Introduction

The term ametropia, in Greek means an irregular eye. It is taken to mean an abnormal refractive condition and may be contrasted with emmetropia, the normal refractive state. Although emmetropes need no corrective lenses to see clearly at near and far distances, they may suffer from other visual disorders not directly related to focusing, but associated, for example, with the control of eye position or eye movements.

In general, visual anomalies occur when some part of the visual system departs from optimal function. This can occur in a very large, although finite, set of ways. For example, the visuomotor control of the two eyes may be inadequate, causing uncoordinated eye movements; the light gathering and focusing elements, such as the cornea and lens, may not have the proper curvature or the eyeball size itself may not be matched to the focusing mechanism, so that light is not brought to a sharp focus at the retina; biochemical inadequacies may cause light transduction at the photoreceptors (rods and cones) to fail so that electrical stimulation of the neural pathways in the brain is compromised, leading to a loss in detection of fine detail or colors or objects in peripheral vision. Furthermore, the neural projection system from retina to brain may be inadequate, thereby leading to problems in object localization. Defects in virtually any portion of the system can and do occur to create thousands of visual disorders.

Here, eleven very common visual disorders are introduced and described, and clues to the underlying causal mechanisms also are traced.