Understanding Nerve Fiber Layer Analysis

The GDx Access nerve fiber layer analyzer.

When researchers discovered that a great amount of retinal ganglion cells are lost prior to the development of visual field defects, they studied the retinal nerve fiber layer, initially with an ophthalmoscope. However, just as automated perimetry has replaced tangent screens, new technology has been developed to study the retinal nerve fiber layer.

Laser Diagnostic Technology introduced the GDx nerve fiber layer analyzer (GDx NFA) to accurately measure the retinal nerve fiber layer. This device is a scanning-laser polarimeter. Laser light double-passes the retinal nerve fiber layer (NFL) and splits into two parallel rays by the birefringent fibers. The two rays travel at different speeds and this difference (called retardation) directly correlates to the thickness of the nerve fiber layer.

This test is completely objective and takes about 10 minutes to complete, though it makes the actual measurements in less than one second. It works best on undilated pupils and can work quite well through cataracts up to 20/60. A normative database compares the patient's measurements to those of normal patients of the same age, sex, and race.

Key Points in Interpretation

A serial analysis printout.

In each analysis, the 1,500 thickest points in superior and inferior quadrants are identified as well as the 1,500 median points within the nasal and temporal quadrants. The GDx NFA bases its calculations on parameters which best differentiate normal patients from those with glaucoma. These parameters include symmetry of various quadrant ratios, nerve fiber layer area, nerve fiber layer thickness above blood vessels, absolute thickness of all measured points, and total volume of nerve fiber layer.

• Color Image. Color has been artificially added to this image of the optic nerve head and peripapillary retina to assist the operator in observing the nerve head and appreciating the quality of the scan performed.

• Thickness (Polarization) Map. This is a map of measured points creating a color-coded image to indicate NFL thickness. Bright colors (red and yellow) are associated with thicker areas, signifying healthy NFL. Dark colors (blue) are associated with thinner areas, indicating less healthy NFL.

• TSNIT (Double Hump) Graph. The right eye is depicted in blue and left in yellow. This displays the normal range and the patient's NFL thickness from the 200 points along the ellipse. It is a quick, easy look at how the patient compares with normal.

• Symmetry Analysis. This is an overlay of O.D. and O.S. TSNIT graphs. It allows for easy comparison of symmetry between the two eyes.

Nerve Fiber Analysis

These values compare the patient's NFL thickness values to an age- and race-matched population of normal patients, and evaluate a series of ratios, averages and other parameters. Any parameter statistically eva luated as "borderline" or "outside normal" indicates that the patient does not match normative values. No single parameter is more or less sensitive to detecting glaucoma. However, these parameters do bear some explanation:

• The Number. A neural network within the GDx analyzes more than 200 parameters from the image. It then assigns a number between zero and 100 (zero means normal and 100 means glaucoma). This indicates the likelihood that a patient has glaucoma. As the number is still experimental, it does not have a color to indicate significance or deviation from normal.

• Symmetry. This represents the ratio of the average of the 1,500 thickest points in the superior quadrant over the average of the 1,500 thickest points in the inferior quadrant. The closer this number is to 1.0, the more symmetry is present, and the more normal the patient.

• Max Modulation. This is an indication of the difference between the thickest part of the NFL and the thinnest part. The higher the number, the greater the difference. In a normal eye in which the superior and inferior region is thicker than the nasal or temporal region, the modulation number will usually be greater than 1.0. Lower numbers are associated with loss of thickness of the NFL and are characteristic of glaucoma.

Birefringence can occur in an analysis from ocular structures other than the nerve fiber layer, introducing artifacts into the analysis. A new development in the form of a Custom Corneal Compensator may address the difficulty.

While early studies showed a high sensitivity and specificity of the GDx NFA for glaucoma, other studies have questioned the device's ability to correctly identify early cases. There is no single parameter (or even combination) that will identify the presence of glaucoma in every case. While the device can give an accurate representation of the status of the nerve fiber layer, it should not form the sole basis for diagnosis.

The authors have no direct financial interest in Laser Diagnostic Technology or the GDx nerve fiber layer analyzer.

Other reports in this section

• Crystalline Lens Subluxation
• Ciliary-Block Glaucoma
• Primary Open-Angle Glaucoma
• Pigment Dispersion Syndrome and Pigmentary Glaucoma
• Pseudoexfoliation Syndrom and Pseudoexfoliative Glaucoma
• Anterior Uveitis
• New Insights Into Treating Ocular Hypertension
• Treating POAG: How Low Should IOP Go?
• Understanding Nerve Fiber Layer Analysis