Apparent Diffusion Coefficient of the Optic Nerve Head in Idiopathic Intracranial Hypertension.

Idiopathic Intracranial Hypertension (IIH) is a condition in which patients have elevated intracranial pressure which does not have an apparent cause. To diagnose IIH, evaluation excluding other causes of elevated pressure must be performed. This typically includes magnetic resonance imaging (MRI) of the brain and venous sinuses. Despite there being known radiographic signs suggestive of IIH on MRI, there currently are no established correlations between radiographic findings and visual outcomes. Previous work revealed diffusion weighted imaging (DWI), a qualitative measurement on MRI, correlated with clinical findings (i.e presence and grade of papilloedema), but not visual outcomes. We hypothesized that the apparent diffusion coefficient (ADC), a quantitative value obtained during clinical MRI, may correlate with visual outcomes. We conducted a retrospective chart review to correlate findings on the ADC sequence on routine brain MRIs in patients with papilloedema with visual outcomes. In 49 patients with IIH, this study shows the ADC in the retrobulbar optic nerve to be 1,487 ± 559 × 10 mm /s, 15% lower than reported value of 1744 ± 413 in healthy controls. This suggests that there is true restricted diffusion in patients with IIH and papilloedema, as previously reported visually by MRI. However, there was no significant correlation with clinical outcomes of papilloedema grade, mean deviation on standard perimetry, and retinal nerve fibre layer (RNFL) on optical coherence tomography (OCT). We discuss reasons why the ADC measurement may be confounded by motion and partial volume and propose methods that may reduce these confounders for future studies.