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The birth prevalence of Noonan syndrome (NS) is estimated between 1:1000 to 1:2500.

NS typically presents in the neonatal period with feeding difficulties and failure to thrive. Characteristic facial features are often more obvious in infancy : high broad forehead, hypertelorism, palpebral ptosis and downward slanting palpebral fissures, low-set, thick, posteriorly rotated ears, deep philtrum, micrognathia, curly hair and a short neck with sometimes a pterygium colli. With age, the face becomes triangular, with marked skinfolds. The most common congenital heart defect is pulmonary valve stenosis (50-60%) with pulmonic valve dysplasia and various types of cardiac malformations (atrial septal defects, ventricular septal defects ect.). Hypertrophic cardiomyopathy of antenatal onset is common (20%) and may be stable or rapidly progressive. Dilation of coronary arteries and moya-moya disease may develop with aging. Growth delay affects 50%, uncommonly associated with growth hormone deficiency. Weight gain is difficult and many patients remain lean throughout life. Major orthopedic manifestations include sternal deformity, talipes equinovarus, and progressive scoliosis (onset at adolescence). Skin is often dry and sometimes hyperkeratotic on hands and feet. Hair is curly and may be thick or sparse. Peripheral lymphedema may be present and may be progressive and extensive in some. Ocular anomalies (strabismus, refractive errors), and dental crowding are common. Hearing loss is present in 10%. Delayed speech and learning difficulties affect 30-40%. Intellectual disability (often mild) is present in 10-20%. Dyspraxia (clumsiness), attention deficit disorder, agitation, mood disorders and emotional disturbances are not rare, as well as difficulties in identifying and expressing emotions, which can lead to more difficult social interactions. Motor development and puberty are delayed and short stature is present in 50%. Unilateral or bilateral cryptorchidism is present in two-thirds of boys, and hypofertility may affect males, but not females. Thyroid dysfunction may occur. Coagulation defects are frequent but rarely clinically significant. In childhood, there is an increased risk of tumors and leukemias (noteworthy juvenile myelomonocytic leukemia), with a cumulative cancer risk of about 4% by age 20. The risk of common adult cancer does not appear increased.

NS is caused by mutations in PTPN11 (12q24.13) seen in 50% of cases, SOS1 (2p22.1) in 15%, RAF1 (3p25.2), RIT1 (1q22) and LZTR1(22q11.21), and less commonly in other genes associated with the RAS/MAPK signaling pathway. The clinical spectrum of NS may differ slightly between causative genes, and some forms have been described as ''Noonan like'' (NS-like disorder with juvenile myelomonocytic leukemia and NS-like disorder with loose anagen hair).

The diagnosis relies on clinical manifestations but may be difficult because of the highly variable presentation. Molecular genetic testing of the causative genes helps diagnosis and genetic counseling. Mild cases may remain undiagnosed and only brought to clinical attention in adulthood after the birth of a more severely affected child.

Differential diagnoses include Cardio-Facio-Cutaneous syndrome, Costello syndrome, Neurofibromatosis type 1, Noonan syndrome with multiple lentigines (all RASopathies), Baraitser-Winter, Aarskog and Escobar syndromes.

Prenatal diagnosis is possible on chorionic villi or amniotic fluid. Prenatal signs of NS are nonspecific: increased nuchal translucency, cystic hygroma and/or ascites (that may lead to fetal demise), polyhydramnios, cardiomyopathy and congenital heart defect.

Inheritance of NS is autosomal dominant, except LZTR1 which can be either dominant or recessive. Genetic counseling should be offered to affected families.

Treatment requires a multidisciplinary approach. Cardiovascular anomalies are treated with standard approaches. Treatment of growth retardation with growth hormone is still controversial. Developmental disabilities should be addressed early.

The prognosis is variable since the presentation ranges from mild/unrecognized manifestations in adulthood to severe disorder with life-threatening heart disease or malignancy in infancy. Severe cardiomyopathy may lead to early demise.