Unveiling Otolith Mystery: Contemporary Testing Approaches and Their Clinical Significance

ABSTRACT

Objective

Otolith is a critical component of the peripheral vestibular system responsible for detecting linear acceleration. Dysfunction of the otolith organs can lead to vertigo, balance disturbances, and gaze instability. By elucidating the physiological foundations and evaluating both current and emerging methods for assessing otolith function, this review aims to provide a comprehensive resource for researchers and clinicians in the field of vestibular science, contributing to improved diagnostic accuracy and the development of more effective therapeutic strategies.

Data Sources

The databases of PubMed, Embase, and Cochrane Library were systematically searched for the articles.

Methods

The information of physiological mechanism and clinical significance were extracted from each article.

Results

Vestibular‐evoked myogenic potentials (VEMPs) are valuable for evaluating saccule and utricle function, with clinical applications in diagnosing several diseases. Subjective visual vertical (SVV) has clinical value in the diagnosis of peripheral and central vestibular diseases, with recent advancements in smartphone Apps and VR devices providing more accessibility. The ocular counter‐roll (OCR) examination has a superior application prospect, but standardization including the normal range and duration of recovery, was incomplete. The off‐vertical axis rotation (OVAR) can be used to explore motion sickness. The test of linear acceleration perception is limited by device, site and result instability, so it is difficult to apply clinically. Balance‐related tests have been applied clinically, but the isolation of otolith components that determine balance is an unfinished task.

Conclusion

The review emphasizes the need for more precise and standardized otolith function tests to enhance diagnostic accuracy and therapeutic strategies, ultimately improving patient outcomes in vestibular disorders.

1. Introduction

The peripheral vestibular system, comprising the semicircular canals and otolith organs (utricle and saccule), is fundamental for maintaining balance, spatial orientation, and gaze stability [1]. The utricle and saccule are specialized to detect linear acceleration and head tilt, with the utricle primarily responding to horizontal plane motion and the saccule to sagittal plane motion [2]. Linear acceleration or head tilt induces deflection of the hair cell cilia within these organs, generating signals transmitted to the central nervous system to mediate ocular reflexes, spinal reflexes, and sensory integration [3].

Dysfunction of the otolith organs can lead to vertigo, balance disturbances, and gaze instability [4, 5, 6]. As such, the evaluation of otolith function is a critical aspect of clinical assessment in patients presenting with vestibular disorders. By elucidating the physiological foundations and evaluating both current and emerging methods for assessing otolith function, this review aims to provide a comprehensive resource for researchers and clinicians in the field of vestibular science, contributing to improved diagnostic accuracy and the development of more effective therapeutic strategies.

2. Vestibular Evoked Myogenic Potentials

Vestibular‐evoked myogenic potentials (VEMPs), including cervical vestibular‐evoked myogenic potential (cVEMP) and ocular vestibular‐evoked myogenic potential (oVEMP), are detection techniques for evaluating the saccule, utricle [7]. cVEMP, first detailed by Colebatch in 1994 [8], records inhibitory ipsilateral myogenic responses in the sternocleidomastoid muscle evoked by high‐intensity short sounds. oVEMP, described by Rosengren in 2005 [9], records excitatory contralateral responses in extraocular muscles. Additionally, beyond the sternocleidomastoid in cVEMP testing [10], other muscles such as the masseter [11], splenius capitis [12], gastrocnemius, soleus [13], triceps [14], and other muscles [15, 16] have been explored as effectors to assess vestibulospinal reflexes.

2.1. Physiological Basis

The physiological basis of VEMP lies in its neural conduction pathways activated by strong acoustic stimulation. The receptors are originated mainly from the striolar macula areas of the otoliths, which are called the transient system [17]. For cVEMP, air or bone conducted sound [18] stimulation of the saccule transmits signals via the inferior vestibular nerve to the vestibular nucleus in the brainstem. These impulses are then conveyed to the sternocleidomastoid muscle through the medial vestibulospinal tract and accessory nerve [4, 5]. For oVEMP, stimulation of the utricle transmits signals via the superior vestibular nerve to the vestibular nucleus, with projections to extraocular muscles through the VOR pathway [6] (Figure 1).

Figure 1.

Pathways of the vestibular evoked myogenic potentials. IO, inferior oblique muscle; Ⅲ, oculomotor nerve; MLF, medial longitudinal fasciculus; SCM, sternocleidomastoid muscle.

The cVEMP waveform is typically bidirectional, with a positive peak at 13 ms (p13) followed by a negative peak at 23 ms (n23). Similarly, oVEMP is characterized by a negative peak at 10 ms (n10) and a positive peak at 15 ms (p15).

2.2. The Principle of Testing

Stimulation parameters significantly impact VEMP waveforms, including transducer type, stimulus type, and frequency. Air conduction stimulation can utilize plug‐in headsets to minimize signal interference with the recording system. Bone conduction methods include reflex hammer [19], bone‐conduction vibrator [20], electro‐mechanical vibrator [21], and so forth. For cVEMP, air conduction is preferred, while oVEMP demonstrates higher amplitudes with bone conduction and higher response rates with air conduction [22]. But for patients with conductive hearing loss, bone conduction is a priority. The saccule is most sensitive to sounds between 500 and 1000 Hz [23], with a decreased amplitude and prolongation latency at higher frequencies [24]. Clinically, a 500 Hz tone burst is recommended for its larger and more stable amplitude. VEMP amplitude variations across frequencies can help diagnose Meniere's disease (MD).

The main parameters of VEMP include latency, amplitude, asymmetry, and threshold. To measure amplitude in cVEMP, the effect of asymmetry in bilateral SCM contraction must be corrected. This is achieved by calculating the ratio of peak‐to‐peak amplitude to SCM amplitude. VEMP amplitude is also associated with the amplitude ratio (A _L/A _S) and interaural asymmetry ratio [(A _L−A _S)/(A _L + A _S)]. Notably, these parameters are influenced by age [25]. Both children [26] and older adults exhibit prolonged latency, reduced amplitude, and increased threshold [27, 28]. Additionally, older adults experience an asymmetric decline in otolith function, reflected by a higher amplitude asymmetry ratio [29].

2.3. Clinical Application

2.3.1. Meniere's Disease

MD is characterized by paroxysmal vertigo, recurrent fluctuating hearing loss, tinnitus, and ear fullness, with endolymphatic hydrops being a key pathological feature [30]. Diagnosis primarily relies on clinical symptoms and pure‐tone audiometry. However, the non‐invasive and specific characteristics of VEMPs make them valuable for the diagnosis of MD diagnosis. In early MD, saccular hydrops is more common than utricular [31], making cVEMP more sensitive than oVEMP (Table 1). This sensitivity is reflected in decreased amplitude and increased thresholds [32, 33]. The interaural asymmetry ratio, a specific index of hydrops, outperforms electrocochleography in sensitivity [34]. Similarly, the inter‐frequency amplitude ratio (IFAR), comparing VEMP amplitudes at 1000 and 500 Hz, is increased in MD patients [35, 36, 37]. Although IFAR increases with age [38, 39], studies show it remains a reliable diagnostic marker when age‐matched comparisons are used [37]. The 500–1000Hz slope, another frequency‐dependent metric, also demonstrates high specificity and sensitivity [40].

Table 1.

Vestibular diseases and corresponding manifestations of otolith function tests.

Vestibular disease	Test
	VEMP
MD	Decrease in amplitude and increase in threshold (cVEMP) Change of IFAR
VN	Abnormal results of cVEMP or oVEMP
SSCDS	Decrease in threshold (at 2000 Hz especially)
	SVV
VN	Tilt to lesioned side (acute period) and recovery (chronic period) Testing while tilted/eccentric rotated increase sensitivity
BVP	Normal results in the upright position Testing while tilted increase sensitivity
CVD	Normal/abnormal results of SVV and OTR
	OCR
VN	Decrease of gain in lesioned side
BVP	Decrease of gain in both sides

Abbreviations: BVP, bilateral vestibulopathy; CVD, central vestibular disease; MD, Meniere's disease; OCR, ocular counter‐roll; OTR, ocular tilt reaction; SSCDS, Superior semicircular canal dehiscence syndrome; SVV, Subjective visual vertical; VEMP, Vestibular‐evoked myogenic potentials; VN, Vestibular neuritis.

Furosemide‐loaded cVEMP (FVEMP) offers additional insight into endolymphatic hydrops and, when combined with inner ear MRI, enhances diagnostic accuracy. However, FVEMP more strongly indicates cochlear rather than saccular or utricular hydrops [41]. Moreover, FVEMP can yield false negatives in patients with recent frequent vertigo episodes [42]. Normal galvanic cVEMP results in MD patients suggest impaired otolith function with preserved vestibular nerve function [43]. VEMPs provide supplementary insights for disease staging [44, 45].

2.3.2. Vestibular Neuritis (VN)

VN is one of the common peripheral acute vestibular syndromes characterized by persistent vertigo, nausea, vomiting, and gait disturbance. Vestibular function examination is essential for VN diagnosis. Accurate classification of VN can be achieved by combining video head impulse test (vHIT) and VEMP results. This approach leverages the anatomical innervation of the anterior and horizontal semicircular canals and the utricle by the superior vestibular nerve, and the posterior semicircular canal and saccule by the inferior vestibular nerve [46]. vHIT assesses three semicircular canals, while VEMP evaluates otolith organs. This combined method improves diagnostic accuracy over caloric tests [47].

Studies show that unilateral superior division involvement is the most common condition in VN, followed by unilateral superior and inferior division, bilateral superior division, and unilateral inferior division [47, 48]. Interestingly, unilateral superior and inferior division involvement may occur more frequently than isolated superior division cases [49]. Adding galvanic VEMP further reveals that VN primarily affects vestibular nerve function rather than isolated otolith organs. The ampullar vestibulo‐ocular reflex is significantly more impacted than the utriculo‐ocular reflex [47]. Follow‐up studies indicate otolith function recovers faster than semicircular canal function [50].

2.3.3. Superior Semicircular Canal Dehiscence Syndrome (SSCDS)

SSCDS is characterized by a bony fissure in the superior semicircular canal, leading to symptoms such as sound‐ or pressure‐induced vertigo, bone‐conduction hyperacusis, and conductive hearing loss. VEMPs are essential in diagnosing SSCDS. Patients with SSCDS exhibit heightened sensitivity. A threshold of cVEMP below 70 dB and a decrease in amplitude of oVEMP are criteria for diagnosis, achieving a specificity and a sensitivity of 90% [51, 52]. If combined with 2000 Hz TWI, it achieves 100% specificity and 92% sensitivity [53]. VEMP is invaluable for both diagnosis and intraoperative assessment of SSCDS [54].

3. Test of Perception

3.1. Subjective Visual Vertical (SVV)

SVV examination assesses whether a patient's perception of the gravitational vertical deviates from that of healthy individuals. This phenomenon was first described by Friednann in 1971 [55]. Traditional methods involve environments with disrupted visual cues, like hemispheric dome systems [56], bucket tests [57], or light beam adjustments in dark rooms [58]. However, these approaches often face limitations, including bulky equipment, high costs, and imprecise angle measurements. Recent advancements in apps and VR devices have made SVV assessment more achievable [59, 60, 61, 62].

3.1.1. Physiological Basis

The physiological mechanism of SVV involves vestibular, visual, and proprioceptive inputs [63]. The receptors of vestibular information are originated from the extra‐striolar macula areas of the utricles, also called sustained systems. This system is responsive to gravitoinertial force (GIF) [17]. Then the vestibular information is afferent and integrated in the vestibular nucleus. The upward signal travels via the gravity‐perception pathway, crossing to the opposite side at the lower pons, and ascends through the medial longitudinal fasciculus to the interstitial nucleus of Cajal, rostral medial longitudinal fasciculus, and thalamus, forming the perception of SVV in the vestibular cortex (Figure 2). This pathway also influences eye movements, contributing to ocular torsion and skew deviation, while the downward signal reaches neck muscles, causing head tilt. Consequently, SVV is closely linked to the ocular tilt reaction (OTR), comprising head tilt, skew deviation, and ocular torsion [64]. Normally, slight head tilt and SVV deviations (< 2.5°) may occur, but skew deviation and ocular torsion are rare [63, 65, 66].

Figure 2.

Pathways of the subjective visual vertical.

Physiologically, trunk tilting induces an OTR to the contralateral side, aligning the line of sight with the horizontal. Pathological OTR and SVV changes can occur in the upright position, with lesion localization often inferred from specific signs due to the crossing of the gravity‐perception pathway in the lower pons. Peripheral vestibular damage results in complete OTR and SVV deviation, while selective anterior or posterior semicircular canal damage causes unconjugated binocular torsion due to distinct muscle activations. Lesions between the midbrain and pons (medial longitudinal fasciculus) present with skewing‐torsion and altered SVV but no head tilt. Damage to the interstitial nucleus of Cajal leads to complete OTR with conjugated ocular torsion and SVV changes, whereas thalamic or vestibular cortex lesions manifest as isolated SVV changes. Cerebellar lesions may produce partial or complete OTR and SVV changes [67, 68].

Visual and proprioceptive inputs also affect SVV perception. Complex visual stimuli reduce accuracy compared to simple lines, and the SVV deflection angle depends on the initial strip alignment relative to head tilt [69, 70]. Neck proprioception plays a compensatory role in SVV judgment [71, 72, 73]. In bilateral vestibular disease, neck input compensates for vestibular deficits, reducing reliance on vision during body tilt [74].

3.1.2. The Principle of Testing

The measurement methods of SVV include static tests (upright and tilt tests) and dynamic tests (on‐axis and eccentric rotation tests). These assess the angle and direction of deviation between the perceived and actual vertical. In upright static tests, normal subjects exhibit slight SVV deflections within 2.5°–3° at rest. During on‐axis rotation, the utricular cilia experience outward shear forces that cancel each other, mimicking upright conditions. However, SVV measured during rotation has higher sensitivity due to enhanced detection of utricular asymmetry [75]. The difference between on‐axis and off‐axis SVV is useful for identifying chronic utricle dysfunction by minimizing baseline bias from asymmetry [76].

Tilt‐SVV and eccentric rotation‐SVV (EC‐SVV) share physiological similarities but different mechanisms. During eccentric rotation, the subject undergoes on‐axis rotation at constant velocity, followed by lateral axis displacement along the interaural axis (3.5–4 cm), positioning the axis through one utricle [77]. In tilt tests, both utricular cilia tilt simultaneously, whereas in EC‐SVV, one utricle tilts laterally while the other remains unaffected [78]. Both tests induce a tilt sensation due to gravity‐inertial force (GIF), a vector synthesis of gravity and inertial forces. For tilt, GIF decomposes into forces along the interaural (Fy) and head‐tilted (Fz) axes. For eccentric rotation, GIF combines Fz (gravity) and Fy (centrifugal force), creating a skewed angle between GIF and the head's long axis, eliciting a tilt sensation. Normal subjects show ocular torsion opposite to the tilt direction in both scenarios.

Notably, tilt and eccentric rotation produce different effects. The shear force F _y correlates linearly with ocular torsion and SVV, but F _z also influences ocular counter‐roll (OCR) and SVV [79]. Subjects undergoing eccentric rotation, with greater F _z components, exhibit larger torsion angles than during tilt [80]. GIF's relationship with ocular torsion is linear during eccentric rotation, and greater torsion of the lateral eye compared to the medial eye suggests utricular cilia's sustained tilt preferentially affects the ipsilateral eye [81, 82]. Clinical evidence, including pronounced SVV shifts in the ipsilateral eye of post‐vestibular resection patients with incomplete compensation, supports these findings [83]. Therefore, tilt‐SVV and EC‐SVV involve distinct mechanisms.

3.1.3. Clinical Application

3.1.3.1. VN

Static SVV and dynamic SVV play an important role in the acute and compensatory phase of VN patients. Patients with VN often show SVV tilt to the affected side in the acute phase [84], and the tilt value varies widely in different literatures. Patients with inferior VN often have normal SVV results because utricle function is not affected [85]. However, the strong proprioceptive, visual and central static compensatory capacity for SVV restored SVV to normal within 3–4 weeks [86]. In addition, otolith function recovers faster than SCCs in patients with acute VN [50]. At this point, the SVV while tilted and eccentric rotated can play a role [87, 88, 89]. When the patient is tilted to the affected side, the SVV has a greater difference compared with normal person and the difference is maintained for more days than when the patient is upright. When the patient was rotated centrifugally, the difference in SVV from the control would have been more pronounced compared to tilt with the same tilt angle, and still showed a difference after 3 months in one patient [90, 91].

3.1.3.2. Bilateral Vestibulopathy

Upright SVV outcomes in patients with bilateral vestibulopathy (BVP) do not differ from normal subjects because patients may exhibit the same degree of decline in bilateral vestibulo‐ocular reflex function. But BVP patients are more sensitive to tilting stimuli. When the patient is tilted to an angle where a normal subject can still manifest SVV of about 0°, the patient's SVV does not remain upright but is tilted in the same direction as the tilt. Furthermore, the visual attraction effect defined by a deviation of the SVV to the side of the initial line presentation appeared to be higher in the BVP than in controls implying higher visual dependence in BVP [74].

3.1.3.3. Central Vestibular Disease

In central vestibular disease, lower brain stem lesions involving the vestibular nucleus led to the same outcome as external vestibular lesions, with SVV skewed to the affected side. Brain stem lesions above the crossing plane of the gravity‐sensing pathway cause SVV to tilt toward the healthy side [64]. Cerebellar lesions may present different signs depending on whether the nodulus are involved. Patients without nodulus involvement had SVV tilt and falling on the affected side without eye torsion and skew deflection, while patients with nodulus involvement had SVV tilt and falling on the healthy side with eye torsion and skew deflection [67, 68]. A study revealed that patients with central vestibular disease have a higher rate of abnormality and a higher tilt angle of SVV than vestibular migraine [92]. SVV is also valuable for the assessment of higher cognitive function and sensory integration function involving the peripheral vestibular system and the central vestibular system [93].

3.2. Perception of Linear Acceleration

The otolithic organs, essential for detecting gravitational tilt and linear acceleration, can be evaluated via the SVV test and linear acceleration perception paradigms. In 1946, Jungkees pioneered the use of a parallel swing to study human linear acceleration perception. Subjects lying on the swing were exposed to foot‐head axis acceleration. The minimum perception threshold was 6–13 cm/s². Subsequent studies employed devices such as weight‐driven trolleys and oscillating platforms.

Although the test of perception of linear acceleration is theoretically feasible, it has many limitations. Past research has shown that humans have different perception thresholds for linear acceleration along different axes, highest in Z‐axis and lowest in Y‐axis [94, 95]. However, the exact magnitude of the obtained threshold for perceived acceleration varies greatly. First, the magnitude of acceleration is not the only factor affecting the magnitude of the threshold. Benson demonstrated frequency‐dependent thresholds in the X‐axis, aligning with otolith sensitivity to jerk (acceleration gradient). This frequency dependency of the thresholds was similar to the frequency response of the irregular otolith units responding to acceleration and jerk [94]. Gianna reported lower thresholds for acceleration steps versus gradual profiles [96]. while Melvill Jones highlighted velocity's role in directional sensitivity, indicating modulation by acceleration, velocity, and jerk [97].

Moreover, whether the components of the proprioceptor are involved in the perception of linear acceleration remains to be determined. Postural modifications change thresholds, suggesting proprioceptive involvement [94]. However, normal thresholds in water‐immersed subjects and spinal cord injury patients imply nonessential contributions, revealing mechanistic inconsistencies [98].

Furthermore, thresholds fluctuate with learning and fatigue, compounded by stringent technical requirements. Studies report overlapping thresholds in bilateral vestibular loss patients versus controls [96], yet labyrinthine‐deficient subjects exhibit 10‐fold elevated thresholds [98]. Interestingly, different positions of subjects with unilateral vestibular loss showed different sensitivity to motion, which may suggest the condition otoliths press on the macula limits function of otoliths, instead of the condition otoliths hang [99].

In summary, while linear acceleration perception testing provides theoretical insights into otolith function, its clinical utility is hindered by methodological variability and multifactorial influences.

4. Test of Otolith‐Ocular Reflex (OOR)

4.1. OCR

The OCR test evaluates torsional eye movement in response to head tilt, counteracting displacement to stabilize retinal imaging. Unlike angular vestibulo‐ocular reflex (aVOR), OCR is driven by linear acceleration or sustained tilt, mediated primarily by the utricle. Traditional measuring methods such as electronystagmography (ENG/EOG), face challenges including electrode placement variability and signal noise. In contrast, videonystagmography (VNG/VOG) enhances accuracy through infrared cameras and image‐processing algorithms [100]. The emergence of VOG gives more practical meaning to the measurement of eye movements, and video‐OCR may become an important clinical test, like vHIT.

4.1.1. Physiological Basis

OCR is an OOR of the otolith organ, mainly the utricle. Unlike the angle vestibulo‐ocular reflex (aVOR) from SCCs, a linear vestibulo‐ocular reflex (lVOR) responds to linear acceleration or a sustained tilt. Acceleration of the head rotation in different dimensions causes the eyeball to move in the opposite direction by aVOR to maintain retinal imaging stability. Similarly, lVOR causes eyeballs to move and roll in response to linear acceleration and tilt [101]. The vestibular information is input by utricle to the vestibular nucleus, ascending to the inferior pons, and then crossing to the contralateral to the medial longitudinal fasciculus to regulate eye movements [64].

When a normal subject tilts his head to the left, bilateral utricles but mainly the left utricle are activated, resulting in a total effect of internal rotation and upward displacement of the left eye and external rotation and downward displacement of the right eye. As the tilt angle increases, the shear force F_y between the ears increases and thus the angle of OCR increases [77]. The gain of OCR in normal subjects is approximately 0.15, with a head tilt angle of 30° and an OCR angle of 4.5° [102, 103, 104]. But increasing age is associated with a decrease in OCR angle, and the decrease is more pronounced in women, so further standardization is needed [105, 106].

When the pathway is impaired on one side of patient, the otolith afferent signals on the affected side are reduced, and the imbalance of the push‐pull effect causes the upper pole of the eyeball to roll toward the affected side. However, because the normal eye can also have a certain degree of physiological external rotation (< 12°), the ocular rolling in the upright subject is not the criterion for judging the abnormality. When the patient tilted toward the lesioned side, the gain of the OCR in both eyes was generally reduced compared with the normal side. The change in gain was roughly symmetrical in both eyes. The mechanism is the utricle responds primarily to lateral forces, so an intact utricle can provide sufficient electrical in response to tilt to align the visual field to the horizontal during tilt to the intact side but not during tilt to the lesioned side [89].

4.1.2. The Principle of Testing

Subjects were required to wear glasses for measuring the angle of eye torsion before the test and to ensure that the glasses did not move relative to the head, which would create errors. The subject was required to either tilt his head on his torso or his head and torso during the test. When the subject tilts his head on the trunk, neck proprioception also plays a role leading to higher gain. In a patient with subacute vestibular loss, the gain can even be greater when the head is tilted to the affected side than to the unaffected side. When the subject tilted his head and torso, the inability of neck proprioception to function resulted in a higher correlation of gain with otolith function. Testing when the head is tilted on the trunk allows evaluation of the compensatory role of cervical inputs during a vestibular loss, while testing when the head and trunk are tilted allows accurate assessment of otolith function loss [71, 72]. Subjects should try to maintain a uniform and slow speed during the tilt process and hold for more time after the end of the tilt process to reduce the increase of the roll angle from the aVOR [107]. Subjects were required to stare at an earth‐fixed target rather than a head‐fixed target throughout the test because staring at a head‐fixed target produced an effect similar to VOR suppression, reducing gain by 40%.

4.1.3. Clinical Application

OCR examination has significance in judging the side and stage of unilateral vestibular loss. In patients with acute unilateral vestibular loss, substantially reduced gain on tilting to the lesioned side can be found, with normal gain on the normal side. However, in patients with chronic unilateral vestibular loss, an equally reduced gain in OCR was observed bilaterally, which was less pronounced than during the acute period tilt to the lesioned side. In contrast, vHIT could consistently and accurately determine the side of unilateral vestibular loss, but not the stage. Considering these differences, vHIT and video‐OCR (vOCR) can be complementary if combined into a single VOG battery (Table 2). In patients with acute vestibular loss, both vHIT and vOCR show unilateral abnormalities, while as the disease progresses, vHIT still shows unilateral abnormalities and vOCR shows equally decreased gain bilaterally because of central compensation. OCR can be combined with oVEMP because they appear to represent the function of different parts of the utricle [108, 109]. In patients with chronic bilateral vestibular defects, OCR can detect a decrease in OCR gain bilaterally, which can be more accurate when combined with vHIT, which shows the same results.

Table 2.

vHIT and vOCR can be complementary if combined into a single VOG battery.

Side and stage	vHIT		vOCR
	Gain	Catch‐up saccades	Gain
Unilateral vestibular loss (acute period)	Decrease in lesioned side	Exist	Decrease in lesioned side
Unilateral vestibular loss (chronic period)	Decrease in lesioned side	Exist	Slight decrease in both sides
Bilateral vestibular loss	Decrease in both sides	Exist	Decrease in both sides

Abbreviations: vHIT, video head impulse test; vOCR, video ocular counter‐roll; VOG, videonystagmography.

4.2. Off‐Vertical Axis Rotation (OVAR)

During test of OVAR, both otoliths are stimulated. While constant gravity relative to the rotating head results in sinusoidally varying linear accelerations along the nasooccipital and interaural axes, thus triggering ocular reflexes [110, 111]. The tilt ocular reflex involves modulation of torsion and vertical eye position to stabilize eye alignment during roll and pitch tilt, respectively [112]. The translational ocular reflex involves modulation of horizontal and convergent slow‐phase eye velocity to minimize retinal slip during linear accelerations along the interaural and nasooccipital axis, respectively [113, 114].

During the test, the velocities of the eyes and the head position were recorded and plotted as functions accordingly. Adjustable variables include the frequency of rotation and the tilt angle of the rotation axis. The velocity of the eyes in all dimensions will increase with the tilt angle of the axis, since tilt will increase the component of gravity in the nasooccipital and interaural axes. The modulation of vertical and torsional eye positions was greater at lower rotational frequencies, whereas the modulation of horizontal and convergent slow‐phase velocities was greater at higher rotational frequencies [111, 115, 116].

OVAR seems to be helpful in the exploration of motion sickness (MS). The modulation of low frequency to tilt ocular reflex and high frequency to translational ocular reflex suggests that the ambiguity of otolith information lies in the cross region. And the crossing area coincides with peak susceptibility to motion sickness [117]. MS susceptibility during OVAR is positively correlated with susceptibility to other forms of MS [118]. The length of the resultant 3D eye velocity vector may be used as a variable to diagnose motion sickness susceptible population [119].

5. Test of Otolith‐Spinal Reflex

The human balance system integrates multisensory inputs from visual, vestibular, and proprioceptive systems. On stable surfaces, proprioception dominates (70%), while vestibular and visual inputs account for 20% and 10%, respectively. Conversely, unstable surfaces (foam pads) shift reliance to the vestibular system (60%), with vision and proprioception reduced to 30% and 10% [120]. Visual dependency diminishes in darkness or visually conflicting (busy traffic) environments, and vestibular reliance decreases during pre‐programmed motor activities (running) [121]. The weight of these can also be changed by functional states of the other two balance senses. Experiments in chronic spinal cats and paraplegic patients have shown false vestibular information is suppressed through sensory‐down and‐up channels [122, 123]. The corresponding physiological message is that a slight shaking of the head does not displace signals from proprioceptive receptors in the legs and produce a sensation of movement when the person is standing [124]. Cortical imaging further reveals reciprocal inhibitory interactions between visual and vestibular systems, improving accurate self‐motion perception [125].

Clinical evaluation of balance begins with the Romberg test. Static posturography quantifies this test by measuring center of pressure trajectories under altered sensory inputs (eye closure, foam surfaces) [126]. Key parameters include sway velocity, trajectory length, and Romberg/foam ratios, reflecting visual and proprioceptive dependencies [127, 128].

Though the tests mentioned above can test balance function well, they cannot distinguish between otolith damage and semicircular canal damage. Patients with otolith disease often experience the following sensations: “like walking on pillows,” “feeling drunk” or “tumbling,” revealing that damage to otolith alone may also affect balance [129]. One study showed a decline in OCR was associated with mediolateral measures of sway in any age group [105]. Another study of patients with inferior VN showed patients with abnormal saccule function but normal SCC function showed an increase in Romberg's ratio of velocity and area with foam rubber [127]. But the separation of otolith components from the balance test is an unfinished task. A study of MD patients showed Romberg ratio of sway area on foam pad was only associated with abnormal oVEMP [130]. Another study showed Romberg's ratio of velocity with foam rubber was the best indicator for cVEMP abnormalities [127].

Current balance assessments provide valuable prognostic insights but lack granularity in isolating otolith contributions. Refining otolith‐specific metrics is critical for precise diagnosis and monitoring recovery trajectories in vestibular disorders.

6. Conclusion

Overall, VEMPs are valuable for evaluating saccule and utricle function, with clinical applications in diagnosing conditions like Ménière's disease, VN, and SSCDS. SVV has clinical value in the diagnosis of peripheral and central vestibular diseases, with advancements in apps and VR devices providing more accessibility. OCR examination has a superior application prospect, but standardization including the normal range and duration of recovery, was incomplete. OVAR can be used to explore motion sickness. The test of linear acceleration perception is limited by device, site and result instability, so it is difficult to apply clinically. Balance‐related tests have been applied clinically, but the isolation of otolith components that determine balance is an unfinished task.

The otolith organs, serving as critical graviceptors in spatial orientation and linear acceleration detection, play an indispensable role in maintaining human postural control and gaze stabilization. Given the importance of otolith, examinations of otolith function are still incomplete. The accuracy and standardization need to be established. More precise otolith function tests will not only refine diagnostic precision but also pave the way for accurate therapies, ultimately improving quality of life for patients with vestibular impairments and advancing neuro‐otological sciences.

Author Contributions

Shun Zhou, E Tian and Hua‐Jing Yang contributed to the conceptual framework, collected literature information and drafted the manuscript. Su‐Lin Zhang, Yi‐Sheng Lu, Xin Ma and Qing Zhang reviewed and revised the manuscript. All authors have approved the final version.

Ethics Statement

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.

Zhou S., Tian E., Yang H‐J., et al., “Unveiling Otolith Mystery: Contemporary Testing Approaches and Their Clinical Significance,” World Journal of Otorhinolaryngology ‐ Head and Neck Surgery 0 (2026): 1‐12. 10.1002/wjo2.70151.

Data Availability Statement

The authors have nothing to report.