Structure Function Correlation in Primary Open Angle Glaucoma

A Cross Sectional Study of Correlation Between Retinal Nerve Fibre Thickness Measured by Optical Coherence Tomography and Humphrey Visual Fields in Early and Moderate Open Angle Glaucoma

Glaucoma is an optic neuropathy characterised by progressive degeneration of retinal ganglion cells and axons that leads to nerve fibre loss, optic disc cupping, and consecutive glaucomatous visual field defects. It is considered to be one of the major causes of blindness worldwide.

It is a well accepted fact least 25 - 40% of retinal ganglion cells need to be lost before statistically detectable visual field defects appear on automated visual field testing, which is also consistent with post-mortem histologic findings in glaucomatous eyes. Since the damage associated with glaucoma is irreversible, and retinal nerve fibre layer loss is considered as an early sign of glaucomatous damage, its early detection and prevention is warranted. Retinal nerve fibre layer studies can be undertaken through non - invasive, reproducible technologies such as optical coherence tomography, scanning laser polarimetry etc.

The purpose of the study is to evaluate the relationship between visual fields and retinal nerve fibre layer thickness as measured by Cirrus spectral domain optical coherence tomography with visual fields by Humphrey Field Analyser (HFA) in early and moderate primary open - angle glaucoma.

It is often said that the structural damage due to glaucoma precedes functional loss, it is not always clear in its interpretation. To define, which test shows up the earliest sign of glaucomatous damage would be difficult to predict, since the outcome can be influenced by many factors, such as the sensitivity of the test, inter - individual difference or the stage of the disease process itself. Studies on post-mortem retinal ganglion cells counts and SAP field loss in humans and monkeys lead to an observation that, on an average about 25% retinal ganglion cells loss leads to development of an afferent pupillary defect, 35% loss for visual field defects and 40% loss leads to worsening of visual acuity. It would be safe to state that a) the most important clinical challenge is early detection of glaucomatous damage and progression over a period of time; b) Both structural and functional tests are important in assessing early damage and progression; c) significant damage to the retinal ganglion cells can occur before standard tests detect a functional loss in vision. These have paved the way to the development of many studies on the structure - function correlation and stating linear models for the same. However these studies present a dilemma; is the structure - function relation on glaucoma, a statistical statement (a structural measure will reach significance before a functional test) or relational statement (statistical correlation between the structural and functional tests).

We propose to evaluate the structure - function correlation in early and moderate open - angle glaucoma by assessing the retinal nerve fibre layer thickness by Spectral Domain Optical Coherence Tomography and areas of decreased visual field sensitivity given by Humphrey 24 - 2 Swedish Interactive Threshold Algorithm Standard protocol of automated perimetry and determine any representational or statistical significance, if any between the two tests.

Inclusion Criteria:

• Subjects visiting a glaucoma clinic diagnosed with primary open - angle glaucoma.
• Subjects should have best corrected visual acuity not less than 6/18.
• Refractive spherical error within the range of -6.00 dioptres to + 3.00 dioptres and cylindrical error within +/- 2.5 dioptres.
• Visual fields examination of the subjects demonstrating field defects secondary to glaucomatous damage with mean deviation ranging from -2 to -10 decibels (db).
• The visual fields of the subjects should be within the reliability criteria (<20% fixation losses, false positives and false negatives).
• The signal to noise ratio for a reliable Optical Coherence Tomograph - Retinal Nerve Fibre Layer scan shall not be less than 6.

Exclusion Criteria:

• History of (H/O) retinal disease or photocoagulation.
• H/O ocular trauma or surgery except an uncomplicated cataract surgery.
• H/O other optic nerve disease except glaucoma
• H/O amblyopia
• Neurological disease which can affect visual fields.
• Visually significant cataracts (Posterior Subcapsular Cataracts, > N.S Grade II).