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📋 Full VNG (Videonystagmography) Interpretation — Complete Guide

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1. What is VNG?

• ENG (Electronystagmography) — older method; uses electrodes at the lateral canthi and above/below one eye to measure the corneoretinal electrical potential (a dipole) indirectly.
• VNG — modern method; uses infrared video cameras to record eye movements directly in real-time and digitally.

VNG replaced ENG because it is more accurate, allows visualization of torsional nystagmus, and produces digital recordings.

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2. Components of VNG

A. Vestibular Subtests

B. Oculomotor Subtests

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3. Key Concept: Nystagmus

• By convention, direction of nystagmus = direction of the FAST phase
• The slow phase velocity (SPV) is the actual measure of vestibular function

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4. Spontaneous Nystagmus

• Tested with eyes open and eyes closed (fixation removed)
• Normal: No nystagmus (or very minimal, < 2°/s)
• Abnormal: Nystagmus > 2°/s

• In peripheral lesions → fixation suppresses nystagmus (normal fixation suppression)
• In central lesions → fixation does NOT suppress nystagmus (failure of fixation suppression)

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5. Gaze Nystagmus

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6. Positional vs. Positioning Nystagmus

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7. The Bithermal Caloric Test — The Core of VNG

Principle

Stimulus

COWS Mnemonic

Cold — Opposite, Warm — Same

• Cold irrigation → endolymph falls → ampullofugal flow → inhibition of that ear → eyes drift toward the stimulated ear → fast (corrective) nystagmus beats to the opposite side
• Warm irrigation → endolymph rises → ampullopetal flow → excitation of that ear → eyes drift away → fast nystagmus beats to the same side

How to Calculate

• RW = Right ear Warm
• RC = Right ear Cool
• LW = Left ear Warm
• LC = Left ear Cool

• Abnormal if > 25–30% (lab-dependent, commonly 25%)
• A high UW% means the ear with the lower sum is the weak (hypoactive) ear
• Indicates peripheral vestibular hypofunction on that side

• Abnormal if > 30%
• Reflects an asymmetry in nystagmus direction regardless of which ear is stimulated
• Weak clinical value on its own, but may point toward the side of a central lesion (preponderance) or away from a peripheral lesion

• Abnormal if LW + LC < 12°/s AND RW + RC < 12°/s (lab-dependent)
• Suggests bilateral peripheral vestibular loss (e.g., ototoxicity, autoimmune)

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8. Oculomotor Subtest Interpretation

Smooth Pursuit

• Patient tracks a sinusoidally moving target
• Normal: Smooth, continuous tracking
• Abnormal: Saccadic (jerky) pursuit = central pathology (cerebellum, brainstem, or age-related)

Saccades

• Patient makes rapid eye movements to jump between targets
• Metrics: Latency, velocity, accuracy
• Undershoot (hypometria): Normal minor variant, also in cerebellar disease
• Overshoot (hypermetria): Cerebellar lesion
• Slow saccades: Brainstem, neurodegenerative disease

Optokinetic Nystagmus (OKN)

• Moving stripes/pattern → induced nystagmus
• Asymmetric OKN: Central pathway lesion (brainstem, parietal lobe)
• Symmetric OKN gain reduction: Diffuse central dysfunction

Fixation Suppression

• If nystagmus is present during caloric, can the patient suppress it with visual fixation?
• Normal: Nystagmus suppressed ≥ 60% (fixation suppression index, FSI > 0.6)
• Abnormal FSI (failure of fixation suppression): Central lesion (cerebellar/brainstem)

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9. Pattern Recognition — Central vs. Peripheral

Red Flags for Central Pathology in VNG:

1. Spontaneous/positional nystagmus with normal caloric responses
2. Direction-changing nystagmus
3. Failure of fixation suppression
4. Bilateral absent caloric responses without history of ear disease or ototoxicity
5. Abnormal saccades or saccadic pursuit with normal calorics
6. Hyperactive caloric responses (loss of cerebellar inhibition) without TM defect or mastoid cavity

Signs of Peripheral Pathology:

1. Unilateral caloric weakness (UW > 25–30%)
2. Bilateral caloric weakness with history of labyrinthine disease or ototoxicity
3. Fatiguing positional nystagmus
4. Intact fixation suppression
5. Direction-fixed nystagmus

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10. Step-by-Step VNG Interpretation Approach

STEP 1: Oculomotor subtests
   → Are saccades, pursuit, OKN normal?
   → If abnormal → suspect CENTRAL

STEP 2: Spontaneous nystagmus
   → Present? Direction? Suppressed by fixation?
   → Failure of fixation suppression → CENTRAL

STEP 3: Gaze testing
   → Direction-changing? → CENTRAL
   → Direction-fixed → peripheral

STEP 4: Positional/positioning tests
   → Fatiguing, latency, direction-fixed → peripheral (BPPV)
   → Persistent, no fatigue, direction-changing → CENTRAL

STEP 5: Caloric test (the KEY step)
   → Calculate UW and DP
   → Unilateral weakness → peripheral hypofunction on that side
   → Bilateral weakness → bilateral peripheral loss
   → Normal calorics but abnormal oculomotor → CENTRAL

STEP 6: Synthesize
   → Match all findings → peripheral vs. central pattern
   → Correlate with clinical history

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11. Common Clinical Correlations

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12. Quick Memory Anchors

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• worked caloric calculation examples (real numbers),
• a one-page rapid interpretation algorithm,
• viva-style practice cases with answers.