Colorblindness-Achromatopsia 4

Clinical Characteristics
Ocular Features: 

The ocular phenotype in ACHM4 is similar to that of other forms of achromatopsia.  Nystagmus, poor visual acuity, photophobia, and defects in color vision are usually present.  Some subjects, however, retain some color discrimination, a condition referred to as incomplete achromatopsia.  The ERG documents the absence of cone function but normal rod responses.  The retina appears normal clinically.

Few families have been reported and the complete phenotype remains undocumented.  For example, it has been reported that visual acuity weakens with age in some patients although it is uncertain if this is true of all cases. 

Systemic Features: 

No systemic abnormalities are associated. 

Genetics

This is an autosomal recessive disorder caused by mutations in GNAT2 located at 1p13.  These mutations account for less than 2% of achromatopsia cases.  The majority are caused by mutations in CNGA3 (25%), responsible for ACHM2 (216900) and CNGB3 (50%), causing ACHM3 (262300).  Mutations in PDE6C (613093 ) causing ACHM5 are responsible for less than 2%. No doubt others will be found as many cases do not have mutations in these genes. 

Treatment
Treatment Options: 

No treatment is available for this disorder but tinted lenses and low vision aids can be helpful.  Red contact lenses can reduce the photophobia and may improve vision. 

References
Article Title: 

References

Rosenberg T, Baumann B, Kohl S, Zrenner E, Jorgensen AL, Wissinger B. Variant phenotypes of incomplete achromatopsia in two cousins with GNAT2 gene mutations. Invest Ophthalmol Vis Sci. 2004 Dec;45(12):4256-62.

PubMedID: 15557429

Aligianis IA, Forshew T, Johnson S, Michaelides M, Johnson CA, Trembath RC, Hunt DM, Moore AT, Maher ER. Mapping of a novel locus for achromatopsia (ACHM4)to 1p and identification of a germline mutation in the alpha subunit of cone transducin (GNAT2). J Med Genet. 2002 Sep;39(9):656-60.

PubMedID: 12205108