In rare cases, cone cells in the eye can be damaged, resulting in a type of cone dystrophy, which is a general term to describe rare disorders that affect cone cells.
Types of Cone Dystrophies
To date, cone dystrophies have been classified into two groups: stationary and progressive disorders. Stationary cone dystrophies are usually present at birth (congenital disorder) or in early childhood and continue to be stable over the lifetime of the patient. The opposite is true for progressive cone dystrophies in which symptoms worsen over time. Progressive dystrophies tend to occur in late adolescence to early adulthood. Health care professionals, however, have differed in how to use the term cone dystrophy. Some use the term only for progressive disorders, while others will use the term for both stable and progressive disorders.
Cone dystrophies can be inherited or occur spontaneously without a specific cause for the disease. Some health care providers also call spontaneous cone dystrophies sporadic dystrophies. Types of cone dystrophies include complete and incomplete achromatopsia—a stationary and inherited disorder in which a person has the inability to see color as well as having a sensitivity to light (photophobia) and decreased vision; blue cone monochromatism—a rare X-linked disorder in which persons have poor or no color discrimination, increased sensitivity to light, and decreased central vision; and X-linked progressive cone dystrophy—an uncommon disorder that may also cause degeneration in rod cells.
Inherited dystrophies are caused by a mutation in one or several genes that are associated with cone proteins and can be inherited as an autosomal dominant (only a single copy of the gene, either from the mother or father, is mutated) or recessive (both copies of the gene are mutated) trait or as an X-linked recessive trait (the abnormal gene is located on the X chromosome). If a cone dystrophy is X-linked, then males are affected more often than females as they only have one X chromosome. Females can either be a carrier—having one mutated X chromosome—or have the disorder if both X chromosomes are affected. In sporadic, autosomal dominant and recessive cone dystrophies, males and females are affected at the same rate.
Signs and Symptoms
Symptoms in persons with cone dystrophies can vary between patients as does the progression and severity of the degeneration. In cone dystrophies, cone cells can degenerate, resulting in loss of color vision (not color blindness as this is generally caused by a different mechanism) and loss of vision acuity particularly in central vision—when looking straight ahead. Often, this also causes an increased sensitivity to light that can be painful. In sporadic dystrophies, the age of onset can vary, as does the amount of visual deficits. In some cases, patients may become legally blind, meaning their vision is 20/200 or worse and cannot be corrected with glasses or contact lenses.
Other symptoms may include patients developing a nystagmus (rapid, involuntary movements of the eyes) associated with their disorder. Generally, persons do not become completely blind as the rod cells are often spared. In cases where the onset of the dystrophy began later in life, rod cells may be affected, reducing peripheral and low-light vision in these patients.
To date, there are no known cures for cone dystrophies. Treatments are provided to alleviate specific symptoms, such as wearing dark sunglasses in brightly lit areas and using magnifying devices to help improve clarity when reading.
Jennifer L. Hellier
See also: Color Blindness; Color Perception; Fovea Centralis; National Organization for Rare Disorders; Retina; Visual System
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