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).

While there is no current cure, the treatments for Chiari malformation are surgery and management of symptoms, based on the occurrence of clinical symptoms rather than the radiological findings. The presence of a syrinx is known to give specific signs and symptoms that vary from dysesthetic sensations to algothermal dissociation to spasticity and paresis. These are important indications that decompressive surgery is needed for patients with Chiari Malformation Type II. Type II patients have severe brain stem damage and rapidly diminishing neurological response.

Decompressive surgery involves removing the lamina of the first and sometimes the second or third cervical vertebrae and part of the occipital bone of the skull to relieve pressure. The flow of spinal fluid may be augmented by a shunt. Since this surgery usually involves the opening of the dura mater and the expansion of the space beneath, a dural graft is usually applied to cover the expanded posterior fossa.

A small number of neurological surgeons believe that detethering the spinal cord as an alternate approach relieves the compression of the brain against the skull opening (foramen magnum), obviating the need for decompression surgery and associated trauma. However, this approach is significantly less documented in the medical literature, with reports on only a handful of patients. It should be noted that the alternative spinal surgery is also not without risk.

Complications of decompression surgery can arise. They include bleeding, damage to structures in the brain and spinal canal, meningitis, CSF fistulas, occipito-cervical instability and pseudomeningeocele. Rare post-operative complications include hydrocephalus and brain stem compression by retroflexion of odontoid. Also, an extended CVD created by a wide opening and big duroplasty can cause a cerebellar "slump". This complication needs to be corrected by cranioplasty.

In certain cases, irreducible compression of the brainstem occurs from in front (anteriorly or ventral) resulting in a smaller posterior fossa and associated Chiari malformation. In these cases, an anterior decompression is required. The most commonly used approach is to operate through the mouth (transoral) to remove the bone compressing the brainstem, typically the odontoid. This results in decompressing the brainstem and therefore gives more room for the cerebellum, thus decompressing the Chiari malformation. Arnold Menzes, MD, is the neurosurgeon who pioneered this approach in the 1970s at the University of Iowa. Between 1984 and 2008 (the MR imaging era), 298 patients with irreducible ventral compression of the brainstem and Chiari type 1 malformation underwent a transoral approach for ventral cervicomedullary decompression at the University of Iowa. The results have been excellent resulting in improved brainstem function and resolution of the Chiari malformation in the majority of patients.

Treatment for individuals with Dandy–Walker Syndrome generally consists of treating the associated problems, if needed.

A special tube (shunt) to reduce intracranial pressure may be placed inside the skull to control swelling. Endoscopic third ventriculostomy is also an option.

Treatment may also consist of various therapies such as occupational therapy, physiotherapy, speech therapy or specialized education. Services of a teacher of students with blindness/visual impairment may be helpful if the eyes are affected.

There is no standard course of treatment for cerebellar hypoplasia. Treatment depends upon the underlying disorder and the severity of symptoms. Generally, treatment is symptomatic and supportive. Balance rehabilitation techniques may benefit those experiencing difficulty with balance. Treatment is based on the underlying disorder and the symptom severity. Therapies include physical, occuptational, speech/language, visual, psych/ behavioral meds, special education.

There is no cure for this condition. Treatment is supportive and varies depending on how symptoms present and their severity. Some degree of developmental delay is expected in almost all cases of M-CM, so evaluation for early intervention or special education programs is appropriate. Rare cases have been reported with no discernible delay in academic or school abilities.

Physical therapy and orthopedic bracing can help young children with gross motor development. Occupational therapy or speech therapy may also assist with developmental delays. Attention from an orthopedic surgeon may be required for leg length discrepancy due to hemihyperplasia.

Children with hemihyperplasia are thought to have an elevated risk for certain types of cancers. Recently published management guidelines recommend regular abdominal ultrasounds up to age eight to detect Wilms' tumor. AFP testing to detect liver cancer is not recommended as there have been no reported cases of hepatoblastoma in M-CM patients.

Congenital abnormalities in the brain and progressive brain overgrowth can result in a variety of neurological problems that may require intervention. These include hydrocephalus, cerebellar tonsillar herniation (Chiari I), seizures and syringomyelia. These complications are not usually congenital, they develop over time often presenting complications in late infancy or early childhood, though they can become problems even later. Baseline brain and spinal cord MRI imaging with repeat scans at regular intervals is often prescribed to monitor the changes that result from progressive brain overgrowth.

Assessment of cardiac health with echocardiogram and EKG may be prescribed and arrhythmias or abnormalities may require surgical treatment.

The prognosis of this developmental disorder is highly based on the underlying disorder. Cerebellar hypoplasia may be progressive or static in nature. Some cerebellar hypoplasia resulting from congenital brain abnormalities/malformations are not progressive. Progressive cerebellar hypoplasia is known for having poor prognosis, but in cases where this disorder is static, prognosis is better.

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.

Because pachygyria is a structural defect no treatments are currently available other than symptomatic treatments, especially for associated seizures. Another common treatment is a gastrostomy (insertion of a feeding tube) to reduce possible poor nutrition and repeated aspiration pneumonia.

Cerebellar agenesis is a rare condition in which a brain develops without the cerebellum. The cerebellum controls smooth movement, and when it does not develop, the rest of the brain must compensate, which it cannot do completely. The condition is not fatal on its own, but people born without a cerebellum experience severe developmental delays, language deficits, and neurological abnormalities. As children with cerebellar agenesis get older, their movements usually improve. It can co-exist with other severe malformations of the central nervous system, like anencephaly, holoprosencephaly, and microencephaly.

The condition was first reported in 1831. 10 cases had been reported as of 1998. Agenesis of one half or another part of the cerebellum is more common than complete agenesis.

Cerebellar agenesis can be caused by mutations in the PTF1A gene.

Dandy–Walker syndrome (DWS) is a rare group of congenital human brain malformations. There are three subtypes which affect multiple organs to varying degrees, but the fundamental abnormalities involve the cerebellum which controls muscle coordination. The adjacent fourth ventricle is often affected, which can alter the flow of cerebrospinal fluid, increase intracranial pressure, and lead to multiple other brain function problems. The degree of disability varies but is typically lifelong. Treatment may involve physical therapy, special education, or surgical placement of a cerebral shunt. It is named for Walter Dandy and Arthur Earl Walker. In the majority of individuals with Dandy–Walker malformation, signs and symptoms caused by abnormal brain development are present at birth or develop within the first year of life. Some children have a buildup of fluid in the brain (hydrocephalus) that may cause increased head size (macrocephaly). Up to half of affected individuals have intellectual disability that ranges from mild to severe, and those with normal intelligence may have learning disabilities. Children with Dandy–Walker malformation often have delayed development, particularly a delay in motor skills such as crawling, walking, and coordinating movements. People with Dandy–Walker malformation may experience muscle stiffness and partial paralysis of the lower limbs (spastic paraplegia), and they may also have seizures. While rare, hearing and vision problems can be features of this condition.

In France, Aymé, "et al." (1989) estimated the prevalence of Fryns syndrome to be 0.7 per 10,000 births based on the diagnosis of 6 cases in a series of 112,276 consecutive births (live births and perinatal deaths).

Treatment involves removal of the etiologic mass and decompressive craniectomy. Brain herniation can cause severe disability or death. In fact, when herniation is visible on a CT scan, the prognosis for a meaningful recovery of neurological function is poor. The patient may become paralyzed on the same side as the lesion causing the pressure, or damage to parts of the brain caused by herniation may cause paralysis on the side opposite the lesion. Damage to the midbrain, which contains the reticular activating network which regulates consciousness, will result in coma. Damage to the cardio-respiratory centers in the medulla oblongata will cause respiratory arrest and (secondarily) cardiac arrest. Current investigation is underway regarding the use of neuroprotective agents during the prolonged post-traumatic period of brain hypersensitivity associated with the syndrome.

Once a patient with neurocutaneous melanosis becomes symptomatic, little can be done to improve prognosis as there is no effective treatment for the disorder. Most therapies are designed to treat the symptoms associated with the disorder, mainly those related to hydrocephalus. A ventriculoperitoneal shunt to relieve intracranial pressure is the preferred method.

Chemotherapy and radiotherapy have been shown to be ineffective in cases of neurocutaneous melanosis where malignancy is present. Additionally, due to the total infiltration of the central nervous system by these lesions, surgical resection is not a viable treatment option.

It has been demonstrated that early embryonic, post-zygotic somatic mutations in the NRAS gene are implicated in the pathogenesis of NCM. Recently, experimental treatment with MEK162, a MEK inhibitor, has been tried in a patient with NCM and progressive symptomatic leptomeningeal melanocytosis. Pathological studies with immunohistochemical and Western Blot analyses using Ki67 and pERK antibodies showed a potential effect of MEK inhibiting therapy. Further studies are needed to determine whether MEK inhibitors can effectively target NRAS-mutated symptomatic NCM.

Microlissencephaly is listed in Orphanet database as a rare disease. There is no much information available about the epidemiology of microlissencepahly in literature. A PhD thesis has estimated the prevalence of microlissencepahly in South–Eastern Hungary between July 1992 and June 2006 to be a case every 91,000 live births (0.11:10,000).

Microlissencephaly (MLIS) is a rare congenital brain disorder that combines severe microcephaly (small head) with lissencephaly (smooth brain surface due to absent sulci and gyri). Microlissencephaly is a heterogeneous disorder i.e. it has many different causes and a variable clinical course. Microlissencephaly is a malformation of cortical development (MCD) that occurs due to failure of neuronal migration between the third and fifth month of gestation as well as stem cell population abnormalities. Numerous genes have been found to be associated with microlissencephaly, however, the pathophysiology is still not completely understood.

The combination of lissencephaly with severe congenital microcephaly is designated as microlissencephaly only when the cortex is abnormally thick. If such combination exists with a normal cortical thickness (2.5 to 3 mm), it is known as "microcephaly with simplified gyral pattern" (MSGP). Both MLIS and MSGP have a much more severe clinical course than microcephaly alone. They are inherited in autosomal recessive manner. Prior to 2000, the term “microlissencephaly” was used to designate both MLIS and MSGP.

Treatment plans will vary depending on the severity of the condition and its evidences in each patient.

Areas that will probably need to be evaluated and assessed include speech, vision, hearing and EEG. Treatment measures may include physical therapy, occupational therapy, Speech therapy, anti-seizure drugs and orthotic devices. Surgery may be needed to assuage spastic motor problems. Various supportive measures such as joint contractures that could prevent complications.

Genetic counseling may also be recommended

The term 'pachygyria' does not directly relate to a specific malformation but rather is used to generally describe physical characteristics of the brain in association with several neuronal migration disorders; most commonly disorders relating to varied degrees of lissencephaly. Lissencephaly is present in 1 of 85,470 births and the life span of those affected is short as only a few survive past the age of 20.

Pachygyria is a condition identified by a type of cortical genetic malformation. Clinicians will subjectively determine the malformation based on the degree of malposition and the extent of thickened abnormal grey differentiation present.

3C syndrome, also known as CCC dysplasia, Craniocerebellocardiac dysplasia or Ritscher–Schinzel syndrome, is a rare condition, whose symptoms include heart defects, cerebellar hypoplasia, and cranial dysmorphism. It was first described in the medical literature in 1987 by Ritscher and Schinzel, for whom the disorder is sometimes named.

Neurocutaneous melanosis is a congenital disorder characterized by the presence of congenital melanocytic nevi on the skin and melanocytic tumors in the leptomeninges of the central nervous system. These lesions may occur in the amygdala, cerebellum, cerebrum, pons, and spinal cord of patients. Although typically asymptomatic, malignancy occurs in the form of leptomeningeal melanoma in over half of patients. Regardless of the presence of malignancy, patients with symptomatic neurocutaneous melanosis generally have a poor prognosis with few treatment options. The pathogenesis of neurocutaneous melanosis is believed to be related to the abnormal postzygotic development of melanoblasts and mutations of the NRAS gene.

In a newborn boy thought to have Fryns syndrome, Clark and Fenner-Gonzales (1989) found mosaicism for a tandem duplication of 1q24-q31.2. They suggested that the gene for this disorder is located in that region. However, de Jong et al. (1989), Krassikoff and Sekhon (1990), and Dean et al. (1991) found possible Fryns syndrome associated with anomalies of chromosome 15, chromosome 6, chromosome 8(human)and chromosome 22, respectively. Thus, these cases may all represent mimics of the mendelian syndrome and have no significance as to the location of the gene for the recessive disorder.

By array CGH, Slavotinek et al. (2005) screened patients with DIH and additional phenotypic anomalies consistent with Fryns syndrome for cryptic chromosomal aberrations. They identified submicroscopic chromosome deletions in 3 probands who had previously been diagnosed with Fryns syndrome and had normal karyotyping with G-banded chromosome analysis. Two female infants were found to have microdeletions involving 15q26.2 (see 142340), and 1 male infant had a deletion in band 8p23.1 (see 222400).

Non-progressive congenital ataxia (NPCA) is a non-progressive form of cerebellar ataxia which can occur with or without cerebellar hypoplasia.

Head circumference measurements should be obtained regularly and monitored carefully to detect hydrocephalus. Neurosurgical procedures to relieve hydrocephalus are important. A ventriculoperitoneal shunt may be required in some infants. A pediatric cardiologist should be consulted to manage high-output failure, if present. Often patients need to be intubated. In most cases, the fistulous arteries feeding into the Vein of Galen must be blocked, thereby reducing the blood flow into the vein. Open surgery has a high morbidity and mortality. Recent advances over the past few decades have made endovascular embolization the preferred method of treatment. These treatments are preferred because they offer little threat to the surrounding brain tissue. However, there have been several reported cases of arteriovenous malformations recurring. The young age of many patients, the complex vascular anatomy, and the sensitive location of the Vein of Galen offer considerable challenges to surgeons. Another treatment option is Radiotherapy. Radiotherapy, also called radiosurgery, involves the use of focused beams to damage the blood vessel. Radiotherapy is often not pursued as a treatment because the effects of the procedure can take months or years and there is risk of damaging adjacent brain tissue.

NPCA is a syndrome and can have diverse causes. It has a genetic basis and inheritance is considered to be autosomal recessive. However, autosomal dominant variety has also been reported. There may be familial balanced translocation t(8;20)(p22;q13) involved.

Bilateral frontoparietal polymicrogyria (BFPP) is a genetic disorder with autosomal recessive inheritance that causes a cortical malformation. Our brain has folds in the cortex to increase surface area called gyri and patients with polymicrogyri have an increase number of folds and smaller folds than usual. Polymicrogyria is defined as a cerebral malformation of cortical development in which the normal gyral pattern of the surface of the brain is replaced by an excessive number of small, fused gyri separated by shallow sulci and abnormal cortical lamination. From ongoing research, mutation in GPR56, a member of the adhesion G protein-coupled receptor (GPCR) family, results in BFPP. These mutations are located in different regions of the protein without any evidence of a relationship between the position of the mutation and phenotypic severity. It is also found that GPR56 plays a role in cortical pattering.

Treatment is not needed in the asymptomatic patient. Symptomatic patients may benefit from surgical debulking of the tumor. Complete tumor removal is not usually needed and can be difficult due to the tumor location.