Aicardi syndrome

Description

Aicardi syndrome is a rare disorder that occurs primarily in women and girls. Aicardi syndrome has typically been characterized by the presence of three main features:

• Absent or underdeveloped tissue that connects the left and right halves of the brain (agenesis or dysgenesis of the corpus callosum)
• Defects in the retina, which is the light-sensitive tissue at the back of the eye (chorioretinal lacunae)
• Seizures that develop within the first year of life (infantile spasms)

Although many affected individuals will have all three of these features, some people with Aicardi syndrome will have only two. Besides these main features, people with Aicardi syndrome typically have additional signs and symptoms.

The folds and grooves on the surface of the brain often do not develop properly in people with Aicardi syndrome. Additional abnormalities that can occur in affected individuals include a difference in the size or shape between the two halves of the brain, cysts in the brain, enlargement of the fluid-filled cavities (ventricles) near the center of the brain, and clusters of cells that are not in the correct location (heterotopias).

Infantile spasms are the most common type of seizure in infants with Aicardi syndrome. However, as they age, many people with Aicardi syndrome will develop additional types of seizures, some of which may not respond well to medication.

People with Aicardi syndrome typically have developmental delays and intellectual disabilities, which can range from mild to severe. For most affected individuals, developmental delays and intellectual disabilities fall within the moderate to severe range.

Chorioretinal lacunae are considered to be a characteristic feature of Aicardi syndrome. However, additional eye abnormalities may also occur. The structure that carries information from the eye to the brain (optic nerve) can be underdeveloped (hypoplasia) or have a gap or hole (coloboma). Another eye abnormality that can occur is called microphthalmia, which is a birth defect in which one or both eyes do not fully develop and are abnormally small. The eye abnormalities in individuals with Aicardi syndrome can cause vision loss.

Some people with Aicardi syndrome have differences between the right and left sides of the face (facial asymmetry), a short area between the upper lip and the nose (philtrum), a flat nose with an upturned tip, large ears, and sparse eyebrows. Other features of this condition include small or malformed hands; spinal and rib abnormalities that lead to progressive abnormal curvature of the spine (scoliosis); and gastrointestinal problems, such as constipation or diarrhea, gastroesophageal reflux, and difficulty feeding.

The number and severity of the signs and symptoms seen in people with Aicardi syndrome can vary. Affected individuals with severe signs and symptoms may not survive past childhood, while those with milder features can survive into adulthood.

Frequency

Aicardi syndrome is a very rare disorder. It occurs in about 1 in 105,000 to 167,000 newborns in the United States. Researchers estimate that there are approximately 4,000 affected individuals worldwide.

Causes

The cause of Aicardi syndrome is unknown. Because it occurs almost exclusively in women and girls, researchers believe that Aicardi syndrome is caused by changes in a gene on the X chromosome. Genetic changes that cause disease are called pathogenic variants.

People normally have 46 chromosomes in each cell. Two of the 46 chromosomes, known as X and Y, are called sex chromosomes because they help determine whether a person will develop male or female sex characteristics. Females typically have two X chromosomes (46,XX), and males have one X chromosome and one Y chromosome (46,XY).

Early in embryonic development in females, one of the two X chromosomes is permanently inactivated in cells other than egg and sperm cells. This process is called X-inactivation. X-inactivation ensures that females, like males, have only one active copy of the X chromosome in each body cell. X-inactivation usually occurs randomly, so that each X chromosome is active in about half the body's cells. Sometimes, however, X-inactivation is not random. This is called skewed X-inactivation.

Skewed X-inactivation sometimes occurs when there is a pathogenic variant in one of the X chromosomes in each cell. Skewed X-inactivation has been identified in some women and girls with Aicardi syndrome. This could explain why some people with Aicardi syndrome are more severely affected than others. Skewed X inactivation also supports the idea that the condition is caused by a pathogenic variant in a gene on the X chromosome. However, the gene that contains this variant has not been identified.

Inheritance

Nearly all known cases of Aicardi syndrome are sporadic, which means that they occur in people with no history of the disorder in their family.

Researchers suspect that the pathogenic variants that cause Aicardi syndrome typically occur as random (de novo) events during the formation of eggs or sperm in an affected individual's parent or during early embryonic development.

Although this condition primarily affects women and girls, cases of men and boys with Aicardi syndrome have been reported.

Other Names for This Condition

• Agenesis of corpus callosum with chorioretinal abnormality

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