What Are the Causes of Cone Rod Dystrophy?

April 10, 2023

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Inherited retinal dystrophies are a large, genetically diverse group of degenerative disorders of the retina. These inherited retinal dystrophies are caused when a gene or two are defective or undergo a harmful change. These mutations lead to different diseases or disorders in our bodies, like Down syndrome and vision loss.

As discussed in our article ‘What is Cone Rod Dystrophy?’, we have helped identify what cone and cone-rod dystrophies are.

Here is a brief overview of it…

Cone and cone-rod dystrophies are a group of 35 inherited diseases that cause the photoreceptors of the retina to deteriorate. The cells degenerate over time and eventually die, which causes vision loss.

How Our Eyes Work

Our eyes transmit light from the world around us into an image for us to perceive. Though small, the eye is a highly complex organ. Every part of the eye must function properly to capture light, focus it, and process messages back to the brain to see a visual image.

The entire process of vision must be completely intact, from light reflected in our field of view and gathered by the cornea to the passage of visual signals to the brain via optic nerves. A break at any point in the process causes visual impairment. From muscles that control the eyes to the parts within the eye to the pathway to the brain, vision impairments result from technical problems during the transitional phases. Other times, the eyes might work perfectly, but there might be a problem with how the brain interprets the signals it receives.

Rods and Cones in the Eye

Photoreceptors, specialized cells located on the retina at the back of the eye, process images. They have very specific roles, to receive and process signals of light and color, which enables our vision.

There are two types of photoreceptors in the human eye: rods and cones, both found in a layer at the back of the eye. Let’s take a look at what these two photoreceptors do for the retina.

What is the Main Function of the Rods in the Eye?

Rods are rod-like structures located throughout the retina except for the fovea (responsible for sharp central vision). They are responsible for vision in low light levels (dim light or twilight), also called scotopic vision. They do not have color vision and have low spatial acuity.

The most basic and crucial function of the rods in the eye is to perceive light. Rods are specialized for picking up light signals to determine light and shadow. There are about 120 million rods in the human eye. They pick up signals from all directions, improving peripheral vision, motion sensing, and depth perception.

What is the Main Function of Cones in the Eye?

Cones are cone-shaped structures that are responsible for vision in low-light conditions, also known as photopic vision. While rods perceive light and dark, cones have the role of color perception. There are about 6 to 7 million cones in the human eye. These cones are concentrated mostly at the center of the retina, around the fovea.

There are three types of cone cells, and each has a different sensitivity to light wavelengths. One perceives red light, the other perceives green light, and the third perceives blue light. When light enters the eye, it stimulates the cone cells. The brain interprets the signals from the cone cells to help determine the color of the object. The three RGB cones work together to create a color spectrum.

A person with normal vision has all three types of cones working properly. Color blindness happens when one or more of the types of cones are damaged. If the green cone is damaged, you won’t be able to see colors that contain blue clearly.

Vision is a miracle… It works over a delicate system of various processes that help us see. Let’s be honest, knowing about how our eyes work and perceive images makes us appreciate what we can see every day.

Now that we know how these rods and cones work together to make us see, let’s get into the causes of cone rod dystrophy. Damage to photoreceptors can become the cause of cone rod dystrophy and lead to vision loss.

Causes of Cone Rod Dystrophy and Cone Dystrophy

As mentioned earlier, cone rod dystrophies are a genetic condition and there are different types of cone rod dystrophies. These types of cone rod dystrophies differ due to their genetic cause and how they are inherited.

There are around 35 genes identified that become the cause of cone rod dystrophy, where mutation in these leads to the loss of cone and rod photoreceptor cells. A few of the genes linked to this condition are ABCA4, CRX, GUCY2D, and RPGR.

As the type of cone rod dystrophy depends on a varied inheritance pattern, it includes autosomal dominant, autosomal recessive, and X-linked inheritance patterns. To learn more about these, you can check out our blog ‘What is Cone Rod Dystrophy?’.

Autosomal recessive is the most common inheritance pattern of cone rod dystrophy. In an autosomal dominant pattern, one copy of the gene does not work properly, becoming the cause of cone rod dystrophy. Here, the affected person receives one copy of the mutated gene from an affected parent.

Whereas in autosomal recessive inheritance, both copies of the gene are mutated and do not work properly, becoming the cause of cone rod dystrophy. Here, the affected person’s mother and father both passed on the mutation of the gene.

In X-linked inheritance, if the male has an X-chromosome with a mutated gene, the condition arises with just one copy of the X-chromosome containing the gene. Since females almost always have an extra X-chromosome functioning, they usually do not develop the condition.

There are cases where a rare phenomenon of non-random X-chromosome activation causes some females to develop the symptoms of cone rod dystrophy.

Some causes of cone rod dystrophy happen sporadically. In such cases, new genetic mutations may occur, exacerbating the condition.

What Are Some of the Common Rod Cone Dystrophies??

Leber’s Amaurosis, Retinitis Pigmentosa (and Usher’s Syndrome) and Batten’s Disease are some of the most common conditions where cone and rod photoreceptors are damaged in the eye and do not work properly.

How Is Vision with Cone Rod Dystrophy?

Cone rod dystrophy usually causes loss of sharp visual acuity, which cannot be corrected with glasses, sensitivity to light (photophobia), central blind spots, and progressive vision loss in dimly lit environments (night blindness), and peripheral vision loss… All these can be progressive.

Currently, there are no treatments available for cone rod dystrophy, which causes vision loss. Yet, there are treatment options available that can help manage and slow down the degenerative process of vision loss, like avoiding brightly lit places, using low vision aids, or taking advantage of some of the greatest advances in technology, like assistive visual aids.

It’s quite challenging to live independently with vision loss or low vision. Assistive technology like IrisVision is a revolutionary find in the world of optical aids for people with visual impairments. This technology uses the functional areas of the eye and enhances the remaining vision of an affected person with cone rod dystrophy. It helps bring some life to your sight that has started deteriorating due to the degeneration of cones and rods. You can learn more about how IrisVision can help by clicking here.