Vestibular Function Testing

Vestibular function testing provides objective, quantitative assessment of the vestibular end organs and their central connections when the bedside examination is insufficient for diagnosis, when quantification is needed for monitoring disease progression or treatment response, or when medicolegal documentation is required. The vestibular testing laboratory has evolved significantly over the past two decades, with video head impulse testing (vHIT) and vestibular evoked myogenic potentials (VEMPs) joining the traditional caloric test and rotary chair to create a comprehensive assessment battery that can evaluate each semicircular canal and each otolith organ independently. Understanding when to order each test, how to interpret results, and how findings correlate with specific clinical conditions is essential for neurologists managing vestibular disorders.

Videonystagmography / Electronystagmography (VNG/ENG)

VNG (using infrared video goggles) has largely replaced ENG (using periorbital electrodes) as the standard vestibular test battery. VNG provides superior spatial resolution and real-time visualization of eye movements. The battery typically includes three components: oculomotor evaluation, positional testing, and caloric testing.

Oculomotor Evaluation

Positional Testing

• Static positional testing: Head in various positions (supine, head right, head left, head hanging) for 30–60 seconds each; nystagmus recorded with and without fixation
• Dix-Hallpike and supine roll: Performed within the VNG suite with goggles recording; provides permanent documentation of nystagmus direction, latency, duration, and fatigue
• Interpretation: Direction-fixed positional nystagmus (same direction in all positions) is usually peripheral; direction-changing positional nystagmus is concerning for central pathology but can also occur with horizontal canal BPPV

Caloric Testing

The caloric test remains the gold standard for detecting unilateral peripheral vestibular hypofunction. It evaluates each horizontal semicircular canal independently by inducing endolymph convection currents through thermal stimulation.

Limitations of Caloric Testing

• Tests only the horizontal semicircular canal — does not evaluate posterior or superior canals, or the otolith organs
• Tests only the very low frequency range (~0.003 Hz) — a patient can have normal calorics but abnormal high-frequency VOR (detected by vHIT)
• Anatomic variations in the temporal bone and external auditory canal can affect heat transfer, producing false results
• Cannot be performed with tympanic membrane perforation (use air caloric) or middle ear disease
• Patient alertness affects results — the patient must perform alerting tasks (serial subtractions, conversation) to prevent habituation

Video Head Impulse Test (vHIT)

vHIT uses lightweight video goggles with a high-speed camera (typically 250 Hz) to precisely measure eye movements during rapid head impulses, providing quantitative assessment of the VOR for each individual semicircular canal.

Technique

• Equipment: Lightweight goggles with infrared camera and gyroscope (measures head velocity); patient fixates on a wall target at ~1 m distance
• Horizontal canals: Tested by horizontal head impulses (as in bedside HIT); head is turned in the plane of the horizontal canals
• Vertical canals: LARP (left anterior / right posterior) and RALP (right anterior / left posterior) planes; the examiner turns the head diagonally in the plane of the canal pair being tested
• Parameters measured: VOR gain (ratio of eye velocity to head velocity at a specific time point, typically at 60–80 ms) and corrective saccades (overt and covert)

Interpretation

Vestibular Evoked Myogenic Potentials (VEMPs)

VEMPs are short-latency electromyographic responses evoked by loud sound or vibration stimuli that assess otolith organ (saccule and utricle) function. There are two types: cervical VEMP (cVEMP) and ocular VEMP (oVEMP).

Cervical VEMP (cVEMP)

Ocular VEMP (oVEMP)

Clinical Applications of VEMPs

Rotary Chair Testing

Rotary chair testing evaluates the VOR across a range of frequencies by rotating the patient on a motorized chair in darkness, measuring the resulting eye movements.

Technique

• Setup: Patient seated in a motorized rotational chair in a light-tight enclosure; eye movements recorded with VNG goggles or EOG electrodes
• Sinusoidal harmonic acceleration (SHA): Chair oscillates sinusoidally at frequencies from 0.01 to 0.64 Hz (or higher); measures VOR gain, phase, and symmetry at each frequency
• Step velocity testing: Rapid rotation to a constant velocity (e.g., 60°/s); measures the decay of per-rotational nystagmus (time constant) and post-rotational responses
• Visual-vestibular interaction: VOR in light, VOR suppression (fixation on a chair-mounted target during rotation)

Interpretation

Computerized Dynamic Posturography (CDP)

CDP evaluates postural stability by measuring sway on a force platform under systematically varied sensory conditions. The most widely used protocol is the Sensory Organization Test (SOT).

Sensory Organization Test (SOT)

Interpretation Patterns

• Vestibular pattern: Falls/excessive sway on conditions 5 and 6 (vestibular-only conditions); conditions 1–4 relatively preserved; consistent with bilateral vestibular hypofunction or uncompensated unilateral loss
• Somatosensory pattern: Falls on conditions 3, 4, 5, 6 (all conditions with sway-referenced platform); suggests peripheral neuropathy or somatosensory deficit
• Visual preference pattern: Poor performance whenever visual input is inaccurate (conditions 3 and 6); patient is overly reliant on vision; seen in visual vertigo and some PPPD patients
• Aphysiologic pattern: Falls on simple conditions (1 and 2) but performs better on more difficult conditions (5, 6) — this paradoxical pattern suggests functional/non-organic dizziness or symptom exaggeration

Audiometry in Vestibular Disorders

Audiometric testing is an essential component of the vestibular evaluation, as specific patterns of hearing loss point to particular diagnoses and localize pathology.

When to Order Audiometry

• All patients presenting with vertigo or dizziness (baseline assessment)
• Any patient with subjective hearing loss, aural fullness, or tinnitus
• Asymmetric symptoms suggesting unilateral pathology (to screen for schwannoma)
• Follow-up for Ménière disease (documenting progression of hearing loss)
• Before and during ototoxic drug therapy (aminoglycosides, cisplatin)
• After acute vestibular syndrome to differentiate vestibular neuritis (normal hearing) from labyrinthitis (hearing loss) or AICA stroke (hearing loss)

Auditory Brainstem Response (ABR)

• Purpose: Evaluates neural conduction along the auditory pathway from the cochlear nerve through the brainstem; useful as a screening tool for retrocochlear pathology when MRI is unavailable or contraindicated
• Key waves: Wave I (distal CN8), wave III (cochlear nucleus/SOC), wave V (lateral lemniscus/inferior colliculus); interpeak latencies (I–III, III–V, I–V) are the primary measures
• Vestibular schwannoma: Prolonged I–III or I–V interpeak latency; absent wave V; interaural latency difference >0.2 ms for wave V is abnormal; sensitivity ~90% for tumors >1 cm, but lower for small intracanalicular tumors
• Limitations: Cannot detect small intracanalicular schwannomas reliably; MRI with gadolinium is the gold standard imaging test and has largely replaced ABR for schwannoma screening in most centers
• Intraoperative monitoring: ABR is used during CPA surgery to monitor auditory nerve function in real time and preserve hearing

Clinical Decision-Making: Which Tests to Order

Emerging and Advanced Testing

Electrocochleography (ECoG)

• Purpose: Detects endolymphatic hydrops in suspected Ménière disease
• Measure: Summating potential to action potential ratio (SP/AP ratio); elevated ratio (>0.4–0.5) suggests endolymphatic hydrops
• Technique: Transtympanic (needle on promontory — most sensitive) or extratympanic (tympanic membrane surface electrode)
• Limitations: Sensitivity varies (60–80%); may be normal interictally; invasive transtympanic approach limits availability

MRI Hydrops Imaging

• Technique: 3D-FLAIR MRI 4–24 hours after intratympanic or intravenous gadolinium injection
• Principle: Gadolinium enters perilymph but not endolymph, allowing visualization of endolymphatic space expansion
• Clinical utility: Directly visualizes hydrops; increasingly used in research and clinical practice; may differentiate Ménière from vestibular migraine

Subjective Visual Vertical (SVV)

• Purpose: Assesses otolith (primarily utricle) function; the patient aligns a luminous bar to perceived vertical in darkness
• Abnormal: Tilt >2–2.5° from true vertical; tilts toward the lesion side in acute peripheral vestibulopathy; may tilt away from lesion in central otolithic lesions (brainstem)
• Time course: Maximally abnormal in the acute phase; normalizes over weeks as central compensation occurs; persistent deviation suggests ongoing vestibular imbalance or central pathology
• Clinical role: Quick bedside or office test; complements VEMPs in assessing otolith function; available as a smartphone application for point-of-care use

Vestibular Test Patterns by Condition

Practical Considerations

References

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