Retinal nerve fiber layer thickness tied to visual field

Patients who were less severely affected by the disease have wider preservation of nerve fiber bundles with a better visual outcome compared with patients who are more severely affected, according to Piero Barboni, MD.

The investigators had observed in patients with Leber's hereditary optic neuropathy that the nerve fiber thickness was preserved in those patients who recovered some visual acuity; they then set out to determine if there was a relationship between the RNFL thickness, the visual acuity, and the visual fields, Dr. Barboni explained.

Twenty-five patients from 20 families were divided into groups based on the degree of impact of the disease on their vision. Type 1 patients had a central scotoma that was greater than 20°, type 2 patients had a central scotoma of from 10° to 20°, and type 3 patients had a central scotoma less than 10°.

The patients all had a pathogenic mtDNA point mutation. The control group included 25 subjects who were matched to the patients in age and the size of the optic nerve head. All participants underwent measurement of the RNFL thickness with Stratus optical coherence tomography (OCT, Carl Zeiss Meditec), Dr. Barboni said. He works in a private practice in Centro Salus, Bologna, Italy, and works in close collaboration with the Department of Neurological Sciences at the University of Bologna.

"We found that the recovery was longer in the patients classified as having type 1 disease. These patients had a decrease in the thickness of the nerve fiber layer that was statistically significant (p < 0.0001) both in the 360° average and in each quadrant measured," Dr. Barboni reported.

Patients with type 1 and type 2 disease had a significant decrease in the average RNFL thickness, but there was no significant difference in the average thickness between type 1 and type 2. When the average RNFL thickness of the patients with type 1 disease was compared with those with type 3 disease, the difference was significant. Those with type 3 disease had a significantly thicker RNFL, except in the temporal quadrant.

When the investigators looked at the three categories of disease together, they found a positive correlation between the 360° measurement of the RNFL thickness and the visual field mean deviation, according to Dr. Barboni.

When they analyzed the visual acuity, they found that patients with type 3 disease had better visual acuity compared with the other two groups.

"Linear regression analysis showed that there was a strong correlation between the RNFL measurement and the visual fields and a slightly weaker correlation between the RNFL and visual acuity," he said.

Based on this study, Dr. Barboni and colleagues concluded that in patients with Leber's hereditary optic neuropathy there is a stronger association between the RNFL thickness and preservation of the visual fields than with the final visual acuity. In patients with type 3 disease, there was a wider preservation of fibers with a better visual acuity outcome compared with patients with type 1 disease in which there was diffuse reduction of nerve fiber thickness with patchy gain of function of a few fibers and intermediate visual acuity. Patients with type 2 disease had a mixed pattern of fiber reduction and intermediate visual acuity.

"We think that the recovery of visual acuity and visual function may have different physiopathologic substrates ranging from preservation of myelinated axons from death (type 3) to the regaining of function in surviving axons (type 1). The recovery of function in surviving axons may be the result of remyelination," he said.

"A limitation of the study is that we have no data on the RNFL thickness from presymptomatic and acute stages of the disease. Future investigations may confirm the prognostic value of the OCT to detect the favorable course of the disease," Dr. Barboni said.