Insular and occipital changes in visual snow syndrome: a BOLD fMRI and MRS study

To investigate the pathophysiology of visual snow (VS), through a combined functional neuroimaging and magnetic resonance spectroscopy (¹ H-MRS) approach.

We applied a functional MRI block-design protocol studying the responses to a visual stimulation mimicking VS, in combination with ¹ H-MRS over the right lingual gyrus, in 24 patients with VS compared to an equal number of age- and gender-matched healthy controls.

We found reduced BOLD responses to the visual stimulus with respect to baseline in VS patients compared to controls, in the left (k = 291; P = 0.025; peak MNI coordinate [-34 12 -6]) and right (k = 100; P = 0.003; peak MNI coordinate [44 14 -2]) anterior insula. Our spectroscopy analysis revealed a significant increase in lactate concentrations in patients with respect to controls (0.66 ± 0.9 mmol/L vs. 0.07 ± 0.2 mmol/L; P < 0.001) in the right lingual gyrus. In this area, there was a significant negative correlation between lactate concentrations and BOLD responses to visual stimulation (P = 0.004; r = -0.42), which was dependent on belonging to the patient group.

As shown by our BOLD analysis, VS is characterized by a difference in bilateral insular responses to a visual stimulus mimicking VS itself, which could be due to disruptions within the salience network. Our results also suggest that patients with VS have a localized disturbance in extrastriate anaerobic metabolism, which may in turn cause a decreased metabolic reserve for the regular processing of visual stimuli.