Vestibular Dysfunction in Parkinson's Disease: A Systematic Review of Signs and Symptoms

Abstract

Background

Despite the growing interest in the link between Parkinson's disease (PD) neuropathology and vestibular system, there is still a lack of consensus about the presence of vestibular dysfunctions in PD.

Objective

This review aims to identify vestibular signs and symptoms reported in PD patients and to summarize and critically appraise the measurement tools used to assess them.

Methods

We searched at MEDLINE, Web of Science, Scopus, and PEDro databases until June 2023. All experimental or observational studies focused on evaluating vestibular signs or symptoms in PD patients were included. For clinical appraise of measurement tools previously published criteria were applied.

Results

In total, 104 studies met the inclusion criteria. A total of 238 vestibular signs were identified, with the most frequently reported being nystagmus (n = 47, 19.7%) central oculomotor abnormalities (n = 47, 19.7%), vestibular evoked myogenic potentials impairments (n = 39, 16.4%), and postural control impairments (n = 37, 15.5%). A total of 27 vestibular symptoms were identified, with dizziness (n = 17, 63.0%) being the most frequently reported. Of the most commonly used, 6 were classified as “recommended,” 14 as “suggested,” and 5 as “listed.”

Conclusions

Our findings raise awareness that vestibular signs and symptoms may be present in PD and should therefore be assessed comprehensively using the most suitable measurement tools. Future studies should focus on investigating the relationship between PD and the vestibular system, the progression of these signs and symptoms across disease stages, and the development of a standardized vestibular assessment protocol for PD to improve clinical management of these symptoms.

Parkinson's disease (PD) is the second most‐common neurodegenerative disorder, primarily caused by the progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta of the basal ganglia. ¹ , ² The cardinal motor symptoms, as defined by the International Parkinson and Movement Disorder Society, include bradykinesia in combination with either rest tremor, rigidity, or both. ³ Although postural instability is not part of the diagnostic criteria, ² it remains a significant concern, often contributing to recurrent falls. Beyond motor symptoms, non‐motor features play a crucial role in the clinical presentation and progression of PD. These include cognitive impairment, mood disorders, sleep disturbances, autonomic dysfunction, and sensory abnormalities. ⁴ , ⁵ When present, vestibular dysfunction may further exacerbate postural instability, ⁶ , ⁷ , ⁸ highlighting the need to assess both motor and non‐motor aspects in the comprehensive management of PD. Given this wide spectrum of manifestations, many experts now consider PD a clinical syndrome rather than a single disease entity. ⁴

Vestibular system is responsible for gaze stabilization during head movements and for position and motion detection of the head in space, which modulated by basal ganglia results in the ability to maintain balance control. ⁶ , ⁷ , ⁸ Four cardinal vestibular symptoms were defined by Bárány Society: dizziness (“disturbed spatial orientation without a false sense of motion”); vertigo (“a false sense of motion”); vestibulo‐visual symptoms (“visual symptoms that result from vestibular pathology or visual‐vestibular interactions”); and postural symptoms (“balance‐related symptoms that occur while in an upright posture—seated, standing, or walking”), including subtypes for each. ⁹ Lately, they also defined nystagmus (“involuntary, rapid, rhythmic, oscillatory eye movement with at least one slow phase”), other nystagmus‐like movements, and their different forms as important vestibular signs. ¹⁰

Currently, the relationship between PD and vestibular dysfunction remains unclear. However, recent studies suggest a potential association between them. ¹ , ¹¹ Based on a detailed neurological and neurotological tests, Venhovens and colleagues ¹² reported that although most PD patients in their study did not present vestibular symptoms, a significant number exhibited central vestibular dysfunctions, and that 18% of those who experienced at least one fall per year had vestibular dysfunctions. However, a more recent study based on a single test found no specific link between falls and vestibular deficits in PD. ¹³

Given the lack of consensus in the literature, this systematic review aims to clarify the presence of vestibular deficits in PD. Specifically, our goal is to identify and list the vestibular signs and symptoms reported in PD patients, while also summarizing and critically evaluating the measurement tools used for their assessment.

Methods

The Preferred Reporting Items for Systematic Reviews and Meta‐Analyses (PRISMA) guidelines were used to structure this review. The protocol of this systematic review was registered at PROSPERO under the code CRD42023439419.

Literature Search

An electronic search was conducted in MEDLINE, Web of Science, Scopus, and PEDro databases from their inception to June 2023, using “dizziness,” “vertigo,” “vestibulo‐visual symptoms,” “postural symptoms,” ⁹ “nystagmus,” ¹⁰ “vestibular,” and “Parkinson's disease” as keywords (Data S1). Reference lists from the identified articles were cross‐checked to identify any further potentially eligible studies.

Study Selection

Studies were eligible for inclusion if they were experimental or observational and focused on evaluating vestibular signs or symptoms in PD patients (based on study aims and specific vestibular assessment tools used). Studies that included conditions other than PD, such as atypical parkinsonism, were included only if they reported data for each population separately, allowing us to extract PD‐specific data. Studies that reported signs or symptoms without a clear vestibular cause as therapeutic adverse events were excluded. Studies published in languages other than English, Portuguese, and Spanish were also excluded. No other restrictions were applied.

All retrieved abstracts were independently screened by two authors (F.R.M., B.S.). The full texts of potentially relevant articles were retrieved for further assessment. Disagreements were resolved by consensus or with the help of a third reviewer (R.B.M.).

Data Extraction

Data from the individual studies were extracted by two authors (F.R.M., B.S.) independently. Four predefined domains were considered: general information (journal of publication, year of publication, study design, and aim of the study), patient characteristics (age, Hoehn & Yahr [HY], and disease duration), methods (measurement tools used and sample size), and signs and symptoms (cause). Disagreements were resolved by a third author (R.B.M.).

Data Analysis

Descriptive data were summarized in frequencies and percentages using a Microsoft Excel spreadsheet (version 16.90.2).

The Bárány Society's consensus was used to classify vestibular signs and symptoms. ⁹ , ¹⁰ Although Bárány Society only published a consensus about nystagmus and nystagmus‐like movements as vestibular signs, additional categories were created based on previously published evidence ¹⁴ , ¹⁵ to categorize other findings.

Despite vestibular signs and symptoms were inclusion criteria for this review, not all studies clearly identified their vestibular causes. For the purposes of this review, signs and symptoms were classified as having a clear vestibular cause if explicitly stated by the authors or if a specialized vestibular assessment tool was used. Those that potentially met the Bárány Society criteria but lacked a definitive cause were categorized as nonspecific.

Additionally, for the purpose of this study, measurement tools were classified into three categories: those that specifically evaluate vestibular signs or symptoms; those that are recommended for vestibular disorders, measuring more than just the vestibular system ¹⁶ ; and those that, though “non‐defined” tools refers to those that could not be reporducible due to the lack of detailed information, allow us to potentially detect them. Assessment tools that could not be reproducible due to the lack of detailed information were classified as “non‐defined.” These classifications were guided by their outcome measure, their recommendation for patients with vestibular disorders, ¹⁶ and, only if the previous data were not available, the authors’ descriptions.

To summarize measurement tools, the Academy of Neurologic Physical Therapy (ANPT) recommendations were applied. This includes assessing postural stability, dynamic stability, gaze stability, vestibulo‐ocular reflex (VOR) function, symptom severity, activity and participation restrictions, and screening for benign paroxysmal positional vertigo (BPPV). ¹⁶ Other categories were created to provide a more specific summary.

To identify the most used tools, we considered those reported as being used as frequently or more frequently than the average of the respective domain group. They were then critically appraised and classified as “recommended,” “suggested,” or “listed,” considering the following criteria: (1) being applied to PD patients, (2) being used in published studies by other people than the developers with available data of use, and (3) having clinimetric/psychometric data in PD that support its use (reliability, validity, and sensitivity to change). Then, tools were classified as “recommended” if they met all criteria, “suggested” if they met two, and “listed” if they met only one. ¹⁷ , ¹⁸

Results

General Features

The electronic search yielded 2208 records, with an additional 40 articles identified through hand searches. After screening titles and abstracts, 206 records were deemed eligible for full‐text analysis. Then, a total of 104 studies met the inclusion criteria (Fig. 1). The general features of included studies are summarized in Table S1. The most frequently identified study designs were cross‐sectional (n = 60, 57.7%), prospective (n = 11, 10.6%), and retrospective cohort studies (n = 10, 9.6%). The 104 included studies were published in 61 different journals, with Movement Disorders (n = 7, 6.7%, impact factor [IF]: 7.4) and Parkinsonism and Related Disorders (n = 6, 5.8%, IF: 3.1) being the most used for publications. The study reports were published between 1969 (n = 1, 0.9%) and June 2023 (n = 3, 2.8%), with 36 (34.6%) publications occurring between 2020 and 2023 (Fig. S1).

Fig. 1.

Flow diagram of study selection process.

From the 104 included studies in the current review, one reported a vestibular symptom as a clear adverse event (AE) from deep brain stimulation (DBS), ¹⁹ and another reported a vestibular sign during DBS‐on. ²⁰ As they were consequences of PD medical intervention rather than of PD itself, we excluded their results from the analyses of signs and symptoms and patient characteristics.

Patient Characteristics

A total number of 4074 PD patients were assessed in the 102 included studies. The study participants had a mean age of 65.0 ± 5.4 years, a mean disease duration of 6.6 ± 2.7 years, and a mean HY stage of 2.3 ± 0.6.

Signs and Symptoms

Out of the 102 considered studies, 81 (79.4%) reported signs, 4 (3.9%) reported symptoms, and 17 (16.7%) reported both.

Of all 264 signs found, 238 (90.2%) were reported as having a clear vestibular cause (explicitly stated by the authors or resulting from an assessment with a specialized vestibular assessment tool) and 26 (9.8%) with a nonspecific etiology (according to Bárány Society and literature classifications without a definite cause) (Table 1). All signs classified as having a nonspecific cause were associated with postural control impairments (n = 26, 100%).

TABLE 1.

Vestibular signs found in Parkinson's disease (PD) patients

Reported signs (type and subtype)		Cause (n [%])
		Clear vestibular	Nonspecific
Nystagmus	‐ Physiological nystagmus impairment	12 (5.0)	‐
	‐ Pathological nystagmus
	Spontaneous nystagmus	17 (7.1)	‐
	Gaze‐evoked nystagmus	2 (0.8)	‐
	Triggered nystagmus	10 (4.2)	‐
	‐ Nystagmus‐like movements	4 (1.7)	‐
	‐ Non‐allocated nystagmus	2 (0.8)	‐
	Total	47 (19.7)	‐
Central oculomotor abnormalities	Abnormal smooth pursuit	16 (6.7)	‐
	Abnormal saccades	27 (11.3)	‐
	Non‐allocated abnormalities*	4 (1.7)	‐
	Total	47 (19.7)	‐
VEMPs impairments	‐ cVEMPs
	Abnormal	11 (4.6)	‐
	Absent	10 (4.2)	‐
	‐ mVEMPs
	Abnormal	3 (1.3)	‐
	‐ oVEMPs
	Abnormal	9 (3.8)	‐
	Absent	5 (2.1)	‐
	Non‐allocated VEMPs*	1 (0.4)	‐
	Total	39 (16.4)	‐
Postural control impairments	Balance impairments	3 (1.3)	3 (11.5)
	Falls	10 (4.2)	17 (65.4)
	Walk impairments	1 (0.4)	‐
	Sensorial organization impairment	19 (8.0)	4 (15.4)
	Sensorial organization and balance impairments	4 (1.7)	2 (7.7)
	Total	37 (15.5)	26 (100)
Global peripheric vestibular function impairments	Total	21 (8.8)	‐
SVP impairments	Total	16 (6.7)	‐
Impaired VOR	Total	12 (5.0)	‐
Impaired VOR suppression	Total	8 (3.4)	‐
Impaired VSR	Total	1 (0.4)	‐
Non‐allocated signs group**	Total	10 (4.2)	‐
Total		238 (100)	26 (100)

Note: * The authors did not detail the signs, so they could not be allocated to other subgroups or ** groups.

Abbreviations: cVEMP, cervical vestibular evoked myogenic potential; mVEMP, masseter vestibular evoked myogenic potential; oVEMP, ocular vestibular evoked myogenic potential; SVP, subjective vertical perception; VEMP, vestibular evoked myogenic potential; VOR, vestibular‐ocular reflex; VSR, vestibulo‐spinal reflex.

Regarding the 31 symptoms found, 27 (87.1%) were reported as having a clear vestibular cause and 4 (12.9%) with a nonspecific etiology (Table 2).

TABLE 2.

Vestibular symptoms found in Parkinson's disease (PD) patients

Symptom (type and subtype)		Cause (n [%])
		Clear vestibular	Nonspecific
Dizziness	Total	17 (63.0)	2 (50.0)
Postural symptoms	Other balance symptoms	5 (18.5)	1 (25.0)
	Total	5 (18.5)	1 (25.0)
Vertigo	Vertigo	3 (11.1)	1 (25.0)
	Triggered vertigo	1 (3.7)	‐
	Total	4 (14.8)	1 (25.0)
Vestibulo‐visual symptoms	Total	‐	‐
Associated symptoms	Vertigo or disequilibrium	1 (3.7)	‐
	Total	1 (3.7)	‐
Total		27 (100)	4 (100)

From all PD patients, 1603 (39.3%) exhibited vestibular dysfunctions. Specifically, 1365 patients (85.2%) reported clear vestibular signs, whereas 238 (14.8%) reported clear vestibular symptoms.

Vestibular Signs

From 238 (90.2%) vestibular signs found, nystagmus (n = 47, 19.7%), central oculomotor abnormalities (n = 47, 19.7%), impairments in vestibular evoked myogenic potentials (VEMPs) (n = 39, 16.4%), and postural control impairments (n = 37, 15.5%) were the most frequently reported (Table 1).

Vestibular Symptoms

From 27 (87.1%) vestibular symptoms found, dizziness (n = 17, 63.0%) was the most frequently reported. One study did not report isolated symptoms but groups of symptoms—vertigo or disequilibrium (Table 2).

Measurement Tools

Across the 104 included studies, a total of 402 measurement tools were used and classified into three different categories.

Measurement Tool Specifically Developed to Evaluate Vestibular Signs or Symptoms

A total of 310 (77.1%) measurement tools specifically evaluated vestibular signs or symptoms (Table S2). Among them, spontaneous nystagmus testing (n = 30, 9.7%), saccades testing (n = 30, 9.7%), cervical VEMPs (cVEMPs) (n = 22, 7.1%), and the sensorial organization test (SOT) (n = 22, 7.1%) were the most frequent tools. Regarding groups, activity/participation restrictions (n = 14, 4.5%) and symptom severity (n = 3, 1.0%) were between the less assessed.

Measurement Tools Recommended for Vestibular Disorders, Measuring more than Just the Vestibular System

A total of 43 (10.7%) measurement tools were not specifically developed to assess vestibular deficits but are part of the outcome measures recommended by ANPT for vestibular disorders ¹⁶ (Table S3). Among them, Activities‐Specific Balance Confidence (ABC) scale (n = 12, 27.9%), Berg Balance Scale (BBS) (n = 9, 20.9%), Time Up and Go (TUG) (n = 4, 9.3%), Mini Balance Evaluating Systems Test (MiniBESTest) (n = 3, 7.0%), Romberg Test (n = 3, 7.0%), and Tandem Walk (TW) (n = 3, 7.0%) were the most frequent.

Non‐defined Measurement Tools

A total of 49 (12.2%) measurement tools were used to assess signs or symptoms, despite not being defined, specialized, or recommended by any scientific society (Table S4). Among them, falls history assessments based on reporting (n = 20, 40.8%), non‐defined postural and dynamic stability assessments (n = 16, 32.7%), and non‐defined questions about vestibular symptoms (n = 9, 18.4%) were the most frequent.

Measurement Tools Critical Appraise

Of all the measurement tools used in the included studies, 6 met the “recommended” level, 14 met the “suggested” level, and 5 met the “listed” level for assessing vestibular signs or symptoms in PD (Table 3 provides detailed data). SOT (n = 22, 7.1%), Dizziness Handicap Inventory (DHI) (n = 13, 4.2%), ABC scale (n = 12, 27.9%), BBS (n = 9, 20.9%), TUG (n = 4, 9.3%), and MiniBESTest (n = 3, 7.0%) achieved the “recommended” level.

TABLE 3.

Classification of the most used measurement tools per group

Group	Most used tools	Applied in PD	Applied beyond original developers	Studied clinimetric properties in PD	Classification
(1) Nystagmus search and testing	Optokinetic nystagmus testing	Yes	Yes	No	Suggested
	Spontaneous nystagmus testing	Yes	Yes	No	Suggested
(2) Central oculomotor testing	Saccades testing	Yes	Yes	No	Suggested
(3) VEMPs testing	cVEMP	Yes	Yes	No	Suggested
	oVEMP	Yes	Yes	No	Suggested
(4) Postural and dynamic stability (direct)	SOT	Yes	Yes	Yes	Recommended
(5) Global peripheral vestibular function testing	Caloric tests	Yes	Yes	No	Suggested
	Head shaking test	Yes	Yes	No	Suggested
(6) SVP testing	SHV test	Yes	Yes	No	Suggested
	SVV test	Yes	Yes	No	Suggested
	Vestibular heading perception task	Yes	No	No	Listed
	Visual heading perception task	Yes	No	No	Listed
(7) BPPV screening	Dix‐Hallpike maneuver	Yes	Yes	No	Suggested
(8) Direct VOR function testing	vHIT	Yes	Yes	No	Suggested
(9) Activity/participation restrictions	DHI	Yes	Yes	Yes	Recommended
(10) VOR cancelation function testing	Fixation suppression testing	Yes	Yes	No	Suggested
(11) Symptom severity	VSS	Yes	No	No	Listed
	Visual analogue scale for dizziness severity	Yes	No	No	Listed
(12) VSR function testing	Unterberger test	Yes	No	No	Listed
(13) Postural and dynamic stability	BBS	Yes	Yes	Yes	Recommended
	MiniBESTest	Yes	Yes	Yes	Recommended
	Romberg test	Yes	Yes	No	Suggested
	TW	Yes	Yes	No	Suggested
	TUG	Yes	Yes	Yes	Recommended
(14) Activity/participation restrictions	ABC scale	Yes	Yes	Yes	Recommended

Abbreviations: ABC scale, Activities‐Specific Balance Confidence scale; BBS, Berg Balance Scale; BPPV, benign paroxysmal positional vertigo; cVEMP, cervical vestibular evoked myogenic potential; DHI, Dizziness Handicap Inventory; MiniBESTest, Mini Balance Evaluating Systems Test; oVEMP, ocular vestibular evoked myogenic potential; SHV, subjective haptic vertical; SOT, sensory organization test; SVP, subjective vertical perception; SVV, subjective visual vertical; TUG, Time Up and Go; TW, Tandem Walk; vHIT, video Head Impulse Test; VOR, vestibular‐ocular reflex; VSR, vestibular‐spinal reflex; VSS, Vestibular Symptom Scale.

Discussion

A total of 264 signs and 31 symptoms in PD patients were found, of which 238 (90.2%) and 27 (87.1%), respectively, were reported as having a clear vestibular cause. To assess all signs and symptoms reported, 402 measurement tools were used across the studies.

Are Vestibular Dysfunctions Present in PD?

According to our results, among the 4074 PD patients, 1603 (39.3%) exhibited clear vestibular dysfunctions. Specifically, 1365 (85.2%) reported clear vestibular signs, whereas 238 (14.8%) reported clear vestibular symptoms.

Out of the 102 analyzed studies, 81 (79.4%) reported signs, 4 (3.9%) reported symptoms, and 17 (16.7%) reported both. Additionally, activity/participation restrictions (n = 14, 4.5%) and symptom severity (n = 3, 1.0%) groups were between the less assessed. Our results revealed that symptoms were specifically less investigated than signs, which could have led to the smaller number of vestibular symptoms reported. Previous studies could also justify our findings by concluding that even when PD patients do not present vestibular symptoms, their vestibular tests may reflect central ¹² or peripheral ²¹ vestibular signs. However, without assessment, it is not possible to fully understand which vestibular symptoms may be present in PD and when they may arise.

Across the 102 analyzed studies, the average of HY stage was 2.3 ± 0.6. Among them, only 8 (7.8%) had a mean HY equal or above than Stage 3, and only another (1.0%) considered PD prodromal period. ²² Although limited in number, these findings alert that vestibular signs or symptoms may be present at all disease stages, even before PD onset.

Although it is important to acknowledge that aging by itself may lead to vestibular dysfunction, ²³ which may confound the interpretation of associations between PD and vestibular dysfunctions, among the 102 analyzed studies, 48 (47.1%) ⁶ , ¹² , ²⁴ , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ²⁹ , ³⁰ , ³¹ , ³² , ³³ , ³⁴ , ³⁵ , ³⁶ , ³⁷ , ³⁸ , ³⁹ , ⁴⁰ , ⁴¹ , ⁴² , ⁴³ , ⁴⁴ , ⁴⁵ , ⁴⁶ , ⁴⁷ , ⁴⁸ , ⁴⁹ , ⁵⁰ , ⁵¹ , ⁵² , ⁵³ , ⁵⁴ , ⁵⁵ , ⁵⁶ , ⁵⁷ , ⁵⁸ , ⁵⁹ , ⁶⁰ , ⁶¹ , ⁶² , ⁶³ , ⁶⁴ , ⁶⁵ , ⁶⁶ , ⁶⁷ , ⁶⁸ , ⁶⁹ reported statistically significant differences between PD and age‐matched healthy controls. The most frequently reported dysfunctions in these studies included postural control impairments, VEMPs impairments, oculomotor abnormalities, nystagmus, and subjective vertical perception impairments. However, not all studies performed these analyses with age‐matched healthy controls or investigated the significance of all the variables included in the studies, which may limit our conclusions.

The presence of vestibular dysfunctions in PD might be further supported by emerging therapeutic approaches targeting the vestibular system. Recent evidence indicates that both galvanic and caloric vestibular stimulation may offer a low‐risk and noninvasive therapeutic approach to improve a broad range of motor and non‐motor symptoms in PD. ⁷⁰ These data highlight the need to further investigate the relationship between the PD neuropathology and the vestibular system.

In summary, vestibular signs or symptoms were found in 1603 (39.3%) PD patients. Our results may show a possible association between PD and these dysfunctions, but it is not possible to establish a definitive causal relationship between them. We emphasize the need to understand the behavior of signs and symptoms during disease progression to increase the specific assessment of symptoms and to compare findings with age‐matched healthy controls.

Which Are the Most Common Vestibular Signs in PD Patients?

Of all 264 signs reported, 238 (90.2%) were reported as having a clear vestibular cause. Among them, nystagmus (n = 47, 19.7%), ¹² , ²¹ , ²⁴ , ²⁶ , ³² , ³⁹ , ⁴⁰ , ⁴⁸ , ⁵⁰ , ⁵¹ , ⁷¹ , ⁷² , ⁷³ , ⁷⁴ , ⁷⁵ , ⁷⁶ , ⁷⁷ , ⁷⁸ , ⁷⁹ , ⁸⁰ , ⁸¹ , ⁸² , ⁸³ , ⁸⁴ , ⁸⁵ , ⁸⁶ central oculomotor abnormalities (n = 47, 19.7%), ¹² , ²¹ , ²⁴ , ²⁶ , ²⁷ , ³² , ³⁹ , ⁴⁴ , ⁴⁷ , ⁶² , ⁶³ , ⁶⁴ , ⁷⁴ , ⁷⁵ , ⁷⁷ , ⁸⁰ , ⁸¹ , ⁸³ , ⁸⁴ , ⁸⁶ , ⁸⁷ , ⁸⁸ , ⁸⁹ , ⁹⁰ , ⁹¹ , ⁹² VEMPs impairments (n = 39, 16.4%), ¹² , ²⁴ , ²⁸ , ³⁰ , ³² , ³³ , ³⁶ , ³⁷ , ⁴¹ , ⁴⁷ , ⁴⁹ , ⁵² , ⁵⁴ , ⁵⁵ , ⁵⁸ , ⁵⁹ , ⁶⁵ , ⁹³ , ⁹⁴ , ⁹⁵ , ⁹⁶ and postural control impairments (n = 37, 15.5%) ²¹ , ²⁵ , ³¹ , ³⁴ , ³⁵ , ³⁹ , ⁴⁵ , ⁵¹ , ⁵³ , ⁵⁶ , ⁶⁸ , ⁶⁹ , ⁸⁰ , ⁸⁶ , ⁹⁷ , ⁹⁸ , ⁹⁹ , ¹⁰⁰ , ¹⁰¹ , ¹⁰² , ¹⁰³ , ¹⁰⁴ , ¹⁰⁵ , ¹⁰⁶ , ¹⁰⁷ , ¹⁰⁸ , ¹⁰⁹ were the most found.

Nystagmus (n = 47, 19.7%) ¹² , ²¹ , ²⁴ , ²⁶ , ³² , ³⁹ , ⁴⁰ , ⁴⁸ , ⁵⁰ , ⁵¹ , ⁷¹ , ⁷² , ⁷³ , ⁷⁴ , ⁷⁵ , ⁷⁶ , ⁷⁷ , ⁷⁸ , ⁷⁹ , ⁸⁰ , ⁸¹ , ⁸² , ⁸³ , ⁸⁴ , ⁸⁵ , ⁸⁶ and central oculomotor abnormalities (n = 47, 19.7%) ¹² , ²¹ , ²⁴ , ²⁶ , ²⁷ , ³² , ³⁹ , ⁴⁴ , ⁴⁷ , ⁶² , ⁶³ , ⁶⁴ , ⁷⁴ , ⁷⁵ , ⁷⁷ , ⁸⁰ , ⁸¹ , ⁸³ , ⁸⁴ , ⁸⁶ , ⁸⁷ , ⁸⁸ , ⁸⁹ , ⁹⁰ , ⁹¹ , ⁹² were the most reported vestibular signs in PD patients. Although this review included data exclusively from patients diagnosed with PD, it is important to acknowledge that central oculomotor abnormalities, such as nystagmus, are also recognized features of atypical parkinsonian syndromes, ¹¹⁰ particularly multiple system atrophy (MSA). ¹¹¹ The presence of these signs should prompt careful diagnostic evaluation in future studies to differentiate PD from atypical parkinsonism.

Despite inclusion criteria, nonspecific signs were still found, exclusively within the postural control impairment group (n = 26, 100%). Both clear vestibular signs and nonspecific ones were most related to sensorial organization impairments and falls in the mentioned group (see Table 1). Even with a lack of consensus about sensorial organization impairments etiology, 19 (7.9%) studies found a vestibular association with it. ³¹ , ³⁴ , ³⁵ , ³⁹ , ⁴⁵ , ⁵¹ , ⁵³ , ⁵⁶ , ⁶⁸ , ⁸⁰ , ⁹⁷ , ⁹⁸ , ⁹⁹ , ¹⁰⁰ , ¹⁰² , ¹⁰⁵ , ¹⁰⁶ , ¹⁰⁷ , ¹⁰⁹ Moreover, a total of 10 studies linked vestibular dysfunctions to falls in PD. ¹² , ²¹ , ³⁵ , ⁵⁶ , ⁸⁰ , ⁸⁶ , ¹⁰³ , ¹⁰⁵ , ¹⁰⁶ , ¹⁰⁷ Also, considering that postural control results from the interaction of visual, somatosensorial, and vestibular systems, ¹¹² it is emphasized that a multifaceted and detailed approach ¹¹³ should be performed. Together, these findings highlight that, although postural instability in PD is mostly associated with impaired postural reactive responses and reduced functional limits of stability, ¹¹⁴ , ¹¹⁵ a vestibular cause should also be considered.

Furthermore, VEMPs (n = 39, 16.4%), ¹² , ²⁴ , ²⁸ , ³⁰ , ³² , ³³ , ³⁶ , ³⁷ , ⁴¹ , ⁴⁷ , ⁴⁹ , ⁵² , ⁵⁴ , ⁵⁵ , ⁵⁸ , ⁵⁹ , ⁶⁵ , ⁹³ , ⁹⁴ , ⁹⁵ , ⁹⁶ subjective vertical perception (SVP) (n = 16, 6.7%), ⁶ , ¹² , ²⁹ , ³⁸ , ⁴² , ⁴³ , ⁴⁶ , ⁴⁷ , ⁵⁹ , ⁶⁰ , ⁶⁵ , ¹¹⁶ , ¹¹⁷ , ¹¹⁸ VOR (n = 12, 5.0%), ¹² , ²⁴ , ²⁶ , ³⁴ , ⁴⁴ , ⁴⁷ , ⁶⁵ , ⁶⁶ , ⁶⁷ , ⁷⁶ , ⁸⁷ and vestibulo‐spinal reflex (VSR) (n = 1, 0.4%) ⁵¹ impairments were also reported as vestibular signs. Our results are in line with a previous review that found VOR and VSR deficits in PD patients, further suggesting an emerging consensus on VEMP abnormalities and a probable relationship between subjective visual vertical (SVV) impairments and Pisa syndrome. ¹¹ However, all of them were reported as vestibular symptoms instead of vestibular signs, ¹¹ which differ from the terminology already defined by Bárány Society. These divergences present an additional challenge to drawing further conclusions.

In summary, nystagmus, central oculomotor abnormalities, VEMPs impairments, and postural control impairments were the most frequently reported vestibular signs in PD patients. We highlight the importance of studying the etiology of signs, specifically related to postural instability and falls, which should not be overlooked in the well‐known approach to PD.

Which Are the Most Common Vestibular Symptoms in PD Patients?

Of all the 31 symptoms reported, 27 (87.1%) were reported as having a vestibular cause based on the defined criteria. Among them, dizziness (n = 17, 63.0%) ¹² , ²¹ , ²² , ²⁵ , ⁴⁷ , ⁷¹ , ⁷² , ⁸⁶ , ⁹⁴ , ⁹⁵ , ⁹⁸ , ¹⁰¹ , ¹⁰⁸ , ¹¹⁹ , ¹²⁰ , ¹²¹ , ¹²² was the most frequently reported, followed by postural symptoms (n = 5, 18.5%) ²¹ , ⁸⁶ , ⁹³ , ¹²¹ and vertigo (n = 4, 14.8%). ²⁴ , ⁷¹ , ⁸⁶ These findings are consistent with previous studies, ²¹ , ²² which also identified dizziness as the most common vestibular symptom in PD.

Despite the studies’ inclusion criteria, nonspecific symptoms (n = 4, 100%) ¹⁰⁰ , ¹²⁰ , ¹²³ , ¹²⁴ were still reported. Dizziness (n = 2, 50%) remained the most frequently reported among them, followed by vertigo and postural symptoms (n = 1, 25% each). In addition to symptoms being underreported and undervalued in clinical practice, ¹¹ , ¹² , ¹²⁵ the lack of investigation into their etiology may be difficult in understanding the vestibular dysfunctions in PD, given other potential confounding causes.

An included case series study highlighted the importance of symptoms study by showing that PD patients can report dizziness in absence of orthostatic hypotension (OH) due to a vestibular dysfunction, whereas in other cases, both OH and vestibular dysfunction can coexist. ⁷¹ OH is a PD non‐motor symptom, affecting 1 in every 3 patients. Its manifestation is related to dizziness, fatigue, unexplained falls, or blurred vision sensation, developing upon standing and recover by lying down. ¹²⁶ , ¹²⁷ Dizziness in PD patients is commonly associated with it; however, considering that some of these complaints overlap with vestibular symptoms, ⁹ a detailed investigation of their cause is required to differentiate them. As suggested by the Bárány Society, ⁹ asking patients who experience dizziness on arising about other situations in which the symptom occurs may help in the study of symptoms. Additionally, OH assessment should be performed to exclude it as a potential etiology, as done by some included studies. ¹² , ⁴⁷ , ⁷¹ , ⁷² , ⁹⁴ , ⁹⁵

A total of 5 (18.5%) postural symptoms were found with a clear vestibular cause. ²¹ , ⁸⁶ , ⁹³ , ¹²¹ Of these, 4 were reported together with another vestibular symptom ²¹ , ⁸⁶ , ¹²¹ and 1 with a vestibular sign. ⁹³ In those ⁹³ , ¹²¹ published after the Bárány Society consensus, even though none mentioned a specific pulse direction, none reported “unsteadiness” as recommended. ⁹ Our findings emphasize the importance of consistently assessing additional vestibular symptoms alongside postural symptoms, as this approach may confirm their vestibular origin. ⁹ Additionally, our review alerts that some studies did not adhere to the Bárány Society consensus terminology, which can raise an additional challenge in drawing further conclusions.

In summary, dizziness was the most frequently reported vestibular symptom in PD patients. We highlight the importance of investigating the etiology of symptoms, alerting that not all dizziness in PD patients should be presumed to result from OH, further suggesting the use of previously defined vestibular terms.

What Are the Most Suitable Measurement Tools to Assess Vestibular Dysfunction in PD Patients?

A total of 402 measurement tools were used across the studies. Among the most used tools (defined as used as frequently or more than the average of the respective domain group) and based on previously published classification criteria, ¹⁷ , ¹⁸ 6 met the “recommended” level, 14 met the “suggested” level, and 5 met the “listed” level for assessing vestibular signs or symptoms in PD (Table 3 provides detailed data).

Those classified as “recommended” were SOT (n = 22, 7.1%), ²⁵ , ³¹ , ³⁵ , ³⁹ , ⁴⁵ , ⁵¹ , ⁵³ , ⁵⁶ , ⁶⁵ , ⁹⁷ , ⁹⁸ , ⁹⁹ , ¹⁰¹ , ¹⁰² , ¹⁰⁵ , ¹⁰⁶ , ¹⁰⁷ , ¹⁰⁸ , ¹⁰⁹ , ¹²⁴ , ¹²⁸ DHI (n = 13, 4.2%), ¹² , ²⁵ , ⁴⁷ , ⁷² , ⁹⁸ , ¹⁰¹ , ¹⁰⁸ , ¹⁰⁹ , ¹¹⁹ , ¹²⁰ , ¹²¹ , ¹²² , ¹²⁴ ABC scale (n = 12, 27.9%), ⁶ , ¹² , ³⁷ , ⁴⁷ , ⁵³ , ⁵⁷ , ⁵⁸ , ¹⁰⁰ , ¹⁰¹ , ¹⁰⁹ , ¹²³ , ¹²⁴ BBS (n = 9, 20.9%), ¹² , ³⁷ , ⁴⁷ , ⁵³ , ⁶⁹ , ⁷⁵ , ¹⁰⁰ , ¹¹⁷ , ¹²¹ TUG (n = 4, 9.3%), ²⁵ , ⁶⁸ , ¹⁰⁰ , ¹⁰⁸ and MiniBESTest (n = 3, 7.0%). ²⁸ , ⁴¹ , ⁴⁹ Among these, only the SOT and DHI directly assess vestibular inputs.

SOT is a complete posturography test to quantitatively assess postural control under various conditions; however, its high cost and space requirements limit its availability in clinical practice. ¹²⁹ , ¹³⁰ The Modified Clinical Test of Sensory Interaction on Balance (mCTSIB) offers a cheaper and quicker alternative to apply. ¹²⁹ However, its application in PD has been limited given that only one included study using it ¹⁰⁰ and the lack of validation in this population. This gap makes it challenging to fully assess postural control impairments in PD patients, making it difficult for clinicians to improve their approaches for one of the patients' main problems. ¹³¹

Furthermore, activity/participation restrictions (n = 14, 4.5%) ¹² , ²⁵ , ⁴⁷ , ⁷² , ⁹⁸ , ¹⁰¹ , ¹⁰⁸ , ¹⁰⁹ , ¹¹⁹ , ¹²⁰ , ¹²¹ , ¹²² , ¹²³ , ¹²⁴ and symptom severity (n = 3, 1.0%) ⁹⁴ , ⁹⁵ , ⁹⁸ groups were between the less assessed, indicating a specific underassessment of vestibular symptoms. Despite being among the most frequently used “non‐defined” tools, questioning about vestibular symptoms (n = 9, 18.4%), ¹² , ¹⁹ , ²¹ , ²² , ⁴⁷ , ⁵⁰ , ⁸⁶ , ⁹³ , ¹²⁰ specifically during interviews, does not yield standardized results. Given the importance of considering patients’ perspectives, ¹³² vestibular symptom reporting should be assessed during patient interviews but complemented with specific vestibular measurement tools.

A comprehensive vestibular assessment is required to draw further conclusions. Then, considering further tools besides those that met the “recommended” level to assess nystagmus, oculomotor integrity, VEMPs, and BPPV might allow to enhance diagnostic accuracy. ¹⁰

In summary, ABC scale, BBS, DHI, MiniBESTest, SOT, and TUG were “recommended” to be included in the vestibular assessment of PD patients. We also highlight that, although vestibular measurement tools have already been validated for vestibular disorders, further validation in PD is needed to help establish a standardized assessment protocol.

What Are the Implications for Clinical Practice?

Vestibular signs and symptoms were found in 1603 (39.3%) PD patients. However, not all of the included studies reported the number of patients with vestibular dysfunctions, neither all described their causes or compared the findings with age‐matched healthy controls, which limit a comprehensive understanding of these dysfunctions.

Our findings highlight that, although postural control impairments, falls, and dizziness are mostly assumed as other causes of PD, they may also be due to a vestibular dysfunction. To improve the reporting of vestibular signs and symptoms in PD patients, healthcare professionals must be aware of their existence to avoid misdiagnosing them with other potential confounding causes. For that, clinicals should consider it during interviews and use the most suitable measurement tools to enhance diagnostic accuracy. Because not everyone may have access to these tools, referring patients to a specialist in the field is crucial.

Because PD is a complex neurodegenerative condition, in which the best way to contribute to the well‐being and quality of life of patients is to manage patients' impairments, further knowledge about the role of the vestibular system in this population may improve patient‐center approaches.

What Are the Implications for Research?

Although research on vestibular dysfunctions in PD is increasing (Fig. S1), the present study should encourage researchers to further investigate the link between PD neuropathology and vestibular system.

It is necessary to investigate the causes of signs and symptoms to easily define which have a vestibular cause. Once they are better known, it will allow to investigate their onset, their behavior across disease stages, and which patients are most likely to report them. For that, it is crucial to ensure consistency in vestibular terms across the studies and to establish a standardized assessment protocol for vestibular system in PD, which should include both validated specific vestibular tests and patient‐reported questionnaires.

Additionally, we highlight the importance of monitoring potential vestibular AE resulting from PD medical interventions, such as DBS, and suggest that futures studies should continue to explore this topic.

Together, these insights may help characterize vestibular dysfunctions in PD patients and consequently contribute to improve intervention targets and future research.

Limitations

A few limitations need to be acknowledged in this systematic review. First, a key limitation in the current study is the inability to establish a causal relationship between PD and those vestibular signs and symptoms found. Furthermore, the assumption of a clear vestibular cause was based on the results of previous studies, which may have limited our findings as there are variables that we cannot control. Lastly, the lack of descriptions on signs and symptoms, terms, and consensus on measurement tools made it challenging to analyze all findings. In consequence, some signs and symptoms had to be classified as having a nonspecific cause due to the lack of their description. As we choose to follow a more comprehensive approach, given the limited evidence and focus on this topic, we included and analyzed them separately to avoid overestimating our findings.

Despite this, we believe that our results may raise awareness for vestibular dysfunctions in PD, helping clinicals to better guide their practice, and highlight the need for further investigation of the link between PD and the vestibular system.

Conclusion

Our findings raise awareness that vestibular signs and symptoms may be present in PD, with nystagmus, central oculomotor abnormalities, VEMPs impairments, postural control impairments, and dizziness being the most reported across studies. In addition, 6 measurement tools were “recommended” to assess vestibular dysfunctions in PD. To our knowledge, this is the first systematic review investigating both vestibular signs and symptoms in PD, also summarizing the measurement tools used to assess them. However, to reach stronger conclusions, future studies should prioritize the investigation of the relationship between the PD neuropathology and the vestibular system, as well as the behavior of signs and symptoms across disease stages, to understand their prevalence in PD. The establishment of a standardized protocol for the assessment of the vestibular system in PD is also essential to improve disease management and refine rehabilitation goals for patients.

Author Roles

(1) Research Project: A. Conception, B. Organization, C. Execution; (2) Statistical analysis: A. Design, B. Execution, C. Review and critique; (3) Manuscript preparation: A. Writing of the first draft, B. Review and critique.

F.R.‐M.: 1A, 1B, 1C, 2A, 2B, 3A

B.S: 1A, 1B, 1C, 2A, 2B

C.M.C.: 1A, 1B, 2A, 2C, 3B

T.E.: 1A, 3B

R.B.‐M.: 1A, 1B, 2A, 2B, 2C, 3B

J.J.F.: 1A, 2C, 3B

Disclosures

Ethical Compliance Statement: This article being a systematic review, no patient data were collected. As such, informed consent was not obtained, and IRB approval was not sought. The authors confirm that they have read the journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines.

Funding Sources and Conflicts of Interest: No specific funding was received for this work. The authors declare that there are no conflicts of interest relevant to this work.

Financial Disclosures for Previous 12 Months: F.R.‐M., B.S., C.M.C., T.E., and R.B.‐M have no additional disclosures to report. Joaquim J. Ferreira has provided consultancy to Bial, Biogen, AbbVie, Sunovion Pharmaceuticals, Roche, Stada, Neuroderm, Lundbeck, Affiris, Lilly and Organon, received speaker fees from Bial, Biogen, AbbVie, Sunovion Pharmaceuticals, Infucure, Zambon, Roche, Stada, ONO Pharma, Britannia, SK Chemicals, Major Pharma and has received grants from AbbVie, Bial, Novartis, GlaxoSmithKline, Medtronic and Angelini.

Supporting information

Data S1. Search strategy.

Figure S1. Publication year of included studies.

Table S1. General features of included studies.

Table S2. Measurement tools specifically developed to evaluate vestibular signs or symptoms.

Table S3. Measurement tools recommended for vestibular disorders, measuring more than just the vestibular system.

Table S4. Measurement tools that even “not defined” allow to potentially detect vestibular signs or symptoms.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.