the zehl files

Cone-rod Dystrophy

To help raise public awareness for low vision and color blindness, I have put together a page with relevant information I have found useful in learning everything I can about cone-rod dystrophy, a rare eye condition that has been a part of my life since birth, but has also shaped my life in unimaginable ways. I sincerely hope that this information can help other patients and their families who may be affected or on a similar journey.

What is cone-rod dystrophy?

Cone-rod dystrophy is a rare, degenerative genetic disease that causes progressive dysfunction of the photoreceptors in the retina of the eye. These cells are responsible for converting visible light into the neuroelectric impulses that are transmitted along the optic nerve to the brain for further processing. The cone cells are responsible for color and central vision, while the rod cells are responsible for night and peripheral vision.

Another similar dystrophy of the photoreceptors, known as retinitis pigmentosa, can be characterized by night blindness, and a narrowed field of vision. The most significant difference between the two diseases lies in which receptors appear to be significantly affected first.

How do I acquire this disease?

Cone-rod dystrophy results from mutations in genes and is passed through one of four mechanisms:

  1. Autosomal dominant (AD): only one mutated copy of the gene is required in order to cause the disease. Each child has a 50% chance of inheriting the mutation from the mother or the father.
  2. Autosomal recessive (AR): two mutated copies will cause the disease, one from each parent. If both parents have a one healthy and one mutated copy, the children have a 25% chance of having cone-rod dystrophy, 25% chance of having no disease, and a 50% chance of being a carrier like the parents.
  3. X-linked (XL): the mutation is located on the X-chromosome. Males are usually affected by the disease and females are the carriers, whose sons have a 50% chance of inheriting the disease.
  4. Mitochondrial: the mutations are in the mitochondrial genes, which pass exclusively through the egg. If the mother has the disease, all her children will inherit it, but only the females will pass it on.

Genetic testing is required to determine the location of the mutation given a specific patient and can help prospective parents make informed decisions.

What are the symptoms?

Cone-rod dystrophy affects the ability of the retina to detect light, and this can cause issues impacting vision:

  1. Sensitivity to light: extreme sensitivity to bright lights and glare, causing the patient to squint, wear sunglasses indoors, and avoid brightly lit environments.
  2. Loss of central vision: the cone cells mostly concentrate in the central vision field near the fovea, so a loss in function causes a profound decrease in visual acuity.
  3. Loss of color vision: colors appear washed out and bright, vividly colored objects appear less saturated.
  4. Loss of night vision: the disease affects primarily cone cells; however, rod cell degradation makes vision at night close to impossible.
  5. Loss of peripheral vision: deterioration of the rod cells also causes progressive peripheral vision loss.

What can I do about this disease?

Currently, there is no cure for cone-rod dystrophy. However, research in areas such as stem cell therapy and viral gene therapy shows potential for future treatments. Despite being controversial, these avenues are actively being explored.

While cone-rod dystrophy typically does not result in total blindness, it can lead to significant functional vision impairment that persists throughout an individual's lifetime.

Here are some accommodations that can make life easier.

  1. High-contrast colors tailored to the vision of the patient (e.g. this website)
  2. Re-mapping colors combinations that are invisible. For example, if a person can’t distinguish dark red from black, change it to a color that is visible (e.g. dark blue or dark green)
    1. Samsung Galaxy phones and tablets can do this with accessibility settings.
    2. Dolphin Supernova screen magnification software can do this natively
    3. Windows 11 Pro allows colors to be remapped
  3. Indoor sunglasses with anti-glare coating (e.g. yellow or amber)
  4. Colored transparency sheets for reading (e.g. blue transparency for sharpness)
  5. Tactile markings (digital braille devices)

What happens to color vision?

Often the only way to answer the question “what color is this” is to ask someone who has normal color vision.

Factors in the environment such as the presence of ambient light, the color and intensity of light, and the source and distance of light all play critical roles in determining whether a patient will be able to identify a color in a specific situation.

Red may blend with black, blue may blend with white, and green and yellow can become indistinguishable, among others, depending on which wavelengths are in question. It is reasonable to assume that a person with mild cone-rod dystrophy is functionally color blind and will probably have a hard time completing activities that require recognition of specific colors.

How do I confirm the diagnosis?

Electroretinography allows for the precise measurement of electrical signals produced by the photoreceptors and other cells and structures within the retina.

The patient has an electrode temporarily affixed to the cornea of the eye and must not blink while varying colors and intensities and frequency patterns are shined into a very dilated, dark-adapted eyes. These signals are recorded and build a waveform that can inform exactly how light is being transduced within the cells of the retina.

Optical coherence tomography is an imaging technique that offers detailed cross-sectional images of the retina, which can give an indication as to the extent of the retinal damage.

Fundus autofluorescence imaging allows for examination of the retinal pigment epithelium and can detect changes in the retina.

What are the prevalence rates of cone-rod dystrophy?

The prevalence rate of cone-rod dystrophy is 1 in 30,000 to 1 in 40,000. This makes the disease rare according to GARD.

Cited: Chase Zehlchen, "Cone-rod Dystrophy," Zehl Files, Release: January 12, 2025. Edit: January 14, 2025. Available: https://zehlchen.com/life/cone-rod-dystrophy







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