Numerous possible risk factors have been identified, including gestational diabetes, transplacental infections (the "TORCH complex"), first trimester bleeding, and a history of miscarriage. As well, the disorder is found twice as often in female babies. However, there appears to be no correlation between HPE and maternal age.

There is evidence of a correlation between HPE and the use of various drugs classified as being potentially unsafe for pregnant and lactating mothers. These include insulin, birth control pills, aspirin, lithium, thorazine, retinoic acid, and anticonvulsants. There is also a correlation between alcohol consumption and HPE, along with nicotine, the toxins in cigarettes and toxins in cigarette smoke when used during pregnancy.

In utero exposure to cocaine and other street drugs can lead to septo-optic dysplasia.

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.

Currently, research is focusing on identifying the role of the genes on 18p in causing the signs and symptoms associated with deletions of 18p. This will ultimately enable predictive genotyping.

TGIF-Mutations and deletions of this gene have been associated with holoprosencephaly. Penetrance is incomplete, meaning that a deletion of one copy of this gene is not in and of itself sufficient to cause holoprosencephaly. Ten to fifteen percent of people with 18p- have holoprosencephaly, suggesting that other genetic and environmental facts play a role in the etiology of holoprosencephaly in these individuals.

Once a mother has given birth to a child with iniencephaly, risk of reoccurrence increases to 1-5%.

Isolated

1. Familial (autosomal recessive) microcephaly

2. Autosomal dominant microcephaly

3. X-linked microcephaly

4. Chromosomal (balanced rearrangements and ring chromosome)

Syndromes

- Chromosomal

1. Poland syndrome

2. Down syndrome

3. Edward syndrome

4. Patau syndrome

5. Unbalanced rearrangements

- Contiguous gene deletion

1. 4p deletion (Wolf–Hirschhorn syndrome)

2. 5p deletion (Cri-du-chat)

3. 7q11.23 deletion (Williams syndrome)

4. 22q11 deletion (DiGeorge syndrome)

- Single gene defects

1. Smith–Lemli–Opitz syndrome

2. Seckel syndrome

3. Cornelia de Lange syndrome

4. Holoprosencephaly

5. Primary microcephaly 4

6. Wiedemann-Steiner syndrome

Acquired

- Disruptive injuries

1. Ischemic stroke

2. Hemorrhagic stroke

3. Death of a monozygotic twin

- Vertically transmitted infections

1. Congenital cytomegalovirus infection

2. Toxoplasmosis

3. Congenital rubella syndrome

4. Zika virus

- Drugs

1. Fetal hydantoin syndrome

2. Fetal alcohol syndrome

Other

1. Radiation exposure to mother

2. Maternal malnutrition

3. Maternal phenylketonuria

4. Poorly controlled gestational diabetes

5. Hyperthermia

6. Maternal hypothyroidism

7. Placental insufficiency

Rare familial recurrence has been reported, suggesting at least one genetic form (HESX1). In addition to HESX1, mutations in OTX2, SOX2 and PAX6 have been implicated in de Morsier syndrome, but in most cases SOD is a sporadic birth defect of unknown cause and does not recur with subsequent pregnancies.

Genetic

- Inborn errors of metabolism

1. Congenital disorder of glycosylation

2. Mitochondrial disorders

3. Peroxisomal disorder

4. Glucose transporter defect

5. Menkes disease

6. Congenital disorders of amino acid metabolism

7. Organic acidemia

Syndromes

- Contiguous gene deletion

1. 17p13.3 deletion (Miller–Dieker syndrome)

- Single gene defects

1. Rett syndrome (primarily girls)

2. Nijmegen breakage syndrome

3. X-linked lissencephaly with abnormal genitalia

4. Aicardi–Goutières syndrome

5. Ataxia telangiectasia

6. Cohen syndrome

7. Cockayne syndrome

Acquired

- Disruptive injuries

1. Traumatic brain injury

2. Hypoxic-ischemic encephalopathy

3. Ischemic stroke

4. Hemorrhagic stroke

- Infections

1. Congenital HIV encephalopathy

2. Meningitis

3. Encephalitis

- Toxins

1. Lead poisoning

2. Chronic renal failure

- Deprivation

1. Hypothyroidism

2. Anemia

3. Congenital heart disease

4. Malnutrition

Genetic factors may play a role in causing some cases of microcephaly. Relationships have been found between autism, duplications of chromosomes, and macrocephaly on one side. On the other side, a relationship has been found between schizophrenia, deletions of chromosomes, and microcephaly. Moreover, an association has been established between common genetic variants within known microcephaly genes ("MCPH1, CDK5RAP2") and normal variation in brain structure as measured with magnetic resonance imaging (MRI)i.e., primarily brain cortical surface area and total brain volume.

The spread of Aedes mosquito-borne Zika virus has been implicated in increasing levels of congenital microcephaly by the International Society for Infectious Diseases and the US Centers for Disease Control and Prevention. Zika can spread from a pregnant woman to her fetus. This can result in other severe brain malformations and birth defects. A study published in The New England Journal of Medicine has documented a case in which they found evidence of the Zika virus in the brain of a fetus that displayed the morphology of microcephaly.

More than 80% of children with Patau syndrome die within the first year of life. Children with the mosaic variation are usually affected to a lesser extent. In a retrospective Canadian study of 174 children with trisomy 13, median survival time was 12.5 days. One and ten year survival was 19.8% and 12.9% respectively.

HPE is not a condition in which the brain deteriorates over time. Although serious seizure disorders, autonomic dysfunction, complicated endocrine disorders and other life-threatening conditions may sometimes be associated with HPE, the mere presence of HPE does not mean that these serious problems will occur or develop over time without any previous indication or warning. These abnormalities are usually recognized shortly after birth or early in life and only occur if areas of the brain controlling those functions are fused, malformed or absent.

Prognosis is dependent upon the degree of fusion and malformation of the brain, as well as other health complications that may be present.

The more severe forms of encephalopathy are usually fatal. This disorder consists of a spectrum of defects, malformations and associated abnormalities. Disability is based upon the degree in which the brain is affected. Moderate to severe defects may cause mental retardation, spastic quadriparesis, athetoid movements, endocrine disorders, epilepsy and other serious conditions; mild brain defects may only cause learning or behavior problems with few motor impairments.

Seizures may develop over time with the highest risk before 2 years of age and the onset of puberty. Most are managed with one medication or a combination of medications. Typically, seizures that are difficult to control appear soon after birth, requiring more aggressive medication combinations/doses.

Most children with HPE are at risk of having elevated blood sodium levels during moderate-severe illnesses, that alter fluid intake/output, even if they have no previous diagnosis of diabetes insipidus or hypernatremia.

Ethmocephaly is a type of cephalic disorder caused by holoprosencephaly. Ethmocephaly is the least common facial anomaly. It consists of a proboscis separating narrow-set eyes with an absent nose and microphthalmia (abnormal smallness of one or both eyes). Cebocephaly, another facial anomaly, is characterized by a small, flattened nose with a single nostril situated below incomplete or underdeveloped closely set eyes.

The least severe in the spectrum of facial anomalies is the median cleft lip, also called premaxillary agenesis.

Although the causes of most cases of holoprosencephaly remain unknown, some may be due to dominant or chromosome causes. Such chromosomal anomalies as trisomy 13 and trisomy 18 have been found in association with holoprosencephaly, or other neural tube defects. Genetic counseling and genetic testing, such as amniocentesis, is usually offered during a pregnancy if holoprosencephaly is detected. The recurrence risk depends on the underlying cause. If no cause is identified and the fetal chromosomes are normal, the chance to have another pregnancy affected with holoprosencephaly is about 6%.

There is no treatment for holoprosencephaly and the prognosis for individuals with the disorder is poor. Most of those who survive show no significant developmental gains. For children who survive, treatment is symptomatic. It is possible that improved management of diabetic pregnancies may help prevent holoprosencephaly, however there is no means of primary prevention.

The Chromosome 18 Registry & Research Society

The Chromosome 18 Registry & Research Society in Europe

Chromosome 18 Clinical Research Center, University of Texas Health Science Center at San Antonio

Unique

Chromosome Disorder Outreach

Affected individuals have a somewhat shortened lifespan. The maximum described lifespan is 67 years. Adults with 13q deletion syndrome often need support services to maintain their activities of daily living, including adult day care services or housing services.

Prognosis is poor. Previous research suggested a 100% mortality rate for those with acrania. This disease is rare, occurring in 1 in 20,000 live births.

In order to better manage an acrania diagnosis, early detection is of extreme importance so that actions may be taken to help the mother and child. Families may choose either to terminate the pregnancy, or to carry the child to term. Acrania may cause a fetus to spontaneously abort before reaching term.

Currently, research is focusing on identifying the role of the genes on 18p and 18q in causing the signs and symptoms associated with deletions of 18p and/or 18q. This will ultimately enable predictive genotyping.Thus far, several genes on chromosome 18 have been linked with a phenotypic effect.

TGIF - Mutations and deletions of this gene, which is located on18p, have been associated with holoprosencephaly. Penetrance is incomplete, meaning that a deletion of one copy of this gene is not in and of itself sufficient to cause holoprosencephaly. Ten to fifteen percent of people with 18p- have holoprosencephaly, suggesting that other genetic and environmental facts play a role in the etiology of holoprosencephaly in these individuals.

TCF4 – In 2007, deletions of or point mutations in this gene, which is located on 18q, were identified as the cause of Pitt-Hopkins disease. This is the first gene that has been definitively shown to directly cause a clinical phenotype when deleted. If a deletion includes the TCF4 gene (located at 52,889,562-52,946,887), features of Pitt-Hopkins may be present, including abnormal corpus callosum; short neck; small penis; accessory and wide-spaced nipples; broad or clubbed fingers; and sacral dimple. Those with deletions inclusive of TCF4 have a significantly more severe cognitive phenotype.

TSHZ1 - Point mutations and deletions of this gene, located on 18q, are linked with congenital aural atresia Individuals with deletions inclusive of this gene have a 78% chance of having aural atresia.

"Critical regions" – Recent research has narrowed the critical regions for four features of the distal 18q- phenotype down to a small segment of distal 18q, although the precise genes responsible for those features remain to be identified.

"Haplolethal Regions" - There are two regions on chromosome 18 that has never been found to be deleted. They are located between the centromere and 22,826,284 bp (18q11.2) and between 43,832,732 and 45,297,446 bp (18q21.1). It is hypothesized that there are genes in these regions that are lethal when deleted.

In utero exposure to cocaine and other street drugs can lead to hydranencephaly.

Triploidy affects approximately 1-2% of pregnancies, but most miscarry early in development. At birth, males with triploidy are 1.5 times more common than females.

As a recessive genetic condition, both parents must carry the asymptomatic gene and pass it along to their child, a chance of 25 percent. Despite determination of cause, hydranencephaly afflicts both males and females in equal numbers.

Triploidy can result from either two sperm fertilizing one egg (60%) or from one sperm fertilizing an egg with two copies of every chromosome (40%).

Patau syndrome is the result of trisomy 13, meaning each cell in the body has three copies of chromosome 13 instead of the usual two. A small percentage of cases occur when only some of the body's cells have an extra copy; such cases are called mosaic Patau.

Patau syndrome can also occur when part of chromosome 13 becomes attached to another chromosome (translocated) before or at conception in a Robertsonian translocation. Affected people have two copies of chromosome 13, plus extra material from chromosome 13 attached to another chromosome. With a translocation, the person has a partial trisomy for chromosome 13 and often the physical signs of the syndrome differ from the typical Patau syndrome.

Most cases of Patau syndrome are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). An error in cell division called non-disjunction can result in reproductive cells with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of the chromosome. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 13 in each of the body's cells. Mosaic Patau syndrome is also not inherited. It occurs as a random error during cell division early in fetal development.

Patau syndrome due to a translocation can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 13 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 13. Although they do not have signs of Patau syndrome, people who carry this type of balanced translocation are at an increased risk of having children with the condition.

13q deletion syndrome is a rare genetic disease caused by the deletion of some or all of the large arm of human chromosome 13. It causes intellectual disability and congenital malformations that affect a variety of organ systems.

Young–Madders syndrome, alternatively known as Pseudotrisomy 13 syndrome or holoprosencephaly–polydactyly syndrome, is a genetic disorder resulting from defective and duplicated chromosomes which result in holoprosencephaly, polydactyly, facial malformations and mental retardation, with a significant variance in the severity of symptoms being seen across known cases. Many cases often suffer with several other genetic disorders, and some have presented with hypoplasia, cleft lip, cardiac lesions and other heart defects. In one case in 1991 and another in 2000 the condition was found in siblings who were the product of incest. Many cases are diagnosed prenatally and often in siblings. Cases are almost fatal in the prenatal stage with babies being stillborn.

Though it is now thought that earlier cases were misdiagnosed as other genetic disorders with similar pathology—such as Smith–Lemli–Opitz syndrome—the earliest publicised recognition of the condition as a new, hitherto unclassified, genetic disorder was made by two British doctors in Leicester in 1987. Though they identified the condition, later named for them, they did not identify the genetic anomalies responsible but suspected a link with trisomy 13 due to the similar symptoms. With only one or two occurrences documented towards the end of the decade, a group of eight doctors published a five-patient case-study in 1991 which identified the likely chromosomal factors that caused the condition, similar to but distinct from trisomy 13, and gave it the name 'holoprosencephaly–polydactyly syndrome' based on its two most prolific presenting conditions. Later research showed that the condition could manifest in patients with normal karyotypes, without duplication of the chromosomes, and the most recent genetic research implicates problems with the gene code FBXW11 as a likely cause.

Ring 18 is a genetic condition caused by a deletion of the two tips of chromosome 18 followed by the formation of a ring-shaped chromosome. It was first reported in 1964.

Little genetic counseling can be offered for acrania because the genetic origins are not fully understood. In order to make genetic counseling for families easier this disease is often differentially diagnosed with other diseases that can occur at the same time such as anencephaly and acalvaria, though these diseases may not always occur simultaneously. While this disease is tragic, reoccurrence rates are extremely low so genetic counseling is not always necessary.

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.