What is cone-dystrophy?

A cone dystrophy is an inherited ocular disorder characterized by the loss of cone cells, the photoreceptors responsible for both central and color vision.

Symptoms:

The symptoms of cone dystrophy may vary from one person to another, even among individuals with the same form of the disorder. The age of onset, specific symptoms, severity, and progression (if any) can vary greatly. The amount of vision loss varies and is difficult to predict. Affected individuals should talk to their physician and medical team about their specific case and associated symptoms.

The most common symptoms of cone dystrophy are vision loss (age of onset ranging from the late teens to the sixties), sensitivity to bright lights, and poor color vision. Therefore, patients see better at dusk. Visual acuity usually deteriorates gradually, but it can deteriorate rapidly to 20/200; later, in more severe cases, it drops to “counting fingers” vision. Color vision testing using color test plates (HRR series) reveals many errors on both red-green and blue-yellow plates.

Diagnosis:

Many cases of cone dystrophy occur randomly for no identifiable reason (sporadically). Some forms are inherited as an autosomal dominant, autosomal recessive or X-linked recessive trait. Inherited forms of cone dystrophy are due to mutations to one of several different genes that have been linked to cone dystrophy. These genes contain instructions for making certain proteins, specifically proteins that play vital roles in the development, function or overall health of cone cells. The exact, underlying mechanisms that cause cone dystrophy are not fully understood.

Mutated genes in cone dystrophy can be inherited as an autosomal dominant, autosomal recessive or X-linked recessive trait. Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother.

Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary for the appearance of the disease. The abnormal gene can be inherited from either parent, or can be the result of a new mutation (gene change) in the affected individual. The risk of passing the abnormal gene from affected parent to offspring is 50 percent for each pregnancy regardless of the sex of the resulting child.

X-linked recessive genetic disorders are conditions caused by an abnormal gene on the X chromosome. Females have two X chromosomes but one of the X chromosomes is “turned off” and all of the genes on that chromosome are inactivated. Females who have a disease gene present on one of their X chromosomes are carriers for that disorder. Carrier females usually do not display symptoms of the disorder because it is usually the X chromosome with the abnormal gene that is “turned off”. A male has one X chromosome and if he inherits an X chromosome that contains a disease gene, he will develop the disease. Males with X-linked disorders pass the disease gene to all of their daughters, who will be carriers. A male cannot pass an X-linked gene to his sons because males always pass their Y chromosome instead of their X chromosome to male offspring. Female carriers of an X-linked disorder have a 25 percent chance with each pregnancy to have a carrier daughter like themselves, a 25 percent chance to have a non-carrier daughter, a 25 percent chance to have a son affected with the disease, and a 25 percent chance to have an unaffected son.

The fundus exam via ophthalmoscopy is essentially normal early on in cone dystrophy, and definite macular changes usually occur well after visual loss. Fluorescein angiography (FA) is a useful adjunct in the workup of someone suspected to have cone dystrophy, as it may detect early changes in the retina that are too subtle to be seen by ophthalmoscope. For example, FA may reveal areas of hyperfluorescence, indicating that the RPE has lost some of its integrity, allowing the underlying fluorescence from the choroid to be more visible. These early changes are usually not detected during the ophthalmoscopic exam.

The most common type of macular lesion seen during ophthalmoscopic examination has a bull’s-eye appearance and consists of a doughnut-like zone of atrophic pigment epithelium surrounding a central darker area. In another, less frequent form of cone dystrophy there is rather diffuse atrophy of the posterior pole with spotty pigment clumping in the macular area. Rarely, atrophy of the choriocapillaris and larger choroidal vessels is seen in patients at an early stage. The inclusion of fluorescein angiography in the workup of these patients is important since it can help detect many of these characteristic ophthalmoscopic features. In addition to the retinal findings, temporal pallor of the optic disc is commonly observed. As expected, visual field testing in cone dystrophy usually reveals a central scotoma. In cases with the typical bull’s-eye appearance, there is often relative central sparing.

Treatment

There is no cure for cone dystrophy. Treatment is directed toward the specific symptoms that are apparent in each individual. Treatment may include using tinted lenses or dark sunglass in bright environments and magnifying devices to assist in reading and other similar activities.

Genetic counseling may be of benefit for affected individuals and their families. Other treatment is symptomatic and supportive.