Research Interests:Understanding the optical properties of the tissues of the eye, especially the retinal nerve fiber layer, and translating basic knowledge into improvements on clinical diagnosis of glaucoma.

XiangRun Huang, Ph.D.
Research Assistant Professor

View published research articles by this doctor in the National Library of Medicine.

Current Research Summary: Glaucoma, the second leading cause of blindness, is characterized by retinal ganglion cell death and optic nerve degeneration. Delays in detecting glaucoma can lead to inadequate treatment and irreversible visual loss. The retina nerve fiber layer (RNFL) in human eyes consists of the axons of retinal ganglion cells. Recognizable loss of the RNFL precedes measurable loss of vision and thus measurement of RNFL may provide early diagnosis of glaucoma. Several quantitative methods for optically assessing the RNFL promise enhanced sensitivity and objectivity over previous methods, such as visual function measurement. All these techniques measure some aspects of the optical properties of the RNFL, however, these properties are only partly known. The goal of Dr. Huang’s research is to provide a comprehensive quantitative description of the optical properties of the RNFL and translate basic knowledge into improvements in clinical diagnosis of glaucoma.

Isolated model retina for studying the optical properties of the RNFL.

The RNFL reflectance arises from light scattering by cylinders. By modeling the RNFL as a thick birefringence slab containing a parallel array of scattering cylinders, the RNFL reflectance has been predicted to be a polarization-preserving reflector. Experimental measurement of degree of polarization in model RNFL reflection confirmed this prediction. Measurement of the diattenuation revealed that the RNFL reflection has very weak intrinsic diattenuation from visible to near infrared wavelengths. Fitting the diattenuation spectrum with the RNFL reflectance model constrains the relative refractive indices of the scattering cylinders to be less than 1.03. Understanding the RNFL reflectance mechanism and its anatomic basis can enhance the interpretation of clinical measurements of the RNFL and improve their ability to detect damage or progression of damage.

RNFL of an isolated model retina viewed with confocal laser scanning microscope.

Glaucoma diagnosis and management center on ways to detect ganglion cell loss. An optical property of the tissue called birefringence depends on details of the microscopic anatomy of ganglion cell axons, such as, the volume fraction and relative refractive index of cylindrical organelles in axons. Differences in birefringence, therefore, must necessarily reflect differences in the density or composition of the axons that make up the RNFL. We’ve studied the birefringence of the RNFL in vitro and found that the birefringence is similar at multiple wavelengths, which is consistent with a mechanism of form birefringence from thin cylindrical organelles in axons. Recent study of the birefringence of the RNFL in normal human subjects reveals that the RNFL birefringence varies with position around the optic nerve head and is constant along bundles. The result confirms that birefringence depends on the ultrastructure of axons in the RNFL. Accurate measurement of birefringence thus may be useful for detecting subcellular changes that might precede cell death in early stage glaucoma.