Structural and functional footprint of visual snow syndrome

Abstract

Patients with visual snow syndrome suffer from a continuous pan-field visual disturbance, additional visual symptoms, tinnitus, and non-perceptional symptoms. The pathophysiology of visual symptoms might involve dysfunctional visual cortex. So far, the extra-visual system has not been investigated. We aimed at identifying structural and functional correlates for visual and non-visual symptoms in visual snow syndrome. Patients were compared to age- and sex-matched controls using 18F-2-fluoro-2-deoxy-d-glucose PET (n = 20 per group) and voxel-based morphometry (n = 17 per group). Guided by the PET results, region of interest analysis was done in voxel-based morphometry to identify structural-functional correspondence. Grey matter volume was assessed globally. Patients had corresponding hypermetabolism and cortical volume increase in the extrastriate visual cortex at the junction of the right lingual and fusiform gyrus. There was hypometabolism in the right superior temporal gyrus and the left inferior parietal lobule. Patients had grey matter volume increases in the temporal and limbic lobes and decrease in the superior temporal gyrus. The corresponding structural and functional alterations emphasize the relevance of the visual association cortex for visual snow syndrome. The broad structural and functional footprint, however, confirms the clinical impression that the disorder extends beyond the visual system.

Keywords: FDG PET; migraine; non-visual symptoms; visual snow; voxel-based morphometry.

Figure 1

Differences in brain metabolism between patients with VSS and controls. When compared to controls, patients with VSS demonstrated hypermetabolism in FDG PET (red) and a corresponding increase of grey matter volume as determined by VBM (yellow) at the junction of the lingual and fusiform gyrus on the right, both in Brodmann area (BA) 19 (A–C). VSS patients showed additional areas of hypometabolism in the right superior temporal gyrus (BA 22, D1) and left inferior parietal lobule (BA 40, D2) without corresponding alterations in grey matter volume. PET statistical parametric maps at a threshold of P < 0.001 uncorrected for multiple comparisons are overlaid on a normalized standard T₁-weighted MRI. The VBM map was obtained using a 10-mm sphere as an explicit mask around the maximum from significant PET clusters and thus represents only a part of the grey matter alterations shown in Fig. 2 (threshold P < 0.05, family wise error rate on cluster level). VS = visual snow.

Figure 2

Grey matter volume differences between patients with VSS and controls. Using VBM, we observed grey matter volume increases (visual snow > control) in the right fusiform gyrus (A), the right middle temporal gyrus (B), the right parahippocampal gyrus (C), the left superior temporal gyrus (D) as well as the right anterior cingulate cortex (F) in patients with visual snow syndrome. Grey matter decreases were found in the left superior temporal gyrus (E). VBM statistical parametric maps at a threshold of P < 0.001 uncorrected for multiple comparisons are overlaid on a normalized standard T₁-weighted MRI. VS = visual snow.