Cone dystrophy
Encyclopedia
A cone dystrophy is an inherited
ocular disorder characterized by the loss of cone cells, the photoreceptors responsible for both central and color vision
.
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.
The pathogenesis of cone dystrophy has yet to be elucidated. It appears that the dystrophy is primary, since subjective and objective abnormalities of cone function are found before ophthalmoscopic changes can be seen. However, the retinal pigment epithelium(RPE) rapidly becomes involved, leading to a retinal dystrophy primarily involving the macula
. The histological examination of the eyes of one such patient showed that the outer nuclear layer of cones and rods had disappeared completely, whereas the RPE showed pronounced pigment changes. There was also atrophy of the temporal disc.
The fundus exam via ophthalmoscope 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.
Because of the wide spectrum of fundus changes and the difficulty in making the diagnosis in the early stages, electroretinography
(ERG) remains the best test for making the diagnosis. Abnormal cone function on the ERG is indicated by a reduced single-flash and flicker response when the test is carried out in a well-lit room (photopic ERG). The relative sparing of rod function in cone dystrophy is evidenced by a normal scotopic ERG, i.e. when the test is carried out in the dark. In more severe or longer standing cases, the dystrophy involves a greater proportion of rods with resultant subnormal scotopic records. Since cone dystrophy is hereditary and can be asymptomatic early on in the disease process, ERG is an invaluable tool in the early diagnosis of patients with positive family histories.
Cone dystrophy in general usually occurs sporadically. Hereditary forms are usually autosomal dominant, and instances of autosomal recessive and X-linked inheritance also occur.
In the differential diagnosis, other macular dystrophies as well as the hereditary optic atrophies must be considered. Fluorescent angiography, ERG, and color vision tests are important tools to help facilitate diagnosis in early stages.
Heredity
Heredity is the passing of traits to offspring . This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve...
ocular disorder characterized by the loss of cone cells, the photoreceptors responsible for both central and color vision
Color vision
Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit...
.
The most common symptoms of cone dystrophy are vision loss
Vision loss
Vision loss or visual loss is the absence of vision where it existed before, which can happen either acutely or chronically .-Ranges of 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
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
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.
The pathogenesis of cone dystrophy has yet to be elucidated. It appears that the dystrophy is primary, since subjective and objective abnormalities of cone function are found before ophthalmoscopic changes can be seen. However, the retinal pigment epithelium(RPE) rapidly becomes involved, leading to a retinal dystrophy primarily involving the macula
Macula
The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells...
. The histological examination of the eyes of one such patient showed that the outer nuclear layer of cones and rods had disappeared completely, whereas the RPE showed pronounced pigment changes. There was also atrophy of the temporal disc.
The fundus exam via ophthalmoscope is essentially normal early on in cone dystrophy, and definite macular changes usually occur well after visual loss. Fluorescein angiography
Fluorescein angiography
Intravenous Fluorescein angiography or fluorescent angiography is a technique for examining the circulation of the retina using the dye tracing method...
(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
Scotoma
A scotoma is an area of partial alteration in one's field of vision consisting of a partially diminished or entirely degenerated visual acuity which is surrounded by a field of normal - or relatively well-preserved - vision.Every normal mammalian eye has a scotoma in its field of vision, usually...
. In cases with the typical bull’s-eye appearance, there is often relative central sparing.
Because of the wide spectrum of fundus changes and the difficulty in making the diagnosis in the early stages, electroretinography
Electroretinography
Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors , inner retinal cells , and the ganglion cells. Electrodes are usually placed on the cornea and the skin near the eye, although it is possible to record the ERG from skin electrodes...
(ERG) remains the best test for making the diagnosis. Abnormal cone function on the ERG is indicated by a reduced single-flash and flicker response when the test is carried out in a well-lit room (photopic ERG). The relative sparing of rod function in cone dystrophy is evidenced by a normal scotopic ERG, i.e. when the test is carried out in the dark. In more severe or longer standing cases, the dystrophy involves a greater proportion of rods with resultant subnormal scotopic records. Since cone dystrophy is hereditary and can be asymptomatic early on in the disease process, ERG is an invaluable tool in the early diagnosis of patients with positive family histories.
Cone dystrophy in general usually occurs sporadically. Hereditary forms are usually autosomal dominant, and instances of autosomal recessive and X-linked inheritance also occur.
In the differential diagnosis, other macular dystrophies as well as the hereditary optic atrophies must be considered. Fluorescent angiography, ERG, and color vision tests are important tools to help facilitate diagnosis in early stages.