Leber's hereditary optic neuropathy differentially affects smaller axons in the optic nerve

Leber's hereditary optic neuropathy (LHON), though known to be due to 1 of 3 pathogenic mtDNA point mutations (nucleotide positions 11,778, 3460, and 14,484), usually manifests itself acutely in young adulthood with a stereotypical presentation of dyschromatopsia, loss of central vision, and loss of the papillomacular bundle nerve fiber layer. Histopathologic investigations have demonstrated devastating losses of axons with relative sparing of the most peripherally placed fibers in the optic nerves. This study was designed to morphometrically investigate the nerve fiber spectrum from ultrastructural studies of optic nerves obtained from 2 patients with LHON.

Two cases of LHON were molecularly characterized and the optic nerves from these cases studied by light microscopy and electron microscopy. Montages were made of electron micrographs cut orthogonal to fibers obtained from the periphery of each optic nerve, and these were then used for the measurement of each axon (short and long axis) and its myelin sheath. From this, a spectrum of nerve fiber layer was generated, yielding axon caliber profiles that could be compared between optic nerves.

The total depletion of optic nerve fiber population in the 2 cases of LHON varied from 95% to 99%. Those fibers that were spared were limited to the peripheral optic nerve. The nerve fiber layer spectra of these remaining fibers showed a marked diminution of the first peak of axons of less than 1 micron in diameter, with relative emphasis of a second peak of axons of about 2 microns in diameter. In comparison to normal controls, this reflected a preferential loss of the smallest axons corresponding to the P-cell population.

The clinical features of dyschromatopsia and central scotoma (with preservation of pupils) in LHON suggests the selective loss of the P-cell population known to subserve these (and not pupil) functions. This also correlates well with the fundus findings of early losses of the papillomacular bundle. The present study extends these findings to demonstrate a relative preservation of the M-cells in the optic nerve as reflected by the nerve fiber spectral profile. This selective loss of smaller fibers and their corresponding smaller retinal ganglion cells may, in addition to explaining the clinical features in LHON, provide valuable insights as to the exact pathophysiologic mechanisms by which mitochondrial impairment may induce apoptosis in vulnerable neurons.