Achromatopsia, also known as rod monochromacy, is a medical syndrome that exhibits symptoms relating to five conditions, most notably monochromacy. Historically, the name referred to monochromacy in general, but now typically refers only to an autosomal recessive congenital color vision condition. The term is also used to describe cerebral achromatopsia, though monochromacy is usually the only common symptom. The conditions include: monochromatic color blindness, poor visual acuity, and day-blindness. The syndrome is also present in an incomplete form that exhibits milder symptoms, including residual color vision. Achromatopsia is estimated to affect 1 in 30,000 live births worldwide.

The five symptoms associated with achromatopsia are:^{[citation needed]}

The syndrome is typically first noticed in children around six months of age due to their photophobia 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 six to seven years of life – but remain near 20/200. Otherwise the syndrome is considered stationary and does not worsen with age.^{[citation needed]}

If the light level during testing is optimized, achromats may achieve corrected visual acuity of 20/100 to 20/150 at lower light levels, regardless of the absence of color.^{[citation needed]} The fundus of the eye appears completely normal.^{[citation needed]}

Achromatopsia can be classified as complete or incomplete. In general, symptoms of incomplete achromatopsia are attenuated versions of those of complete achromatopsia. Individuals with incomplete achromatopsia have reduced visual acuity with or without nystagmus or photophobia. Incomplete achromats show only partial impairment of cone cell function.^{[citation needed]}

Achromatopsia is sometimes called rod monochromacy (as opposed to blue cone monochromacy), as achromats exhibit a complete absence of cone cell activity via electroretinography in photopic lighting. There are multiple genetic causes of achromatopsia, including mutations in:

The hemeralopic aspect of achromatopsia can be diagnosed non-invasively using electroretinography. The response at low (scotopic) and median (mesopic) light levels will be normal but the response under high light level (photopic) conditions will be absent. The mesopic level is approximately a hundred times lower than the clinical level used for the typical high level electroretinogram. When as described; the condition is due to a saturation in the neural portion of the retina and not due to the absence of the photoreceptors per se.^{[citation needed]}

In general, the molecular pathomechanism of achromatopsia is either the inability to properly control or respond to altered levels of cGMP; particularly important in visual perception as its level controls the opening of cyclic nucleotide-gated ion channels (CNGs). Decreasing the concentration of cGMP results in closure of CNGs and resulting hyperpolarization and cessation of glutamate release. Native retinal CNGs are composed of 2 α- and 2 β-subunits, which are CNGA3 and CNGB3, respectively, in cone cells. When expressed alone, CNGB3 cannot produce functional channels, whereas this is not the case for CNGA3. Coassembly of CNGA3 and CNGB3 produces channels with altered membrane expression, ion permeability (Na⁺ vs. K⁺ and Ca²⁺), relative efficacy of cAMP/cGMP activation, decreased outward rectification, current flickering, and sensitivity to block by L-cis-diltiazem.^{[citation needed]}