The 5 Most Common Rare Diseases in Children

The 5 most common rare diseases in children: by prevalence and rate of diagnosis

Down syndrome

Widely recognised as the most prevalent and most commonly diagnosed genetic disorder, Down syndrome is not an inherited genetic syndrome. It is caused by the presence of an extra copy, or extra partial copy, of Chromosome 21. It is also sometimes known as Trisomy 21. 

This duplication occurs spontaneously, and is not inherited from either parent. There are known risk factors for this cause however, prime among them maternal age at the time of conception. 

Down syndrome occurs in around 1 in every 700 live births in the US today. This makes it one of the most common, if not the most common genetic condition based on prevalence.

Advancements in prenatal genetic screening have made screening for Down syndrome in a fetus a part of standard prenatal care in many developed countries. As early as the first trimester, parents-to-be can opt for genetic screening for down syndrome. They may also undergo invasive screening, known as chorionic villus sampling to understand with a high degree of accuracy if their unborn child will be born with Down syndrome. 

Genetic counseling for Down syndrome is very well developed and common. Any genetic screening during pregnancy which suggests a baby has a higher risk for being born with the syndrome, should be followed by genetic counseling. This is to ensure the parents-to-be are fully informed about their options in terms of further genetic testing, and any decisions concerning the continuation of the pregnancy. 

The symptoms of Down syndrome include unique facial features characteristic of the syndrome, including a flattened face, and wide bridge of the nose. People with Down syndrome may also be born with heart defects, and other medical conditions. 

Turner syndrome

Turner syndrome occurs in 1 in every 2000 life female births, making it a more common genetic disorder but only amongst females. The syndrome is not inherited, and is the result of a spontaneous mutation which means affected females are born with a missing X chromosome. This mutation is considered to be completely random, with no known cause.

This rare disease is diagnosed in children, but more often than not is diagnosed during puberty when it’s main symptoms – delayed puberty, infertility- become apparent. 

However symptoms of the syndrome, including a short stature, an accumulation of liquid in the hands and neck, and congenital heart defects, can lead to a diagnosis earlier in an individual’s childhood. 

Turner syndrome has very clear symptoms, making it easier to diagnose than some rare diseases. Genetic counseling for Turner syndrome in a child will focus on recommending the right targeted genetic testing to diagnose the condition, and the management of health issues related to the syndrome. 

Fragile X syndrome

This rare disease occurs in anywhere between 1 in 4-6,000 live births. This makes it fairly common as rare diseases go, but generally it occurs more often in males, and symptoms amongst males are more severe, leading to a higher rate of diagnosis amongst them. 

Genetic mutations in the FMR1 gene cause the syndrome. This particular gene is responsible for the production of a protein which helps with brain development. The specific mutation in the gene which causes Fragile X prevents the production of this protein. 

As a genetic disorder inherited in an X-linked dominant pattern, a mutation in just one copy of the gene on the X chromosome triggers it. With only one X chromosome, males tend to have more severe symptoms. 

Common symptoms of this genetic disorder in children include intellectual disability and developmental delay which can range from mild to moderate. Children with Fragile X are also considered higher risk for an autism, ADD, ADHD or other behavioral and neurological diagnosis. 

In some children the initial symptoms of Fragile X may in fact be developmental delay and/or behavioral issues. These may then lead to a recommendation for genetic testing to understand better the precise cause of these. 

Unique facial features of the syndrome include a long and narrow face, and large ears. These may be obvious in a child, or may become so with age. 

Tay-Sachs syndrome

Tay-Sachs is a genetic disorder which becomes apparent in infants from around 3 to 6 months when their physical development appears to regress, and their muscle tone appears affected and weaker. This regression is often accompanied by a startled reaction to loud stimuli. Other symptoms in infants include seizures along with vision and hearing loss, and eventual paralysis. Infants with the syndrome also often develop a cherry-red spot in their eye. 

Tay-Sachs is a degenerative and regressive genetic disorder, and children with the most common and the most severe form of the disease do not live beyond early childhood. 

This genetic disorder is more common in children with parents who are both of Ashkenazi descent, and genetic testing for Tay-Sachs specifically is recommended for these specific parents-to-be. A genetic counselor would be able to guide them through the testing options, and help them to understand any results of this process. 


This genetic disorder is screened for during the newborn screening that occurs shortly after an infant is born. This testing allows affected infants to receive prompt and early treatment which treats the infant and ensures their long term quality of life.

Phenylketonuria is an inherited genetic disorder which leads to increased levels of phenylalanine in the blood. If this is allowed to build up in the blood it can lead to intellectual disability and other serious symptoms affecting the health of someone with the syndrome. It is due to the seriousness of the disorders’ symptoms, and the ability of early treatment to treat it, that it is a part of newborn screening. 

Any parent or guardian who suspects that their infant or child may be affected by a genetic disorder, should consult with a doctor and a genetic counselor. Navigating the diagnosis of rare diseases in children is not easy, and genetic counseling provides an important genetic and emotional support service. 

A one-on-one session with a genetic counselor can be incredibly helpful in detecting rare symptoms and syndromes. This process will allow families to get one step closer to finding a diagnosis, and can  potentially prevent the severity of a rare genetic condition including Apert syndrome and others.

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