What are the causes of decreased optokinetic nystagmus?

Causes of Decreased Optokinetic Nystagmus

Decreased optokinetic nystagmus (OKN) is primarily caused by lesions in the central nervous system, particularly in the brainstem, cerebellum, and parietal lobe regions, requiring MRI imaging for proper diagnosis and management. 1, 2

Classification of Causes

Neurological Causes

• Brainstem and Cerebellar Lesions

  • Cerebellar tumors - found in 6 patients with abnormal OKN when other tests were normal 3
  • Cerebellar atrophy 3
  • Brain stem lesions - disrupt central vestibular pathways 1, 3
  • Multiple sclerosis (MS) - characterized by eye-speed fatigability as drum speed increases 3

• Cortical Lesions

  • Parietal lobe tumors - impair ipsilateral foveal pursuit and full-field pursuit 4
  • Occipital lobe lesions - may cause OKN asymmetry due to associated visual field defects 5
  • Stroke affecting parietal regions - unilateral loss of OKN correlates with poorer functional recovery 6

Visual Pathway Disorders

• Anterior Visual Pathway Lesions

  • Optic pathway gliomas - found in 2% of children with nystagmus 1
  • Chiasmal lesions with bitemporal hemianopia - can cause asymmetric OKN gain 5

• Visual Field Defects

  • Hemianopia secondary to occipital lesions - contributes to OKN asymmetry 5
  • Visual field defects from cortical lesions - 7 patients showed asymmetric OKN gain 5

Metabolic and Systemic Disorders

• Leukodystrophies 1
• Mitochondrial diseases 1
• Harada syndrome 3

Diagnostic Approach

Imaging Recommendations

• MRI of the head without and with IV contrast is the recommended initial imaging for evaluating decreased OKN, especially with:

  • Late onset nystagmus
  • Concurrent neurological symptoms
  • Decreased visual acuity
  • Asymmetric/unilateral or progressive nystagmus 1, 2

• A retrospective review found 15.5% of children who underwent MRI for isolated nystagmus had abnormal intracranial findings, including:

  • Abnormal T2 hyperintense signal in white matter (4%)
  • Chiari 1 malformation (3.4%)
  • Optic pathway glioma (2%) 1, 2

OKN Testing Considerations

• Testing should include:
  • Both horizontal and vertical planes
  • Five progressively faster drum speeds to detect eye-speed fatigability (characteristic of MS) 3
  • Assessment of asymmetry between directions, which is the most consistent finding in neurological conditions 3

Clinical Pearls and Pitfalls

Important Clinical Considerations

• Vertical OKN asymmetry, though less common than horizontal, should be evaluated as 14 patients had only this finding 3
• Unilateral loss of OKN correlates with poorer functional recovery after stroke and longer rehabilitation periods 6
• Distinguishing between direct injury to OKN pathways versus effects of visual field defects is crucial for proper diagnosis 5

Common Pitfalls

• Using CT scans instead of MRI can lead to missed diagnoses 2
• Failing to test both horizontal and vertical OKN may miss important diagnostic findings 3
• Not considering OKN testing in patients with normal physical exams, brain scans, and EEGs may delay diagnosis of cerebellar tumors 3

Functional Implications

• Patients with unilateral loss of OKN after right hemisphere stroke showed:
  • Significantly less independence in upper extremity dressing
  • 40% greater inpatient length of stay
  • Higher likelihood of nursing home placement after rehabilitation 6

OKN testing remains a valuable clinical tool that can provide critical neurodiagnostic information, even in the era of modern neuroimaging 7.