According to the National Organization for Rare Disorders, Noonan syndrome is thought to affect approximately 1 in 1,000 to 1 in 2,500 people.
Some reports suggest it may be more common than this, but misdiagnosis or under-diagnosis makes it difficult to determine the true prevalence.
Noonan's syndrome is a genetic condition.
Noonan syndrome is caused by an abnormality or mutation in one of several genes.
The faulty gene associated with Noonan syndrome can be:
- Inherited from a parent. Approximately 50 percent of people with the condition have an affected parent. The parent carrying the genetic abnormality may or may not have any obvious features of the disorder.
- A new mutation that randomly occurs in children without any genetic predisposition.
Having a parent with Noonan syndrome is the greatest risk factor for development of the condition. The risk of passing the defective gene from parent to child is 50 percent for each pregnancy.
Signs and symptoms
The signs and symptoms associated with Noonan syndrome vary greatly from person to person. They can be mild, moderate, or severe and may vary depending on the gene at fault. However, there are a number of common signs and symptoms.
Unusual facial features
This is one of the most obvious signs of Noonan syndrome, and this is a key consideration in diagnosis.
Facial features tend to be more obvious in children, but they grow more subtle as people move into adulthood.
The most common features of Noonan syndrome are:
- Wide-set eyes
- Drooping eyelids
- Pale irises
- Large head
- Broad forehead
- A short, broad nose
- Drooping face that appears to lack expression
- A small jaw
- A short neck
- Excess skin on the neck
- Ears that are low-set and turned toward the back of the head
Issues with growth and development
Noonan syndrome may involve growth and developmental issues.
People with the condition may:
- Have a normal birthweight, but experience delayed growth over time
- Show low levels of growth hormones
- Go through puberty a few years later than the average
- Fail to experience normal growth spurts during puberty
Some people with Noonan syndrome reach normal height by adulthood, but the average height is 5 feet and 3 inches for men with the condition, and 5 feet for women.
Most people with Noonan syndrome have some form of congenital heart disease.
Examples of the types of heart defects they may have are:
Pulmonary stenosis: A narrowing of the pulmonary valve, which helps control blood flow from the heart to the lungs. The heart is under greater pressure to ensure blood is pumped into the lungs. This may occur alone in those with Noonan syndrome, or with other heart defects.
Hypertrophic cardiomyopathy: The muscles of the heart become enlarged, placing strain on the heart.
Septal defects: A hole in the heart which separates the two lower chambers of the heart. Small defects don't cause symptoms, but larger ones cause the heart and lungs to work harder.
Irregular heartbeat: This occurs in most people with Noonan syndrome, with or without abnormal heart structures.
Other signs and symptoms
A variety of other issues can arise.
Some children experience feeding difficulties as it is harder for them to suck and chew.
Eye problems may include strabismus, or crossed eyes, where the eyes point in different directions, lazy eye, where one eye is less able to focus, and slightly blurred vision.
Hearing problems may arise due to structural abnormalities or nerve issues.
Learning difficulties sometimes occur. Intelligence is not normally affected, but some people may have learning difficulties or mild intellectual disabilities. Issues with hearing and eyesight may contribute to these.
Feeding issues can affect infants, who may struggle to suck and chew or may vomit shortly after eating.
Bruising and bleeding can result from blood clotting or platelet issues.
Behavioral issues may occur, including difficulties with attention and emotional challenges. A child may appear immature compared with their peers.
Decreased muscle tone may cause developmental milestones to be reached later than expected. People with Noonan syndrome report decreased muscle strength and clumsiness.
Male infertility may occur, particularly if one or both testicles fail to drop into the scrotum, and this is not corrected at an early age. Female fertility is usually unaffected.
Lymphatic conditions can occur if excess fluid builds up in the lymphatic system. This may be noticeable on the back of the hands or top of the feet.
Kidney conditions sometimes arise, but these are rare and mild.
Other complications that may need special attention and medical or other interventions include:
Increased risk of cancer: According to the National Cancer Institute, people with Noonan syndrome have a higher risk of certain types of cancer, including soft tissue tumors, cancer of the nerve cells, and some forms of leukemia.
Developmental delays: If learning difficulties, intellectual disabilities, or other developmental challenges are significant, children with Noonan syndrome may need a special plan to meet their educational and developmental needs.
Bleeding problems: Excessive bleeding can be common, but it may not be discovered until dental work or surgery is being performed. It is important to be aware of this risk prior to any procedures.
Excess fluid build-up: The excess fluid that collects from lymphatic conditions may pool around the heart and lungs, and this can be dangerous.
Diagnosis and treatment
A doctor will diagnose Noonan syndrome based on key signs and symptoms. Diagnosis can often be difficult because of the subtlety of some of the features associated with the condition.
In some cases, the disorder isn't diagnosed until someone with Noonan syndrome has a child with more obvious signs and symptoms. A diagnosis can be confirmed through molecular genetic testing.
The earlier Noonan syndrome is diagnosed, the earlier treatment and prevention strategies can be implemented, leading to better outcomes for patients.
As Noonan syndrome stems from a genetic defect, there is no cure. Treatment aims to manage the associated symptoms and complications, so it varies for each individual.
Children with Noonan syndrome may need treatment for heart problems.
Potential treatments include:
- Heart conditions: Medication or surgery, or both, may be necessary to manage heart defects. Regular check-ups may also be recommended.
- Growth issues: Height and weight should be routinely monitored during childhood and adolescence. Blood tests may be requested, and growth hormone therapy suggested as a treatment option.
- Learning difficulties: Infant stimulation programs, physical therapy, speech therapy, and educational interventions may be necessary to address learning problems in childhood.
- Excessive bleeding and bruising: Drugs to help blood clotting may be prescribed, and it may be recommended to avoid aspirin and products which contain it.
- Undescended testicle: Surgery may be required if one or both testicles fail to move into position.
- Eye and ear issues: Regular vision and hearing examinations of eye health and hearing may be advised. Glasses, contact lenses, and hearing aids will be recommended as appropriate.
Prevention and outlook
As many cases of Noonan syndrome occur randomly, there is currently no known way to prevent the condition developing.
Those with a family history of the condition should discuss it with their doctor or a genetic counselor before trying to conceive.
According to the National Center for Advancing Translational Sciences, people with Noonan syndrome have a threefold higher mortality rate than the general population.
However, symptoms vary widely from person to person, and the outlook varies accordingly. Long-term prognosis tends to depend on the presence and severity of heart defects.
The Genetic and Rare Diseases Information Center advise that, with early intervention and proper management of symptoms, most people with Noonan syndrome reach adulthood and lead independent lives.