Achromatopsia (ACHM), also known as total color blindness, is a medical syndrome that exhibits symptoms relating to at least five conditions. The term may refer to acquired conditions such as cerebral achromatopsia, also known as color agnosia, but it typically refers to an autosomal recessive congenital color vision condition, the inability to perceive color and to achieve satisfactory visual acuity at high light levels (typically exterior daylight). The syndrome is also present in an incomplete form which is more properly defined as dyschromatopsia. It is estimated to affect 1 in 40,000 live births worldwide.
There is some discussion as to whether achromats can see color or not. As illustrated in The Island of the Colorblind by Oliver Sacks, some achromats cannot see color, only black, white, and shades of grey. With five different genes currently known to cause similar symptoms, it may be that some do see marginal levels of color differentiation due to different gene characteristics. With such small sample sizes and low response rates, it is difficult to accurately diagnose the ‘typical achromatic conditions’. If the light level during testing is optimized for them, they may achieve corrected visual acuity of 20/100 to 20/150 at lower light levels, regardless of the absence of color. One common trait is hemeralopia or blindness in full sun. In patients with achromatopsia, the cone system and fibres carrying color information remain intact. This indicates that the mechanism used to construct colors is defective.
Total color blindness can be classified as:
Acquired achromatopsia (Cerebral achromatopsia)
Complete typical achromatopsia
Incomplete atypical achromatopsia or incomplete atypical dyschromatopsia
Achromatopsia–The complete lack of the perception of color in a subject, seeing only in black, white, and shades of grey.
Amblyopia–Defined conceptually by Duke-Elder (1973) as a monocular acuity deficit which is not due to refractive error or any organic abnormality. A neural condition. Poor spatial performance of the precision optical servomechanism of the eyes at nominal illumination levels without any morphological cause. One form of lazy eye.
Hemeralopia–Reduced visual capacity in bright light. Colloquially, day-blindness.
Nystagmus–This term is used variously to describe both normal and pathological conditions related to the oculomotor system. In the current context, it is a pathological condition involving an uncontrolled oscillatory movement of the eyes during which the amplitude of oscillation is quite noticeable and the frequency of the oscillation tends to be quite low.
Photophobia–The avoidance of bright light by those suffering from hemeralopia.
Signs and symptoms
The syndrome is frequently noticed first in children around six months of age by their photophobic activity and/or their nystagmus. The nystagmus becomes less noticeable with age but the other symptoms of the syndrome become more relevant as school age approaches. Visual acuity and stability of the eye motions generally improve during the first 6–7 years of life (but remain near 20/200). The congenital forms of the condition are considered stationary and do not worsen with age.
The five symptoms associated with achromatopsia/dyschromatopsia are:
Amblyopia (reduced visual acuity)
Hemeralopia (with the subject exhibiting photophobia)
Iris operating abnormalities
The syndrome of achromatopsia/dyschromatopsia is poorly described in current medical and neuro-ophthalmological texts. It became a common term following the popular book by the neuroscientist Oliver Sacks, “The Island of the Colorblind” in 1997. Up to that time most color-blind subjects were described as achromats or achromatopes. Those with a lesser degree of color perception abnormality were described as either protanopes, deuteranopes or tetartanopes (historically tritanopes).
Achromatopsia has also been called rod monochromacy and total congenital color blindness. Individuals with the congenital form of this condition show complete absence of cone cell activity via electroretinography at high light levels. There are at least four genetic causes of congenital ACHM, two of which involve cyclic nucleotide-gated ion channels (ACHM2/ACHM3), a third involves the cone photoreceptor transducin (GNAT2, ACHM4), and the last remains unknown.
Aside from a complete inability to see color, individuals with complete achromatopsia have a number of other ophthalmologic aberrations. Included among these aberrations are greatly decreased visual acuity (