OCT in MOGAD
The quantitative and qualitative assessment of the retinal changes over
time can be performed in close-to-cellular resolution using spectral
domain optical coherence tomography (OCT) (23,115). Improvement of OCT
techniques in the past decade has allowed the retina to be examined in
greater detail. The unprecedented resolution of down to 3.9 µm enables
measurement of retinal ganglion cell loss, evaluated by the combined
macular ganglion cell layer and inner plexiform layers (mGCIPL) and
their axons, as measured by the peripapillary retinal nerve fiber layer
(pRNFL). These OCT metrics have been shown to correlate well with visual
function and the damage that occurs in NMOSD and MS patients (116,117)
Thus, OCT is a valuable tool for monitoring many neuro-ophthalmological
and neurological conditions, including NMOSD and MOGAD (Figure 4)
(68,118,119).
Acute ON in MOGAD is often bilateral and localised in the anterior optic
nerve inducing severe and characteristic retinal edema (120). Initially
covered by the edema, the neuroaxonal layers of the retina (pRNFL,
mGCIPL) degenerate significantly in the following months (Figure 5)
(23,69,115,120,121). These losses accumulate with each additional ON
episode, which occur frequently in MOGAD (69,70). Therefore, although a
single episode does not often lead to disastrous damage (122,123), the
highly recurrent ON attacks accumulate with pRNFL and mGCIPL loss. This
is comparable to patients with AQP4-IgG seropositive NMOSD, which is
characterised by less frequent, but more damaging ON episodes (70). In
comparison with MS, MOGAD patients are described as undergoing more
severe retinal neurodegeneration after ON, however a final consensus on
this topic has not been reached (115,122,124).
Further studies are warranted to investigate retinal neurodegeneration
independent of ON in MOGAD. One study performed a first exploratory
analysis in a small dataset recording pRNFL loss without associated
GCIPL reduction (125). Apart from true retinal neurodegeneration, this
could potentially be explained by a remission of attack-associated
edema, which commonly affects the RNFL more than the ganglion cell layer
(126). If the absence of ON-independent GCIPL loss is confirmed, this
would not only stress the importance of ON attack-prevention in MOGAD
but also allow a better separation from MS and AQP4-IgG seropositive
NMOSD, which are both affected by ON-independent retinal neuroaxonal
loss.
OCT data in paediatric MOGAD is scarce. The results in paediatric
cohorts generally mirror those in adults with measurable post-ON
swelling and associated reduction and thinning of the pRNFL (69,127).
There are, however, conflicting reports concerning unilateral ON cases
with subclinical involvement of the contralateral, clinically healthy
eye, an area which would benefit from further research given the
potential detrimental impact on the otherwise healthy eye (127,128).
In both paediatric and adult presentations, and notwithstanding the high
relapse rates and severe neuroaxonal degeneration, high-contrast visual
acuity is surprisingly preserved in MOGAD patients compared with
AQP4-IgG seropositive NMOSD patients although both groups have
comparable neuroaxonal loss (70,122,129–131). How visual acuity is
preserved in MOGAD remains unclear, but data suggests an influence of a
primary retinal astrocytopathy in AQP4-IgG seropositive NMOSD
accumulating in additional retinal changes with functional consequences
(Giegengack et al. in preparation ) (132). Nevertheless, MOGAD
patients with their high prevalence of ON attacks, are at risk of
irreversible visual impairment when deprived of a timely diagnosis and
immunosuppression therapy.