic symptom disorder also called conversion disorder or functional neurological disorder (FND). MUS is a term that implies physical complaints without a clear physiological explanation. However, in its diagnostic criteria, the DSM-5 de-emphasizes the issue of pathogenesis to avoid grounding the diagnosis in the absence of such explanation.
Per DSM-5 criteria, somatic symptom disorder is diagnosed when a physical symptom is the primary complaint and there is accompanying psychobehavioral criteria (excessive thinking, behaviors, and/or emotional responses) related to the primary somatic symptom. To meet DSM-5 diagnostic criteria, patients need to have one or more somatic symptoms for a duration of at least 6 months. There may or may not be a medically diagnosed condition to explain the somatic symptoms.2 Further, these symptoms are distressing and result in significant impairment.3 It has been estimated that somatic symptom disorders account for an estimated 256 billion dollars/year in medical expenditures in the United States.4
FNSD is a disorder in which the somatic complaint consists of neurologic symptoms. Per DSM-5 diagnostic criteria, FNSD symptoms involve altered voluntary motor or sensory function. Cognitive complaints of a functional nature (also called “cogniform” symptoms) are not included in the DSM-5 diagnostic criteria for FNSD, although they are frequently encountered in clinical practice and have been described in the neuropsychology literature.5 The diagnosis of FNSD is based on demonstrating that the neurologic symptom is not consistent with an alteration in the normal physiology of the nervous system, and not better explained by another medical or mental disorder. Presence of a psychological stressor or psychobehavioral criteria is not required for a diagnosis of FNSD.2
In this chapter, we will discuss the epidemiology, pathophysiology, clinical presentation as well as the appropriate assessment, differential diagnosis, and treatment for patients with FNSD. The discussion will include FNSDs that present with continuous symptoms, such as functional movement disorder or functional weakness, and paroxysmal “spells” known as psychogenic non-epileptic seizures (PNES), the most common subset of FNSD. Although patients with FNSD with episodic neurologic complaints share significant disability and features with those who suffer from constant symptoms, they present differently to a unique set of clinicians requiring evaluations and treatments that are distinct. There is no evidence that patients with FNSD are feigning their symptoms and there is increasingly convincing evidence that FNSD is a disorder that arises from changes in the normal function of the brain.
It is difficult to obtain accurate incidence rates due to the varied definitions for FNSD and the differences ...