Macular Ganglion cell layer imaging in glaucoma using Spectralis Optical Coherence Tomography

Study objectives -

To evaluate macular ganglion cell layer (GCL) changes in patients with glaucoma using Spectralis Optical Coherence Tomography (HRA-OCT) and to assess its utility as an early structural biomarker for disease detection and progression.

This cross sectional observational study will be approved by the Institutional Ethics Committee of Nethradhama Eye Hospital and will adhere to the tenets of the Declaration of Helsinki. Verbal and written informed consent will be obtained from all participants enrolled in the study.

All subjects will undergo a complete ophthalmic examination including best corrected visual acuity assessed using the Snellen chart refraction slit lamp biomicroscopy intraocular pressure measurement using applanation tonometry gonioscopy using a Sussman goniolens and dilated fundus examination using a ninety diopter lens and indirect ophthalmoscopy after obtaining a complete clinical history.

Visual field testing will be performed using the SITA standard thirty two program of the Humphrey Field Analyzer model two seven hundred fifty. Glaucomatous visual field loss will be defined by the consistent presence of a cluster of three or more non edge points on the pattern deviation plot with a probability of occurring in less than five percent of the normal population with at least one point having a probability of less than one percent. Additional criteria include a pattern standard deviation probability of less than five percent or a glaucoma hemifield test result outside normal limits. Visual field defects must be repeatable on at least two consecutive tests.

Patients will be categorized into three glaucoma severity subgroups using the Hodapp Parrish Anderson classification based on the mean deviation index of the visual field. Early glaucoma will be defined as mean deviation less than or equal to minus six decibels moderate glaucoma as mean deviation between minus six and minus twelve decibels and severe glaucoma as mean deviation worse than minus twelve decibels.

Optical coherence tomography is a non invasive non contact imaging technique based on the principle of optical reflectometry which allows precise anatomical localization of ocular structures. It functions on the principle of low coherence interferometry using broadband light. Infrared light of approximately eight hundred thirty nanometers is scanned across ocular tissue and focused using an internal lens. In spectral domain optical coherence tomography a broad spectrum light source such as a super luminescent diode is used and a spectrometer in the detector arm separates the reflected light into a spectrum which is detected by a line scan camera. Optical coherence tomography is currently the preferred method for measuring retinal thickness and retinal nerve fiber layer thickness.

All recruited eyes will undergo macular imaging using the Spectralis HRA optical coherence tomography system manufactured by Heidelberg Engineering Germany. The Spectralis volume scan consists of sixty one B scans each containing seven hundred sixty eight A scans acquired along the fovea disc centroid axis. Each scan will be repeated nine to eleven times to reduce speckle noise and improve image quality.

The following ganglion cell inner plexiform layer thickness parameters will be analyzed including average minimum and sectoral thickness values comprising superonasal superior superotemporal inferotemporal inferior and inferonasal sectors. The inbuilt software will analyze the data compare it with the internal normative database and generate thickness and deviation maps.