Optic Nerve Size May Reflect Pattern of Axonal Degeneration in Leber’s Hereditary Optic Neuropathy

Abstract

Purpose: : To perform a morphometric analysis of four optic nerves from two related Leber’s Hereditary Optic Neuropathy (LHON) patients and study the pattern of degeneration in relationship to different states of disease progression.

Methods: : Optic nerves from two LHON (11778/ND4 mtDNA mutation) affected brothers with moderate and severe optic atrophy respectively were studied and compared. Both patients had been followed as part of a large systematic prospective ophthalmological investigation of LHON. The optic nerves were fixed, plastic embedded, and stained with p-phenylenediamine to allow for axon counting. For total axon counts, a nerve cross section was partitioned into five different regions. Photos were taken at 1000X magnification and axons were counted by hand. Total counts and axon densities were calculated. Total cross sectional areas were also calculated for the optic nerves using the "Spot II Advanced" software package.

Results: : Two different degeneration patterns were observed. One brother (patient 1) had a focal area of degeneration in the temporal zone of both eyes with sparing of the periphery, while the other brother (patient 2) had a diffuse pattern of degeneration. Total axon counts per optic nerve for patient 1 were 333,192 (OD) and 269,319 (OS) while patient 2 had counts of 22,889 (OD) and 22,376 (OS). The cross sectional area of the optic nerves were 5.79(OD) and 7.97 (OS) mm² for patient 1, and 3.29(OD) and 3.35(OS) mm² for patient 2.

Conclusions: : This study provides morphometric evidence of the relationship between different stages of LHON and optic nerve size. Since both patients showed extensive levels of degeneration well below the approximately 1.0 million axon per optic nerve average for this age group (Johnson, et. al; Age 1987), it is not likely that the entire difference in nerve size can be attributed to the level of degeneration. Hence, even taking into account that increased degeneration leads to more atrophic shrinkage of the optic nerve, a smaller, and thus more crowded, optic nerve may be associated with more severe and diffuse patterns of degeneration.

Keywords: optic nerve • pathology: human