The differential diagnosis for chronic dizziness is broad and includes bilateral vestibulopathy, drug-induced dizziness, neurogenetic disorders (e.g., cerebellar ataxia with neuropathy and vestibular areflexia syndrome [CANVAS], spinocerebellar ataxia), neurodegenerative disorders such as multiple systems atrophy (cerebellar variant), and functional dizziness syndromes such as persistent postural-perceptual dizziness (PPPD) and mal de débarquement syndrome (MdDS). In many patients suffering from functional chronic dizziness, an anecdotal link to migraine headaches and VM has been identified ^[1][25]. Thus, these disorders may share pathophysiological aspects and may represent different entities aligned on a spectrum of migraine-associated disorders. 

Diagnostic criteria for PPPD ^[2][16] characterize this as a chronic vestibular disorder that manifests with waxing and waning symptoms of dizziness, unsteadiness, or non-spinning vertigo causing significant distress or functional impairment (Table 1). Minimal symptom duration is three months with symptoms being present on most days. An exacerbation of symptoms in an upright posture, when exposed to passive or active motion and when exposed to moving visual stimuli and complex visual patterns is mandatory. There is a broad range of triggers for PPPD, i.e., conditions that result in vestibular symptoms or disrupt or threaten balance function, including neuro-otologic (e.g., acute unilateral vestibulopathy, BPPV, VM) and other medical conditions and psychological distress (including panic attacks and generalized anxiety). The reported incidence of PPPD 3–12 months after such an acute or episodic vestibular trigger (despite compensation or recovery from the initial disease) is about 25% ^[3][26]. Pathophysiological mechanisms leading to PPPD are not fully understood, but current concepts suggest abnormal bottom-up central processing of self and external motion signals that influence top-down postural behavior in predisposed individuals (see ^[4][27] for a detailed review of current evidence). Abnormal sway patterns on posturography were noted in 50% of PPPD patients in one study ^[5][28], whilst others have identified postural misperception as a possible clinical biomarker that better distinguishes PPPD from other chronic vestibulopathies ^[6][29].

In a retrospective survey study including 36 consecutive patients with PPPD, 19 patients (53%) also met diagnostic criteria for migraine headaches, and 6 of those 19 patients (17% of all patients) met diagnostic criteria for definitive VM ^[1][25]. An additional 31% of PPPD patients studied fulfilled three or four out of five criteria for migraine headaches, thus underlining an association between PPPD and migraine. In other studies, 26% ^[7][30] and 17% ^[8][31] of PPPD patients also met diagnostic criteria for migraine headaches, where VM seemed to act as a trigger and/or perpetuating factor for PPPD ^[1][5][25,28].

Indeed, VM seems to be a frequent trigger for PPPD. In a retrospective database analysis, VM was the second most frequent somatic trigger for PPPD (24%) after BPPV (27%) ^[5][28]. Likewise, in a retrospective review of 198 PPPD cases, the most common vestibular precipitating condition was VM, being present in 25% of patients ^[9][32]. When following-up patients diagnosed with BPPV for about 3.5 months, the risk for developing PPPD was significantly higher in those patients that also suffered from VM, whereas no such increase was observed in patients with migraine headaches (but no VM) ^[10][33].

Treatment of PPPD is often multimodal, combining vestibular rehabilitation therapy, cognitive behavioral therapy and antidepressants (SSRIs or SNRIs). While there have been no high-quality, randomized controlled trials published so far, more than 1000 PPPD patients have been included in treatment studies since 2002 ^[4][27]. Unlike treatment for VM where prophylactic agents are used to reduce the frequency and severity of attacks, therapeutic strategies for PPPD (and perhaps, by extension, for chronic migraine variants) work to reduce bodily hypervigilance and normalize mis-processing of self and visual motion signals through gradual re-exposure, thus emphasizing the value of cognitive physical therapy approaches early on ^[3][26] or even their preferential use over medications (see ^[11][34] and [Popkirov et al. in press]).

The pivotal symptom of mal de débarquement syndrome (MdDS) is a persistent subjective sensation of self-motion (typically a rocking sensation like being on a boat). While transient mal de débarquement is common and usually resolves within 48 h (and never persists for more than one month), MdDS exceeds one month duration ^[12][35]. Recently, diagnostic criteria have been proposed by the classification committee of the Bárány Society ^[13][17] (Table 2). MdDS may emerge (a) after exposure to passive motion (sea travel, air travel, land travel) or (b) spontaneously. When re-exposed to passive motion, patients typically report transient improvement. This transient amelioration is the defining feature of MdDS and allows a distinction between MdDS and PPPD, with the latter leading to almost constant dizziness that becomes worse during passive motion ^[2][16], although most patients with PPPD will also report being symptom free when driving, making this distinction less clear. A link between migraine headache and VM and MdDS has been reported. In a retrospective study including 62 MdDS patients (87% women; spontaneous-onset MdDS: 55%), 39 patients (63%) met diagnostic criteria for VM as well ^[12][35]. While air travel and sea travel were the most frequent triggers for motion-triggered MdDS (22/28), a VM attack was the most commonly reported trigger for spontaneous-onset MdDS (8/22). Other symptoms reported in this cohort of MdDS patients (n = 62) included visually induced dizziness (68%) and head motion-induced dizziness (73%). Notably, clinical characteristics of the subgroup of MdDS with VM were substantially different from the characteristics of the subgroup without VM. Specifically, in the MdDS group with VM, age at symptom onset was lower, interictal visually-induced dizziness and head motion-induced dizziness were more frequent, and the dizziness handicap inventory (DHI) score was higher. Optimal disease control was achieved with monotherapy in 80% of patients, with venlafaxine (27%) and antiepileptics (11%) most frequently used, although in this publication, which specific medication(s) were given was not reported ^[12][35].

One area of contention is whether PPPD and MdDS share a common neuropathological mechanism, and thus, whether there is redundancy in using two separate terms, accepting that PPPD and MdDS may have different triggers, and some differing clinical characteristics (greater symptoms when standing still (MdDS) and on self-movement (PPPD)). However, the treatment approach for both conditions is in fact very similar, reflecting these share underlying mechanisms, with subtle individualized changes required for any given patient.