Purpose: To analyze the relationship between normal-tension glaucoma (NTG) and serum bilirubin levels.

Materials and Methods: This research included 38 patients with NTG and 38 healthy controls with similar age and sex distribution, for a total of 76 subjects. Serum samples were taken from all of the subjects, direct serum bilirubin, indirect serum bilirubin and the total serum bilirubin were measured.

This is a retrospective study of cases with recently or previously diagnosed NTG who were admitted to our clinic between 2010 and 2017. This study included 38 patients with NTG and 38 healthy controls with similar age and sex distribution, for a total of 76 subjects. Written informed consent was obtained from all of the participants. The study was approved by the institutional review board of the hospital and adhered to the tenets of the Declaration of Helsinki.

A complete ophthalmic examination including best-corrected visual acuity, slit-lamp biomicroscopy, Goldmann applanation tonometry (AT 900, Haag-Streit Diagnostics, Koeniz, CH), corneal thickness evaluation (Pachymeter SP-3000, Tomey Corporation, Nagoya, JP), gonioscopy and ophthalmoscopy was performed in all NTG cases. Additionally, optical coherence tomography (OCT RS-3000 Lite, NIDEK Corporation, Tokyo, JP) measurement of the peripapillary retinal nerve fibre layer thickness, and Humprey visual field testing (Carl-Zeiss Meditec, Dublin, CA) were also performed.

NTG was diagnosed based on a clinical examination showing a normal IOP (corrected for corneal thickness), thinning of the peripapillary retinal nerve fibre layer, optic nerve damage, and characteristic glaucomatous visual field defects. The exclusion criteria included other forms of glaucoma such as primary open-angle glaucoma, primary closed-angle glaucoma, pigmentary glaucoma and exfoliative glaucoma. Patients who had active systemic diseases such as those affecting the liver, biliary system, pancreas and kidney and infectious diseases or malignant tumours that could influence the serum bilirubin concentrations were also excluded.

Fasting blood samples were taken from each participant to measure direct bilirubin, indirect bilirubin, and the total serum bilirubin concentrations. The direct bilirubin and total bilirubin concentrations were measured with an Architect C16000 Clinical Chemistry Analyzer (Abbott Laboratories, Abbott Park, IL, USA), using its original kits, and the indirect bilirubin concentrations were calculated by the following formula: indirect bilirubin = total bilirubin - direct bilirubin.

All the data were compiled into a computer file, and Statistical Package for Social Sciences for Windows 22.0 was used to perform the statistical analysis. Student's t test or the Mann-Whitney U test were used to compare the mean values between the two groups, after the evaluation of the data for a normal distribution using the Kolmogorov-Smirnov test. A p value less than 0.05 was considered statistically significant.

Inclusion Criteria:

• Healthy volunteers

Exclusion Criteria:

• Patients who had active systemic diseases such as those affecting the liver, biliary system, pancreas and kidney and infectious diseases or malignant tumours that could influence the serum bilirubin concentrations were excluded.

NTG was diagnosed based on a clinical examination showing a normal IOP (corrected for corneal thickness), thinning of the peripapillary retinal nerve fibre layer, optic nerve damage, and characteristic glaucomatous visual field defects. The exclusion criteria included other forms of glaucoma such as primary open-angle glaucoma, primary closed-angle glaucoma, pigmentary glaucoma and exfoliative glaucoma. Patients who had active systemic diseases such as those affecting the liver, biliary system, pancreas and kidney and infectious diseases or malignant tumours that could influence the serum bilirubin concentrations were also excluded.