Achromatopsia

Definition and Clinical Testing

Achromatopsia, or dyschromatopsia, is an inability or impaired ability to perceive colors. This may be ophthalmological or neurological in origin, and either congenital or acquired. A key distinction is that only patients with acquired achromatopsia typically complain of impaired color vision, often describing the world as looking "gray" or "washed out."

Achromatopsia is most conveniently tested for clinically using pseudoisochromatic figures (e.g., Ishihara plates). More quantitative testing can be done with color-sorting tasks like the Farnsworth-Munsell 100 Hue test. However, difficulty with these tests does not always reflect true achromatopsia (see Pseudoachromatopsia).

Congenital Achromatopsia

The most common cause of achromatopsia is inherited "color blindness," which affects the cone photoreceptors in the retina. Several types are recognized: in monochromats, only one of the three cone classes is affected; in dichromats, two are affected. Anomalous sensitivity to specific wavelengths of light may also occur (anomalous trichromat). These inherited dyschromatopsias are characteristically binocular, symmetrical, and do not change over time.

Acquired Achromatopsia

Acquired achromatopsia may result from damage to the optic nerve or the cerebral cortex. Unlike inherited conditions, these deficits are noticeable to the patient and may be confined to only part of the visual field (e.g., hemiachromatopsia).

• Optic Nerve Damage: Optic neuritis typically impairs color vision (affecting red-green more than blue-yellow). This defect may persist even after other acute features, like impaired visual acuity, have resolved.
• Cerebral Achromatopsia: This results from cortical damage (most usually infarction) to the inferior occipitotemporal area. Specifically, area V4 of the visual cortex, located in the fusiform and lingual gyri, is devoted to color processing. Unilateral lesions can cause a homonymous hemiachromatopsia. Lesions in this region may also produce prosopagnosia, alexia, and a superior quadrantanopia. Transient achromatopsia has also been reported in the context of vertebrobasilar ischemia.

Differential Diagnosis

True achromatopsia must be distinguished from:

• Color Agnosia: A loss of color knowledge (e.g., cannot say what color a banana is) despite intact color perception.
• Color Anomia: An inability to name colors despite being able to perceive and sort them correctly.

 

References

Orrell RW, James-Galton M, Stevens JM, Rossor MN. Cerebral achromatopsia as a presentation of Trousseau’s syndrome. Postgraduate Medical Journal 1995; 71: 44-46

Zeki S. A century of cerebral achromatopsia. Brain 1990; 113: 1721-1777

 

Cross References

Agnosia; Alexia; Anomia; Prosopagnosia; Pseudoachromatopsia; Quadrantanopia; Scotoma; Xanthopsia