Editor’s Column May 2024
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This issue marks the second time the phrase ‘retrograde degeneration’ has made an appearance in the Journal of Medical Optometry. In fact, it’s present in two cases this issue: the Neuro Nuggets column by Dr. Kane and the Dolichoectasia article by Dr. Terry. These cases show loss of retinal ganglion cells (RGCs) caused by damage posterior to the globe; in both cases, the damage occurred near the optic chiasm but before the synapse in the lateral geniculate nucleus (LGN). Trans-synaptic retrograde degeneration is one step further and occurs when there is loss of retinal ganglion cells caused by damage posterior to the LGN. Some of our readers might not be completely familiar with this concept, but it’s by no means a new one; it’s simply one that’s come to the forefront in the last ten years with spectral domain OCT.
As a refresher, let’s ask some questions. Say you suspect your patient has had a stroke in their occipital lobe. Besides a good case history and visual field testing, what tests can you do to prove this? Will there be an APD? No. Will there be retinal emboli? Not necessarily. Will there be optic nerve pallor? It won’t be obvious, and it would only be nasal or temporal. Will there be OCT changes? Yes…. depending on how long ago the stroke was. But would neurons in the retina degenerate if the damage was to the occipital lobe? The damage would have to cross over the synapse in the LGN. This happens in trans-synaptic degeneration (TSD).
Let’s dig deeper into TSD, starting with definitions. Anterograde (ascending, or Wallerian) degeneration of a neuron proceeds posteriorly along the axon. Say you sever the posterior optic nerve 3 mm posterior to the globe. Anterograde degeneration is the change in the rest of the optic nerve posterior to the lesion and the optic tracts leading up to the LGN. This happens fast, and in the case of optic nerve damage, changes in the LGN are noticed within 24 hours, and the degeneration is complete within about 7 days.1 Retrograde (descending) degeneration occurs in the opposite direction – in the proximal segment of the axon that remains in contact with the cell body – and takes much longer. In the case of the severed optic nerve, it takes about 6 to 8 weeks for the RGCs to become unviable.1 TSD is when a neuron on one side of a synapse degenerates as a consequence of the loss of a neuron on the other side.
There has been past evidence of anterograde trans-synaptic degeneration (ATSD) in the visual system. More than 100 years ago, Cajal described changes in the occipital lobe in blind or one-eyed patients.2 But evidence of retrograde trans-synaptic degeneration (RTSD) in the visual system was difficult to prove before OCT was available. Take a look at the visual fields in the Neuro Nuggets pencil injury case and the visual fields in the dolichoectasia case. Now take a look at the ganglion cell layer segmentation. Don’t they correlate perfectly? And to go even further, take a look at the RNFL circle scans. Can you tell the story using only that scan? Not nearly as well as examining the ganglion cell layer.
Why does this matter? First, it’s a much more accurate way to follow changes over time. Quantifying a ten micron change over a year via OCT is hard to beat with automated visual fields. Second, it’s an objective test compared to the subjectivity of visual field testing. I’ve used OCT to diagnose a homonymous hemianopia in a brain injury patient who couldn’t move her arms.
Reader, think critically when going over these articles. Don’t just read them to check out interesting cases (although the case in Neuro Nuggets is quite interesting and unique), use them to change the way you practice and care for your patients. I know I take something away from every issue.
REFERENCES
- Millington RS, et al. J Neurol Neurosurg Psychiatry. 2014;85(4):379-386;doi:10.1136/jnnp-2013-3065
- Cajal S, et al. Histologie du systeme nerveux de l’homme et des verte’bres.Vol 2. Paris: Maloine: 1911.
Dr. Rett is the Editor-in-Chief of the Journal of Medical Optometry and the secretary for the American Board of Certification in Medical Optometry. He is the Chief of Optometry at VA Boston and sits on the national Field Advisory Board for Tele Eyecare at the VA. He is adjunct clinical faculty at several optometry schools, lectures nationally and enjoys writing about eyecare whenever and wherever.
