Achromatopsia refers to a complete inability to discriminate colors. As a result, the patient must distinguish different colors primarily on the basis of intensity.
Manifestations: Achromatopsia + congenital nystagmus + ↓VA + photophobia
- Rod Monochromatism
- VA: 20/80–20/200
- Complete cone dysfunction, including the S-cones
- Blue-Cone (S-Cone) Monochromatism
- VA: >20/80 Because a relatively greater proportion of cones remains functional, visual acuity is better than in rod monochromatism.
Important differential diagnoses:
- Congenital Motor Nystagmus
- Ocular Albinism
Unlike achromatopsia, both congenital motor nystagmus and ocular albinism have a normal Cone-ERG.
A 9-year-old boy is referred with reduced vision since childhood. His parents report nystagmus since early childhood, and one of his maternal uncles also has poor visual acuity. Visual acuity is 20/60 in both eyes. Fundus examination is normal. ERG shows normal scotopic responses but reduced photopic responses. Which of the following is the most likely diagnosis?
(C, D): Ocular albinism and congenital motor nystagmus typically have a normal cone ERG. (B): In rod monochromatism, visual acuity is usually worse than 20/80.
Quick Clue: Visual acuity can provide a rapid way to distinguish the causes of achromatopsia.
- Rod monochromatism: VA <20/80
- S-cone monochromatism: VA >20/80