Interpret the VNG findings showing a right‑sided peripheral vestibular weakness with a 30 % unilateral caloric weakness, no spontaneous or positional nystagmus, negative vertebral vascular insufficiency test, and normal ocular motor tests, and what are the likely causes?

VNG Interpretation: Right Peripheral Vestibular Hypofunction

This VNG demonstrates a right-sided peripheral vestibular weakness (30% unilateral caloric weakness) consistent with partial or compensated peripheral vestibular dysfunction on the right side.

Key Findings Analysis

The 30% unilateral weakness on caloric testing indicates reduced function of the right horizontal semicircular canal and/or superior vestibular nerve, while the absence of spontaneous nystagmus suggests this is either a compensated lesion or a partial deficit 1. The normal ocular motor tests (saccades, pursuit, optokinetic) effectively rule out central pathology 2.

Critical Interpretation Points:

• Caloric weakness of 25-30% represents a "gray zone" - this is abnormal but indicates partial rather than complete vestibular loss 3
• Absence of spontaneous nystagmus does NOT rule out peripheral vestibular disease - it typically indicates central compensation has occurred or the lesion is incomplete 1, 4
• Normal central ocular motor tests strongly favor peripheral localization over brainstem or cerebellar pathology 2
• The negative Dix-Hallpike excludes BPPV as a concurrent or alternative diagnosis

Most Likely Causes (in order of probability):

1. Compensated Vestibular Neuritis

• Most common cause of unilateral caloric weakness without acute symptoms 5, 4
• Patients often present weeks-to-months after acute episode when compensation has occurred
• 67% of definitive Ménière's patients show unilateral caloric weakness, but this typically presents with episodic vertigo and hearing loss 6

2. Labyrinthitis (resolved acute phase)

• Similar presentation to vestibular neuritis but may have had associated hearing loss during acute phase
• Caloric weakness persists even after clinical recovery

3. Early/Atypical Ménière's Disease

• Can present with unilateral caloric weakness (67% of cases) 6
• Critical caveat: Requires episodic vertigo attacks (20 minutes to 12 hours) plus fluctuating hearing loss/tinnitus for diagnosis
• VNG alone cannot diagnose Ménière's - this is a clinical diagnosis 6

4. Vestibular Schwannoma (less likely given normal central tests)

• Would warrant MRI if asymmetric hearing loss present 6
• Normal ocular motor findings make this less probable

5. Chronic Labyrinthine Ischemia

• Vertebrobasilar insufficiency test was negative, making this unlikely
• Would expect additional neurologic findings

Clinical Action Algorithm:

Step 1: Obtain detailed history focusing on:

• Acute vertigo episode in past weeks-to-months (suggests vestibular neuritis)
• Episodic vertigo attacks with hearing changes (suggests Ménière's)
• Asymmetric hearing loss or tinnitus (warrants MRI for schwannoma) 6

Step 2: Perform audiometry

• If asymmetric sensorineural hearing loss → obtain MRI with contrast 6
• If fluctuating low-frequency hearing loss with episodic vertigo → clinical diagnosis of Ménière's 6

Step 3: If history suggests old vestibular neuritis (single acute episode, now compensated):

• No further vestibular testing needed 6
• Focus on vestibular rehabilitation if residual symptoms persist

Step 4: If considering ablative therapy (for refractory Ménière's):

• Additional vestibular testing (vHIT, VEMP) warranted to assess contralateral function before permanent ablation 6

Important Caveats:

• Do not over-interpret this VNG result - vestibular testing correlates poorly with patient-perceived disability 6
• Caloric testing can show false negatives early in disease - conversion from normal to abnormal can occur within days in acute vestibular neuritis 5
• The Jongkees formula (used for unilateral weakness calculation) underestimates true paresis, particularly when the weaker ear is around 41% of the stronger ear 3
• Normal caloric testing does not rule out vestibular disease - 24% of vestibular neuritis patients show normal caloric paresis at compensated stages 1

6, 1, 5