Axonal Energy Crisis and Calcium Phosphate Dysregulation as Pathogenesis of Optic Disc Drusen

Soft tissue ectopic calcification is due to abnormal accumulation of calcium and phosphate outside the bone. It is the result of the failure of a complex, active, and highly regulated process - much of which is still not well understood. Some of our understanding of ectopic calcification come from studies on diseases such as atherosclerosis, aortic valve disease and kidney stone disease. In the eye, the most common causes of visual defects due to ectopic calcification include optic disc drusen (ODD) and age-related macular degeneration (AMD). In ODD, ectopic calcification occurs only in the most anterior, unmyelinated portion of optic nerve - the sole output of the eye and particularly susceptible to hypoxic and metabolic stress. In this article, we review the effects of hypoxia on mitochondrial function and calcium regulation and delineate the key processes likely involved in ectopic calcification. We propose a working hypothesis for ODD centering on the role of hypoxia-induced calcium and phosphate overload, mitochondrial dysfunction and osteogenic differentiation, leading to hydroxyapatite deposition and biomineralization.