Objective To quantify the rate of diagnostic error amongst patients with IIH. Additionally to identify factors contributing to diagnostic error.

Methods Sequential patients referred with a diagnosis of IIH to the Birmingham tertiary neuro-ophthalmology IIH clinic were prospectively included (October 2013- February 2017) A diagnostic error taxonomy tool was applied to cases referred as ‘definite’ or ‘possible’ IIH. Discrepancy between referred and final diagnosis were recorded.

Results 212 patients were referred, (96.2% female), 138/212 (65%) with definite IIH and 74/212 (35%) with possible IIH. Of those diagnosed with definite IIH 25% were not IIH and out of those diagnosed with possible IIH 57% were not IIH. Reasons for diagnostic error included incorrectly identifying papilloedema where in fact pseudopapilloedema existed and diagnosing IIH following an isolated lumbar puncture (LP) pressure >25 cmCSF (but in the absence of other diagnostic criteria for IIH). Misdiagnosis lead to 43% receiving unnecessary acetazolamide (or other diuretics) and 14% having multiple LPs.

Conclusions We noted a high diagnostic error rate amongst IIH patients referred to a tertiary centre for ongoing management. Where there is doubt about the presence of true papilloedema early specialist review may reduce unnecessary treatment and LP’s.