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 fails to close, usually between the 23rd and 26th day following conception. Strictly speaking, the Greek term translates as "without a brain" (or totally lacking the inside part of the head), but it is accepted that children born with this disorder usually only lack a telencephalon, the largest part of the brain consisting mainly of the cerebral hemispheres, including the neocortex, which is responsible for  cognition. The remaining structure is usually covered only by a thin layer of membrane—skin, bone, meninges, etc., are all lacking. With very few exceptions, infants with this disorder do not survive longer than a few hours or days after birth. Signs and symptoms
The National Institute of Neurological Disorders and Stroke (NINDS) describes the presentation of this condition as follows: "A baby born with anencephaly is usually blind, deaf, unaware of its surroundings and unable to feel pain. Although some individuals with anencephaly may be born with a main brain stem, the lack of a functioning cerebrum permanently rules out the possibility of ever gaining awareness of their surroundings. Reflex actions such as breathing and responses to sound or touch may occur. "Due to the presence of the brainstem, children with anencephaly have almost all the primitive reflexes of a newborn, responding to auditory, vestibular and painful stimuli.
Known ciliopathies include primary ciliary dyskinesia, Bardet–Biedl syndrome, polycystic kidney and liver disease, nephronophthisis, Alström syndrome, Meckel–Gruber syndrome, and some forms of retinal degeneration. Diagnosis
Anencephaly can often be diagnosed before birth through an ultrasound examination. The maternal serum alpha-fetoprotein (AFP screening) and detailed fetal ultrasound can be useful for screening for neural tube defects such as spina bifida or anencephaly. Meroanencephaly
Meroanencephaly is a rare form of anencephaly characterized by malformed cranial bones, a median cranial defect, and a cranial protrusion called area cerebrovasculosa. Area cerebrovasculosa is a section of abnormal, spongy, vascular tissue admixed with glial tissue ranging from simply a membrane to a large mass of connective tissue, hemorrhagic vascular channels, glial nodules, and disorganized choroid plexuses. Holoanencephaly
The most common type of anencephaly, where the brain has entirely failed to form, except for the brain stem. Infants rarely survive more than one day after birth with holoanencephaly. Craniorachischisis
The most severe type of anencephaly where area cerebrovasculosa and area medullovasculosa fill both cranial defects and the spinal column. Craniorachischisis is characterized by anencephaly accompanied by bony defects in the spine and the exposure of neural tissue as the vault of the skull fails to form. Craniorachischisis occurs in about 1 of every 1000 live births, but various physical and chemical tests can detect neural tube closure during early pregnancy. Prognosis
There is no cure or standard treatment for anencephaly.