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The Basics of JHD

January 16, 2022

Huntington's Disease Youth Organization

HDYO has more information about HD available for young people, parents and professionals on our site:


This section explains the basics of Juvenile-onset Huntington’s disease (JoHD) and provides some insight into the kind of support available for families impacted by JoHD.

What is JoHD?

When George Huntington first identified Huntington’s Disease (HD) in 1872, he suggested that the disease affects adults only. It is now known that HD can occur in individuals at any age - young or old. When someone develops symptoms of HD before the age of 20, they are classed as having JoHD. They will be classed as having JoHD no matter what their current age is. When someone with JoHD is currently under the age of 18, this is referred to as pediatric HD (PHD). So effectively ‘PHD/JoHD’ means a young person with HD.

Why do people get JoHD rather than adult-onset HD?

HD is hereditary - meaning it can be passed down, from a parent to child, through genes in our DNA. Features we have are passed to you from your parents via these genes - that’s why you might have hair like your dad, or brown eyes like your mum. Each gene comes in ‘pairs’ because we inherit one copy of a gene our mum and one copy from our dad.

CAG repeat

In 1993, a team of researchers discovered the gene responsible for HD (Huntingtin gene) is on chromosome 4. Everybody has two copies of this gene which make a normal protein called huntingtin. We know our bodies need this huntingtin protein. The researchers found that one part of the gene repeats 3 letters of its genetic code: C, A and G, and is known as a the ‘CAG repeat’ – like a stutter, or when you hold a key down too long on a computer keeeeeeeeeeeeeeeeeyboard.

Having more repetitions than usual of this CAG repeat is what causes the development of HD. Put simply, people who develop HD have longer CAG repeats than those that don’t and the exact numbers we explain below:

26 And lower (Unaffected range) - As we mentioned, everybody has a CAG repeat in their Huntingtin gene (even those in families not affected by HD). People who don’t have HD usually have around 10-20 CAG repeats. Any number of CAG repeats up to 26 is fine and will not cause HD.

40 Repeats and higher (disease range) - Anyone with 40 CAG repeats or above is, unfortunately, certain to develop HD in their lifetime. They also have a 50% risk of passing the mutated gene on to each of their children as well. CAG repeats in the range between 27 and 39 are less straightforward…

36 - 39 Repeats (reduced penetrance or risk of disease range) - Anyone with 36-39 CAG repeats may or may not develop HD in an average lifespan. As scientists do not understand this range fully, it is sometimes referred to as the ‘gray area’.

27 - 35 Repeats (high unaffected range) - Anyone with 35 CAG repeats or lower will not develop HD themselves. However, someone who is in the 27-35 CAG repeat range has a possible risk of their children developing in HD.

Most people who have HD have a CAG repeat of between 40-50. For these people, it is hard to tell when their symptoms will begin as there is a lot of variability. For example, two people with the same CAG repeat of 45 could start to see symptoms of HD begin at completely different ages. But generally, a higher CAG repeat is linked to people showing HD symptoms earlier in life. Kids with JoHD tend to have higher CAG repeats, usually 50 and above, leading to their symptoms starting before the age of 21.

Is JoHD common?

JoHD is even more rare than adult onset. Around 5% of all people with HD develop JoHD. Most people (around 80%) who have JoHD inherit the gene from their father. This is thought to be due to CAG repeats being less stable, meaning they can increase by larger numbers when inherited from a male. In general, CAG repeats can increase or decrease a little when passed from generation to generation. However, in a small percentage of people there is a larger increase which can lead to JoHD. It’s important to remember, however, that JoHD is very rare.

Symptoms of JoHD

Support for young people with JoHD is vital. Their needs differ from typical adult-onset HD in both the symptoms and the ways that it changes the lives of the individuals and families. JoHD presents unique challenges to young people, their caregivers, and the various professionals who are called upon to assist them.

Typical early symptoms of JoHD

  • Stiffness or rigidity
  • Clumsiness
  • Problems with speech
  • Decline in cognitive function (how a person thinks, remembers, and learns)
  • Changes in behavior
  • Seizures (in ~30-35% of cases there is a tendency to have epileptic seizures - something almost never seen in adult-onset HD)
  • Progression of JoHD is typically faster than that of adult-onset HD. However, as with adult-onset HD, there is a lot of variability in each individuals’ symptoms and how their condition progresses.

Diagnosing JoHD


Diagnosing JoHD can be very difficult. As mentioned previously, JoHD is rare and not many physicians have seen patients with JoHD before. This can lead to a great deal of time being spent eliminating other possible causes of symptoms. Even the best physicians may need to examine a child several times before being confident of a diagnosis of JoHD. A pediatric neurologist can usually determine that the child has a disorder affecting a portion of the brain called the basal ganglia (which is affected in HD) but it can be difficult to distinguish HD from other similar diseases. It can also be difficult to be sure that early symptoms (such as changes in behavior, difficulty in school, and being clumsy) are caused by JoHD. If a child shows these symptoms, it’s important to look into other causes rather than assuming it is JoHD just because the condition is in the family.

Testing for JoHD is not straightforward. Most young people displaying symptoms of JoHD have high repeat counts which are suggestive of those symptoms being JoHD. Other young people may show symptoms of JoHD but don’t have high CAG repeats. The risk then is that if you test a young person for the gene the result could be in the adult-onset range. In that case, any symptoms the young person has might be due to other causes and not necessarily JoHD. National and international guidelines state that unless a healthcare professional feels symptoms are due to JoHD, a person should not be tested until they are at least 18 years old. Testing before the age of 18 takes away the person's right to decide to be tested, and, in some countries, may cause the young person to be declined from obtaining certain benefits when they are older (such as life insurance or long-term care insurance).

As a result of all these difficulties, it can take a while for a young person to be diagnosed with JoHD. This can be frustrating for everyone. Although finding support can be a bit harder during this time, HDYO encourages young people and families to reach out to us. While we cannot provide medical advice, HDYO will try to connect you to people who might be able to help.

Support for JoHD

Below you will find a list of health care professionals HDYO considers key in the care of someone with JoHD.

Neurologist: Having a neurologist (ideally a pediatric neurologist) who specializes in HD can be a huge help with understanding and treatment of JoHD.

Physiotherapist: A physiotherapy program for children with JoHD focuses on maintaining range of motion and independent mobility. Although many children with JoHD have stiffness, physical therapy and activity can help to prevent muscle contractures (spasms).

Occupational Therapist: As JoHD progresses, equipment and mobility support may be required. An occupational therapist can assess what support is required to help keep young people with JoHD as independent as possible.

Speech and Language Therapist: A speech and language therapist can create strategies to help maintain speech, communication, and swallowing for a young person with JoHD. One way speech therapists can help is by use of devices which allow the user to “speak” a word or phrase by pressing squares on a computer or surface. Specific words and phrases can be programmed into the computer according to the child’s needs, for example, “I need a drink” and “I need a hug.”

Dietician/Nutritionist: A dietician or nutritionist can provide guidance to help a young person with JoHD to maintain their weight.

HHD Non-profits, Charities and associations: The Huntington’s Disease Non-profits, Charities and Associations (HDNCAs) provide support to families impacted by HD in their region. Many countries have a HDNCA, but the support available in different regions varies. Many HDNCA’s have good information available on JoHD. A good first step is to speak with a healthcare professional and to contact your nearest HDNCA. You can find contact details of your local HDNCA here.

JoHD is a difficult disease to cope with. If you are dealing with or have a family member with JoHD, please contact HDYO by clicking this link. We are here to help.

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