ON is a rare condition. US studies estimate the annual incidence to be 6.4 per 100,000 in population. The occurrence of ON tends to be the highest in populations located in higher latitudes (in the northern US and Western Europe), and is the lowest in regions closer to the equator. ON is more common in women (predominance ratio of 3:1), and develops in most patients between the ages of 20 and 45. Additionally, ON typically occurs more frequently in Caucasians than African Americans. It has been reported that whites with northern European descent develop ON eight times more frequently than blacks and Asians.