What is Noonan syndrome?
Noonan syndrome (NS) is a hereditary condition that occurs in 1/1,000 to 1/2,500 births, in both boys and girls. The name “Noonan” comes from an American paediatric cardiologist, Dr Jacqueline Noonan. In 1963, she described nine patients with narrowing of the pulmonary artery, similar facial features, chest deformities and short stature.
What causes Noonan syndrome?
The syndrome is caused by a change in one of the following seven genes involved in Noonan syndrome: PTPN11, SOS1, KRAS, RAF1, SHOC2, BRAF and NRAS.
In half of patients, the condition is familial, so there are also other family members with Noonan syndrome. Inheritance is then dominant: there is a 50% chance of passing the disease on to the children.
How is Noonan syndrome diagnosed?
Diagnosis is primarily based on recognition of specific physical characteristics. These characteristics can differ greatly from one patient to another. Young children in particular often have a triangular face with a large forehead. The eyes are wide apart and the upper eyelid sometimes hangs down (ptosis). Some also have a fold of skin in the inner corner of the eye. The eyes sometimes have a light blue to blue-green colour. The ears are set low, tilt back a bit and have a thick rim. The middle groove in the upper lip is strongly pronounced. Often the neck is short and the hairline on the neck is very low.
These symptoms can vary greatly from patient to patient. A clinical scoring system was therefore developed.
What are the signs and symptoms of Noonan syndrome?
- Growth lag
Children with Noonan syndrome are usually born with a low birth weight and small length. During the first 3 years of life, growth is slower than normal and many children with NS grow on or below the bottom lines of the growth curve. During puberty, the growth spurt is smaller than that of other children in puberty. The average height for men in adulthood is 160-165 cm, and 150-155 cm for women.
- Late puberty development
Boys and girls with NS have delayed pubertal development, which is reflected in a delayed bone age. They enter puberty on average 2-3 years later than their peers, i.e. at the age of 14 years in boys and 13 years in girls on average.
- Undescended testes (cryptorchidism) and fertility
In 80% of boys with NS, the testes do not fully descend into the scrotum and surgical correction is often necessary. This may reduce the fertility of men with NS.
- Heart defects
Eighty percent of patients with NS have a heart defect. It is usually pulmonary stenosis: a narrowing of the pulmonary artery of the heart, generally just above the pulmonary heart valve. The valve itself can also be misshapen.
- Increased bleeding tendency
Patients with NS often bruise more easily, have more nosebleeds or have profuse periods. There is also a risk of increased blood loss during surgery.
- Blood cells and spleen enlargement
Young children with NS often have a high number of specific white blood cells (monocytes) and a low number of platelets. This abnormality is often associated with an enlarged spleen and/or liver. This situation usually remains stable and improves with age.
- Kidney abnormalities
Kidney abnormalities are less common (10% of patients) and are usually of little clinical importance: one kidney instead of two, or more ureters than normal.
- Nutritional problems
Infants with NS frequently have feeding problems (50-75%), probably due to a weak sucking reflex and delayed development of bowel function. They suck weakly, drink slowly, often have reflux and vomit frequently. As a result, these children do not gain enough weight and grow slowly in the first year of life.
- Mouth and dental problems
Patients with NS often have a high palate and a narrow lower jaw. The upper and lower teeth are not always aligned, and some children have articulation disorders.
- Psychomotor development
Children with Noonan syndrome often have delayed motor development: sitting without support at around 10 months, walking at 21 months. They are often described as clumsy. Due to the difficulty developing fine motor skills, there are sometimes problems with learning to draw and write.
- Articulation problems often occur (70%)
- Between 15-35% of children with Noonan syndrome have a mild intellectual disability and need special education.
- Children with Noonan syndrome show more behavioural problems than other children. They can be stubborn and often act younger than they are. They are often restless, nervous children who can react very strongly and demand a lot of attention, but there are rarely any real psychiatric problems.
How is Noonan syndrome treated?
NS cannot be cured; however it is possible to manage the various symptoms and problems caused by the syndrome. A child might need different treatments and receive support to manage certain problems. For example, treatment with growth hormone might improve growth problems, surgery can help repair heart defects, professional guidance by a speech therapist can help infants with feeding problems, etc.