Blue Cone Monochromacy
Patient Information
Characteristics
Background and History
The light sensitive cells in the retina are called rods (useful in dim vision) and cones (used for color vision). Gene mutations can impact either or both types. Those that cause a dysfunction in cones result in defective color vision of various types, sometimes called colorblindness.
There are 3 types of cones that individually respond to one of three colors: red, green, and blue. Only 1-2% of these cones respond to blue light and none of them (and no rods) are located in the fovea which is the area of the retina that we usually use for the highest visual resolution. Hence, when the red and green cones are not functioning properly vision is especially poor.
Clinical Correlations
In this disorder (BCM), the red and green pigments do not function properly as a result of gene changes (mutations) leaving only the blue-responding cones and the rods for seeing. Individuals with the relevant mutation on the X chromosome can see blue targets on a yellow background but lack the capacity to see red or green colors. Extreme light sensitivity and poor vision are cardinal features. Visual acuity may be near normal at 20/60 or as low as legal blindness (20/200). ‘Dancing’ eyes (nystagmus) are common and usually becomes apparent by 6 months of age but there may be some decrease with age.
The exact nature of the genetic defect remains to be worked out although mutations in several genes have been implicated. In most individuals, the disease is stationary but in a few families there seems to be progressive disease. Such families have pigmentary changes in the retina with worsening vision.
Genetics
Inheritance
This is a recessive X-linked disorder in which males are primarily affected. The carrier mother has normal color vision but carries the defective gene on one of her two X chromosomes and can expect on average that half of her male offspring will inherit blue cone monochromacy. The male with only this single mutant X chromosome has the disorder but half his sisters can be expected to be carriers like their mother. Affected fathers will not have affected children but all his daughters will be carriers.
Pedigree
X-linked recessive, father affected
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the father, he has the disease since his only X chromosome is mutant and he has no normal X to blunt the effects of the abnormal gene. His sons only receive his Y chromosome and thus are all normal. However, all his daughters receive his one and only X chromosome and will be healthy 'carriers'. Thus such males will have no affected children but half their grandsons from those daughters will have the same disease as he does.
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the father, he has the disease since his only X chromosome is mutant and he has no normal X to blunt the effects of the abnormal gene. His sons only receive his Y chromosome and thus are all normal. However, all his daughters receive his one and only X chromosome and will be healthy 'carriers'. Thus such males will have no affected children but half their grandsons from those daughters will have the same disease as he does.
X-linked recessive, carrier mother
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the mother, she usually does not have the disease since the normal X chromosome without the mutation neutralizes the mutation in the abnormal X chromosome. However, half her sons will inherit the mutation-containing X chromosome and therefore have the X-linked disease. Half the daughters will inherit the mutation-bearing X chromosome and are usuallly healthy 'carriers'.
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the mother, she usually does not have the disease since the normal X chromosome without the mutation neutralizes the mutation in the abnormal X chromosome. However, half her sons will inherit the mutation-containing X chromosome and therefore have the X-linked disease. Half the daughters will inherit the mutation-bearing X chromosome and are usuallly healthy 'carriers'.
Diagnosis and Prognosis
The diagnosis can be made by an ophthalmologist. No systemic abnormalities are associated. Low vision aids can be helpful as can tinted lenses. These work best with a magenta tint since they are protective of the rods while allowing the maximum light to stimulate the blue responding cones.
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