Eleven episodes of recurrent optic neuritis of the same eye for 22 years eventually diagnosed as neuromyelitis optica spectrum disorder

• It is difficult to predict the location of a particular NMOSD attack.
• We report a patient with 11 episodes of recurrent ON of the same eye for 22 years.
• There was no involvement of opposite eye and no myelitis.
• Provides OCT evidence of severe RNFL thinning for the recurrent ON eye.
• For the opposite non-ON eye, RNFL thickness was within normal range after 22 years.

It is difficult to predict whether a particular attack of neuromyelitis optica spectrum disorder (NMOSD) will affect the optic nerve [optic neuritis (ON): unilateral or bilateral], spinal cord (myelitis), brain or brainstem, or a combination of the above. We report an interesting case of recurrent ON of the same eye for a total of 11 episodes in a Chinese woman. Over a period of 22 years, the attacks only involved the left eye, and never the right eye and also no myelitis. For a prolonged duration, she was diagnosed as recurrent idiopathic ON. Only until she was tested positive for aquaporin 4 antibody that her diagnosis was revised to NMOSD. Optical coherence tomography revealed thinning of the retinal nerve fibre layer (RNFL) for the affected left eye, while the RNFL thickness was within normal range for the unaffected right eye. The disability accrual in NMOSD is generally considered to be attack-related – without a clinical attack of ON, there shall be no visual impairment, and no significant subclinical thinning of RNFL. Our case is in agreement with this notion. This is in contrast to multiple sclerosis where subclinical RNFL thinning does occur. This case highlights the importance of revisiting and questioning a diagnosis of recurrent idiopathic ON particularly when new diagnostic tools are available.