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 this is absent, a diagnosis can be made.

The most appropriate diagnostic tools are:

– 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, a Berson Test (Test Plates) which makes it possible to distinguish BCM from achromatopsia, because although the electric response is significantly reduced in both illnesses, blue cone response is well preserved in BCM patients;

– the reconstruction of the family history of the disease.

Differential diagnosis

Differential diagnosis covers other diseases of the retina: Achromatopsia, X-linked Ocular Albinism, Cone Dystrophy, which is also 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 often non-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.

Although it belongs to the Achromatopsia family and is known as X-linked Achromatopsia, BCM is a different disease and different treatment approaches have been studied for it.

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;
• infantile nystagmus or 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 diseases, and affects both males and females;
• Cases of BCM are linked to mutations in the X chromosome, position Xq28, and involve the genes OPM1LW and OPM1MW. The mutations of this gene are known;
• Almost 80 % of cases of achromatopsia are linked to independent mutations on 4 genes in the chromosomes 1, 2, 8 and 10. Mutations in both copies of each of these genes cause the disease. Researchers are carrying out studies in order to understand whether there are other possible mutations and other genes involved;
• The mutations in individuals affected by BCM are responsible for the lack of the protein Opsin in the external segments of the red and green cones;
• The mutations in individuals affected by achromatopsia are responsible for the lack of the proteins that build the ciclo nucleotide gate in all the cones (red, green and blue);

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