Dynamic topography of visual evoked potentials and extrageniculate projection in case of Riddoch phenomenon

Area of Science:

• Neuroscience
• Ophthalmology
• Neurology

Background:

• Cortical blindness can occur following severe physiological stress like hemorrhage and shock.
• The Riddoch phenomenon describes vision only with moving objects in individuals with cortical blindness.

Observation:

• A 34-year-old woman with cortical blindness, presenting with the Riddoch phenomenon, underwent dynamic topography of visual evoked potentials (VEP).
• Her visual acuity was limited to hand movements, with preserved light and color perception but no form recognition.
• Computed tomography (CT) revealed diffuse low density in the occipital lobe's white matter.

Findings:

• VEPs were detectable with flash and checkerboard stimuli.
• Dynamic VEP topography showed an abnormal pathway: a brainstem negative deflection at 30ms, expanding to the parietal region by 90ms, followed by a fronto-occipital positive deflection from 100-150ms.
• This aberrant visual processing differs significantly from normal VEP patterns.

Implications:

• The abnormal VEPs suggest visual information is conducted through the extrageniculate pathway, bypassing the typical geniculostriate route.
• This highlights an alternative visual processing route in cases of occipital lobe damage and cortical blindness.
• Dynamic VEP topography offers insights into visual pathway function in complex neurological conditions.