What is the difference between ocular clonus and nystagmus?

Differences Between Ocular Clonus and Nystagmus

Ocular clonus and nystagmus are distinct eye movement disorders with different characteristics, etiologies, and clinical implications.

Key Differences

Definition and Characteristics

• Nystagmus: Involuntary rhythmic oscillations of the eyes that can be horizontal, vertical, and/or torsional 1, 2
• Ocular Clonus: Rapid, rhythmic contractions and relaxations of the extraocular muscles, typically less organized and more irregular than nystagmus

Classification

Nystagmus can be classified into three main types 1, 2:

• Manifest nystagmus: Present constantly under all viewing conditions
• Latent nystagmus: Appears only when one eye is occluded; reverses direction with change in fixation
• Manifest-latent nystagmus: Present under binocular viewing but increases with monocular occlusion

Waveform Patterns

• Nystagmus:
  • Jerk nystagmus: Has slow and quick phases
  • Pendular nystagmus: Equal velocity in both directions
• Ocular Clonus:
  • More irregular, less predictable pattern
  • Often appears as rapid, repetitive jerking movements

Etiology

• Nystagmus:
  • Infantile (congenital) nystagmus: Typically caused by albinism, retinal disease, low vision, and congenital cataracts 2
  • Acquired nystagmus: Caused by lesions in the brainstem/cerebellum, anterior optic pathway, or metabolic diseases 2
• Ocular Clonus:
  • Often associated with seizure disorders
  • Can be a manifestation of serotonin syndrome
  • May occur with certain medication toxicities

Clinical Significance

• Nystagmus:
  • Often associated with decreased visual acuity due to inability to maintain stable foveal vision 3
  • Acquired forms may cause oscillopsia (subjective sensation of visual world movement) 3
  • May require specific head postures to find the "null point" where nystagmus is minimized 2
• Ocular Clonus:
  • Often a sign of acute neurological emergency
  • May indicate drug toxicity or metabolic disturbance
  • Usually requires immediate medical intervention

Treatment Approaches

• Nystagmus:
  • Pharmacological:
    • Downbeat nystagmus: 4-aminopyridine, gabapentin, baclofen, clonazepam 2, 3
    • Periodic alternating nystagmus: Baclofen 3
    • Acquired pendular nystagmus: Gabapentin, memantine 2, 3
  • Optical devices and prisms 2
  • Surgical procedures for abnormal head posture 2
• Ocular Clonus:
  • Treatment of underlying cause (e.g., medication withdrawal, treatment of serotonin syndrome)
  • Acute symptomatic management

Diagnostic Approach

Nystagmus Evaluation

1. Detailed characterization:
   • Direction (horizontal, vertical, torsional)
   • Amplitude and frequency
   • Effect of gaze position
   • Presence of null point
2. Age of onset (infantile vs. acquired)
3. Associated visual symptoms
4. Neuroimaging when indicated:
   • MRI recommended for asymmetrical/unilateral nystagmus
   • Required for late onset, neurological symptoms, decreased visual acuity, or progressive nystagmus 2

Ocular Clonus Evaluation

1. Medication history (particularly serotonergic agents)
2. Toxicology screening
3. Neurological examination for other signs of serotonin syndrome or seizure activity
4. Urgent neurological consultation

Clinical Pearls and Pitfalls

• Asymmetrical or unilateral nystagmus is a neurological red flag requiring urgent evaluation 2
• Nystagmus blockage syndrome should be distinguished from infantile esotropia; children use excessive convergence to dampen nystagmus amplitude 1
• Documentation of preferred head posture is critical for surgical planning in patients with nystagmus 1, 2
• Infantile nystagmus typically does not cause oscillopsia, while acquired forms usually do 4
• Always consider drug-induced causes when evaluating new-onset ocular movement disorders, particularly ocular clonus

By understanding these differences, clinicians can better diagnose and manage these distinct ocular movement disorders, leading to improved patient outcomes.