The prevalence of congenital Chiari I malformation, defined as tonsilar herniations of 3 to 5 mm or greater, was previously believed to be in the range of one per 1000 births, but is likely much higher. Women are three times more likely than men to have a congenital Chiari malformation. Type II malformations are more prevalent in people of Celtic descent. A study using upright MRI found cerebellar tonsillar ectopia in 23% of adults with headache from motor-vehicle-accident head trauma. Upright MRI was more than twice as sensitive as standard MRI, likely because gravity affects cerebellar position.

Cases of congenital Chiari malformation may be explained by evolutionary and genetic factors. Typically, an infant's brain weighs around 400g at birth and triples to 1100-1400g by age 11. At the same time the cranium triples in volume from 500 cm to 1500 cm to accommodate the growing brain. During human evolution, the skull underwent numerous changes to accommodate the growing brain. The evolutionary changes included increased size and shape of the skull, decreased basal angle and basicranial length. These modifications resulted in significant reduction of the size of the posterior fossa in modern humans. In normal adults, the posterior fossa comprises 27% of the total intracranial space, while in adults with Chiari Type I, it is only 21%. If a modern brain is paired with a less modern skull, the posterior fossa may be too small, so that the only place where the cerebellum can expand is the foramen magnum, leading to development of Chiari Type I. H. neanderthalensis had platycephalic (flattened) skull. Some cases of Chiari are associated with platybasia (flattening of the skull base).

Basilar invagination can be present at birth. If the condition develops after birth, it is usually the result of injury or diseases. If due to injury, about half the time it is caused by vehicle or bicycle accidents; 25% of the time by falls and 10% of the time by recreational activities such as diving accidents.

It also occurs in patients with bone diseases, such as osteomalacia, rheumatoid arthritis, Paget's disease, Ehlers-Danlos syndrome, Marfan syndrome, and osteogenesis imperfecta.

The prevalence of Klippel–Feil syndrome is unknown due to the fact that there was no study done to determine the true prevalence.

Although the actual occurrence for the KFS syndrome is unknown, it is estimated to occur 1 in 40,000 to 42,000 newborns worldwide. In addition, females seem to be affected slightly more often than males.

Iniencephaly is thought to make up around 1% of all fetal abnormalities, with an incidence rate estimated at 0.1 to 10 in 10,000 deliveries.

For unknown reasons, this disease seems to occur most often in newborn females (about 90%).

Studies have shown that obesity of the mother increases the risk of neural tube disorders such as iniencephaly by 1.7 fold while severe obesity increases the risk by over 3 fold.

The most widely accepted pathophysiological mechanism by which Chiari type I malformations occur is by a reduction or lack of development of the posterior fossa as a result of congenital or acquired disorders. Congenital causes include hydrocephalus, craniosynostosis (especially of the lambdoid suture), hyperostosis (such as craniometaphyseal dysplasia, osteopetrosis, erythroid hyperplasia), X-linked vitamin D-resistant rickets, and neurofibromatosis type I. Acquired disorders include space occupying lesions due to one of several potential causes ranging from brain tumors to hematomas.

Head trauma may cause cerebellar tonsillar ectopia, possibly because of dural strain. Additionally, ectopia may be present but asymptomatic until whiplash causes it to become symptomatic. Posterior fossa hypoplasia causes reduced cerebral and spinal compliance.

The heterogeneity of the Klippel–Feil syndrome has made it difficult to outline the diagnosis as well as the prognosis classes for this disease. Because of this, it has complicated the exact explanation of the genetic cause of the syndrome.

The prognosis for most individuals with KFS is good if the disorder is treated early on and appropriately. Activities that can injure the neck should be avoided, as it may contribute to further damage. Other diseases associated with the syndrome can be fatal if not treated, or if found too late to be treatable.

Vein of Galen malformations are devastating complications. Studies have shown that 77% of untreated cases result in mortality. Even after surgical treatment, the mortality rate remains as high as 39.4%. Most cases occur during infancy when the mortality rates are at their highest. Vein of Galen malformations are a relatively unknown affliction, attributed to the rareness of the malformations. Therefore, when a child is diagnosed with a faulty Great Cerebral Vein of Galen, most parents know little to nothing about what they are dealing with. To counteract this, support sites have been created which offer information, advice, and a community of support to the afflicted (, ).

Lymphangiomas are rare, accounting for 4% of all vascular tumors in children. Although lymphangioma can become evident at any age, 50% are seen at birth, and 90% of lymphangiomas are evident by 2 years of age.

The prognosis for lymphangioma circumscriptum and cavernous lymphangioma is generally excellent. This condition is associated with minor bleeding, recurrent cellulitis, and lymph fluid leakage. Two cases of lymphangiosarcoma arising from lymphangioma circumscriptum have been reported; however, in both of the patients, the preexisting lesion was exposed to extensive radiation therapy.

In cystic hygroma, large cysts can cause dysphagia, respiratory problems, and serious infection if they involve the neck. Patients with cystic hygroma should receive cytogenetic analysis to determine if they have chromosomal abnormalities, and parents should receive genetic counseling because this condition can recur in subsequent pregnancies.

Complications after surgical removal of cystic hygroma include damage to the structures in the neck, infection, and return of the cystic hygroma.

Acalvaria usually occurs in less than 1 of every 100,000 births. By way of epidemiological data, it is thought that females are more prone to have this defect. Currently, acalvaria is not thought to have much of a risk of recurrence.

Some studies suggest a hormonal link. Specifically, the hormone relaxin has been indicated.

A genetic factor is indicated since the trait runs in families and there is an increased occurrence in some ethnic populations (e.g., Native Americans, Lapps / Sami people). A locus has been described on chromosome 13. Beukes familial dysplasia, on the other hand, was found to map to an 11-cM region on chromosome 4q35, with nonpenetrant carriers not affected.

A traumatic neuroma (also known as "amputation neuroma" or "pseudoneuroma") is a type of neuroma which results from trauma to a nerve, usually during a surgical procedure. The most common oral locations are on the tongue and near the mental foramen of the mouth. They are relatively rare on the head and neck.

The main risk is intracranial hemorrhage. This risk is difficult to quantify since many patients with asymptomatic AVMs will never come to medical attention. Small AVMs tend to bleed more often than do larger ones, the opposite of cerebral aneurysms. If a rupture or bleeding incident occurs, the blood may penetrate either into the brain tissue (cerebral hemorrhage) or into the subarachnoid space, which is located between the sheaths (meninges) surrounding the brain (subarachnoid hemorrhage). Bleeding may also extend into the ventricular system (intraventricular hemorrhage). Cerebral hemorrhage appears to be most common.

One long-term study (mean follow up greater than 20 years) of over 150 symptomatic AVMs (either presenting with bleeding or seizures) found the risk of cerebral hemorrhage to be approximately 4% per year, slightly higher than the 2-3% seen in other studies. A simple, rough approximation of a patient's lifetime bleeding risk is 105 - (patient age in years), assuming a 3% bleed risk annually. For example, a healthy 30-year-old patient would have approximately a 75% lifetime risk of at least one bleeding event. Ruptured AVMs are a significant source or morbidity and mortality; post rupture, as many as 29% of patients will die, and only 55% will be able to live independently.

Usually babies with this malformation do not survive past birth. However, there have been cases of survival. As of 2004, there were only two reported living cases. Of these two, one was severely cognitively impaired and physically disabled. The status of the other was unreported. If the fetus progresses to full term, there is the risk that it will have head trauma from the pressure applied to the head while being delivered. A few other cases of acalvaria have been reported, which did not progress to birth. In addition to the lack skull cap, there were brain malformations present in each case, and all of the pregnancies were terminated either electively or the fetuses were spontaneously aborted.

In most cases, a fetus with CPAM is closely monitored during pregnancy and the CPAM is removed via surgery after birth. Most babies with a CPAM are born without complication and are monitored during the first few months. Many patients have surgery, typically before their first birthday, because of the risk of recurrent lung infections associated with CPAMs. Some pediatric surgeons can safely remove these lesions using very tiny incisions using minimally invasive surgical techniques (thoracoscopy). However, some CPAM patients live a full life without any complication or incident. It is hypothesized that there are thousands of people living with an undetected CPAM. Through ultrasound testing employed in recent years, many more patients are aware that they live with this condition. Rarely, long standing CPAMs have been reported to become cancerous.

Very large cystic masses might pose a danger during birth because of the airway compression. In this situation, a special surgical type of delivery called the EXIT procedure may be used.

In rare extreme cases, where fetus's heart is in danger, fetal surgery can be performed to remove the CPAM. If non-immune hydrops fetalis develop, there is a near universal mortality of the fetus without intervention. Fetal surgery can improve the chances of survival to 50-60%. Recently, several studies found that a single course of prenatal steroids (betamethasone) may increase survival in hydropic fetuses with microcystic CPAMs to 75-100%. These studies indicate that large microcystic lesions may be treated prenatally without surgical intervention. Large macrocyst lesions may require in utero placement of a Harrison thoracoamniotic shunt.

Prognosis varies widely depending on severity of symptoms, degree of intellectual impairment, and associated complications. Because the syndrome is rare and so newly identified, there are no long term studies.

a combination of various vascular malformations. They are 'complex' because they involve a combination of two different types of vessels.

- CVM: capillary venous malformation

- CLM: capillary lymphatic malformation

- LVM: lymphatic venous malformation

- CLVM: capillary lymphatic venous malformation. CLVM is associated with Klippel-Trenaunay syndrome

- AVM-LM: Arteriovenous malformation- lymphatic malformation

- CM-AVM: capillary malformation- arteriovenous malformation

The annual new detection rate incidence of AVMs is approximately 1 per 100,000 a year. The point prevalence in adults is approximately 18 per 100,000. AVMs are more common in males than females, although in females pregnancy may start or worsen symptoms due the increase in blood flow and volume it usually brings. There is a significant preponderance (15-20%) of AVM in patients with hereditary hemorrhagic telangiectasia (Osler-Weber-Rendu syndrome).

Hip dysplasia is considered to be a multifactorial condition. That means that several factors are involved in causing the condition to manifest.

The cause of this condition is unknown; however, some factors of congenital hip dislocation are through heredity and racial background. It is also thought that the higher rates in some ethnic groups (such as some Native American groups) is due to the practice swaddling of infants, which is known to be a potential risk factor for developing dysplasia. It also has a low risk in African Americans and southern Chinese.

Kaposiform hemangioendothelioma (KHE) is a rare vascular neoplasm that is locally aggressive but without metastatic potential. It occurs particularly in the skin, deep soft tissue, retroperitoneum, mediastinum, and rarely in bone. Although lesions occur solitary, they often involve large areas of the body, such as the head/neck region (40%), trunk (30%), or extremity (30%).

Usually, it is present at birth as a flat, reddish-purple, tense and edematous lesion.

Although half of lesions are congenital, 58% of KHE develop during infancy, 32% between age 1 and 10 years (32%) and 10% after 11 years of age. Moreover, adult onset has been described too with mainly males being affected. Both sexes are affected equally in children.

Lesions are often greater than 5 cm in diameter and can cause visible deformity and pain. During early childhood, KHE may enlarge and after 2 years of age, it may partially regress. Though, it usually persists longterm. In addition, 50% of patients suffer from coagulopathy due to thrombocytopenia (<25,000/mm3), presenting with petechiae and bleeding. This is called the Kasabach-Merritt Phenomenon, which is caused by trapping of platelets and other clotting factors within the tumor. Kasabach-Merritt Phenomenon is less likely in patients with lesions less than 8 cm. As two-thirds of adult-onset KHE tumors are less than 2 cm, KHE in adults is rarely associated with Kasabach-Merritt Phenomenon.

Patients with KHE and Kasabach-Merritt Phenomenon present with petechiae and ecchymosis.

Most KHE tumors are diffuse involving multiple tissue planes and important structures. Resection of KHE is thus often difficult. Treatment of kaposiform hemangioendothelioma is therefore medical. The primary drug is interferon alfa, which is successful in 50% of children. Another option is vincristine, which has lots of side-effects, but has a response rate of 90%. Drug therapy is often used in shrinking the tumor and treating the coagulopathy. However, many of these kaposiform hemangioendotheliomas do not completely regress and remain as a much smaller asymptomatic tumor. However, KHE still has a high mortality rate of 30%. Although complete surgical removal with a large margin has the best reported outcome, it is usually not done because of the risk of bleeding, extensiveness, and the anatomic site of the lesion.

Operative management may be possible for small or localized lesions. Removal of larger areas also may be indicated for symptomatic patients or for patients who have failed farmacotherapy. Resection is not required for lesions that are not causing functional problems, because KHE is benign and because resection could cause deformity.

The complications that are usually associated with vein of Galen malformations are usually intracranial hemorrhages. Over half the patients with VGAM have a malformation that cannot be corrected. Patients frequently die in the neonatal period or in early infancy.

Mosaic mutations in PIK3CA have been found to be the genetic cause of M-CM. Genetic testing for the mutation is currently only available on a research basis. Other overgrowth conditions with distinct phenotypes have also been found to be caused by mosaic mutations in PIK3CA. How different mutations in this gene result in a variety of defined clinical syndromes is still being clarified. Mutations in PIK3CA have not been found in a non-mosaic state in any of these disorders, so it is unlikely that the conditions could be inherited.

Basilar invagination is invagination (infolding) of the base of the skull that occurs when the top of the C2 vertebra migrates upward. It can cause narrowing of the foramen magnum (the opening in the skull where the spinal cord passes through to the brain). It also may press on the lower brainstem.

This is similar to Chiari malformation. That, however, is usually present at birth.

Recent research has found that Dandy–Walker syndrome often occurs in patients with PHACES syndrome.