Optic Nerve Imaging:

Imaging the optic nerve can be done in a number of ways.

Ophthalmoscopy

You’ve all seen medical shows in which a physician picks up a hand held device, holds it
about an inch away from the eye and examines the nerve looking through the instrument monocularly (with one eye only).  This is called ophthalmoscopy and gives a magnified, yet one dimensional view of the nerve.  

For glaucoma assessment, it is preferable to look at the nerve stereoscopically, or in three dimensions. This requires most often that the pupil of the eye be dilated, or enlarged with eyedrops.

Once dilated, using special lenses, the doctor using a device called a slit lamp with a 90 Diopter lens.  The slit lamp has two eyepieces (oculars) so a stereoscopic view of the nerve is seen, rather than a flat perspective. Since the degree and the extent of optic nerve cupping is being sought, the stereo-view is advantageous.  

Photography

A conventional one-dimensional photograph or drawing is often made of the optic  disk at the time of the initial examination.

To take stereo-photos of the nerve, two pictures must be taken, each separated 8 degrees from the other, to recreate the stereo-effect seen at the time of the actual exam.  To view images in 3 dimensions or stereoscopically, a special viewer must be used.

Serial comparison of photos or drawings in useful in detecting signs that the optic cup is enlarging. When the optic cup enlarges (with simultaneous thinning of the optic nerve rim tissue), glaucomatous progression has occurred.

GDX & HRT Technology

New technologies have emerged to assess optic nerve structure. In addition, tests have been developed to measure the thickness and pattern of the nerve fiber layer. The nerve fiber layer (sometimes referred to as ganglion layer) is the outer-most part of the retina and directly feeds into the optic nerve, Glaucoma specialists use both nerve fiber layer and optic nerve structure to diagnose and follow glaucoma patients.

The GDX (nerve fiber layer analyzer) uses scanning laser polarimetry to analyze the nerve fiber layer. It depends upon the polarization property of the nerve fiber layer (NFL) to which is organized into columns. These columns have special properties to both absorb light and reflect it back. Some of the light reflected back through this layer is delayed. This allows direct measurement of the thickness of the NFL, which is directly related to glaucomatous damage to the eye. The amount of light reflected back is measured, and gives a graphic image of the nerve fiber layer. The thinner the nerve fiber layer, the more damage that’s present.

The newest generation of the GDX has a special feature called a corneal compensator; the compensation adjusts for the polarization properties of the cornea, which would otherwise interfere with interpretation the nerve fiber layer images.

A normal GDX shows an hourglass type pattern with bright red and yellow colors. (Image1) an abnormal pattern shows a “dropout” of new fiber layer.

We often diagnose glaucoma on the basis of an abnormal NFL study BEFORE loss of visual field changes have occurred. The GDX is our preferred instrument in patients with elevated intraocular pressure and normal visual field testing when we are trying to determine whether or nor the patient should be treated.

In contrast to the HRT-3 which measures the topography of the optic disk, the GDX measures the nerve layer which is adjacent to the nerve. In many cases the optic nerve may appear normal, but the GDX will demonstrate “dropout” of the nerve layer. Often though, the two tests are confirmative, with the GDX showing thinning of the nerve fiber layer, and the HRT-3 showing thinning of the optic nerve rim tissue in the corresponding quadrant.

OCT Technology 

We recently added the Cirrus HD-OCT (Carl Zeiss, Meditec, Dublin, CA) to our glaucoma early detection testing armamentarium.

We have been thrilled with the unit and find it to be immensely helpful in the decision making paradigm for patients with ocular hypertension, large cup disk ratios, and other patients where the diagnosis of glaucoma is being entertained.

For its glaucoma module, The Cirrus HD-OCT features RNFL Thickness map, RNFL Thickness Deviation and RNFL TSNIT normative data. The test for glaucoma is a cube scan, measuring 6 mm x 6 mm area. A 200 x 200 testing strategy is used (200 B scans with 200 A-scans per B-scan).  The Cirrus delivers 27,000 A-scans per second of time domain OCT’s. Because of this fast acquisition time and improved optics, patients with pupils 2.5 or larger with clear media do not need have to be dilated to obtain a good scan.  

The scans—which can be completed within four minutes including data entry--are both patient and user friendly.  The ergonomically designed unit allows the technician to sit at the side of the patient. This optimizes positioning by allowing direct visualization of the patient’s head and body.