Incidence and Mimickers of Acute Idiopathic Optic Neuritis: Analysis of 291 Consecutive Patients from Southern Finland.

PURPOSE: To estimate the population-based incidence of acute idiopathic optic neuritis (ON) and analyse its differential diagnosis in patients referred with symptoms suggestive of ON.

METHODS

Patients with suspected ON referred to the Helsinki University Hospital, serving a population of 1.5 million in Southern Finland, were reviewed between 1st May 2008 and 14th April 2012. Brain and optic nerve magnetic resonance imaging (MRI) was performed within 24 hours in 83% of patients.

RESULTS

Of 291 referred patients, 184 (63%; 95% confidence interval [CI], 57-69%) were diagnosed with ON whereas 107 (37%) had another condition. The estimated crude incidence of ON in Southern Finland was 3.0 (95% CI 2.8-3.3) per 100,000 (females, 4.6 and males, 1.4). Mean age was 34 years (range 15-61), 76% were female. Two (1%) were diagnosed with neuromyelitis optica. ON as the first demyelinative episode was diagnosed in 108 (59%) patients, and MRI showed demyelinating lesions (MRI+) in 82% (95% CI, 75-89) of them. MRI+ predicted the development of multiple sclerosis (MS): 54% of MRI+ vs. 5% MRI- patients were diagnosed as MS during a mean follow-up of 7.7 years. The most common differential diagnosis was non-arteritic anterior ischemic optic neuropathy (12%). Six (2%) intracranial compressive lesions were found upon MRI scan.

CONCLUSIONS

More than a third of patients with symptoms suggestive of ON had another condition. Demyelinative lesions on MRI indicated higher risk of developing MS. We recommend the use of MRI to improve the differential diagnostic accuracy of ON and to identify patients with high risk of MS.