Disease Etiology
The World Health Organization has described four levels of visual function (1):

• normal vision
• moderate visual impairment
• severe visual impairment
• blindness.

The literature refers to individuals with moderate or severe visual impairment as having low vision while those with moderate or severe visual impairment or blindness are referred to as having visual impairment.  

The main causes of visual impairment are uncorrected refractive  errors (vision problems that have