Video Head Impulse Test (vHIT), VEMP (cVEMP/oVEMP) & Dynamic Posturography (CDP)

Video Head Impulse Test (vHIT), VEMP & Dynamic Posturography (CDP)

Comprehensive assessment of the balance system requires evaluating the vestibular apparatus across its full operational frequency spectrum and testing all five sensory end-organs in each ear: the three semicircular canals (horizontal, superior/anterior, posterior) and the two otolith organs (utricle and saccule). Combining the Video Head Impulse Test (vHIT), Vestibular Evoked Myogenic Potentials (cVEMP and oVEMP), and Computerized Dynamic Posturography (CDP) provides a comprehensive functional profile of the vestibular periphery and central postural integration.

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Video Head Impulse Testing (vHIT)

The Video Head Impulse Test (vHIT) is an objective clinical implementation of the manual Halmagyi-Curthoys head impulse test. While caloric testing evaluates the horizontal canal at ultra-low frequencies ($0.003\,\text{Hz}$) and rotational chair assesses mid frequencies ($0.01-0.64\,\text{Hz}$), vHIT evaluates the VOR at its true physiological operating range:

• Stimulus Frequency: $2.0\,\text{Hz}$ to $5.0\,\text{Hz}$
• Head Angular Acceleration: $>2000^\circ/\text{s}^2$ to $4000^\circ/\text{s}^2$
• Head Angular Velocity: $150^\circ/\text{s}$ to $250^\circ/\text{s}$
• Displacement Amplitude: Small, passive, unpredictable angular rotations of only $10^\circ$ to $20^\circ$

Coplanar Semicircular Canal Testing Planes

Because semicircular canals operate as functional push-pull pairs, vHIT can isolate and quantify all six individual semicircular canals by delivering head thrusts along specific anatomical planes:

1. Horizontal Plane (Yaw): Rapid lateral head thrusts to the right and left.
2. RALP Plane: Right Anterior - Left Posterior canals. The patient's head is turned $35^\circ-45^\circ$ to the left; pitch impulses (downward and upward) are delivered along the plane of the right anterior and left posterior canals.
3. LARP Plane: Left Anterior - Right Posterior canals. The patient's head is turned $35^\circ-45^\circ$ to the right; pitch impulses are delivered along the plane of the left anterior and right posterior canals.

vHIT Interpretation: VOR Gain and Catch-Up Saccades

During testing, high-speed lightweight video goggles ($250\,\text{Hz}$) track pupil position while integrated triaxial gyroscopes measure instantaneous head velocity.

• VOR Gain: Calculated as the area under the curve of eye velocity divided by the area under the curve of head velocity (or instantaneous velocity ratios). Normal horizontal gain ranges between $0.80$ and $1.00$; vertical gain (RALP/LARP) ranges between $0.70$ and $1.00$.
• Corrective Refixation Saccades: In a paretic canal, the eyes fail to stay fixed on the visual target during the head thrust, drifting with the head. To maintain target visualization, the central nervous system generates compensatory catch-up saccades:
  • Covert Saccades: Triggered during the head movement itself (latency $<100\,\text{ms}$). Because they occur while the head is actively rotating, covert saccades are invisible to the clinician's naked eye during manual bedside examination. High-speed vHIT goggles are strictly required to detect them.
  • Overt Saccades: Triggered after head movement has completely ceased (latency $>100-150\,\text{ms}$). Overt saccades are visible to the naked eye at the bedside.
  • Diagnostic Significance: The presence of organized, repetitive catch-up saccades (even with borderline gain values) confirms peripheral VOR pathology.

Triage of Acute Vestibular Syndrome: The HINTS Exam

vHIT plays a critical role in the emergency differentiation of acute vestibular syndrome (AVS). Clinicians employ the HINTS protocol (Head Impulse, Nystagmus, Test of Skew):

• Acute Peripheral Vestibular Neuritis: Abnormal head impulse test (reduced gain + catch-up saccades on the affected side), unidirectional horizontal nystagmus obeying Alexander's law, and absent vertical skew deviation.
• Posterior Circulation Stroke (for example, PICA cerebellar infarct): Normal head impulse test (gain $\ge 0.85$ without catch-up saccades), direction-changing gaze-evoked nystagmus, and/or vertical ocular skew deviation on alternate cover testing. In acute continuous vertigo with nystagmus, a normal head impulse test is a major red flag for stroke. Caution: AICA infarcts can also damage the labyrinth and produce an abnormal head impulse, so new unilateral hearing loss during acute vertigo (the HINTS-plus finding) is itself a stroke warning sign.

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Vestibular Evoked Myogenic Potentials (VEMP)

VEMP testing evaluates the otolith organs (utricle and saccule) and their respective divisions of the vestibular nerve. While the semicircular canals sense angular acceleration, the otolith organs detect linear acceleration and gravitational tilt.

1. Cervical VEMP (cVEMP): Saccule & Inferior Vestibular Nerve

• Reflex Arc: The vestibulocollic reflex (VCR). The reflex originates in the saccular macula, travels along the inferior vestibular nerve to the vestibular nuclei, descends through the medial vestibulospinal tract (MVST), and synapses on motor neurons of the accessory nerve (CN XI) to inhibit the ipsilateral sternocleidomastoid (SCM) muscle.
• Neurophysiology: The cVEMP is an ipsilateral, inhibitory myogenic reflex recorded from a tonically contracted muscle. Because it is an inhibitory potential, the SCM muscle must maintain sustained isometric contraction throughout testing (achieved by lifting the head while supine or turning the head against resistance).
• Waveform Morphology: A biphasic potential characterized by an initial positive peak at $\approx 13\,\text{ms}$ (P13 or p1) followed by a negative peak at $\approx 23\,\text{ms}$ (N23 or n1).
• Stimulus: High-level acoustic stimuli (typically $500\,\text{Hz}$ tone bursts or acoustic clicks at $95-100\,\text{dB nHL}$) presented via insert earphones, or bone-conduction vibration at the mastoid or forehead.
• EMG Rectification: Because raw peak-to-peak amplitude scales directly with muscle contraction force, modern systems monitor pre-stimulus electromyographic (EMG) background levels and calculate an EMG-rectified amplitude to prevent false asymmetries.
• Amplitude Asymmetry Ratio (AR): $\text{AR} = \frac{|\text{Amp}_{\text{Right}} - \text{Amp}_{\text{Left}}|}{\text{Amp}_{\text{Right}} + \text{Amp}_{\text{Left}}} \times 100\%$ An asymmetry ratio $>30\%$ to $40\%$ confirms unilateral saccular or inferior vestibular nerve hypofunction on the lower-amplitude side.

2. Ocular VEMP (oVEMP): Utricle & Superior Vestibular Nerve

• Reflex Arc: The vestibulo-ocular reflex (VOR) otolith-ocular pathway. The reflex originates in the utricular macula, travels along the superior vestibular nerve to the vestibular nuclei, ascends through the medial longitudinal fasciculus (MLF), decussates, and synapses in the contralateral oculomotor nucleus (CN III) to excite the inferior oblique extraocular muscle.
• Neurophysiology: The oVEMP is a contralateral, excitatory myogenic reflex recorded from surface electrodes placed beneath the contralateral eye. To bring the inferior oblique muscle close to the surface electrode, the patient must maintain maximal upward gaze ($20^\circ-30^\circ$) throughout recording.
• Waveform Morphology: A biphasic potential characterized by an initial negative peak at $\approx 10\,\text{ms}$ (N10 or n1) followed by a positive peak at $\approx 15\,\text{ms}$ (P15 or p1).
• Clinical Significance: An amplitude asymmetry ratio $>30\%$ to $35\%$ indicates utricular or superior vestibular nerve pathology on the lower-amplitude side.

3. Superior Semicircular Canal Dehiscence (SSCD / SCDS)

Superior Semicircular Canal Dehiscence (SSCD) is an otic capsule defect characterized by a bony dehiscence overlying the superior semicircular canal into the middle cranial fossa. This creates an abnormal "third mobile window" (in addition to the oval and round windows).

The Third Mobile Window Phenomenon

Normally, acoustic energy is channeled exclusively into the cochlea. In SSCD, acoustic and hydraulic energy escapes through the dehiscence. This manifests as a unique diagnostic profile:

• Audiometry: Low-frequency pseudo-conductive hearing loss with an apparent air-bone gap ($10-30\,\text{dB}$) despite completely normal middle ear immittance (Type A tympanometry) and intact acoustic reflexes. Bone conduction thresholds are frequently hypersensitive (better than $0\,\text{dB HL}$, e.g., $-5$ to $-10\,\text{dB HL}$).
• VEMP Findings (Pathognomonic): The third mobile window shunts acoustic energy across the vestibular labyrinth, dramatically lowering acoustic impedance. Consequently, both cVEMP and oVEMP acoustic thresholds are abnormally reduced ($<70-80\,\text{dB nHL}$, often present at $55-65\,\text{dB nHL}$), accompanied by giant, supranormal amplitudes ($>2-3\times$ normal limits).
• Clinical Signs:
  • Tullio Phenomenon: Dizziness, oscillopsia, or vertical-torsional nystagmus induced by loud acoustic stimuli (e.g., telephone rings, musical instruments).
  • Hennebert Sign: Vertigo and nystagmus induced by changes in external auditory canal pressure (tragal compression or pneumatic otoscopy) or intracranial pressure (Valsalva against a closed glottis or pinched nostrils).
  • Autophony: Heightened sensitivity to internal bodily sounds (hearing one's own voice echoing, heartbeat, chewing, or even eye movements scratching in the orbit).

Computerized Dynamic Posturography (CDP)

While VNG, rotary chair, vHIT, and VEMPs evaluate sensory reflexes in isolation, Computerized Dynamic Posturography (CDP) assesses how the central nervous system integrates visual, somatosensory (proprioceptive), and vestibular inputs to maintain postural balance. CDP measures functional equilibrium rather than localizing an isolated ear lesion.

The Sensory Organization Test (SOT)

The core protocol of CDP is the Sensory Organization Test (SOT). The patient stands strapped into a safety harness on a dual-force plate surrounded by a visual enclosure. By systematically providing accurate, absent, or misleading visual and somatosensory information, the SOT isolates the patient's reliance on each sensory modality.

Center-of-Gravity Sway & Equilibrium Score

Force plate sensors measure anterior-posterior (AP) center-of-gravity (COG) sway. In healthy individuals, the maximum theoretical limit of AP stability without taking a step is $12.5^\circ$ ($8.0^\circ$ anteriorly and $4.5^\circ$ posteriorly). The Equilibrium Score (ES) compares the patient's actual peak-to-peak AP sway angle ($\theta$) to this theoretical limit: $\text{ES} = \left(1 - \frac{\theta_{\text{peak-to-peak}}}{12.5^\circ}\right) \times 100$ A score of $100$ indicates perfect stability (zero sway), while a score of $0$ indicates a fall (requiring the harness or taking a protective step).

The Six SOT Test Conditions

1. Condition 1 (Baseline): Eyes open, fixed support platform, fixed visual surround. All three sensory systems (somatosensory, visual, vestibular) are available and accurate.
2. Condition 2 (Vision Denied): Eyes closed, fixed platform, fixed visual surround. Vision is eliminated; the patient must rely on accurate somatosensory and vestibular inputs.
3. Condition 3 (Vision Inaccurate): Eyes open, fixed platform, sway-referenced visual surround. The visual surround tilts synchronously with the patient's AP body sway, providing inaccurate visual cues. Tests ability to ignore misleading vision and rely on somatosensory and vestibular systems.
4. Condition 4 (Somatosensory Inaccurate): Eyes open, sway-referenced platform, fixed visual surround. The platform tilts directly with patient sway, eliminating reliable ankle joint proprioception. Tests reliance on vision and vestibular cues.
5. Condition 5 (Vestibular Isolated - Vision Denied): Eyes closed, sway-referenced platform. Vision is removed, and somatosensory input is disrupted. Postural stability depends entirely on the vestibular system.
6. Condition 6 (Vestibular Isolated - Severe Sensory Conflict): Eyes open, sway-referenced platform, sway-referenced visual surround. Both somatosensory and visual inputs are simultaneously disrupted. The patient must rely strictly on vestibular cues to prevent falling.

Diagnostic Sensory Patterns & Pathology Profiles

• Normal Balance Profile: Equilibrium scores $>75-90$ across Conditions 1-4, and $>60-70$ on Conditions 5 and 6.
• Vestibular Dysfunction Pattern: Normal performance on Conditions 1, 2, 3, and 4, with significant instability or repeated falls on Conditions 5 and 6. Confirms that the patient cannot maintain balance when vestibular input is required.
• Visual Preference (Visual Dependency) Pattern: Normal scores on Conditions 1, 2, 4, and 5, but abnormally low scores on Conditions 3 and 6. The patient relies heavily on visual information even when it is inaccurate, swaying in lockstep with the visual surround. Notably, these patients score worse with eyes open in an inaccurate surround (Condition 3) than with eyes closed (Condition 2).
• Somatosensory / Surface Dependency Pattern: Normal performance when the platform is stable (Conditions 1-3), but abnormal scores whenever the platform is sway-referenced (Conditions 4, 5, and 6), seen in peripheral neuropathy or cervical myelopathy.
• Aphysiologic (Non-Organic / Functional) Pattern: Suggests non-organic balance impairment, malingering, or functional movement disorder. Characterized by:
  • Scores on difficult conditions (Condition 5 or 6) that exceed scores on simple conditions (Condition 1 or 2).
  • Wide, exaggerated circular or lateral sway without falling or taking a protective step.
  • High inter-trial variability with erratic, inconsistent performance.
  • Physiological "near-falls" where the patient recovers violently without activating the safety harness.