Bietti Crystalline Corneoretinal Dystrophy
Patient Information
Characteristics
Background and History
This disorder was first described by an Italian ophthalmologist, G.B.Bietti, in 1937. It belongs to a group of disorders which cause night blindness.
Clinical Correlations
Night blindness has its onset about the third decade of life at about the time that some vision loss is noted. Vision may remain near normal, though, until midlife. The disease is progressive and some patients suffer severe vision loss by the 5th decade. There is also loss of side vision, sometimes severely. The rate of progression varies widely and there may be considerable difference in vision between the two eyes. The disease is limited to the eye.
Genetics
Inheritance
This is an autosomal recessive disorder which requires the inheritance of a mutation from each parent.
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.
Diagnosis and Prognosis
The diagnosis is usually made by an ophthalmologist based on the symptoms, the appearance of the retina, and the results of an electroretinogram (ERG) test. Night blindness becomes progressively worse as does daytime vision. Color vision is lost. Some patients experience severe visual problems by their 40s but others may have good vision until their 50s or 60s. There is no associated systemic disease.
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