---

Full VNG (Videonystagmography) Interpretation

Sources: K.J. Lee's Essential Otolaryngology; Cummings Otolaryngology Head & Neck Surgery; Bradley & Daroff's Neurology in Clinical Practice; Harrison's Principles of Internal Medicine 22e

---

1. What is VNG?

• Horizontal, vertical, and (with 3D systems) torsional eye movements
• Real-time or recorded nystagmus
• Quantified slow-phase eye velocity (SPV) for all subtests

---

2. The VNG Battery — Overview

---

3. Oculomotor Subtests

3a. Saccades

• What it tests: Voluntary rapid eye movements to jump between targets
• Normal: Accurate, fast, symmetric
• Abnormal patterns:
  • Hypometric (undershoots) → cerebellar or brainstem disease
  • Hypermetric (overshoots) → cerebellar dysfunction
  • Slow saccades → brainstem pathology, drugs, progressive supranuclear palsy
  • Asymmetric latency → frontal lobe lesion on the side with prolonged latency

3b. Smooth Pursuit

• What it tests: Ability to smoothly track a slow-moving target
• Normal: Smooth, symmetric gain ~1.0
• Abnormal: Cogwheeling / saccadic intrusions (pursuit "breaks up") → central pathology (cerebellum, brainstem, diffuse CNS disease, medications, age)

3c. Optokinetic Nystagmus (OKN)

• What it tests: Reflexive nystagmus in response to a full-field moving pattern
• Normal: Symmetric OKN in both directions
• Abnormal: Asymmetric or absent OKN → parieto-occipital or brainstem lesion

3d. Gaze Testing

• What it tests: Stability of gaze held at eccentric positions (left, right, up, down)
• Normal: No nystagmus at gaze
• Gaze-evoked nystagmus (GEN):
  • Beats in the direction of gaze
  • Unilateral GEN → ipsilateral cerebellar or brainstem lesion
  • Bilateral GEN (both directions) → drugs (alcohol, anticonvulsants, sedatives), bilateral cerebellar disease
  • Direction-changing GEN = strongly central

3e. Fixation Suppression (Critical differentiator!)

• The normal VOR is suppressed when the eyes fix on a target moving with the head
• Intact fixation suppression = peripheral lesion
• Failure of fixation suppression (nystagmus continues despite fixation) = central lesion (vestibulocerebellum, especially flocculus/nodulus)

---

4. Vestibular Subtests

4a. Spontaneous Nystagmus

---

4b. Positional Nystagmus (Static)

---

4c. Positioning (Dynamic) Nystagmus — Dix-Hallpike

• Latency: 1–5 seconds before nystagmus starts
• Direction: Upbeat + torsional (top pole beats toward the affected/lower ear)
• Duration: < 60 seconds (usually 20–30 s)
• Fatigues with repeated testing
• Reversal: Nystagmus reverses when patient sits back up

• No latency (immediate onset)
• No fatigue
• Pure vertical (downbeat or upbeat) without torsional component
• Duration > 60 seconds
• Nystagmus in direction opposite to expected

---

4d. Bithermal Caloric Testing — The Core of VNG

Principle

• Each ear is irrigated with warm (44°C) and cool (30°C) water (or air: 58°C / 24°C) for 30 seconds (water) or 60 seconds (air).
• The temperature gradient creates convection currents in the endolymph of the lateral (horizontal) semicircular canal, producing a controlled, measurable nystagmus.

Cold Opposite, Warm Same

• Cool irrigation → nystagmus beats AWAY from the irrigated ear
• Warm irrigation → nystagmus beats TOWARD the irrigated ear

Why this works (physiology):

• Cool → endolymph falls → ampullofugal flow → hair cell inhibition → slow eye drift toward irrigated ear → compensatory fast phase away
• Warm → endolymph rises → ampullopetal flow → hair cell excitation → slow drift away from irrigated ear → fast phase toward irrigated ear

The Four Measurements

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

---

4e. Caloric Formulas

Unilateral Weakness (UW) — Jongkees Formula

• Normal: < 20–25% (lab-dependent; most use 20–25%)
• Abnormal: > 20–25% → reduced response on the weaker side = peripheral vestibular hypofunction
• Points to the side with lower total response (i.e., the side contributing less to the numerator being positive = right side weak if UW is positive and right values are low)

Directional Preponderance (DP)

• Normal: < 30%
• Abnormal DP = nystagmus beats more strongly in one direction
• DP toward the side of lesion suggests central; DP away from lesion suggests peripheral (still debated)
• DP alone without UW is non-localizing

Bilateral Weakness (BW)

• Total response of both sides < 12°/s (lab-dependent)
• i.e., LW + LC < 12°/s AND RW + RC < 12°/s
• Suggests bilateral vestibular loss (ototoxicity, bilateral Menière's, autoimmune labyrinthopathy)

---

5. Caloric Trace Interpretation — Real Images

---

---

6. Pattern Recognition — Central vs. Peripheral Summary

---

7. Findings That Are ALWAYS Central

1. Failure of fixation suppression
2. Direction-changing spontaneous nystagmus
3. Spontaneous or positional nystagmus with NORMAL calorics (central nystagmus generator, not the labyrinth)
4. Abnormal saccades or broken pursuit with normal calorics
5. Bilateral caloric weakness without history of ototoxicity or labyrinthine disease
6. Hyperactive caloric responses (absent cerebellar inhibition) without TM perforation
7. Purely vertical positioning nystagmus (no torsional component)
8. Caloric inversion

---

8. Findings That Are ALWAYS Peripheral

1. Unilateral caloric weakness (abnormal UW formula)
2. Bilateral caloric weakness WITH history of ototoxicity/labyrinthine disease
3. Intact fixation suppression on nystagmus
4. Direction-fixed nystagmus that obeys Alexander's law
5. Fatiguing positional nystagmus
6. Classic Dix-Hallpike response (latency + torsional + fatigue)

---

9. Clinical Correlation — Common Diagnoses on VNG

---

10. Practical Considerations Before/During Testing

• Otoscopy first — always inspect TM before calorics
• Water is contraindicated with TM perforation → use air calorics instead (perforated side will show an artificially heightened response)
• Excess cerumen must be cleared — it impedes heat transfer and gives falsely reduced responses
• Medications that suppress the CNS (benzodiazepines, antihistamines, anticonvulsants) must be stopped 48–72 hours before testing — they depress nystagmus responses and create false findings
• Patient must be alert and mentally stimulated (asked to count or name items) during caloric testing — mental alertness maximizes nystagmus amplitude

---

11. Quick Summary Flowchart

VNG Result
│
├── Abnormal CALORICS?
│   ├── Unilateral weakness → Peripheral (ipsilateral)
│   ├── Bilateral weakness + ototoxicity hx → Peripheral (bilateral)
│   ├── Bilateral weakness, no hx → CENTRAL
│   └── Hyperactive / inverted → CENTRAL
│
├── Normal CALORICS but NYSTAGMUS present?
│   └── → CENTRAL (nystagmus generator, not labyrinth)
│
├── Fixation suppression ABSENT?
│   └── → CENTRAL (vestibulocerebellum)
│
├── Abnormal SACCADES / PURSUIT with normal calorics?
│   └── → CENTRAL (brainstem/cerebellum)
│
└── UW + direction-fixed + fixation suppresses + normal oculomotor?
    └── → PERIPHERAL

---

Key Formula Card

---

1. exam-style MCQs + answer key, or
2. a 1-page rapid-reference sheet for clinic use.