Optical coherence tomography and neuro: What and when?

Calgary, Alberta-Retinal nerve fiber layer (RNFL) measurements may be a surrogate marker of axonal damage in the anterior visual pathway that will enhance the understanding of mechanisms of optic nerve damage and, by association, central nervous system damage.

However, further prospective data on this hypothesis are needed, said Fiona E. Costello, MD, FRCP, co-director of the Neuroprotection and Repair Evaluation Unit and clinical associate professor, Departments of Clinical Neurosciences and Surgery, University of Calgary, Alberta.

The toll of retrobulbar optic neuropathy on the axonal bed can be seen because the RNFL lacks myelin, Dr. Costello explained. In theory, axonal damage that can be measured quantitatively can predict and correlate to visual recovery after a variety of modalities of injuries.

Optic neuritis

The first case and findings from several studies suggest that RNFL measurements may potentially be used to predict outcomes after optic nerve injury. The case was that of a previously healthy 42-year-old woman who presented with painful vision loss in the right eye over 3 days. Her visual acuity in the right eye was down to counting fingers, while it was 20/20 in the left eye. A fundus examination showed that optic nerves were normal in appearance. These findings were consistent with the clinical diagnosis of optic neuritis.

At baseline, optical coherence tomography (OCT) revealed slight thickening of the RNFL in the right eye, while at 3 months the thickness had declined by about 40% to 62 µm; the left eye RNFL thickness had changed by only 1 µm.

At 6 months, thickness in the right eye had decreased to 45 µm, while RNFL measurements in the left eye were unchanged.

"This case illustrates what subsequent studies have shown us, and that is that RNFL measurements are decreased post-acutely in optic neuritis-affected eyes, both in patients with multiple sclerosis and in patients presenting with optic neuritis as their first manifestation of multiple sclerosis," Dr. Costello said. "There are also differences between eyes with optic neuritis and the so-called 'normal' eyes of these patients, and similarly, macular volumes are decreased in these eyes as well."

She and colleagues have shown that 74% of patients who had recovered from optic neuritis still had significant RNFL loss-about 20%- in their affected eye after 1 year. They also learned that below the threshold of 75 µm, visual recovery could be predicted in linear fashion; below that threshold, patients seldom have persistent visual deficits. They subsequently showed that changes could be tracked out to about 6 months. Vision then plateaus, which may mean that a window of therapeutic opportunity exists in which clinicians could try a different approach to enhancing visual recovery.