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Diagnosis of Blue Cone Monochromacy

In a male child, from 2 months upwards, an aversion to light and nystagmus may lead to the suspicion of a case of achromatopsia, but it does not provide sufficient indications to establish the form of the condition.

To identify a case of BCM, it is necessary to reconstruct the family history, with the condition linked to the transmission of the X chromosome, if there are other cases in the family.

A colour vision test may then be conducted, and if color vision is abnormal, a diagnosis can be made.

The most appropriate diagnostic tools are:

  • DNA Test;
  • the electroretinogram (ERG), which can demonstrate the loss of cone function with retained rod function;
  • a color test like a Farnsworth D-15, a Farnsworth Munsell 100 Hue test;
  • the reconstruction of the family history of the disease.

Differential diagnosis

Differential diagnosis covers other diseases of the retina: Achromatopsia, X-linked Ocular Albinism, and Cone Dystrophy or Cone Rod Dystrophy, which can also be X-linked.

It is very important to obtain an accurate diagnosis, because the evolution of the disease (progressive, as in the case of Cone Dystrophy, or slowly progressive, as in the case of BCM), possible treatments and necessary treatment aids (for example glasses with screening filters, which are important for BCM, but indispensable for achromatopsia), differ depending on the disease.

BCM is different from Achromatopsia, they are unrelated diseases.

Similarities between BCM and Achromatopsia

The two forms have many elements in common:

  • intolerance to light;
  • weak central vision, low visual acuity;
  • poor ability to distinguish colours;
  • nystagmus;
  • retina with a normal appearance.

Differences between BCM and Achromatopsia

  • BCM is X-linked and affects male individuals only;
  • Achromatopsia is an autosomal recessive disease, and affects both males and females equally;
  • The mutations that cause BCM result in the lack of functional opsin photopigments in the red and green cones;
  • The mutations that cause achromatopsia result in disruption of the phototransduction signaling cascade. So while the photopigments may be present and can absorb light, the cones cannot signal this to the brain and are thus all non-functional (red, green and blue).

To learn more please read the Clinical utility gene card for: Blue Cone Monochromatism.