OMIM ID:
Colorblindness-Achromatopsia 2
Alternate Names
Defective Genes
Clinical Characteristics
Ocular Features
Patients with this congenital, nonprogressive condition often have nystagmus as infants which may improve later. Eccentric fixation secondary to a small central scotoma is often present. Visual acuity is 20/200 or worse. Hyperopia is common. Photophobia is extreme and vision under daylight conditions improves in dim light. Patients are unable to distinguish any colors. However, there is considerable variability in symptoms and some individuals retain some color perception and have better visual acuity (sometimes 20/80) than others suggesting some residual cone function. The term ‘incomplete achromatopsia’ is sometimes applied to such cases but the molecular basis for this variation is unknown. Optical coherence tomography reveals the central retina to be thinner than in normal controls. The fundus appearance is normal, however.
ERG responses indicate an absence of cone function with no photopic responses.
Systemic Features
There are no associated systemic abnormalities.
Genetics
Inheritance
Mutations in CNGA3 account for approximately 25% of cases of achromatopsia. ACHM2 is an autosomal recessive disorder caused by mutations in CNGA3 (2q11). Mutations in this gene also have been found in rare patients with progressive cone dystrophies. A clinically similar but genetically distinct disorder, ACHM3, results from mutations in CNGB3 (262300). Mutations in GNAT2 (ACHM4; 139340) and PDE6C (ACHM5; 613093) also cause achromatopsia.
Pedigree
Autosomal recessive
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.