Imaging reveals optic tract degeneration in hemianopia

Area of Science:

• Neuroscience
• Ophthalmology
• Radiology

Background:

• Hemianopia, a visual field defect, can result from damage to the primary visual cortex (V1), lateral geniculate nucleus (LGN), or optic tract.
• Transsynaptic degeneration is a process where neuronal damage in one area leads to degeneration in connected areas.

Purpose of the Study:

• To investigate transsynaptic degeneration in the human optic tract in hemianopia.
• To determine how degeneration varies with the duration and location of the visual pathway insult.

Main Methods:

• Structural MRI scans were performed on seven hemianopic patients with damage to V1, LGN, or optic tract.
• Optic tract volume and cross-sectional area were computed using T1-weighted image intensity values.
• A laterality index was calculated to compare optic tract size between hemispheres at different intensity thresholds.

Main Results:

• Hemianopic patients with longstanding V1 or LGN damage showed significant optic tract degeneration (laterality index > 0.5) at high intensity values.
• Recent optic tract damage resulted in even higher laterality indices due to direct degeneration.
• Correlation between cross-section and volume indices was significant 18 months post-V1 lesion, unlike in controls.

Conclusions:

• Transsynaptic degeneration in the optic tract commences by 18 months after V1 lesion, compromising white matter integrity.
• Visible volume decrease in the optic tract requires longer durations of hemianopia.
• This MRI-based method offers a noninvasive approach to monitor optic tract degeneration timelines.