Understanding Nerve Fiber Layer Analysis
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
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:
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.
reports in this section
& Eyelashes | Conjunctiva
& Sclera | Cornea
Uvea | Vitreous & Retina | Optic Nerve & Brain | Oculosystemic Disease
Handbook Main Page