Introduction
Rods and cones are the two types of photoreceptors located in the retina. Photoreceptors are special cells in the retina (a thin, light-sensitive layer of the eye) that convert light into electrical signals and are then sent to the brain. Rods are cylindrical-shaped cells, extremely sensitive, and are responsible for vision at low light levels, also known as scotopic vision. Cones are clustered around the center of the retina, known as the macula, and are responsible for color perception and also for seeing fine details. Cone dystrophy is an eye disorder characterized by the loss of cone cells resulting in abnormalities of central and color vision or visual loss.
What Is Cone Dystrophy?
Cone dystrophy also known as retinal cone dystrophy is a common term to describe a group of rare disorders of the eyes that affect the cone cells of the retina. In cone-rod dystrophy, both the rods and cones are affected, whereas in cone dystrophy, only the cone cells undergo degeneration. There are mainly two forms of cone dystrophy:
- Stationary Cone Dystrophy: In this type, the symptoms are usually present during infancy (from birth) or early childhood, and the condition remains stable throughout life.
- Progressive Cone Dystrophy: In this type of cone dystrophy, the symptoms develop in late childhood or early adulthood and gradually worsen over time. The age of onset, severity, and progression of the condition vary significantly among individuals.
What Are the Causes of Cone Dystrophy?
Cone-rod dystrophy is a rare condition affecting about one in 30,000 to 40,000 individuals. It is an inherited disorder that equally affects males and females and is mainly dependent on the part of the gene that is mutated. Studies have shown that mutations in more than 30 genes are known to cause this condition. Cone dystrophy may be less often inherited in an autosomal recessive pattern. Mutations in genes such as GUCY2D and CRX account for about 50 percent of cases with cone-rod dystrophy. In some rare cases, the mutation can occur sporadically (randomly) or as a first appearance in the individual's genes without any inheritance factors.
About 30 cone-rod dystrophies are differentiated by their inheritance pattern and genetic cause. Genes associated with this condition play a major role in the structure and functioning of photoreceptor cells. A progressive condition can lead to the gradual loss of these cells, resulting in vision loss. Cones are known to break down before the rods; hence, sensitivity to light or impairment in color vision are the first signs of this condition. As the genes associated with cone-rod dystrophy may also be linked with other eye problems, cone dystrophy can occur alone or as a part of a syndrome affecting multiple body areas.
What Are the Signs and Symptoms of Cone Dystrophy?
The symptoms of cone dystrophy vary among individuals, even though it may be the same disorder. The first signs and symptoms of cone dystrophy are noticed during early childhood, such as a decrease in the sharpness or clarity of vision (visual acuity), followed by other signs or symptoms at a later stage. Decreased visual acuity can cause difficulty in reading and writing, affecting the child’s academic performance.
The most common symptoms of cone dystrophy include poor color vision (dyschromatopsia), sensitivity to bright light (photophobia), blind spots in the center of the visual field, and loss of vision, with an age of onset from late teenage up to 60 years. Patients complain of better vision during the day than at night. The healthcare professional can notice many errors, such as red-green or blue-yellow differentiation, during the color vision examination of patients with cone dystrophy. Over time, the patients experience night blindness or peripheral vision (partial vision loss) or develop nystagmus (involuntary eye movements) and drifting or wandering eyes. Usually, the visual acuity deteriorates gradually in cone dystrophy; however, in some severe cases, the deterioration can be rapid after a certain age.
How Is Cone Dystrophy Diagnosed?
The diagnosis of cone dystrophy is based on identifying characteristic symptoms, a complete family history, and a thorough clinical evaluation of the patient. Ophthalmological examinations (eye tests) measure visual acuity, color perseverance, and field of vision. The retina of patient with cone dystrophy may sometimes look relatively normal, and it might be challenging for the doctor to diagnose the exact condition.
An electroretinogram (ERG) is the most useful test to confirm the diagnosis of cone dystrophy. It is performed once in a bright room and then in a dark room. During the procedure, eye drops are applied to anesthetize (numb) the eye, and a special contact lens electrode is placed on the eye. The patient is then directed to watch flashing lights to stimulate the retina. The doctors then measure the electrical signals made by the photoreceptors to determine the functioning of the rods and cones. A weak or an absence of signal indicates cone dystrophy.
How Is Cone Dystrophy Managed?
There is no treatment available for cone dystrophy, and the management of the condition is mainly done by treating the specific symptoms experienced by each individual. Some individuals with cone dystrophy may have better vision at night or in low-light conditions if the rod cells are unaffected. However, complete blindness is uncommon as the peripheral vision (side vision) is usually unaffected. Genetic counseling is recommended for the patients and caregivers to help them understand the disorder and adapt to its implications. Dark sunglasses or tinted lenses can be used in bright environments, and magnifying devices can be helpful while reading. Animal studies have suggested that gene therapy can be beneficial to some extent in cone dystrophy. However, adequate research is required to demonstrate the same in humans.
Conclusion
Cone dystrophy is a rare inherited eye disorder in which the photoreceptor cells called cones are impaired leading to loss of central and color vision. The exact cause of this condition has yet to be completely understood. Some forms are autosomal dominant or recessive, whereas some may occur sporadically without any identifiable reason. Symptomatic treatment and genetic counseling can help in the management of cone dystrophy. Patients or caregivers must consult an eye specialist to prevent further complications when they notice the symptoms.
