What are the causes of lateral nystagmus?

Causes of Lateral Nystagmus

Lateral (horizontal) nystagmus is most commonly caused by benign paroxysmal positional vertigo (BPPV) affecting the lateral semicircular canal, which accounts for 5-15% of all BPPV cases. 1

Types of Lateral Canal BPPV

Lateral canal BPPV presents with two main variants that can be identified through the supine roll test:

1. Geotropic Type (Most Common)

   • Characterized by horizontal nystagmus beating toward the undermost (affected) ear
   • When rolled to the opposite side, nystagmus changes direction but remains geotropic (beating toward the ground)
   • The affected ear is the one that produces the most intense nystagmus 1

2. Apogeotropic Type (Less Common)

   • Characterized by horizontal nystagmus beating toward the uppermost ear
   • When rolled to the opposite side, nystagmus changes direction but remains apogeotropic (beating away from the ground)
   • The affected ear is opposite to the side that produces the most intense nystagmus 1

Other Causes of Lateral Nystagmus

Central Nervous System Disorders

• Brainstem and cerebellar stroke - Can present with direction-changing nystagmus without changes in head position 1
• Vertebrobasilar insufficiency - May cause transient episodes of nystagmus 1
• Multiple sclerosis - Can produce various forms of nystagmus including lateral nystagmus 1
• Demyelinating diseases - May affect central vestibular pathways 1
• Lateral medullary syndrome - Can cause torsional nystagmus with a horizontal component 2
• Gaze-evoked nystagmus - Often seen with cerebellar disease 3
• Periodic alternating nystagmus - Direction-changing nystagmus occurring without changes in head position 1, 3

Peripheral Vestibular Disorders

• Vestibular neuritis/labyrinthitis - Can cause spontaneous horizontal nystagmus 1
• Ménière's disease - May present with horizontal nystagmus during acute attacks 1
• Post-traumatic vertigo - Head trauma can cause BPPV or direct damage to vestibular structures 1
• Superior canal dehiscence syndrome - Can cause pressure-induced nystagmus 1
• Perilymphatic fistula - May cause episodic nystagmus with pressure changes 1

Other Entities

• Medication side effects - Various medications can induce nystagmus 1
• Alcohol intoxication - Common cause of horizontal gaze-evoked nystagmus 4
• Toxic exposures - Certain toxins can affect vestibular function 1
• Metabolic disorders - Electrolyte abnormalities, particularly hyponatremia, can cause nystagmus 4

Diagnostic Approach

The supine roll test is the preferred diagnostic maneuver for lateral canal BPPV:

1. Position patient supine with head in neutral position
2. Quickly rotate head 90 degrees to one side and observe for nystagmus
3. Return head to neutral position and wait for nystagmus to subside
4. Rotate head 90 degrees to opposite side and observe for nystagmus 1

Additional diagnostic methods to determine the affected ear in lateral canal BPPV include:

• Bow and lean test - Observing nystagmus direction when the patient bends forward (bowing) and backward (leaning) 1
• Lying-down nystagmus - Observing transient horizontal nystagmus when moving from sitting to supine position 1
• Head-bending nystagmus - Observing nystagmus when the patient sits up from supine position with head bent down 1

Important Clinical Considerations

• When evaluating lateral nystagmus, it's crucial to differentiate between peripheral and central causes, as central causes may indicate serious neurological conditions requiring urgent intervention 1, 4

• The HINTS exam (Head Impulse test, Nystagmus, Test of Skew) is recommended for patients with continuous vertigo, nystagmus, and nausea/vomiting (Acute Vestibular Syndrome) and has higher sensitivity (96.7%) and specificity (94.8%) than MRI for detecting stroke in early presentations 4

• Nystagmus findings that suggest a central cause rather than peripheral BPPV include:

  • Direction-changing nystagmus occurring without changes in head position
  • Gaze-holding nystagmus
  • Direction-switching nystagmus (beats right with right gaze, left with left gaze)
  • Baseline nystagmus without provocative maneuvers 1

• Posterior circulation stroke can present with isolated vertigo in up to 25% of cases, with prevalence increasing to 75% in high vascular risk cohorts 4

• Failure to respond to canalith repositioning procedures should raise concern that the underlying diagnosis may not be BPPV 1