A baby has poor vision, nystagmus and strong discomfort in bright light, but the fundus looks normal. Think of achromatopsia. This article explains what achromatopsia is, and how to read the OCT and ERG that make the diagnosis.

Think of achromatopsia when
  1. Poor vision, nystagmus and photophobia are present from infancy.
  2. The patient sees no colors at all.
  3. The fundus looks normal.
  4. On the ERG, the cone response and 30 Hz flicker are absent, while the rod response is normal.

What is achromatopsia?

Achromatopsia is a congenital disease in which all three types of cones (red, green and blue) do not work from birth. The rods work normally. In a milder form, incomplete achromatopsia, some cone function and color vision remain. It is also called rod monochromatism or total color blindness.

Inheritance

Achromatopsia is autosomal recessive. Autosomal recessive inheritance is common among inherited retinal diseases. It affects about 1 in 30,000 people, and two genes, CNGA3 and CNGB3, cause most cases[1].

Symptoms

The main symptoms are poor vision, photophobia (discomfort in bright light), day blindness (hemeralopia; worse vision in bright light), nystagmus and a lack of color vision. Corrected visual acuity is around 20/200 (6/60). All colors look like shades of gray[2].

Nystagmus appears in diseases where the macula does not work from birth. Cones are concentrated in the macula, so achromatopsia causes nystagmus too.

On the Ishihara test, patients can read only the first plate, a demonstration plate that people with color vision defects can also read.

Fundus

The fundus is usually normal. Some patients develop atrophy at the macula over time.

OCT

OCT can look normal, so a normal OCT does not rule out achromatopsia. OCT findings vary. The ellipsoid zone (EZ) at the fovea can be continuous, broken, or replaced by a dark gap (a hyporeflective zone). Some patients show outer retinal atrophy[1].

Fundus photographs and OCT of two patients with achromatopsia. The fundus looks almost normal in both. On OCT, the upper patient has a continuous ellipsoid zone, and the lower patient has a dark gap in the ellipsoid zone at the fovea.
Achromatopsia. Top: CNGA3, OCT with a continuous ellipsoid zone. Bottom: PDE6C, OCT with a hyporeflective zone at the fovea. The fundus looks almost normal in both. From Choi et al. 2023, CC BY 4.0, two of four rows.

ERG findings

The ERG is the key test. The cone response and the 30 Hz flicker are absent or severely reduced, while the rod response is normal.

DA means dark-adapted and LA means light-adapted. The numbers 0.01 and 3.0 are the flash strength.

ERG stepAchromatopsia
Rod (DA 0.01)Normal
Mixed response (DA 3.0)Normal or mildly reduced
Cone (LA 3.0)Absent or minimal
30 Hz flickerAbsent or minimal

The 30 Hz flicker tests the cones only, because rods cannot follow 30 flashes a second.

The mixed response comes from rods and cones together, but after dark adaptation most of it comes from the rods. So it stays normal or is only mildly reduced[2].

Full-field ERG of a child with achromatopsia. The dark-adapted rod and mixed responses are recorded, but the light-adapted cone response and 30 Hz flicker are flat.
Full-field ERG in achromatopsia (CNGA3). Left: dark-adapted rod response (Scotopic 0.01) and mixed response (Scotopic 3.0), both recorded. Right: light-adapted cone response (Photopic 3.0) and 30 Hz flicker, both flat. From Choi et al. 2023, CC BY 4.0, four of six panels rearranged.

On the ERG alone, achromatopsia looks much like cone dystrophy. The history usually tells them apart. In cone dystrophy, vision is good in early childhood and falls later, usually before age 20. In achromatopsia, vision is poor from birth.

Blue cone monochromatism looks similar too, but it is X-linked, affects boys, and the blue (S) cone ERG is preserved[1].

A child with poor vision, nystagmus and a normal fundus is sometimes labeled as amblyopia of unknown cause. Ask about photophobia and color vision, and consider a full-field ERG.

Treatment and prognosis

There is no approved treatment. Tinted glasses help with photophobia and day blindness. Deep red tints are often used[1]. Gene therapy for CNGA3 and CNGB3 is being tested in early clinical trials. Only patients with a variant in one of these genes can join, so genetic testing is important[3].

Vision usually stays about the same, but slow cone loss over time has been reported in some patients[1].

Summary

  • In achromatopsia, all three types of cones do not work from birth. The rods are normal.
  • Poor vision, photophobia, day blindness, nystagmus and no color vision start in infancy.
  • The fundus is usually normal, and OCT can be normal too.
  • ERG: the cone response and 30 Hz flicker are absent, while the rod response is normal.
  • There is no approved treatment. Tinted glasses help with photophobia.

Figure credits

Fundus, OCT and ERG: Figures 2 and 3 from Choi YJ, Joo K, Lim HT, et al. Genes 2023;14(2):519 (https://doi.org/10.3390/genes14020519), licensed under CC BY 4.0. Figure 2 cropped to two of four rows, with the row letters removed. Figure 3 cropped to four of six panels and rearranged.

References

[1] Aboshiha J, Dubis AM, Carroll J, Hardcastle AJ, Michaelides M. The cone dysfunction syndromes. Br J Ophthalmol. 2016;100(1):115–121. PMID 25770143.

[2] Salmon JF. Kanski’s Clinical Ophthalmology: A Systematic Approach. 10th ed. Elsevier; 2025. Chapter 15, Inherited retinal degenerations: Congenital achromatopsia.

[3] Baxter MF, Borchert GA. Gene therapy for achromatopsia. Int J Mol Sci. 2024;25(17):9739. PMID 39273686.

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