Ehlers-Danlos syndrome is a group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Defects in connective tissues cause the signs and symptoms of these conditions, which range from mildly loose joints to life-threatening complications.
The various forms of Ehlers-Danlos syndrome have been classified in several different ways. Originally, 11 forms of Ehlers-Danlos syndrome were named using Roman numerals to indicate the types (type I, type II, and so on). In 1997, researchers proposed a simpler classification (the Villefranche nomenclature) that reduced the number of types to six and gave them descriptive names based on their major features. In 2017, the classification was updated to include rare forms of Ehlers-Danlos syndrome that were discovered more recently. The 2017 classification describes 13 types of Ehlers-Danlos syndrome.
An unusually large range of joint movement (hypermobility) occurs in most forms of Ehlers-Danlos syndrome, and it is a hallmark feature of the hypermobile type. Infants and children with hypermobility often have weak muscle tone (hypotonia), which can delay the development of motor skills such as sitting, standing, and walking. The loose joints are unstable and prone to dislocation and chronic pain. In the arthrochalasia type of Ehlers-Danlos syndrome, infants have hypermobility and dislocations of both hips at birth.
Many people with the Ehlers-Danlos syndromes have soft, velvety skin that is highly stretchy (elastic) and fragile. Affected individuals tend to bruise easily, and some types of the condition also cause abnormal scarring. People with the classical form of Ehlers-Danlos syndrome experience wounds that split open with little bleeding and leave scars that widen over time to create characteristic "cigarette paper" scars. The dermatosparaxis type of the disorder is characterized by loose skin that sags and wrinkles, and extra (redundant) folds of skin may be present.
Some forms of Ehlers-Danlos syndrome, notably the vascular type and to a lesser extent the kyphoscoliotic, classical, and classical-like types, can cause unpredictable tearing (rupture) of blood vessels, leading to internal bleeding and other potentially life-threatening complications. The vascular type of Ehlers-Danlos syndrome is also associated with an increased risk of organ rupture, including tearing of the intestine and rupture of the uterus during pregnancy.
Other types of Ehlers-Danlos syndrome have additional signs and symptoms. The cardiac-valvular type causes severe problems with the valves that control the movement of blood through the heart. People with the kyphoscoliotic type experience severe curvature of the spine that worsens over time and can interfere with breathing by restricting lung expansion. A type of Ehlers-Danlos syndrome called brittle cornea syndrome is characterized by thinness of the clear covering of the eye (the cornea) and other eye abnormalities. The spondylodysplastic type features short stature and skeletal abnormalities such as abnormally curved (bowed) limbs. Abnormalities of muscles, including hypotonia and permanently bent joints (contractures), are among the characteristic signs of the musculocontractural and myopathic forms of Ehlers-Danlos syndrome. The periodontal type causes abnormalities of the teeth and gums.
The combined prevalence of all types of Ehlers-Danlos syndrome appears to be at least 1 in 5,000 individuals worldwide. The hypermobile and classical forms are most common; the hypermobile type may affect as many as 1 in 5,000 to 20,000 people, while the classical type probably occurs in 1 in 20,000 to 40,000 people. Other forms of Ehlers-Danlos syndrome are rare, often with only a few cases or affected families described in the medical literature.
Mutations in at least 19 genes have been found to cause the Ehlers-Danlos syndromes. Mutations in the COL5A1 or COL5A2 gene, or rarely in the COL1A1 gene, can cause the classical type. Mutations in the TNXB gene cause the classical-like type and have been reported in a very small percentage of cases of the hypermobile type (although in most people with this type, the cause is unknown). The cardiac-valvular type and some cases of the arthrochalasia type are caused by COL1A2 gene mutations; mutations in the COL1A1 gene have also been found in people with the arthrochalasia type. Most cases of the vascular type result from mutations in the COL3A1 gene, although rarely this type is caused by certain COL1A1 gene mutations. The dermatosparaxis type is caused by mutations in the ADAMTS2 gene. PLOD1 or FKBP14 gene mutations result in the kyphoscoliotic type. Other rare forms of Ehlers-Danlos syndrome result from mutations in other genes.
Some of the genes associated with the Ehlers-Danlos syndromes, including COL1A1, COL1A2, COL3A1, COL5A1, and COL5A2, provide instructions for making pieces of several different types of collagen. These pieces assemble to form mature collagen molecules that give structure and strength to connective tissues throughout the body. Other genes, including ADAMTS2, FKBP14, PLOD1, and TNXB, provide instructions for making proteins that process, fold, or interact with collagen. Mutations in any of these genes disrupt the production or processing of collagen, preventing these molecules from being assembled properly. These changes weaken connective tissues in the skin, bones, and other parts of the body, resulting in the characteristic features of the Ehlers-Danlos syndromes.
Some genes associated with recently described types of Ehlers-Danlos syndrome have functions that appear to be unrelated to collagen. For many of these genes, it is not clear how mutations lead to hypermobility, elastic skin, and other features of these conditions.
The inheritance pattern of the Ehlers-Danlos syndromes varies by type. The classical, vascular, arthrochalasia, and periodontal forms of the disorder, and likely the hypermobile type, have an autosomal dominant pattern of inheritance. Autosomal dominant inheritance means that one copy of the altered gene in each cell is sufficient to cause the disorder. In some cases, an affected person inherits the mutation from one affected parent. Other cases result from new (de novo) gene mutations and occur in people with no history of the disorder in their family.
The classical-like, cardiac-valvular, dermatosparaxis, kyphoscoliotic, spondylodysplastic, and musculocontractural types of Ehlers-Danlos syndrome, as well as brittle cornea syndrome, are inherited in an autosomal recessive pattern. In autosomal recessive inheritance, two copies of a gene in each cell are altered. Most often, the parents of an individual with an autosomal recessive disorder are carriers of one copy of the altered gene but do not show signs and symptoms of the disorder.
The myopathic type of Ehlers-Danlos syndrome can have either an autosomal dominant or autosomal recessive pattern of inheritance.