ACHROMATOPSIA: WHAT IS ACHROMATOPSIA?
Definition: What is "Achromatopsia"?
Achromatopsia or ACHM is a condition where the patient is unable to perceive color. Although this name may also be used to refer to acquired defects such as cerebral achromatopsia or color agnosia, it normally refers to a congenital disorder of the vision which is autosomal recessive in nature. It can also be called rod monochromacy or total inborn color blindness. Patients with this kind of disorder manifest total absence of the cone cell activity through electroretinography. At least 4 causes are listed as congenital ACHM: ACHM2 & ACHM3; the third type is a cone photoreceptor transducin called GNAT2, ACHM4; the last cause is still unknown.
Symptoms & Signs
Complete achormatopsia is characterized by the inability to tell one color apart from the other. Those with this type have many other ophthalmologic defects. Among these aberrations are nystagmus, visual acuity & acute photophobia. Also, the fundus may be normal but photopic ERG response is non-existent. The function of the rod cell is still normal. Incomplete achormatopsia may have most of the symptoms of complete achormatopsia but they are in the reduced form. Those with this type have diminished visual acuity either with or without photophobia or nystagmus. Also, these patients manifest incomplete impairment of the function of the cone cell. Rod cell function, just like the first type, still remains normal. Visual stability & acuity get better on the first six to seven years of the child's life.
Cerebral achromatopsia is a type of color blindness that can be acquired. It is caused by an injury to the brain (at the cerebral cortex). This is not an abnormality of the retinal cells. Congenital achromatopsia is caused by the inborn types of achromatopsia. They are all due to the retinal phottransduction pathway defect. Solely, this type of ACHM appears to be a result of cone cell malfunction. This is commonly caused by genetic mutations of the cyclic nucleotide-gated ion channels cone cell CHGA3 & CNGB3 also GNAT2.
Achromatopsia is a common disease & its prevalence is at 1 in every 30,000 individuals. The disease afflicts about 100,000 people in the United States of America. On Pingelap, a tiny Micronesian atoll, about 5% of the 3,000 people that are living there have achromatopsia. The history behind the rampant cases in Pingelap began at about 1775 when the typhoon Lengkieki devastated the area. The incident resulted into a famine which left only twenty survivors. One of these survivors was a heterozygous of the disorder. This person started the disorder that is now widespread within their area. The term for the disorder at Pingelap is 'maksun' which literally means 'not see'.