Exencephaly is a type of

cephalic disorder Cephalic disorders () are Congenital disorder, congenital conditions that stem from damage to, or abnormal development of, the budding nervous system. Cephalic disorders are not necessarily caused by a single factor, but may be influenced by her ...

wherein the brain is located outside of the skull. This condition is usually found in embryos as an early stage of

anencephaly Anencephaly is the absence of a major portion of the brain, skull, and scalp that occurs during embryonic development. It is a cephalic disorder that results from a neural tube defect that occurs when the rostral (head) end of the neural tube ...

. As an exencephalic pregnancy progresses, the neural tissue gradually degenerates. The prognosis for infants born with exencephaly is extremely poor. It is rare to find an infant born with exencephaly, as most cases that are not early stages of anencephaly are usually

stillborn Stillbirth is typically defined as fetal death at or after 20 or 28 weeks of pregnancy, depending on the source. It results in a baby born without signs of life. A stillbirth can often result in the feeling of guilt or grief in the mother. T ...

. Those infants who are born with the condition usually die within hours or minutes. The disorder is caused by the failure of cranial

neuropore Neurulation refers to the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube. The embryo at this stage is termed the neurula. The process begins when the notochord induces the formati ...

to properly fuse between the 3rd and 4th week post conception. Because of this, the

calvarium The calvaria is the top part of the skull. It is the superior part of the neurocranium and covers the cranial cavity containing the brain. It forms the main component of the skull roof. The calvaria is made up of the superior portions of the f ...

doesn't develop/fuse properly and the brain extrudes from the cranium.

Pathophysiology

Until recently, the medical literature did not indicate a connection among many

genetic disorder A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosome abnormality. Although polygenic disorders ...

s, both

genetic syndromes A syndrome is a set of medical signs and symptoms which are correlated with each other and often associated with a particular disease or disorder. The word derives from the Greek σύνδρομον, meaning "concurrence". When a syndrome is paired ...

and

genetic diseases A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosome abnormality. Although polygenic disorders are ...

, that are now being found to be related. As a result of new genetic research, some of these are, in fact, highly related in their root cause despite the widely varying set of medical

symptoms Signs and symptoms are diagnostic indications of an illness, injury, or condition. Signs are objective and externally observable; symptoms are a person's reported subjective experiences. A sign for example may be a higher or lower temperature ...

that are clinically visible in the disorders. Exencephaly is one disease that has recently been identified as part of an emerging class of diseases called cilopathies. The underlying cause may be a dysfunctional molecular mechanism in the primary

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

structures of the

cell Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...

organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

s which are present in many cell types throughout the

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

body. The cilia defects adversely affect "numerous critical developmental signaling pathways" essential to cellular development and thus offer a plausible hypothesis for the often multi-symptom nature of a large set of syndromes and diseases. Known ciliopathies include

primary ciliary dyskinesia Primary ciliary dyskinesia (PCD) is a rare, autosomal recessive genetic ciliopathy, that causes defects in the action of cilia lining the upper and lower respiratory tract, sinuses, Eustachian tube, middle ear, fallopian tube, and flagella of spe ...

Bardet–Biedl syndrome Bardet–Biedl syndrome (BBS) is a ciliopathic human genetic disorder that produces many effects and affects many body systems. It is characterized by rod/cone dystrophy, polydactyly, central obesity, hypogonadism, and kidney dysfunction in ...

polycystic kidney Polycystic kidney disease (PKD or PCKD, also known as polycystic kidney syndrome) is a genetic disorder in which the renal tubules become structurally abnormal, resulting in the development and growth of multiple cysts within the kidney. These cy ...

and

liver disease Liver disease, or hepatic disease, is any of many diseases of the liver. If long-lasting it is termed chronic liver disease. Although the diseases differ in detail, liver diseases often have features in common. Liver diseases File:Ground gla ...

nephronophthisis Nephronophthisis is a genetic disorder of the kidneys which affects children. It is classified as a medullary cystic kidney disease. The disorder is inherited in an autosomal recessive In genetics, dominance is the phenomenon of one variant ...

Alström syndrome Alström syndrome (AS), also called Alström–Hallgren syndrome, is a very rare autosomal recessive genetic disorder characterised by childhood obesity and multiple organ dysfunction. Symptoms include early-onset type 2 diabetes, cone-rod dyst ...

Meckel–Gruber syndrome Meckel-Gruber syndrome is a rare, lethal ciliopathic genetic disorder, characterized by renal cystic dysplasia, central nervous system malformations (occipital encephalocele), polydactyly (postaxial), hepatic developmental defects, and pulmonary ...

and some forms of retinal degeneration.

Notes

{{Reflist

External links

NINDS Overview

Congenital disorders of nervous system

Pathophysiology

See also

Notes

External links