Achromatopsia

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Achromatopsia is a disorder that causes problems with color vision. The retina, which is the light-sensitive tissue at the back of the eye, contains two types of light receptor cells called rods and cones. Rods provide vision in low light (night vision), while cones provide vision in bright light (daytime vision). Cones are also responsible for color vision.

There are two types of achromatopsia: complete and incomplete. People with complete achromatopsia cannot perceive any color; they see only black, white, and shades of gray. People with incomplete achromatopsia can see some color.

People with achromatopsia may have additional problems with their vision, including an increased sensitivity to light and glare (photophobia), involuntary back-and-forth eye movements (nystagmus), and significantly reduced sharpness of vision (low visual acuity). Affected individuals may also have a small visual blind spot (scotoma), farsightedness (hyperopia), or nearsightedness (myopia).

Achromatopsia is different from the more common forms of color vision deficiency (also called color blindness), in which people can perceive color but have difficulty distinguishing between certain colors, such as red and green. One form of color blindness, called blue cone monochromacy (BCM), is sometimes considered a form of incomplete achromatopsia.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Variants (also called mutations) in several different genes can cause achromatopsia. Most of the genes that are associated with achromatopsia are known to play important roles in the normal functioning of cones. Variants in the CNGA3 and CNGB3 genes are the most common causes of achromatopsia.

The CNGA3 and CNGB3 genes provide instructions for making different parts (subunits) of the cyclic nucleotide-gated (CNG) channel. These channels are found exclusively in cones and play an important role in a process called phototransduction. During phototransduction, rods and cones translate light into electrical signals. To help with this process, CNG channels transport positively charged atoms (ions) across the cell membrane and into cells. This movement of ions helps generate an electrical signal that is sent to the brain for interpretation.

Some of the variants in the CNGA3 and CNGB3 genes that are associated with achromatopsia cause cells to produce fewer CNG channel subunits, while other variants cause cells to produce subunits that do not function properly. These changes impair the function of the CNG channels, which disrupts the flow of ions across cell membranes and impairs the cell's ability to generate electrical signals.

Some people with achromatopsia do not have one of the gene variants known to cause the condition. In these individuals, the cause of the disorder is unknown.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell must have a variant to cause the disorder. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

Achromatopsia affects approximately 1 in 30,000 people worldwide. Complete achromatopsia is more common than incomplete achromatopsia.

Complete achromatopsia occurs more frequently among Pingelapese islanders, who live in parts of Micronesia in the western Pacific Ocean. Between 4 and 10 percent of people in this population have a total absence of color vision.

Reported clinical features and what the terms mean

The following findings are associated with this condition in Orphanet. They are not a checklist for diagnosing yourself, and they do not all occur in every affected person. Some are examination, imaging or laboratory findings that cannot be recognised at home.

The frequency labels describe how often a finding was reported among people with the condition in the source. They do not give the chance that a person with that symptom has the condition. Definitions below reproduce HPO terminology; they explain the term, not the likely severity in an individual.

Abnormality of refraction · Very frequent (99-80%)

An abnormality in the process of focusing of light by the eye in order to produce a sharp image on the retina.

Color vision defect · Very frequent (99-80%)

An anomaly in the ability to discriminate between or recognize colors.

Monochromacy · Very frequent (99-80%)

Complete color blindness, a complete inability to distinguish colors. Affected persons cannot perceive colors, but only shades of gray.

Pendular nystagmus · Very frequent (99-80%)

Rhythmic, involuntary sinusoidal oscillations of one or both eyes. The waveform of pendular nystagmus may occur in any direction.

Photophobia · Very frequent (99-80%)

Excessive sensitivity to light with the sensation of discomfort or pain in the eyes due to exposure to bright light.

Undetectable light-adapted electroretinogram · Very frequent (99-80%)

No detectable response to the light-adapted 3.0 ERG (single-flash cone response). This type of ERG measures responses of the cone system; a-waves arise from cone photoreceptors and cone off-bipolar cells; the b-wave comes from On- and Off-cone bipolar cells.

Color vision test abnormality · Very frequent (99-80%)

Inner retinal layer loss on macular OCT · Very frequent (99-80%)

Other findings in the same source

From: Orphanet

Additional reported features include Absent foveal reflex (Frequent (79-30%)); Central scotoma (Frequent (79-30%)); Hypermetropia (Frequent (79-30%)); Hypoplasia of the fovea (Frequent (79-30%)); Myopia (Frequent (79-30%)); Reduced visual acuity (Frequent (79-30%)); Abnormal macular morphology (Occasional (29-5%)); Abnormal pupillary light reflex (Occasional (29-5%)); Attenuation of retinal blood vessels (Occasional (29-5%)); Eccentric visual fixation (Occasional (29-5%)). This is a selected summary, not a complete description of the condition.

Which doctor should you see?

The suggested department for discussing Achromatopsia is Ophthalmology, with a ophthalmologist as the relevant type of clinician. Ophthalmologist; paediatric services for children as appropriate.

Additional services that may be relevant, depending on the findings, include: Clinical Genetics.

This is an editorial referral starting point. The appropriate clinic depends on the person’s age, symptoms, previous diagnosis and local services. The first clinician can decide whether another specialty or a team is needed; a department label does not confirm the diagnosis.

How to prepare for an assessment

Bring a short timeline of the main symptoms: when they first appeared, whether they are constant or episodic, what seems to change them, and how they affect daily activities. Include previous reports, discharge summaries, current medicines and supplements, allergies, and any relevant family history. A dated record is more useful than trying to match every feature in an online article.

Ask the clinician what is already established and what remains uncertain. If a test is suggested, ask what question it answers, what its limitations are and how the result would change the next step. The information here is not an instruction to arrange every possible test. In children, bring growth, developmental and school information if it is relevant to the concern.

• Which part of the eye or visual pathway is affected?
• What change in vision requires immediate contact with the eye service?
• What are the aims and alternatives of any proposed eye treatment?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Achromatopsia. That gap does not mean that treatment is unavailable. A clinician needs to establish the diagnosis and review current guidance before recommending medicines, procedures, rehabilitation or other support.

Before leaving the appointment, clarify the next review date, who will communicate results, and whom to contact if the situation changes. Discuss difficulties with sleep, work, school, mobility, eating or emotional wellbeing when these are relevant. Practical support may require coordination between the treating clinician and other services.

The collected references do not establish a complete prevention or long-term outlook section for this entry. Missing information should not be interpreted as proof that prevention is impossible or that a particular outcome is inevitable. Ask what is known for the exact subtype, stage and personal circumstances, and which uncertainties remain.

When to seek emergency help

Severe breathing difficulty, collapse, new stroke-like symptoms, a seizure that is prolonged or repeated without recovery, uncontrolled major bleeding, or an immediate risk of self-harm require emergency help. In India, call 112 or reach the nearest emergency department. This is a general, non-exhaustive warning list; it is not a condition-specific triage tool.