The following summary is from Orphanet, a European reference portal for information on rare diseases and orphan drugs.

A rare developmental defect during embryogenesis that is characterized by growth dysregulation with overgrowth of the brain and multiple somatic tissues, with capillary skin malformations, megalencephaly (MEG) or hemimegalencephaly (HMEG), cortical brain abnormalities (in particular polymicrogyria), typical facial dysmorphisms, abnormalities of somatic growth with asymmetry of the body and brain, developmental delay and digital anomalies.

Over 200 patients have been reported without sex predominance.

Symptoms are usually recognizable at birth. Their severity varies widely among patients. Megalencephaly is a major clinical feature (MEG: occipitofrontal circumference [OFC] greater than or equal to 3 SD above the mean), which sometimes progresses to hydrocephaly, malformations of cortical development with polymicrogyria and Chiari malformation. Cutaneous capillary anomalies are often scattered over the limbs, palms, soles and trunk, are frequently pink/red and are aggravated by crying and emotions. Facial dysmorphism is observed with frontal bossing, full cheeks, prominent lips and nevus flammeus of the nose and/or philtrum and upper lip. There is a delay in speech and motor skills. Patients may present neurological symptoms, mainly neonatal hypotonia, and, less frequently, seizures. Additional clinical manifestations include prenatal overgrowth, limb asymmetry, joint laxity, soft skin and thick, ''doughy'' subcutaneous tissue, postaxial polydactyly and/or syndactyly of toes 2-3 or fingers 3-4. Some patients develop neoplasias (risk of tumor development estimated at 2-3%). There is a slight increased risk for congenital heart defects and/or cardiac rhythm abnormalities. Adult OFCs range from +2 to +10 SDs above the mean.

Somatic mutations of the PIK3CA gene (3q26), with evidence of postzygotic mosaicism, were found in several patients. Two individuals had a de novo germline pathogenic variant in PIK3CA. The gene PIK3CA encodes the alpha catalytic subunit of phosphatidylinositol-4,5-bisphosphate 3-kinase. PIK3CA mutations are found in several benign overgrowth syndromes, collectively known as PIK3CA-related overgrowth spectrum (PROS). The mutational spectrum in children with the disorder is broader than other PIK3CA-related overgrowth disorders.

The disorder can be diagnosed based on clinical findings in individuals with classic features of MEG or HMEG (major finding 1) associated with neurologic findings of hypotonia, seizures, and mild to severe intellectual disability and characteristic capillary malformations (major finding 2) with focal or generalized somatic overgrowth.. Mosaic mutations of the PIK3CA gene were mainly identified with the advent of massively parallel or next-generation sequencing (NGS) methods. that facilitate detection of low-frequency variation. The level of mosaicism is often lowest in blood‐derived DNA, and higher in saliva and fibroblast‐derived DNA: multiple tissue samples should be tested, prioritizing samples other than blood.

Differential diagnoses include Hemimegalencephaly (HMEG), Megalencephaly polymicrogyria post-axial polydactyly hydrocephalus (MPPH), Klippel-Trénaunay syndrome (KTS), Beckwith-Wiedemann syndrome (BWS), PTEN-related overgrowth disorders.

Findings of prenatal ultrasound include marked fetal overgrowth and progressive macrocephaly in the absence of maternal hyperglycemia or fetal hyperinsulinemia, ventriculomegaly, hydrocephalus, frontal bossing, polydactyly, limb asymmetry, polyhydramnios, hydrops fetalis and pleural effusions.

The risk to sibs of a proband with somatic mosaicism for a pathogenic variant in PIK3CA would be expected to be the same as in the general population. However, low-level germline mosaicism may theoretically be present in a parent of a very rare child with a germline PIK3CA pathogenic variant.

Management requires a multidisciplinary approach (involving pediatrician, neurologist, ophthalmologist, cardiologist, orthopedist, physiatrist, ENT, and dermatologist). Neurologic complications (obstructive hydrocephalus, increased intracranial pressure, cerebellar tonsillar ectopia or Chiari malformation; epilepsy in those with HMEG) may warrant neurosurgical intervention. Regular surveillance is recommended (brain MRI in the first 8 years of life, kidney ultrasound for Wilms tumor screening in the first 8 years of life). However, tumor risk in the disorder appears to be lower than in BWS.

Prognosis depends on the severity of symptoms. Early death, due to complex cardiac heart disease and arrhythmia, has been reported in rare occasions.

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