Anencephaly

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Ayesha Javid, MBBS[2]

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Anencephaly
Anencephaly side.jpg
The side view of the head of an anencephalic fetus.
Source: https://en.wikipedia.org/
ICD-10 Q00.0
ICD-9 740.0
OMIM 206500
DiseasesDB 705
MeSH C10.500.680.196

Overview

Anencephaly is a cephalic disorder in which there is the partial or total absence of the brain. It is an open neural tube defect occurs when the rostral (head) end of the neural tube fails to close resulting in the absence of a major portion of the brain, skull and spinal cord. Children with this disorder are born without a forebrain, the largest part of the brain consisting mainly of the cerebral hemispheres which is responsible for higher cognitive functions and cerebellum which control balance and movement. However, hindbrain is developed. The remaining brain tissue is often exposed - not covered by bone or skin.

Historical Perspective

Classification

Type of Anencephaly Clinical features
Meroanencephaly[3] Meroanencephaly is a rare form of anencephaly. Fetus born have clinical features including misshapen skull bones and a vascular tissue protuberance called cerebrovasculosa.
Holoanencephaly[3] It is the most common type of anencephaly. In this condition, the entire fetus brain fails to develop except for the brain stem. Usually, infant survives for only one day after birth.
Craniorachischisis[4] It is the most severe type of anencephaly. Along with the cranial defect, fetus also has a defect in the vertebral column exposing the underlying neural tissue. Area cerebrovasculosa and area medullovasculosa fill the areas with cranial and spinal defects.

Pathophysiology

  • The craniofacial abnormalities in anencephaly are caused by abnormal induction by the neural crest cells.[7]

Clinical Features

Differentiating Anencephalopathy from other Disorders

At times, anencephaly could be misdiagnosed with other similar diagnosis such as;

Epidemiology and Demographics

Incidence

In the United States, approximately 1,000 to 2,000 babies are born with anencephaly each year. In 2001, the National Center for Health Statistics reported 9.4 cases among 100,000 live births.[10] Anually, more than 300,000 babies are born with neural tube defects throughout the world.[11]

Demographics

In United States, the highest prevalence has been seen among the Hispanic [12]. Female babies, whites and children born to mothers who are at extreme of ages are more likely to be affected by the disorder. Worldwide, Ireland and British Islands has higher prevalence as compared to Asia and Africa which has a lower prevalence rate.[13]

Recurrence rate

Like any other neural tube defect, the recurrence rate of anencephaly is 2-4 percent if one sibling is affected and 10 percent if two siblings are affected.[14][15] This familial tendency is due to genetics, environmental factors, or both.[16]

Risk Factors

Folate deficiency

Genetics

Neural tube defects do not follow direct patterns of heredity, though there is some indirect evidence of inheritance.[19] Recent animal models indicate a possible association with deficiencies of the transcription factor TEAD2.[20]

The motivation behind studying genetic patterns is the following:

  1. NTDs are consistently prevalent among monozygotic twins as compared to dizygotic twins.[21]
  2. There is a high recurrence rate within families. Statistics show the recurrence risk of 1/20 if one previous pregnancy is affected and 1/10 if two pregnancies are affected in a family.[22]
  3. There is higher female preponderence as compared to the males. [23]

Syndromes

Fever/hyperthermia

Amniotic bands

Pregestational diabetes

Maternal Obesity

Toxins

  • Anencephaly and other physical and mental deformities have also been blamed on high exposure to such toxins as lead, chromium, mercury, and nickel. [31]

Natural History, Complications, Prognosis

  • Anencephalic infants are not aggressively resuscitated as there is very little chance of the infant to achieve a full level of consciousness. Instead, clinicians prefer to provide them with adequate hydration, nutrition and comfort.[33] Artificial ventilation, surgery (to fix any co-existing congenital defects), and medications are not considered an option as they have no role in any improvement.[34] Due to this poor prognosis, couple are discussed about the option of the terminating the pregnancy soon after the prenatal diagnosis is established.[35]

Diagnosis

Diagnostic Criteria

Symptoms

Physical Examination

  • Physical examination may be remarkable for:

Laboratory Findings

Alpha-Fetoprotein

Acetylcholinesterase

AChE is an enzyme contained in blood cells, muscle, and nerve tissue. Its levels can be checked from the amniotic fluid via amniocentesis. An elevation of both AFP and AChE values is highly suggestive and accurate for fetal NTD.

Triple screen

Triple screen is a prenatal screening test which includes 3 biomarkers; AFP, hCG and estradiol. It helps to detect certain congenital anomalies, including neural tube defect. The following table shows the triple screening of neural tube defect and how it is differentiated from other anomalies.[46]

Anomaly AFP hCG Estradiol
NTD Increased Normal Normal
Trisomy 21 Decreased Increased Decreased
Trisomy 18 Decreased Decreased Decreased

Imaging Findings

Ultrasound

Ultrasound provides a definitive diagnosis of anencephaly before birth. It can often be diagnosed through a transvaginal ultrasound performed at 12th week postmenstrual.[47]

The ultrasound findings include:

Anencephaly - frog like appearance on prenatal USG. Source: https://radiopaedia.org

.

Management

Medical Therapy

  • There is no known medical therapy available for anencephaly.

Surgery

Prevention

  • Recent studies have shown that the addition of folic acid to the diet of women of child-bearing age may significantly reduce, although not eliminate, the incidence of neural tube defects. Therefore, it is recommended that all women of child-bearing age consume 0.4 mg of folic acid daily, especially those attempting to conceive or who may possibly conceive, as this can reduce the risk to 0.03%.[49]
  • It is not advisable to wait until pregnancy has begun, since by the time a woman knows she is pregnant, the critical time for the formation of a neural tube defect has usually already passed. A physician may prescribe even higher dosages of folic acid (4 mg/day) for women who have had a previous pregnancy with a neural tube defect.[50]

References

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  2. Bard JS (1999). "The diagnosis is anencephaly and the parents ask about organ donation: now what? A guide for hospital counsel and ethics committees". West New Engl Law Rev. 21 (1): 49–95. PMID 12774804 PMID 12774804 Check |pmid= value (help).<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
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  4. 4.0 4.1 Greene ND, Copp AJ (2014). "Neural tube defects". Annu Rev Neurosci. 37: 221–42. doi:10.1146/annurev-neuro-062012-170354. PMC 4486472. PMID 25032496 PMID: 25032496 Check |pmid= value (help).<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
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  49. Template:NINDS
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