Editors' Note: In reference to the article “In vivo identification of morphologic retinal abnormalities in neuromyelitis optica,” Dr. Borruat reports his own experience in diagnosing microcystic macular edema in patients with noninflammatory optic neuropathies, supporting the hypothesis of retrograde transsynaptic degeneration of inner retinal cells as etiology. Dr. Khoo espouses the benefits of Sniffin’ Sticks, an inexpensive, noninvasive test of olfactory dysfunction, in diagnosing idiopathic intracranial hypertension. Authors Kunte et al. explain the drawbacks of this approach. —Megan Alcauskas, MD, and Robert C. Griggs, MD
Sotirchos et al.1 used optical coherence tomography (OCT) and found microcystic macular edema (MME) in a cohort of patients with neuromyelitis optica (NMO). The authors considered the possible mechanisms leading to MME in NMO: a primary retinal inflammatory process, a retinopathy related to the high expression of aquaporin-4 in the inner retina, possible unrecognized episodes of uveitis, or a retrograde transsynaptic degeneration of inner retinal cells.
The authors recommended further OCT studies in other types of optic neuropathies to exclude or confirm a transsynaptic etiology of MME. During the past year, I diagnosed MME in 3 patients who presented with noninflammatory optic neuropathies: optic disc drusen, optic nerve compression from craniopharyngioma, and unilateral traumatic optic neuropathy. Similar to others,2,3 I believe that MME does not reflect a specific underlying inflammatory process but rather I consider it a marker of the severity of optic neuropathies. MME probably reflects a certain stage of retrograde transsynaptic degeneration,4,5 which can occur in optic neuropathies whatever the etiology.
Author Response
We thank Dr. Borruat for his comments on our study, in which we reported MME of the inner nuclear layer in 30% of eyes that had experienced NMO-associated optic neuritis.1 MME was associated with severe retinal axonal and neuronal loss and visual disability. We proposed that MME may develop as a consequence of either retrograde transsynaptic degeneration or a retinal inflammatory process. MME has also been reported in various noninflammatory optic neuropathies,2,6 and Dr. Borruat suggests this provides evidence supporting a transsynaptic etiology. We agree that this is a possibility, but this hypothesis does not satisfactorily explain why MME may fluctuate over time7,8 or why MME develops in only a subset of patients with severe optic neuropathies.1,5,6
We believe that severe optic atrophy is necessary but not sufficient to cause MME and that the possibility that secondary factors contribute to the development of MME must be considered. Secondary factors could include retinal inflammation1,7,8 with a humoral or cell-mediated immune response gaining access to the immune-privileged retina after breakdown of the blood–retinal barrier, a mechanical process such as vitreoretinal traction due to severe retinal atrophy resulting in schesis,6 or a genetic susceptibility to more extensive neurodegeneration.
- © 2014 American Academy of Neurology
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