Cranio-Oculography (Extended Videonystagmography) Report

1. Background: What is VNG?

2. Saccade Testing

2.1 Horizontal Saccades

0.3 Hz Bidirectional Horizontal

0.45 Hz Bidirectional Horizontal

2.2 Vertical Saccades

0.3 Hz Bidirectional Vertical

0.45 Hz Bidirectional Vertical

3. Smooth Pursuit Testing

3.1 Horizontal Pursuit

0.2 Hz Horizontal

0.4 Hz Horizontal

3.2 Vertical Pursuit

4. Optokinetic Testing (OKN)

5. Gaze Testing

Summary of Gaze Results

6. Spontaneous Nystagmus

6.1 In Light (Fixation Present)

6.2 In Dark (Fixation Removed)

• Direction -90° corresponds to downbeat nystagmus (fast phase downward).
• Downbeat nystagmus that appears in darkness and is suppressed by fixation is a recognized sign of cerebellar floccular/parafloccular dysfunction or pathology at the cervicomedullary junction.
• The relatively high SPV of ~30 °/s is clinically significant.
• The fact that it appears only in the LE vertical channel may reflect asymmetric recording sensitivity or a predominantly torsional/vertical component difficult to capture bilaterally.

7. Head-Shake Nystagmus

8. Caloric Testing

9. Summary of Findings

10. Clinical Impression

1. Downbeat nystagmus in darkness (LE vertical channel, SPV ~30 °/s, direction -90°) - the primary abnormal finding, consistent with floccular/dorsal vermis cerebellar dysfunction or a lesion at the cervicomedullary junction (e.g., Chiari malformation, demyelination, drug toxicity, or degenerative condition).
2. Severely reduced smooth pursuit gain across all frequencies and both eyes, a hallmark of central (cerebellar or brainstem) oculomotor dysfunction.
3. Reduced but symmetric OKN gain, consistent with the pursuit findings.
4. Normal saccades and no gaze-evoked nystagmus, which argues against a gross structural brainstem/cerebellar lesion and makes an acute peripheral labyrinthine lesion less likely.

11. Recommendations

1. Complete caloric testing (bithermal, both ears) to evaluate peripheral vestibular function of the lateral semicircular canals.
2. MRI of the posterior fossa and brainstem with gadolinium to exclude structural causes of downbeat nystagmus (Chiari malformation, cerebellar atrophy, demyelinating lesion, tumor).
3. Medication review - rule out drug-induced causes of downbeat nystagmus (lithium, anticonvulsants, alcohol).
4. Consider rotational chair testing for a more physiologic evaluation of bilateral VOR function.
5. Clinical correlation with symptoms (dizziness, vertigo, oscillopsia, ataxia) is essential.
6. Neurotology or neurology referral is advised given the central pattern.

Diagnosis Based on VNG Findings

Primary Diagnosis: Central Vestibulo-Cerebellar Dysfunction

The Three Key Diagnostic Clues

What is Downbeat Nystagmus?

• Impaired cerebellar inhibition (flocculus/nodulus) of vestibular circuits responsible for upward eye movement - eyes drift upward unopposed, with corrective fast phases downward
• OR interruption of posterior semicircular canal projections that control the downward VOR

Differential Diagnoses - Ranked by Likelihood for a 30-Year-Old Male

1. Chiari Malformation Type I (Most Likely)

• Most common structural cause of DBN in a young adult
• Cerebellar tonsil herniation through foramen magnum compresses the flocculus/brainstem
• May be clinically silent for years or present with occipital headache, neck pain, oscillopsia, ataxia
• MRI posterior fossa is mandatory to exclude this

2. Episodic Ataxia Type 2 (EA-2) (High Priority)

• CACNA1A channelopathy - causes paroxysmal cerebellar ataxia + persistent interictal DBN
• Age of onset: teens to 30s - fits this patient perfectly
• Presents with episodes of vertigo, imbalance, nausea lasting hours
• Responds to acetazolamide
• Genetic testing (CACNA1A gene) recommended

3. Spinocerebellar Ataxia Type 6 (SCA-6)

• CAG repeat expansion in CACNA1A
• Progressive cerebellar degeneration; DBN is a hallmark
• Usually onset 30s-50s - possible at age 30
• Family history may be present

4. Multiple Sclerosis / Demyelination

• DBN + pursuit failure is a recognised MS presentation
• Age 30 male is within the MS demographic
• May be the first demyelinating event
• MRI brain/spine with gadolinium would clarify

5. Drug / Toxic Causes (Must Exclude First)

• Lithium, anticonvulsants (phenytoin, carbamazepine), amiodarone, opioids, alcohol
• These produce identical VNG patterns and are reversible
• Medication history is essential before further workup

6. Wernicke Encephalopathy / Thiamine Deficiency

• Can cause DBN + oculomotor abnormalities
• Relevant if nutritional risk factors exist (alcohol use, vomiting, malnutrition)

7. Paraneoplastic Cerebellar Degeneration

• Anti-Yo, anti-Ta, anti-Hu antibodies
• Less likely at 30 without known malignancy but must be considered if no structural cause found

8. Idiopathic Downbeat Nystagmus

• Diagnosis of exclusion; up to 50% of cases remain idiopathic after full workup
• Presumed to represent subclinical cerebellar degeneration

What This is NOT

• NOT BPPV - BPPV produces peripheral positional nystagmus, normal pursuit, and no spontaneous DBN in darkness
• NOT Labyrinthitis/Vestibular Neuritis - peripheral lesion; would show unilateral caloric weakness, intact pursuit, and horizontal rather than vertical nystagmus
• NOT a normal finding - DBN with SPV of 30°/s is clearly pathological

Immediate Action Plan

1. Rule out drugs/toxins first (medication review, alcohol history) - cheapest and fastest
2. MRI posterior fossa + brainstem with gadolinium - exclude Chiari, MS, tumor
3. Blood tests: B12, thiamine, magnesium, anti-GAD antibodies, paraneoplastic panel (anti-Yo, anti-Hu, anti-Ri)
4. Genetic testing: CACNA1A (EA-2/SCA-6) if structural MRI is normal
5. Complete the caloric testing to characterise any coexisting peripheral component
6. Neurology/Neurotology referral - this patient needs subspecialty evaluation

Bottom line: The VNG of Mr. Takreem shows a central vestibular syndrome with downbeat nystagmus as the cardinal feature. The most important first step is MRI of the posterior fossa. The most likely diagnoses in a 30-year-old male are Chiari Malformation Type I, Episodic Ataxia Type 2, or an early demyelinating disease - all of which are actionable.

(Sources: Bradley & Daroff's Neurology in Clinical Practice, Box 18.14; Localization in Clinical Neurology 8e, p.649; K.J. Lee's Essential Otolaryngology, pp.351-352)