Is slight lateral nystagmus (abnormal eye movement) considered normal?

Lateral Nystagmus: Normal or Abnormal?

Slight lateral nystagmus is not normal and requires evaluation as it may indicate underlying neurological or vestibular pathology that could affect morbidity and mortality.

Types of Nystagmus and Clinical Significance

Nystagmus is defined as rhythmic, involuntary eye movements that typically consist of a slow drift followed by a fast corrective movement (saccade). When evaluating lateral nystagmus, it's important to understand its various forms:

Types of Nystagmus:

• Manifest nystagmus: Present constantly under binocular viewing conditions; may be horizontal, vertical, or torsional 1
• Latent nystagmus: Only appears when one eye is occluded; characterized by horizontal jerk oscillations 1
• Manifest-latent nystagmus: Present under binocular viewing but increases with monocular occlusion 1

Diagnostic Approach

When lateral nystagmus is detected, a systematic evaluation should be performed:

1. Characterize the nystagmus:

   • Direction (horizontal, vertical, torsional)
   • Amplitude (slight vs. pronounced)
   • Frequency
   • Effect of gaze position
   • Effect of visual fixation

2. Neurological assessment:

   • MRI of the brain is recommended as the initial imaging study for children with isolated nystagmus 1
   • 15.5% of children with isolated nystagmus have abnormal intracranial findings on MRI, including white matter abnormalities (4%), Chiari 1 malformation (3.4%), and optic pathway glioma (2%) 1

3. Vestibular assessment:

   • Perform the supine roll test to assess for lateral semicircular canal BPPV if positional vertigo is present 1
   • Evaluate for vestibular nystagmus, which may result from dysfunction of peripheral or central vestibular pathways 1

Clinical Implications

Nystagmus can cause significant symptoms and functional impairment:

• Blurred vision
• Oscillopsia (jumping images)
• Reduced visual acuity
• Dizziness and postural imbalance
• Gait disorders 2

Treatment Considerations

Treatment depends on the underlying cause and type of nystagmus:

• Downbeat/upbeat nystagmus: 4-aminopyridine may be effective 2, 3
• Pendular nystagmus: Gabapentin, memantine, clonazepam, or valproate 4, 3
• Periodic alternating nystagmus: Baclofen is the therapy of choice 4, 3

Important Caveats

1. Never assume nystagmus is normal: Even slight lateral nystagmus warrants investigation as it may be the first sign of serious neurological conditions.

2. Consider age of onset: Nystagmus is more common in patients with earlier-onset strabismus than in those with later-onset strabismus 1.

3. Watch for associated symptoms: Head turns or tilts may be compensatory mechanisms to dampen nystagmus and improve vision 1.

4. Differentiate from saccadic intrusions: True nystagmus must be distinguished from other oscillatory eye movements 5.

5. Consider imaging: MRI of the brain (with or without contrast) is appropriate for evaluating patients with nystagmus to rule out intracranial pathology 1.

In summary, slight lateral nystagmus should always be considered abnormal and warrants thorough evaluation to identify potential underlying conditions that could impact patient morbidity and mortality.