There are two categories of disease of the cone system, stationary diseases, and progressive dystrophies.

Slowly progressive visual loss, photophobia, and poor color vision, with onset during the first 3 decades of life. Vision is worse in bright than dim light.

Critical

• Early: Essentially normal fundus examination, even with poor visual acuity. Abnormal cone  function on ERG (e.g., a reduced single-flash photopic response and a reduced flicker response). OCT can show absence of the ellipsoid zone but the external limiting membrane might still be present in the fovea.

• Late: Bull’s eye macular appearance or central GA of the RPE and choriocapillaris.

Other

Nystagmus, temporal pallor of the optic disc, spotty pigment clumping in the macular area, tapetal-like retinal sheen. Rarely rod degeneration may ensue, leading to an RP-like picture (e.g., a cone–rod degeneration, which may have an autosomal dominant inheritance pattern).

Inheritance

Usually sporadic. Hereditary forms are usually autosomal dominant or less often X-linked.

• Stargardt disease: Especially in early stage when yellowish lesions are absent and ERG is usually normal. See 11.30, Stargardt Disease (Fundus Flavimaculatus).

• Chloroquine/hydroxychloroquine maculopathy: May produce a bull’s eye macular appearance and poor color vision. History of medication use, no family history of cone degeneration, no nystagmus. See 11.32, Chloroquine/Hydroxychloroquine Toxicity.

• Central areolar choroidal dystrophy: GA of the RPE with normal photopic ERG.

• AMD: Can have GA of the RPE, but with normal color vision and photopic ERG. See 11.16, Nonexudative (Dry) Age-Related Macular Degeneration and 11.17, Neovascular or Exudative (Wet) Age-Related Macular Degeneration.

• Congenital color deficiency: Normal visual acuity, onset at birth, not progressive.

• RP: Night blindness and peripheral visual field loss. Often with peripheral retinal bone spicules. Can be distinguished by dark-adaptation testing and ERG. See 11.28, Retinitis Pigmentosa and Inherited Chorioretinal Dystrophies.

• Optic neuropathy or atrophy: Decreased acuity, impaired color vision, temporal or diffuse optic disc pallor, or both. See 10.17, Arteritic Ischemic Optic Neuropathy (Giant Cell Arteritis), 10.18, Nonarteritic Ischemic Optic Neuropathy, and 10.20, Miscellaneous Optic Neuropathies.

• Nonphysiologic visual loss: Normal results on ophthalmoscopic examination, IVFA, OCT, ERG, and electrooculogram (EOG). Patients can often be tricked into seeing better by special testing. See 10.25, Nonphysiologic Visual Loss.

1. Family history and genetic testing (see ncbi.nlm.nih.gov/gtr/tests for full list).

2. Complete ophthalmic examination, including formal assessment for dyschromatopsia (e.g., Farnsworth–Munsell 100-hue test).

3. Formal visual field test.

4. Full-field ERG: abnormal photopic response with normal rod-isolated response.

5. OCT can show disruption of outer retinal layers but may be normal even in patients with electrophysical abnormalities.

6. FAF can be useful in the diagnosis (particularly sensitive to disturbances in the RPE), as well as for monitoring these diseases.

There is no proven cure for cone dystrophy. The following measures may be palliative:

1. Heavily tinted glasses or contact lenses may help maximize vision.

2. Miotic drops (e.g., pilocarpine 0.5% to 1% q.i.d.) are occasionally tried to improve vision and reduce photophobia.

3. Genetic counseling.

4. Low-vision aids as needed.

Yearly.