[[File:Bilateral222.jpg|Achromatopsia vs normal vision|thumb]]
[[File:Bilateral222.jpg|Achromatopsia vs normal vision|thumb]]
'''Achromatopsia''', 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]], but it typically refers to an [[autosomal recessive]] congenital [[color blindness|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 30,000 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 cell|cone system]] and fibres carrying color information remain intact. This indicates that the mechanism used to construct colors is defective.
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'''Achromatopsia''' (also known as '''total color blindness''') is a rare, congenital [[vision disorder]] characterized by the complete or nearly complete absence of [[color vision]]. It is typically inherited in an [[autosomal recessive]] manner and is associated with significant visual impairment, including markedly reduced [[visual acuity]] and extreme [[light sensitivity]] ([[photophobia]]). Although the term may also refer to acquired forms such as '''[[cerebral achromatopsia]]''', it most commonly describes a congenital condition involving dysfunction of the retinal [[cone cells]].
== Signs and symptoms ==
== Classification ==
The syndrome is frequently noticed first in children around six months of age by their photophobic activity 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).
Achromatopsia exists in two primary forms:
The congenital forms of the condition are considered stationary and do not worsen with age.
* '''Complete achromatopsia''': Also known as '''rod monochromacy''', this form results in total color blindness and is associated with severe visual impairment in bright lighting conditions.
* '''Incomplete achromatopsia''' (or '''dyschromatopsia'''): In this form, some residual cone function may remain, allowing limited color discrimination and better visual acuity.
The five symptoms associated with achromatopsia or dyschromatopsia are:
== Epidemiology ==
# Achromatopsia
Achromatopsia is estimated to affect approximately 1 in 30,000 live births worldwide. The prevalence may vary slightly by geographic and ethnic population. Both sexes are equally affected due to its autosomal recessive inheritance pattern.
# [[Amblyopia]] (reduced visual acuity)
# [[Hemeralopia]] (with the subject exhibiting photophobia)
# [[Pathologic nystagmus|Nystagmus]]
# [[Iris (anatomy)|Iris]] operating abnormalities
The syndrome of achromatopsia or dyschromatopsia is poorly described in current medical and neuro-ophthalmological texts. It became a common term following the release of neuroscientist Oliver Sacks' book, ''[[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 [[locus (genetics)|genetic causes]] of congenital achromatopsia, two of which involve [[cyclic nucleotide-gated ion channel]]s ([[#ACHM2|ACHM2]], [[#ACHM3|ACHM3]]), a third involves the cone [[photoreceptor cell|photoreceptor]] transducin (GNAT2, ACHM4), and the last remains unknown.
=== Complete achromatopsia ===
Aside from a complete inability to see color, individuals with complete achromatopsia have a number of other [[ophthalmology|ophthalmologic]] aberrations. Included among these [[optical aberration]]s are greatly decreased [[visual acuity]] (<0.1 or 20/200) in daylight, [[hemeralopia]], [[Pathologic nystagmus|nystagmus]], and severe [[photophobia]]. The [[fundus (eye)|fundus]] of the eye appears completely normal.
=== Incomplete achromatopsia ===
In general, symptoms of incomplete achromatopsia (dyschromatopsia) are similar to those of complete achromatopsia except in a diminished form. Individuals with incomplete achromatopsia have reduced visual acuity with or without nystagmus or photophobia. Furthermore, these individuals show only partial impairment of cone cell function but again have retained rod cell function.
== Cause ==
=== Acquired ===
Acquired achromatopsia or dyschromatopsia is a condition associated with damage to the diencephalon—primarily the [[thalamus]] of the mid brain—or the [[cerebral cortex]] (the new brain), specifically the fourth visual association area, V4 which receives information from the parvocellular pathway involved in colour processing.
Thalamic achromatopsia or dyschromatopsia is caused by damage to the thalamus; it is most frequently caused by tumor growth since the thalamus is well protected from external damage.
[[Cerebral achromatopsia]] is a form of acquired [[color blindness]] that is caused by damage to the [[cerebral cortex]] of the brain, rather than abnormalities in the cells of the eye's [[retina]]. It is most frequently caused by physical trauma, hemorrhage or tumor tissue growth.
=== Congenital ===
== Genetic Causes ==
The known causes of the congenital forms of achromatopsia are all due to malfunction of the [[Photoreceptor cell#Phototransduction|retinal phototransduction pathway]]. Specifically, this form of achromatopsia seems to result from the inability of [[cone cell]]s to properly respond to light input by [[membrane potential|hyperpolarizing]]. Known genetic causes of this are mutations in the cone cell [[cyclic nucleotide-gated ion channel]]s CNGA3 (ACHM2) and CNGB3 (ACHM3) as well as the cone cell [[transducin]], GNAT2 (ACHM4).
At least five genes have been identified in association with achromatopsia, including:
* '''[[CNGA3]]''' – encoding the alpha subunit of the cyclic nucleotide-gated channel
* '''[[CNGB3]]''' – encoding the beta subunit
* '''[[GNAT2]]'''
* '''[[PDE6C]]'''
* '''[[PDE6H]]'''
A fourth genetic cause (ACHM5, OMIM 613093) was discovered in 2009.{{sfn|Thiadens|Den Hollander|Roosing|Nabuurs|2009|pp=240–247}} It is a mutation of gene PDE6C, located on chromosome locus 10, 10q24. It is estimated that less than two percent of achromatopsias are caused by a mutation in this gene.
Mutations in these genes lead to impaired or absent function of retinal cone photoreceptors, which are responsible for color detection and high acuity vision in bright light.
== Pathophysiology ==
== Pathophysiology ==
The hemeralopic aspect of achromatopsia can be diagnosed non-invasively using electroretinography. The response at low (scotopic) and median (mesotopic) light levels will be normal but the response under high light level (photopic) conditions will be absent. The mesotopic level is approximately a 100 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.
Achromatopsia results from dysfunctional or absent cone cell activity in the [[retina]]. Unlike normal vision, which integrates input from three types of cones (L-, M-, and S-cones), individuals with achromatopsia rely exclusively on [[rod cells]] for vision. Rods function in low light and do not contribute to color perception, explaining the grayscale vision reported by affected individuals.
In general, the molecular pathomechanism of achromatopsia is either the inability to properly control or respond to altered levels of [[cyclic guanosine monophosphate|cGMP]]; particularly important in [[visual perception]] as its level controls the opening of [[cyclic nucleotide-gated ion channel]]s (CNGs). Decreasing the concentration of cGMP results in closure of CNGs and resulting [[membrane potential|hyperpolarization]] and cessation of [[glutamate]] [[Chemical synapse#Signaling across chemical synapses|release]].
Native retinal CNGs are composed of 2 α- and 2 β-subunits, which are CNGA3 and CNGB3, respectively, in [[cone cell]]s. 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 ([[sodium|Na<sup>+</sup>]] vs. [[potassium|K<sup>+</sup>]] and [[calcium|Ca<sup>2+</sup>]]), relative efficacy of cAMP/cGMP activation, decreased outward [[Image rectification|rectification]], current flickering, and sensitivity to block by [[diltiazem|L-cis-diltiazem]].
Mutations tend to result in the loss of CNGB3 function or gain of function (often increased affinity for cGMP) of CNGA3. cGMP levels are controlled by the activity of the [[cone cell]] [[transducin]], GNAT2. Mutations in GNAT2 tend to result in a truncated and, presumably, non-functional protein, thereby preventing alteration of cGMP levels by [[photon]]s. There is a positive correlation between the severity of mutations in these proteins and the completeness of the achromatopsia [[phenotype]].
Molecular diagnosis can be established by identification of biallelic variants in the causative genes. Molecular genetic testing approaches used in achromatopsia can include targeted analysis for the common CNGB3 variant c.1148delC (p.Thr383IlefsTer13), use of a multigenerational panel, or comprehensive genomic testing.
Interestingly, in congenital achromatopsia, the anatomical structures of cones and associated neural pathways may remain intact, suggesting a defect in the molecular or biochemical mechanisms necessary for color processing.
=== ACHM2 ===
== Clinical Presentation ==
While some mutations in CNGA3 result in truncated and, presumably, non-functional channels this is largely not the case. While few mutations have received in-depth study, at least one mutation does result in functional channels. Curiously, this mutation, T369S, produces profound alterations when expressed without CNGB3. One such alteration is decreased affinity for [[Cyclic guanosine monophosphate]]. Others include the introduction of a sub-conductance, altered single-channel gating kinetics, and increased [[calcium]] permeability.
Typical signs and symptoms include:
* '''Total color blindness''' – vision limited to black, white, and shades of grey
* '''Reduced visual acuity''' – usually in the range of 20/100 to 20/150 under optimal lighting conditions
* '''Photophobia''' – severe sensitivity to light, particularly daylight
* '''Nystagmus''' – involuntary eye movements often present from infancy
* '''Hemeralopia''' – difficulty seeing in bright light or sunlight
When mutant T369S channels coassemble with CNGB3, however, the only remaining aberration is increased calcium permeability.{{sfn|Tränkner|Jägle|Kohl|Apfelstedt-Sylla|2004|pp=138–147}} While it is not immediately clear how this increase in Ca<sup>2+</sup> leads to achromatopsia, one hypothesis is that this increased current decreases the signal-to-noise ratio. Other characterized mutations, such as Y181C and the other S1 region mutations, result in decreased current density due to an inability of the channel to traffic to the surface.{{sfn|Patel|Bartoli|Fandino||2005|pp=2282–2290}} Such loss of function will undoubtedly negate the [[cone cell]]'s ability to respond to visual input and produce achromatopsia. At least one other missense mutation outside of the S1 region, T224R, also leads to loss of function.{{sfn|Tränkner|Jägle|Kohl|Apfelstedt-Sylla|2004|pp=138–147}}
=== Variability in Color Perception ===
Some individuals with incomplete achromatopsia may report vague or inconsistent color differentiation. The variability may reflect differences in gene mutations, with certain genotypes allowing residual cone function. Due to the rarity of the condition and the small number of confirmed cases, it remains challenging to define a “typical” achromat experience conclusively.
=== ACHM3 ===
== Diagnosis ==
While very few mutations in CNGB3 have been characterized, the vast majority of them result in truncated channels that are presumably non-functional. This will largely result in [[haploinsufficiency]], though in some cases the truncated proteins may be able to coassemble with wild-type channels in a [[dominant negative]] fashion. The most prevalent ACHM3 mutation, T383IfsX12, results in a non-functional truncated protein that does not properly traffic to the [[cell membrane]].{{sfn|Peng|Rich|Varnum||2003|pp=34533–34540}}{{sfn|Bright|Brown|Varnum||2005|pp=1141–1150}}
Diagnosis is based on clinical features, supported by:
* '''Electroretinography (ERG)''' – shows absent or severely reduced cone response with preserved rod function
The three missense mutations that have received further study show a number of aberrant properties, with one underlying theme. The R403Q mutation, which lies in the pore region of the channel, results in an increase in outward current rectification, versus the largely linear current-voltage relationship of wild-type channels, concomitant with an increase in cGMP affinity.{{sfn|Bright|Brown|Varnum||2005|pp=1141–1150}} The other mutations show either increased (S435F) or decreased (F525N) surface expression but also with increased affinity for cAMP and cGMP.{{sfn|Peng|Rich|Varnum||2003|pp=34533–34540}}{{sfn|Bright|Brown|Varnum||2005|pp=1141–1150}} It is the increased affinity for cGMP and cAMP in these mutants that is likely the disorder-causing change. Such increased affinity will result in channels that are insensitive to the slight concentration changes of cGMP due to light input into the retina.
* '''Color vision tests''' – such as the Ishihara plates, typically indicate complete color blindness
* '''Genetic testing''' – confirms mutations in associated genes
=== ACHM4 ===
* '''Optical coherence tomography (OCT)''' – may reveal thinning or structural abnormalities of the cone-rich foveal region
Upon activation by light, [[cone opsin]] causes the exchange of GDP for GTP in the guanine nucleotide binding protein ([[G-protein]]) α-[[transducin]]g activity polypeptide 2 (GNAT2). This causes the release of the activated α-subunit from the inhibitory β/γ-subunits. This α-subunit then activates a [[phosphodiesterase]] that catalyzes the conversion of cGMP to GMP, thereby reducing current through CNG3 channels.
As this process is absolutely vital for proper color processing it is not surprising that mutations in GNAT2 lead to achromatopsia. The known mutations in this gene, all result in truncated proteins. Presumably, then, these proteins are non-functional and, consequently, cone opsin that has been activated by light does not lead to altered cGMP levels or [[photoreceptor protein|photoreceptor]] [[membrane potential|membrane hyperpolarization]].
== Management ==
== Management ==
There is generally no treatment to cure achromatopsia. However, dark red or plum colored filters are very helpful in controlling light sensitivity.{{sfn|Corn|Erin|2010|p=233}}
There is currently no cure for achromatopsia. Management focuses on symptom relief and vision support:
* '''Tinted lenses or red contact lenses''' – reduce light sensitivity
Since 2003, there is a cybernetic device called [[eyeborg]] that allows people to perceive color through sound waves.{{sfn|Ronchi|2009|p=319}} Achromatopsic artist [[Neil Harbisson]] was the first to use such a device in early 2004, the eyeborg allowed him to start painting in color by memorizing the sound of each color.{{sfn|Pearlman|2015|pp=84–90}}
* '''Low vision aids''' – magnifiers, high-contrast text, and digital devices
* '''Avoidance of bright light''' – use of hats, visors, or indoor filters
Moreover, there is some research on gene therapy for animals with achromatopsia, with positive results on mice and young dogs, but less effectiveness on older dogs. However, no experiments have been made on humans. There are many challenges to conducting [[gene therapy for color blindness]] on humans.
* '''Genetic counseling''' – recommended for affected families
== Epidemiology ==
Achromatopsia is a relatively uncommon disorder, with a prevalence of 1 in 30,000 people.{{sfn|Thiadens|2011|p=59}}
However, on the small [[Federated States of Micronesia|Micronesia]]n atoll of [[Pingelap]], approximately five percent of the atoll's 3,000 inhabitants are afflicted.{{sfn|Brody|Hussels|Brink|Torres|1970|pp=1253–1257}}{{sfn|Hussels|Morton|1972|pp=304–309}} This is the result of a [[population bottleneck]] caused by a typhoon and ensuing famine in the 1770s, which killed all but about twenty islanders, including one who was heterozygous for achromatopsia.
The people of this region have termed achromatopsia "maskun", which literally means "not see" in [[Pingelapese]].{{sfn|Morton|Hussels|Lew|Little|1972|pp=277–289}} This unusual population drew neurologist [[Oliver Sacks]] to the island for which he wrote his 1997 book, ''[[The Island of the Colorblind]]''.
== Terminology ==
{{term|Acquired achromatopsia}}
{{defn|[[Cerebral achromatopsia]]}}
{{term|Congenital or inherited achromatopsia}}
{{defn|no=1|Complete typical achromatopsia}}
{{defn|no=2|Incomplete atypical achromatopsia, or incomplete atypical [[Color blindness|dyschromatopsia]]}}
=== Related ===
{{term|Achromatopsia}}
{{defn|Complete lack of the perception of color in a subject, seeing only in black, white, and shades of grey. This is different from color agnosia, in which a person can perceive color—as measured by a matching task—but cannot recognize different colors.}}
{{term|[[Amblyopia]]}}
{{defn|Defined conceptually by Sir [[Stewart Duke-Elder]] in 1973 as a monocular acuity deficit which is not due to refractive error or any organic abnormality.{{sfn|Duke-Elder|Wybar|1976}} Poor spatial performance of the precision optical servomechanism of the eyes at nominal illumination levels without any morphological cause. One form of [[amblyopia|lazy eye]].}}
{{term|[[Hemeralopia]]}}
{{defn|Reduced visual capacity in bright light, i.e. day-blindness.}}
{{term|[[Nystagmus]]}}
{{defn|Term 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.}}
{{term|[[Photophobia]]}}
{{defn|Avoidance of bright light by those suffering from [[hemeralopia]].}}
== References ==
=== Footnotes ===
{{reflist|22em}}
=== Sources ===
'''Books'''
{{refbegin|30em}}
*{{cite book
|last = Duke-Elder
|first = S.
|authorlink1 = Stewart Duke-Elder
|last2 = Wybar
|first2 = K. C.
|date = 1976
|title = Ocular Motility and Strabismus
|volume = 6
|series = System of Ophthalmology
|location = London
|publisher = Kimpton
|isbn = 9780853137764
|ref = harv
}}
*{{cite book
|last = Corn
|first = A. N.
|last2 = Erin
|first2 = J. N.
|date = 2010
|title = Foundations of Low Vision: Clinical and Functional Perspectives
|location = Arlington
|publisher = [[American Foundation for the Blind|AFB Press]]
|isbn = 9780891288831
|ref = harv
}}
*{{cite book
|last = Ronchi
|first = A. M.
|date = 2009
|title = eCulture: Cultural Content in the Digital Age
|title = Genetic Etiology and Clinical Consequences of Cone Disorders
|publisher = [[Erasmus University Rotterdam]]
|isbn = 9789461690579
|ref = harv
}}
{{refend}}
'''Journals'''
{{refbegin|30em}}
*{{cite journal
|last = Bright
|first = S. R.
|last2 = Brown
|first2 = T. E.
|last3 = Varnum
|first3 = M. D.
|displayauthors = 1
|date = 2005
|title = Disease-associated mutations in CNGB3 produce gain of function alterations in cone cyclic nucleotide-gated channels
|journal = [[Mol. Vis.]]
|volume = 11
|pages = 1141–1150
|issn = 1090-0535
|pmid = 16379026
|ref = harv
}}
*{{cite journal
|last = Brody
|first = J. A.
|last2 = Hussels
|first2 = I. E.
|last3 = Brink
|first3 = E.
|last4 = Torres
|first4 = J.
|displayauthors = 1
|date = 1970
|title = Hereditary blindness among Pingelapese people of Eastern Caroline Islands
|journal = [[The Lancet|Lancet]]
|volume = 295
|issue = 7659
|pages = 1253–1257
|doi = 10.1016/S0140-6736(70)91740-X
|pmid = 4192495
|ref = harv
}}
*{{cite journal
|last = Hussels
|first = I. E.
|last2 = Morton
|first2 = N. E.
|displayauthors = 1
|date = 1972
|title = Pingelap and Mokil Atolls: achromatopsia
|journal = [[Am. J. Hum. Genet.]]
|volume = 24
|issue = 3
|pages = 304–309
|pmc = 1762260
|pmid = 4555088
|ref = harv
}}
*{{cite journal
|last = Morton
|first = N. E.
|last2 = Hussels
|first2 = I. E.
|last3 = Lew
|first3 = R.
|last4 = Little
|first4 = G. F.
|displayauthors = 1
|date = 1972
|title = Pingelap and Mokil Atolls: historical genetics
|journal = [[Am. J. Hum. Genet.]]
|volume = 24
|issue = 3
|pages = 277–289
|pmc = 1762283
|pmid = 4537352
|ref = harv
}}
*{{cite journal
|last = Patel
|first = K. A.
|last2 = Bartoli
|first2 = K. M.
|last3 = Fandino
|first3 = R. A.
|displayauthors = 1
|date = 2005
|title = Transmembrane S1 mutations in CNGA3 from achromatopsia 2 patients cause loss of function and impaired cellular trafficking of the cone CNG channel
|journal = [[Investig. Ophthalmol. Vis. Sci.]]
|volume = 46
|issue = 7
|pages = 2282–2290
|doi = 10.1167/iovs.05-0179
|pmid = 15980212
|ref = harv
}}
*{{cite journal
|last = Pearlman
|first = E.
|date = 2015
|title = I, Cyborg
|journal = [[PAJ (journal)|PAJ]]
|volume = 37
|issue = 2
|pages = 84–90
|doi = 10.1162/PAJJ_a_00264
|ref = harv
}}
*{{cite journal
|last = Peng
|first = C.
|last2 = Rich
|first2 = E. D.
|last3 = Varnum
|first3 = M. D.
|displayauthors = 1
|date = 2003
|title = Achromatopsia-associated mutation in the human cone photoreceptor cyclic nucleotide-gated channel CNGB3 subunit alters the ligand sensitivity and pore properties of heteromeric channels
|journal = [[J. Biol. Chem.]]
|volume = 278
|issue = 36
|pages = 34533–34540
|doi = 10.1074/jbc.M305102200
|pmid = 12815043
|ref = harv
}}
*{{cite journal
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|first = A. A. H. J.
|last2 = Den Hollander
|first2 = A. I.
|last3 = Roosing
|first3 = S.
|last4 = Nabuurs
|first4 = S. B.
|displayauthors = 1
|date = 2009
|title = Homozygosity mapping reveals PDE6C mutations in patients with early-onset cone photoreceptor disorders
|journal = [[Am. J. Hum. Genet.]]
|volume = 85
|issue = 2
|pages = 240–247
|doi = 10.1016/j.ajhg.2009.06.016
|pmc = 2725240
|pmid = 19615668
|ref = harv
}}
*{{cite journal
|last = Tränkner
|first = D.
|last2 = Jägle
|first2 = H.
|last3 = Kohl
|first3 = S.
|last4 = Apfelstedt-Sylla
|first4 = E.
|displayauthors = 1
|date = 2004
|title = Molecular basis of an inherited form of incomplete achromatopsia
|journal = [[J. Neurosci.]]
|volume = 24
|issue = 1
|pages = 138–147
|doi = 10.1523/JNEUROSCI.3883-03.2004
|pmid = 14715947
|ref = harv
}}
{{refend}}
== External links ==
== Research and Experimental Therapies ==
{{wikt}}
Gene therapy is under investigation as a potential treatment for certain genetic subtypes (e.g., CNGA3 and CNGB3 mutations). Early-phase clinical trials have shown promise in partially restoring cone function, although widespread clinical use remains under development.
{{div col|colwidth=20em}}
* [https://www.medicinenet.com/script/main/art.asp?articlekey=11286 Achromatopsia] at [[MedicineNet]]
* [https://www.merriam-webster.com/medical/achromatopsia Achromatopsia] at [[Merriam-Webster]]
* [https://www.ncbi.nlm.nih.gov/gtr/all/tests/?term=achromatopsia Achromatopsia] at [[National Center for Biotechnology Information|NCBI]]
* [https://www.sciencedirect.com/topics/neuroscience/achromatopsia Achromatopsia] at [[ScienceDirect]]
{{div end}}
{{Medical resources
== Cultural and Literary References ==
|ICD10 = {{ICD10|H53.5}}
Achromatopsia was brought to public attention by neurologist [[Oliver Sacks]] in his book, ''[[The Island of the Colorblind]]'', which describes the lives of individuals with the condition living on the Micronesian island of Pingelap, where the condition has an unusually high prevalence due to a genetic bottleneck.
Achromatopsia (also known as total color blindness) is a rare, congenital vision disorder characterized by the complete or nearly complete absence of color vision. It is typically inherited in an autosomal recessive manner and is associated with significant visual impairment, including markedly reduced visual acuity and extreme light sensitivity (photophobia). Although the term may also refer to acquired forms such as cerebral achromatopsia, it most commonly describes a congenital condition involving dysfunction of the retinal cone cells.
Complete achromatopsia: Also known as rod monochromacy, this form results in total color blindness and is associated with severe visual impairment in bright lighting conditions.
Incomplete achromatopsia (or dyschromatopsia): In this form, some residual cone function may remain, allowing limited color discrimination and better visual acuity.
Achromatopsia is estimated to affect approximately 1 in 30,000 live births worldwide. The prevalence may vary slightly by geographic and ethnic population. Both sexes are equally affected due to its autosomal recessive inheritance pattern.
Mutations in these genes lead to impaired or absent function of retinal cone photoreceptors, which are responsible for color detection and high acuity vision in bright light.
Achromatopsia results from dysfunctional or absent cone cell activity in the retina. Unlike normal vision, which integrates input from three types of cones (L-, M-, and S-cones), individuals with achromatopsia rely exclusively on rod cells for vision. Rods function in low light and do not contribute to color perception, explaining the grayscale vision reported by affected individuals.
Interestingly, in congenital achromatopsia, the anatomical structures of cones and associated neural pathways may remain intact, suggesting a defect in the molecular or biochemical mechanisms necessary for color processing.
Some individuals with incomplete achromatopsia may report vague or inconsistent color differentiation. The variability may reflect differences in gene mutations, with certain genotypes allowing residual cone function. Due to the rarity of the condition and the small number of confirmed cases, it remains challenging to define a “typical” achromat experience conclusively.
Gene therapy is under investigation as a potential treatment for certain genetic subtypes (e.g., CNGA3 and CNGB3 mutations). Early-phase clinical trials have shown promise in partially restoring cone function, although widespread clinical use remains under development.
Achromatopsia was brought to public attention by neurologist Oliver Sacks in his book, The Island of the Colorblind, which describes the lives of individuals with the condition living on the Micronesian island of Pingelap, where the condition has an unusually high prevalence due to a genetic bottleneck.
