Introduction
These types of lesions primarily affects the eye by causing visual field defects. Typically axons proximal to site of degeneration affect the retinal ganglion cells within 6-8 weeks.
Retrograde Degeneration:
- When the injury to an axon leads to the successive damage of the cell body. This process is primarily intra-neuronal.
- Injury propagates intra-neuronally, from the axon to the soma
- Since the ganglion cell axon first synapse at the LGN, any damage before the LGN will lead to retrograde degeneration to directly damage the ganglion cell layer and affect vision.
Retrograde Trans-synaptic Degeneration:
- Can be thought of as a propagating degeneration in a retrograde manner
- The initially damaged neuron will spread it's degeneration to pre-synaptic neurons, causing damage via neural connections.
- The post-synaptic neurons disrupt trophic support due to excitotoxicity from excessive stimulation from glutamate which can lead to metabolic deficiencies.
- Caused by conditions such as multiple sclerosis which demyelinates.
- 3 synapses. RGC -> LGN -> V1 -> Onwards. Damage after LGN that manifests at the retina will be trans-synaptic.
Dead Giveaways
Post-Chiasmal Lesions
Leads to homonymous hemianopia
Optic atrophy
RNFL and GCL loss
OCT Loss:
Usually in retrograde degeneration, GCL loss will respect the vertical midline, similar to VF losses. This can be a key differentiator from glaucoma which respects the horizontal midline
GCL is also more sensitive to retrograde degenerations than RNFL, as the temporal RNFL contains retinal fibres from both temporal and nasal retina in the papillomacular bundle.
GCL scan through the macular represents a physiological divide between crossed and uncrossed fibres within the visual cortex.
diagnostic features
GCL Loss and VF Deficit
Whilst the GCL loss can be attributed to the VF deficits, sometimes functional loss can precede structural loss, with VF deficits appearing despite normal GCL.
Othertimes, the GCL and VF deficits do not fully overlap, and can even be contradictory, such as a nasal GCL loss causing temporal VF loss