20 Chapter 1 Figure 3. Resting state functional magnetic resonance imaging (fMRI) studies investigating functional connectivity in central disorders of hypersomnolence. Regions were generally included in case there were at least two independent studies indicating changes within this region in people with a central disorder of hypersomnolence compared to controls. Reported differences could reflect any fMRI outcome measure, such as within and between network connectivity, seed-based connectivity and fractional low-frequency amplitude of low-frequency fluctuations (fALFF). The numbers between brackets correspond to the reference numbers of the studies. HC = Healthy controls; NT1 = Narcolepsy type 1; IH = Idiopathic hypersomnia. Figure was created with Biorender. Neuroimaging in narcolepsy type 2 and idiopathic hypersomnia Most neuroimaging studies to date have been conducted in small samples (typically 10-30 individuals per group), while narcolepsy type 2 and idiopathic hypersomnia remain relatively understudied. From the limited number of structural neuroimaging studies performed in narcolepsy type 2 and idiopathic hypersomnia, it appears that people with narcolepsy type 2 [99, 110] and idiopathic hypersomnia [120] have a smaller hippocampus and larger precuneus, respectively. Structural brain differences in narcolepsy type 2 and idiopathic hypersomnia exhibit some similar but weaker differences compared to narcolepsy type 1. Even though direct cross-disorder comparisons are lacking, functional resting-state network connectivity differences appear less striking in idiopathic hypersomnia as compared to narcolepsy type 1 [111-115, 120] and no resting-state fMRI studies have been performed in narcolepsy type 2. Given that this field is still relatively new and no single MRI study has thus far included people with narcolepsy type 1, type 2, and idiopathic hypersomnia, it remains difficult to draw robust conclusions on differences between diagnoses.
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