Achromatopsia is a hereditary visual disorder which is characterised by the absence of colour vision. It must not be confused with colour vision deficiency (red-green blindness or blue-yellow deficiency).
Patients with achromatopsia have only rods (for black and white vision) and no functioning cones (for colour vision). This means they only see dark and bright contrasts. They are very sensitive to light due to the lack of cones and often have reduced visual acuity.
One of the main symptoms of achromatopsia are fast ‘to-and-fro’ movements of the eyes (nystagmus). The eye tries to protect the area of sharpest vision (yellow spot), which contains only rods, from excessive light exposure. Since the density of rods present in the centre is below that of cones, which are responsible for daylight vision, patients also have a very poor visual acuity of often less than 0.1.
In addition, since rod cells handle vision in low light (twilight/night vision), patients also experience high glare sensitivity (photopsia).