Functional Vestibular Disorders (FVDs)

The late 20th century and early 21st century have seen greater interest in describing functional syndromes by positive diagnostic criteria (i.e., defining them for what they are rather than what they are not). This is accompanied by a growing recognition that illness may result not only from structural or cellular defects in organs and tissues, but from abnormal functioning of physiologic systems that are structurally intact, including networks in the brain (hence renewed use of the term “functional”). The field of gastroenterology is furthest along in this endeavor with its Rome III Criteria for Functional Gastrointestinal Disorders (Drossman, 2006). That includes more than 40 disorders defined solely by their symptoms. Diagnostic signs and test results are absent. Headache neurology (Headache Classification Committee of the International Headache Society, 2013) and psychiatry (American Psychiatric Association, 2013) also have officially sanctioned manuals of disorders defined by symptom-based diagnostic criteria, though neither field uses the term functional for its illnesses. Importantly, studies using these sets of diagnostic criteria have generated large quantities of data on potential pathophysiologic mechanisms, including results that have informed the development of approved pharmaceuticals, multiple off-label medication options, and effective nonpharmacologic interventions.

In neurotology, one functional vestibular syndrome has been formally defined, namely persistent postural-perceptual dizziness (PPPD), which is supported by 30 years of clinical and physiologic research (ICD-11 beta draft: World Health Organization, 2015). A second functional vestibular syndrome has been suggested on the basis of clinical observations of atypical and invariant vestibular symptoms (e.g., kaleidoscopic motion in multiple directions, chronic unchanging vertigo), but this has not yet gelled into a formal definition (Dieterich et al., 2016). PPPD is included in the beta draft version of the International Classification of Diseases, 11th edition (ICD-11 beta draft: World Health Organization, 2015) under the category of chronic vestibular syndromes. It was defined by the Behavioral Subcommittee of the Committee for the Classification of Vestibular Disorders of the Bárány Society and vetted through the ICD-11 Neurology Workgroup before being added to the ICD-11 beta draft. The origins of PPPD can be traced to the first description of phobic postural vertigo in 1986 (Brandt and Dieterich, 1986), through investigations of space–motion discomfort (Jacob et al., 1993) and visual vertigo (Bronstein, 1995) in the 1990s, to chronic subjective dizziness (Staab and Ruckenstein, 2007) in the 2000s. The evolution of PPPD and reviews of possible treatments are detailed elsewhere (Staab, 2013; Dieterich et al., 2016). The ICD-11 beta draft definition (World Health Organization, 2015) of PPPD is:

persistent non-vertiginous dizziness, unsteadiness, or both lasting three months or more. Symptoms are present most days, often increasing throughout the day, but may wax and wane. Momentary flares may occur spontaneously or with sudden movement. Affected individuals feel worst when upright, exposed to moving or complex visual stimuli, and during active or passive head motion. These situations may not be equally provocative. Typically, the disorder follows occurrences of acute or episodic vestibular or balance-related problems. Symptoms may begin intermittently, and then consolidate. Gradual onset is uncommon.

PPPD is the most common cause of chronic vestibular symptoms. Incidence and prevalence estimates derived from research on phobic postural vertigo (Brandt and Dieterich, 1986), chronic subjective dizziness (Staab and Ruckenstein, 2007), and long-term outcomes of acute vestibular syndromes suggest that it is the second most common diagnosis (after benign paroxysmal positional vertigo) encountered among patients who present to tertiary neurotology centers for evaluation of dizziness. It is the most common diagnosis in middle-aged patients. PPPD may develop in as many as 25% of individuals who experience acute or episodic vestibular syndromes, even if they compensate well for their initial illnesses. Historically, patients with persistent dizziness following acute vestibular events have been diagnosed with “chronic vestibulopathy” or “psychogenic dizziness.” Neither designation is accurate, precise, or supported by research data. Both are so outmoded that they should be abandoned. In contrast, PPPD has a clear definition and offers the potential for symptomatic recovery through treatment options described below.

A second functional vestibular disorder has not been formally defined, but the Behavioral Subcommittee of the Bárány Society's classification project (Staab et al., 2014) and others (Dieterich et al., 2016) have considered a collection of symptoms that are unique, not associated with other structural, functional, or psychiatric conditions, and capable of causing significant distress and disability. One such symptom is vertigo, unsteadiness, or dizziness that is present constantly and chronically without variability in response to changes in posture, position, activity level, or exposure to external motion stimuli. Another is vertigo that involves sensations of simultaneous motion in multiple planes or about multiple rotational axes or full-field swirling or kaleidoscopic motion of the visual surround. Clinical observations in the absence of formal studies of these phenomena suggest that they are often present in patients with multiple other somatic symptoms, especially chronic pain and fatigue. They may, therefore, be vestibular presentations of psychiatric illnesses that manifest primarily with somatic symptom burden. These are designated bodily distress disorder in the ICD-11 beta draft (World Health Organization, 2015) and somatic symptom disorder in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5: American Psychiatric Association, 2013). A third functional vestibular presentation is voluntary nystagmus. All three of these are much less common than other psychiatric vestibular disorders or PPPD. They await better elucidation in the future.

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