Brain iron deposition in primary insomnia—An in vivo susceptibility‐weighted imaging study

Abstract Background To study the brain iron deposition and its relationships with cognitive impairment and sleep quality in primary insomnia (PI). Methods Thirty‐five patients with PI and 35 volunteers underwent MRI scanning using high‐resolution susceptibility‐weighted imaging sequence. Bilateral anterior cingulate cortices, posterior cingulate cortex, hippocampus, caudate nucleus, globus pallidus, putamen, thalamus, red nucleus, substantia nigra, parietal cortex, and frontal white matter were selected as regions of interest. The phase shift values of the above areas were compared between the two groups. Partial correlations between phase shifts values and neuropsychological scale scores including Pittsburgh Sleep Quality Index, Insomnia Severity Index, Mini Mental State Examination (MMSE), Montreal Cognitive Assessment (MoCA), Activities of Daily Living Scale, and Clinical Dementia Rating of the PI patients were analyzed. Results Compared with the normal controls, the PI patients showed significant lower MMSE and MoCA scores and increased phase shift values in the left caudate nucleus, left putamen, left hippocampus, and bilateral thalamus (p < 0.05). Close correlation was found between the phase shift value of the left hippocampus and the MMSE scores of the PI patients (R = −0.447, p < 0.01). Conclusion The PI patients exhibited significant cognitive impairment and increased iron deposition in several brain regions. The iron concentration of the left hippocampus is a biomarker of cognitive impairment and may play an important role in the pathophysiological mechanism.

Electroencephalogram studies have found that PI patients showed elevated spectral power values in the beta and sigma frequency band (Spiegelhalder et al., 2012). Greater global cerebral glucose metabolism during sleep was also found using positron emission tomography in PI (Nofzinger et al., 2004). As a noninvasive technique, resting-state functional magnetic resonance imaging can detect spontaneous neural activity in the brain. Patients with insomnia exhibit decreased frontoparietal cortex activation during working memory task performance (Drummond et al., 2013;. However, the mechanism by which the PI influences the brain function of the specific brain regions has not been fully elucidated. Iron contributes to many biological processes, including oxygen transport, protein expression regulation, and cell growth. Previously, it has been found that excessive brain iron deposition plays an important role in the arise and development of brain function disorders and cognitive impairment (Haller et al., 2010;Liu et al., 2015;Smith, Harris, Sayre, & Perry, 1997). Susceptibility-weighted imaging (SWI), which is combined by the phase images and the magnitude images, is sensitive to the paramagnetic effect of iron particles (Haacke, Mittal, Wu, Neelavalli, & Cheng, 2009;). It has been proved reliable to measure tissue iron concentration, which is consistent with the autopsy examination results. We hypothesized that (a) there is brain iron deposition in PI patients in multiple brain regions and (b) individual cortical brain iron deposition in specific regions could correlate with the brain function alterations of PI patients.

| Subjects
Thirty-five patients with PI were recruited from the sleep disorder clinic of our hospital ( Depression Scale. Exclusion criteria were as follows: (a) the patient who had an abnormal brain in conventional CT or MRI; (b) the patient who had serious organic disease or severe mental disease. Thirtyfive age, gender, and education levels matched normal controls were recruited from the community. The exclusion criteria including: nervous system diseases such as brain trauma, tumor or hemorrhage, alcohol or drug abuse, systemic disease or other MRI contraindication.
All subjects with severe depression (Hamilton Depression Rating Scale ≥18) or dementia (MMSE < 24) were also excluded. All of the participants were right-handed. Written informed consent was signed by every participant. This study was conducted in accordance with the Declaration of Helsinki. All the procedures were approved by the Medical Ethics Committee of our institution.

| Magnetic resonance imaging
All subjects were scanned on a 3.0 T whole body MRI scanner (Magnetom Trio, Siemens Healthcare, Erlangen, Germany).
Susceptibility-weighted images were obtained with the following pa-

| RE SULTS
The PI patients and normal control volunteers had no significant difference in age, sex, and education years (p > 0.05).  Figure 3).
The intraclass correlation coefficients were >0.90 for all the ROIs. TA B L E 2 Regional phase shift values of healthy controls and primary insomnia patients (mean ± SD) examinations of 81 normal brains performed by Hallgren and Sourander (1958). In this study, a close correlation was found be- Recently it has been found that insomnia patients show decreased hippocampal volume and atrophy in the cornu ammonis and dentate gyrus which associated with impaired cognitive functions (Joo, Kim, Suh, & Hong, 2014). In this study, we also found iron deposition in thalamus, caudate nucleus, and putamen of the PI patients. The thalamus plays a major role in arousal, awareness level, and activity regulation. It can control the sleep and wakefulness states (Steriade & Llinás, 1988). The caudate nucleus and putamen are parts of the basal ganglia. They are responsible for voluntary movement control and took part in many cognitive functions including memory, emotion, and learning (Bastos-Leite et al., 2007;Kantarci et al., 2010).

| D ISCUSS I ON
The precise mechanisms of the abnormal iron deposition in these brain regions of PI are not understood.
Using SWI technology, brain iron deposition has been reported in several other diseases previously. The iron concentrations of the substantia nigra in PD patients have been proved significantly increased and closely correlated with the UPDRS motor score . Patients with AD demonstrated increased iron concentration in the temporoparietal, frontal, and parietal regions. High iron content has been proved to play an important role for formation of senile plaques in animal studies, which is the main pathologic manifestation of AD (McCrea, Harder, Martin, Buist, & Nichol, 2008;Nakada, Matsuzawa, Igarashi, Fujii, & Kwee, 2008). Several other diseases including multiple sclerosis, mild cognitive impairment, and amyotrophic lateral sclerosis were also proved associated closely with regional high deposition of iron (Chawla et al., 2016;Schweitzer et al., 2015).
In conclusion, in this study we found the PI patients have significant cognitive impairment and increased iron deposition in several brain regions. The iron concentration of the left hippocampus is a biomarker of cognitive impairment and may play an important role in the pathophysiological mechanism. This study had several limitations. Firstly, there is a relatively small sample size in this study. Secondly, as in previous literature, we considered that increasing susceptibility reflects increasing iron concentration.
However, sometime several other metals such as manganese and copper may have the potential to cause susceptibility (Schenck & Zimmerman, 2004).

ACK N OWLED G M ENT
This work was supported by the National Natural Science Foundation of China (81471194)