What causes horizontal nystagmus?

Causes of Horizontal Nystagmus

Horizontal nystagmus is most commonly caused by lateral (horizontal) semicircular canal BPPV, accounting for 5-15% of all BPPV cases, though it can also indicate serious central pathology requiring urgent neuroimaging when specific red flags are present. 1

Peripheral Vestibular Causes

Lateral Canal BPPV (Most Common Benign Cause)

• Geotropic horizontal nystagmus (beating toward the ground on both sides) is the most common form, with the strongest nystagmus indicating the affected ear 2
• Apogeotropic horizontal nystagmus (beating away from the ground on both sides) occurs less frequently, with the side opposite the strongest nystagmus being the affected ear 2
• Diagnosed using the supine roll test, which is essential to avoid missing lateral canal involvement 2
• Paroxysmal geotropic nystagmus in canalolithiasis has rapid onset (0.8 ± 1 second), peaks at 5-20 seconds, and resolves by 60 seconds with peak SPV of 79 ± 67°/s 3
• Persistent apogeotropic nystagmus in cupulolithiasis has similar onset (0.7 ± 1.4 seconds) but maintains 65-81% of peak velocity at 40 seconds, distinguishing it from canalolithiasis 3

Posterior Canal BPPV Converting to Horizontal

• Posterior canal BPPV can convert to horizontal canal BPPV during positioning maneuvers, with the same ear remaining affected 2
• This conversion emphasizes the need to perform both Dix-Hallpike and supine roll testing in all patients 2

Other Peripheral Vestibular Disorders

• Acute vestibular neuritis produces unidirectional horizontal nystagmus that is strongly inhibited by fixation 4, 5
• Ménière's disease can cause persistent apogeotropic horizontal nystagmus, maintaining 28.6-49.5% of peak SPV at 40 seconds 3
• Vestibular schwannoma may present with horizontal positional nystagmus 3

Central Nervous System Causes (Critical Red Flags)

Direction-Changing Horizontal Nystagmus with Central Features

• Direction-changing horizontal nystagmus that does not follow typical BPPV patterns strongly suggests central pathology and requires urgent MRI 6, 2, 7
• Baseline horizontal nystagmus present in primary position indicates CNS involvement rather than peripheral BPPV 2, 7
• Gaze-evoked horizontal nystagmus is most commonly produced by drug ingestion but can indicate brainstem or cerebellar pathology 4, 8

Cerebellar Lesions

• Small lesions in the lower cerebellum can cause unidirectional horizontal nystagmus directed toward the side of the lesion, enhanced when lying on the non-affected side 5
• Lower cerebellar lesions may also produce direction-changing apogeotropic positional nystagmus through disinhibition of semicircular-ocular and otolith-ocular reflexes 5
• Associated cerebellar signs (ataxia, dysmetria, dysdiadochokinesia) confirm central etiology 6, 2, 7

Vestibular Migraine

• Produces persistent geotropic or apogeotropic horizontal nystagmus, maintaining 61% of peak SPV at 40 seconds 3
• Should be distinguished from BPPV based on temporal profile and associated migraine features 6

Other Central Causes

• Posterior circulation stroke affecting brainstem or cerebellum 6
• Demyelinating diseases involving vestibular pathways 6
• Intracranial tumors affecting central vestibular structures 6, 2

Diagnostic Algorithm

Initial Positional Testing

• Perform Dix-Hallpike maneuver first to assess for posterior canal BPPV 2, 7
• Perform supine roll test to evaluate for lateral canal involvement—this is mandatory to avoid missing horizontal canal BPPV 2
• Check for baseline nystagmus in primary position before any maneuvers 2, 7

Temporal Characteristics to Distinguish Peripheral from Central

• Paroxysmal nystagmus (onset <5 seconds, resolution <60 seconds) suggests canalolithiasis 3
• Persistent nystagmus maintaining >60% of peak velocity at 40 seconds suggests cupulolithiasis, vestibular migraine, or central pathology 3
• Immediate onset without latency may indicate central cause 7

Red Flags Requiring Urgent MRI

• Direction-changing nystagmus not following BPPV patterns 6, 2, 7
• Baseline nystagmus in primary position 2, 7
• Associated cerebellar signs (ataxia, dysmetria) 6, 2, 7
• Concurrent neurological symptoms (motor palsy, sensory deficit, dysarthria) 5
• Progressive course or decreased visual acuity 7

When to Avoid Routine Imaging

• Do not order routine neuroimaging in patients meeting diagnostic criteria for BPPV without additional vestibular signs or symptoms inconsistent with BPPV 7
• Classic lateral canal BPPV with appropriate temporal profile and no red flags does not require imaging 1

Common Pitfalls

• Misdiagnosing central positional nystagmus as BPPV occurs when proper diagnostic maneuvers are not performed or central features are overlooked 2, 7
• Failing to perform supine roll testing leads to missed lateral canal BPPV diagnoses 2
• Using CT imaging is inappropriate as it inadequately visualizes posterior fossa structures and brainstem 7
• Confusing geotropic and apogeotropic forms of lateral canal BPPV—they have opposite rules for determining the affected ear 2