Diffusion tensor imaging for in vivo detection of degenerated optic radiation

Glaucoma-inducing retinal ganglion cell degeneration alters diurnal rhythm of key molecular components of the central clock and locomotor activity in mice

Glaucoma is a chronic optic neuropathy characterized by the progressive degeneration of retinal ganglion cells (RGC). These cells play a crucial role in transmitting visual and non-visual information to brain regions, including the suprachiasmatic nucleus (SCN), responsible for synchronizing biological rhythms. To understand how glaucoma affects circadian rhythm synchronization, we investigated potential changes in the molecular clock machinery in the SCN. We found that the progressive increase in intraocular pressure (IOP) negatively correlated with spontaneous locomotor activity (SLA). Transcriptome analysis revealed significant alterations in the SCN of glaucomatous mice, including downregulation of genes associated with circadian rhythms. In fact, we showed a loss of diurnal oscillation in the expression of vasoactive intestinal peptide (Vip), its receptor (Vipr2), and period 1 (Per1) in the SCN of glaucomatous mice. These findings were supported by the 7-h phase shift in the peak expression of arginine vasopressin (Avp) in the SCN of mice with glaucoma. Despite maintaining a 24-h period under both light/dark (LD) and constant dark (DD) conditions, glaucomatous mice exhibited altered SLA rhythms, characterized by decreased amplitude. Taken altogether, our findings provide evidence of how glaucoma affects the regulation of the central circadian clock and its consequence on the regulation of circadian rhythms.

Transneuronal Degeneration in the Brain During Glaucoma

The death of retinal ganglion cells (RGCs) is a key factor in the pathophysiology of all types of glaucoma, but the mechanism of pathogenesis of glaucoma remains unclear. RGCs are a group of central nervous system (CNS) neurons whose soma are in the inner retina. The axons of RGCs form the optic nerve and converge at the optic chiasma; from there, they project to the visual cortex via the lateral geniculate nucleus (LGN). In recent years, there has been increasing interest in the dysfunction and death of CNS and retinal neurons caused by transneuronal degeneration of RGCs, and the view that glaucoma is a widespread neurodegenerative disease involving CNS damage appears more and more frequently in the literature. In this review, we summarize the current knowledge of LGN and visual cortex neuron damage in glaucoma and possible mechanisms behind the damage. This review presents an updated and expanded view of neuronal damage in glaucoma, and reveals new and potential targets for neuroprotection and treatment.

Diffusion tensor imaging of the visual pathway in dogs with primary angle-closure glaucoma

OBJECTIVE

To describe measurements of in vivo structures of the visual pathway beyond the retina and optic nerve head associated with canine primary angle-closure glaucoma (PACG).

METHODS

A prospective pilot study was conducted using magnetic resonance diffusion tensor imaging (DTI) to obtain quantitative measures of the optic nerve, chiasm, tract, and lateral geniculate nucleus (LGN) in dogs with and without PACG. 3-Tesla DTI was performed on six affected dogs and five breed, age- and sex-matched controls. DTI indices of the optic nerve, optic chiasm, optic tracts, and LGN were compared between normal, unilateral PACG, and bilateral PACG groups. Intra-class correlation coefficient (ICC) was calculated to assess intra-observer reliability.

RESULTS

Quantitative measurements of the optic nerve, optic tract, optic chiasm, and LGN were obtained in all dogs. There was a trend for reduced fractional anisotropy (FA) associated with disease for all structures assessed. Compared to the same structure in normal dogs, FA, and radial diffusivity (RD) of the optic nerve was consistently higher in the unaffected eye in dogs with unilateral PACG. Intra-observer reliability was excellent for measurements of the optic nerve (ICC: 0.92), good for measurements of the optic tract (ICC: 0.89) and acceptable for measures of the optic chiasm (ICC: 0.71) and lateral geniculate nuclei (ICC: 0.76).

CONCLUSION

Diffusivity and anisotropy measures provide a quantifiable means to evaluate the visual pathway in dogs. DTI has potential to provide in vivo measures of axonal and myelin injury and transsynaptic degeneration in canine PACG.