Myelin Oligodendrocyte Glycoprotein (MOG) Optic Neuritis
Myelin Oligodendrocyte Glycoprotein (MOG) Optic Neuritis is an antibody mediated demyelinating disease of the central nervous system (CNS) that is a distinct entity from other demyelinating processes of the CNS such as Multiple Sclerosis (MS) or AQP4-Ab-associated neuromyelitis optica spectrum disorder (NMOSD). Typical optic neuritis (ON) presents with acute, unilateral, onset of variable visual acuity/visual field loss, retrobulbar pain (worse with eye movement), loss of color vison, a relative afferent pupillary defect (RAPD), and a normal fundus exam (retrobulbar ON). ON in MOG however is often “atypical” and may be markedly steroid responsive, bilateral rather than unilateral, and may be associated with optic disc edema rather than a retrobulbar ON. Although ON is the most common symptom in MOG-Ab seropositive disease it can present with acute disseminated encephalomyelitis, myelitis, or an NMOSD like presentation. Children less than 9 years old who are positive for MOG-Antibodies (Ab) more frequently present with acute disseminated encephalomyelitis (ADEM) that may be relapsing or recurrent and may present with ON later in life.
MOG related ON may be associated with certain HLA haplotypes. There is wide variability in the epidemiology, severity of acute vision loss, extraocular neurologic deficits, and prognosis seen in various different studies of MOG however.
MOG ON can be seen in any age, any ethnicity, and in either gender. In contrast to NMOSD which is more frequently seen in African and Asian ancestry. Children with MOG ADEM had up to an 80% chance of developing MOG ON in adolescence or adulthood in one study.  Other studies have shown contradictory associations between sex and MOG ON. One study found that 50% of pregnant women with MOG-ab’s developed their first episode of MOG-ON during or shortly after pregnancy. There was no association between MOG-Ab ON and other autoimmune disorders and MOG ON is not typically associated with oligoclonal bands, a test specific for MS.
The pathophysiology of MOG ON is incompletely understood and the precise mechanism of MOG-Ab remains ill defined.  In addition, human MOG-Ab’s isolated from patient serum do not bind to MOG proteins expressed in rodents, therefore an in vivo study of the pathogenic role of these proteins has not been completely established. MOG protein is found on the surface of oligodendrocytes and known to bind to C1q, IgG Ab’s, DC-SIGN, a c-type lectin receptor expressed on the surface of dendritic cells and macrophages, rubella virus, and nerve growth factor. Disease-causing MOG antibodies are IgG1, which are known to activate the classic complement pathway. Complement deposition has also been seen in neuroimaging studies during acute attacks of MOG-Ab ON.
Moderate to severe optic disc edema is a classic differentiating feature of MOG ON from MS and other forms of ON seen in up to 80% of patients. Although MOG ON in older patients has some overlapping features consistent w/ non-arteritic anterior ischemic neuropathy (NAION) usually the differentiation can be made clinically or radiographically. Bothe NAION and MOG ON may present with optic disc edema and peripapillary hemorrhage but NAION is much more commonly seen in older, vasculopathic patients compared to MOG-Ab positive ON which is seen in younger and healthier adults.
Acute unilateral or bilateral, steroid responsive, recurrent episodes of ON are the hallmark of MOG-Ab ON. MOG ON episodes are often recurrent and the presentation with MOG antibody at the first attack had a positive predictive value on the presentation of subsequent attacks.
MOG ON can be associated with rapidly progressive and severe vision loss which may or may not recover. Commonly reported other neurologic signs and symptoms include myelitis (e.g., paralysis and paresthesia) as well as encephalopathy, seizures, incontinence, and dysarthria.
Symptoms that are highly suggestive of MOG-Ab ON include bilateral and recurrent episodes of ON especially in the setting of steroid responsiveness and steroid dependence. A recent study demonstrated that children with MOG are most likely to present with ADEM, adults aged 20 to 45 are most likely to present with unilateral ON, and adults older than 45 are most likely to present with bilateral ON.  Other neurologic deficits are seen in about 50% of cases but the presentation is highly variable from study to study (e.g., transverse myelitis, sensory nerve loss, incontinence, and ataxia).
Definitive diagnosis of MOG ON is made in the appropriate clinical setting by seropositivity of MOG-Ab (cell-based assays are the current gold standard). One prospective cohort study of patients with ON showed that by testing all cases of ON with bilateral ON, recurrent ON, or optic disc swelling on fundoscopy for MOG antibody, all cases of MOG ON would be detected and only 50% of ON cases in the cohort would be tested overall. An absence of each of the atypical ON features that were considered to be higher risk for MOG had a negative predictive value of over 90%. MRI findings (e.g., OPN, lack of demyelinating white matter lesions for MS, longitudinally extensive enhancement) are also highly suggestive of MOG ON
Longitudinally, extensive (involving >50% of the length of the optic nerve) enhancement and /or T2 weighted hyperintensities in the anterior visual pathways can be seen in MOG-Ab ON. Enhancing lesions are usually seen in the orbital and intracranial region of the optic nerve. Optic perineuritis (OPN) with enhancement the optic nerve sheath is a finding that is common in MOG ON as compared to MS related ON (parenchymal optic nerve enhancement).  Spinal MRI findings include swelling of the spinal cord and contrast enhancement that can resemble the transverse myelitis of NMOSD. In contrast to the ovoid, periventricular, white matter lesions (including corpus callosum) seen in MS, non-specific white matter lesions or a normal brain MRI may be seen in MOG
- NMOSD (caused by AQP4 IgG)
- Lyme Disease
- Granulomatosis with polyangiitis (Wegner granulomatosis)
The widely accepted treatment for acute attacks has been intravenous, high dose corticosteroid therapy (e.g., methylprednisolone) followed by intravenous immunoglobulin (IVIG) or plasma exchange in patients who do not respond to IV steroid treatment.  Methylprednisolone has been shown to increase recovery by 10-20% compared to no treatment. Of the patients who were not responsive to methylprednisolone, 40% showed improvement with IVIG, although partial recovery was more common than full recovery with IVIG. While treatment efficacy may be low in acute attacks, there has been substantial success in preventing recurrent attacks. In one study, 95% of patients receiving doses of at least 20 mg of prednisone for 6 months following a MOG ON attack had no recurrent episodes of ON at follow up of over a year. High dose and longer length of treatment was strongly associated with remittance, and patience who were given a tapered dose, or discontinued therapy earlier had relapse rates comparable to those with no treatment. Randomized, controlled, treatment trials are limited for MOG ON, but observational open-label work suggests a role for high-dose steroids and plasma exchange in the treatment of acute attacks, and immunosuppressive therapy (e.g., steroids, oral immunosuppressants and rituximab) as maintenance treatment.
In one study, 25% of patients suffering an acute attack had permanent visual disability. In other studies, patients experienced complete or partial recovery in up to 90% of cases. Although MOG ON attacks have been show to permanently damage the neural retinal ganglion and peripheral nerve fiber layers of the retina, the majority of patients fully or partially regain vision. The majority of individuals in all studies have a recurrence within the first year of attack. Increased risk of permanent neurologic deficit with subsequent attacks is much worse than after the first attacks. Younger age at first attack is associated with increased risk of permanent vision loss. Persistent MOG-Ab titers have been associated with relapse compared with patients without persistent MOG-Ab. 
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