Optic Nerve Computer Imaging

In recent years three new techniques of optic nerve imaging have become widely available. These are scanning laser polarimetry (GDx), confocal laser ophthalmoscopy (Heidelberg Retinal Tomography or HRT II), and optical coherence tomography (OCT).

The GDx machine does not actually image the optic nerve but rather it measures the thickness of the nerve fiber layer on the retinal surface just before the fibers pass over the optic nerve margin to form the optic nerve. The HRT II scans the retinal surface and optic nerve with a laser. It then constructs a topographic (3-D) image of the optic nerve including a contour outline of the optic cup. The nerve fiber layer thickness is also measured. The OCT instrument utilizes a technique called optical coherence tomography which creates images by use of special beams of light. The OCT machine can create a contour map of the optic nerve, optic cup and measure the retinal nerve fiber thickness. Over time all three of these machines can detect loss of optic nerve fibers.

Your intraocular eye pressure (IOP) is important to determining your risk for glaucoma. If you have high IOP, careful management of your eye pressure with medications can help prevent vision loss. Recent discoveries about the cornea, the clear part of the eye’s protective covering, are showing that corneal thickness is an important factor in accurately diagnosing eye pressure. In response to these findings, the Glaucoma Research Foundation has put together this brief guide to help you understand how your corneal thickness affects your risk for glaucoma, and what you can do to make sure your diagnosis is accurate.

Corneal Thickness

In 2002, the five-year report of the Ocular Hypertension Study (OHTS) was released. The study’s goal was to determine if early intervention with pressure lowering medications could reduce the number of ocular hypertensive (OHT) patients that develop glaucoma. During the study, a critical discovery was made regarding corneal thickness and its role in intraocular eye pressure and glaucoma development.

Why is Corneal Thickness Important?

Corneal thickness is important because it can mask an accurate reading of eye pressure, causing doctors to treat you for a condition that may not really exist or to treat you unnecessarily when are normal. Actual IOP may be underestimated in patients with thinner CCT, and overestimated in patients with thicker CCT. This may be important to your diagnosis; some people originally diagnosed with normal tension glaucoma may in fact be more accurately treated as having regular glaucoma; others diagnosed with ocular hypertension may be better treated as normal based on accurate CCT measurement. In light of this discovery, it is important to have your eyes checked regularly and to make sure your doctor takes your CCT into account for diagnosis.

A Thin Cornea—The Danger of Misreading Eye Pressure

Many times, patients with thin corneas (less than 555 µm) show artificially low IOP readings. This is dangerous because if your actual IOP is higher than your reading shows, you may be at risk for developing glaucoma and your doctor may not know it. Left untreated, high IOP can lead to glaucoma and vision loss. It is important that your doctor have an accurate IOP reading to diagnose your risk and decide upon a treatment plan.

A Thicker Cornea May Mean Less Reason to Worry About Glaucoma

Those patients with thicker CCT may show a higher reading of IOP than actually exists. This means their eye pressure is lower than thought, a lower IOP means that risk for developing glaucoma is lowered. However, it is still important to have regular eye exams to monitor eye pressure and stay aware of changes.

Pachymetry—A Simple Test to Determine Corneal ThicknessA pachymetry test is a simple, quick, painless test to measure the thickness of your cornea. With this measurement, your doctor can better understand your IOP reading, and develop a treatment plan that is right for your condition. The procedure takes only about a minute to measure both eyes.

Corneal Topography