CSDH patients recruited from neurosurgical department at 4 medical centres in China, which have apparent clinical symptoms and are confirmed by computed tomography or magnetic resonance imaging.

non-CSDH, age- and gender-matched patients recruited from ophthalmology, otolaryngology, and physical examination department at 4 medical centres in China.

Nutrition assessment, nutrient intake, dietary inflammatory index, and frailty status will be examined using questionnaires.

Difference in dietary intakes assessed by Food Frequency Questionnaire, a common method for measuring dietary intake in large epidemiological studies.

The Dietary Inflammatory Index (DII) score of the current nutritional status of patients with and without CSDH will be evaluated. The amounts of nutrients will be calculated using last 3-month nutrient analysis program. DII score will calculate from these foods. The z-score must first be calculated. To calculate the z-score, the standard global consumption amount is subtracted from the average consumption amount of the individual. The result obtained is divided by the standard deviation value. The z score found with this result is converted to the percentile score. The percentile value is multiplied by the full inflammatory effect score. DII is obtained as a result of summing the scores calculated for all nutrients. Found DII is an indicator of the inflammatory load of the individual's daily diet.

Difference in nutritional status measured by Mini-Nutritional Assessment.

Difference in frailty status measured by Modified Frailty Index. Modified Frailty Index ranges from score 0 to 1, and frail status is defined as a score of ≥0.27.

Fecal sample will be measured by 16S rRNA sequencing. The alpha and beta diversity of gut microbiota between the groups will be analyzed, including a series of statistical analysis indexes such as Chao, Shannon, Simpsonace, Simpson and Coverage, in order to reflect the microbial community diversity. Based on the results of species annotation, the community histogram, the community heatmap and the Wayne map will be used to analyze the species composition of the sample, in order to explore the relationship between the species and the sample, and analyze the key microflora in the sample. The PCA, PCoA and NMDS analysis will be used to assess the similarities and differences in species composition between groups. To further screen the species with significant differences, the LEfSe difference discriminant analysis will be used.