Overlap of optic neuritis and anterior ischemic optic neuropathy

Dr Horton’s letter, “Mistaken treatment of anterior ischemic optic neuropathy with interferon -1a” is an important reminder of the sometimes difficult distinction between optic neuritis (ON) and anterior ischemic optic neuropathy (AION). Although he touches on several distinguishing characteristics, the distinction is clinically important and merits further expansion. Both ON and AION represent an optic neuropathy with vision loss, visual field defect, dyschromatopsia, and a relative afferent pupil defect. Dr Horton does not present enough information for the reader to confidently differentiate AION from ON, a woman with a painless optic neuropathy, disc edema, and nonspecific magnetic resonance imaging (MRI) white matter lesions. Both ON and AION can progress subacutely over 3 to 4 days with a similar visual nadir. Pain with palpation or eye movement occurs in approximately 90% of ON patients, but only 10% of AION patients. The optic nerve in acute AION is always swollen. However, only one third of optic discs in ON are swollen. Therefore, if the nerve is normal acutely, then it cannot be AION. Other fundus characteristics that argue against ON include severe disc edema, hemorrhages, and macular exudates. Whereas 30% of all patients in the Optic Neuritis Treatment Trial developed clinically definite multiple sclerosis after 5 years, no patient with these fundus characteristics developed definite multiple sclerosis. This finding suggests that these characteristics may not be associated with the demyelinating optic neuritis associated with multiple sclerosis. A brain MRI demonstrating the typical white matter lesions of multiple sclerosis would strongly suggest ON, although would not exclude AION. Recently, Rizzo and colleagues studied the orbital MRI characteristics of acute ON versus AION. Of 32 patients with ON, 31 had either abnormal enhancement or increased short T1 inversion recovery signal. In contrast, only 5 of the 32 AION patients had an abnormal orbital MRI. The degree of vision recovery helps to distinguish ON and AION because ON, unlike AION, is typically associated with significant visual recovery. However, assessment of visual recovery requires 4 to 6 weeks of observation. Cerebrospinal studies can be useful to confirm ON, because oligoclonal bands are detected in 72% of cases, elevated IgG index in 41%, and pleocytosis in 48%. In the acute phase of vision loss, we believe that a dilated fundus examination and orbital MRI should be considered to establish an accurate diagnosis. Cerebrospinal fluid studies could be considered if the diagnosis remains unclear.