Susan Riordan, RN, RDMS and Jonathan Weeks M.D.
Objectives: Upon the completion of this CNE article, the reader will be able to
- Discuss the incidence of anencephaly and the theories behind its development.
- Discuss the ultrasound appearance of anencephaly and other anomalies associated with this congenital defect.
- Describe the psychological adaptation that couples experience once this diagnosis is made.
Anencephaly is a lethal congenital anomaly where most of the brain is absent and what remains is not covered by the calvarium or skull. Frequently, the cerebral hemispheres and midbrain are absent. The defect involves the abnormal closure of the anterior portion of the neural groove.
Neural tube defects are one of the major category’s of congenital birth defects, which includes such entities as anencephaly, encephalocele, and myelomeningocele (spina bifida aperta). Anencephaly comprises approximately 60% of all neural tube defects. It is the most common central nervous system anomaly that occurs with an incidence of approximately 1 per 1,000 deliveries in the United States. There appears to be an increased risk of about 0.5% in diabetic pregnancies. Ninety percent of anencephaly occurs as a first-time event. However, the recurrence risk is approximately 4% following a single affected fetus and this increases with each subsequent affected fetus. In addition, the incidence is increased in children born to families with a history of neural tube defects.
The incidence of anencephaly is also highly variable and is dependant upon geographic location, race, and sex. It is more common in Great Britain and the British Isles and is more common in the White population when compared to African-Americans or Asians. It is also twice as common in female fetuses. Since this condition is a lethal anomaly, the majority of fetuses does not survive to the time of birth and are either spontaneously aborted or delivered as a stillbirth. Another finding is that the incidence of anencephaly in spontaneously aborted pregnancies has been found to be five times greater than the incidence of those observed at birth.
Etiology and Embryology
Neural tube defects result from a combination of genetic and environmental factors and therefore are classified as having a multifactorial etiology. Anencephaly has been induced experimentally in animals with the use of radiation, trypan blue, salicylates, sulfonamides, and excess carbon dioxide with anoxia. The main theory behind the origin of this birth defect is that it is caused by a failure of the anterior neuropore or neural groove to close correctly. A second theory that has been proposed, as a cause for anencephaly, is excess cerebrospinal fluid causing the disruption of normally formed cerebral hemispheres. In addition, decreased blood levels of some vitamins, most notably folic acid, has been identified in the blood of some women who have had a pregnancy complicated by the presence of a neural tube defect.
Anencephaly results from a failure of the neural tube to close completely at the cranial end of the fetus, which results in a lack of formation of the forebrain. The neural groove of the fetus begins to close at about 20 to 21 days after conception (which is about 5 weeks gestation when using the last menstrual period – LMP). The closure of this neural tube usually begins in the upper thoracic region and then extends upward toward the head and downward toward the sacrum. The cranial portion or calvarium usually closes around day 24 or 25 (5½ weeks gestation by LMP) and the sacral end closes around days 27 to 29 (6 weeks gestation by LMP). Therefore, the two most common neural tube defect anomalies are anencephaly and lumbosacral spina bifida (myelomeningocele). Thoracic and cervical spina bifida are the least common.
With anencephaly, the cranial vault and cerebral hemispheres are usually absent, while the brainstem and portions of the midbrain are usually present. A mass of thin-walled vascular channels known as the "cerebrovasculosa" or "angiomatous stroma" may be seen protruding from the base of the skull in place of the cerebral hemispheres. This material may actually be a derivative of the choroid plexus and is usually covered by a membrane that is contiguous with the scalp.
The following physical findings are commonly seen in anencephalic fetuses:
- Eyes = protrude because the orbits are very shallow
- Nose = appears flattened
- Tongue = is enlarged
- Mouth = cleft lip and / or palate may be present
- Mandible = appears better developed than the mid-facial structure
- Neck = is usually shortened due to a decrease in the number of vertebrae or fusion of the cervical vertebrae
- Chest = sometimes there is reduced thoracic volume with secondary pulmonary hypoplasia. Small lung size may also result from decreased fetal breathing movements.
- Limbs = usually well developed but may appear disproportionately long when compared with the head and trunk.
- Pituitary = absent or hypoplastic
- Adrenals = absent or hypoplastic; if seen they are usually very small and weigh less than 1 gram. The normal adrenal gland weighs about 9 grams at 16 to 18 weeks gestation.
Anencephaly was the first fetal anomaly to be diagnosed by antenatal ultrasound in 1972. There are several antenatal indications that may lead to the suspicion of diagnosing anencephaly; however, the diagnosis is often made when patients are referred for a routine ultrasound to determine gestational age. The anomaly can be seen as early as 10 to 12 weeks gestation by ultrasound based on an abnormal shape of the head and absence of the skull. At this early gestational age, it is usually better visualized with a transvaginal ultrasound scan. The anomaly can usually be visualized by an abdominal scan at 14 to 15 weeks gestation when a "frog-like appearance" of the face is seen due to the presence of the poorly formed cranial bones (figures 1, 2 & 3). The anomaly is usually identified when trying to obtain a BPD measurement of the fetal head.
One prenatal condition that could lead to the suspicion of anencephaly is when the patient chooses to have a screening genetic blood test referred to as an MSAFP test (maternal serum alpha-fetoprotein) that is obtained between 15 and 20 weeks gestation. The result is usually elevated in cases of anencephaly. Alpha-fetoprotein is a glycoprotein that is synthesized predominantly in the fetal liver, but also in the yolk sac and fetal gastrointestinal tract. The maternal circulation normally receives small quantities of this protein from the amniotic fluid compartment and across the placental barrier. Maternal serum levels of AFP normally increase from the 7th week of gestation through the 32nd week and then decline. Since there is no skin covering the cranial anomaly, this allows for large quantities of AFP to leak into the amniotic fluid, which then leads to an increase in the maternal serum level.
Another condition that might lead to the diagnosis of anencephaly is the presence of polyhydramnios, which occurs in 40% to 50% of cases. The mechanism for the development of polyhydramnios is unclear, however, several hypotheses are suggested. These include a failure of normal swallowing by the fetus because of a brainstem lesion, excessive micturition (excessive urination), and / or a failure of normal reabsorption of cerebrospinal fluid.
In some cases of anencephaly, there is increased fetal activity seen with an unknown explanation. It has been proposed that irritation of the exposed meninges and neural tissue by cerebrospinal fluid and / or other external factors may be the cause.
Associated Malformations and Potential Pitfalls
There are a few conditions that sometimes can be confused with anencephaly, which include severe microcephaly, acrania, and amniotic band syndrome involving the head. With severe microcephaly, the head size is small but a complete cranium is present. Therefore, this disorder can usually be distinguished from anencephaly. On the other hand, some cases of amniotic band syndrome that involve the head may be more difficult to differentiate. However, with close observation, thin membranes or "strings" can usually be seen attached to the surface of the fetal head, leading to a diagnosis of amniotic band syndrome. Finally, acrania is a disorder where there is a normally formed brain but an absent skull (figures 4 & 5). Again, this is different from anencephaly where normal cerebral hemispheres are lacking.
Other associated anomalies that may be found in the presence of anencephaly include spina bifida in another location of the spine (17% of cases), cleft lip and or palate (2% of cases), clubfoot (2% of cases), and rarely omphalocele.
Most cases of anencephaly are stillborn or if delivered alive, die shortly after birth. There has been a single case report that described an infant who survived 5½ months after delivery. In pregnancies that extend beyond 24 weeks gestation, 53% are delivered prematurely and 15% go postterm. Of those that go beyond 24 weeks gestation, only 32% are live births.
Obstetrical Management and Psychological Adaptation
Since this is a lethal anomaly, termination is an ethical option at any gestational age during the pregnancy and may be viewed as a "late miscarriage". Therefore, it is imperative to make a correct diagnosis before informing the parents of the facts. It is very important to involve them in the decision making process of how to proceed with the pregnancy giving them the option of no medical intervention if explained from a developmental viewpoint. This is often a very difficult moral-ethical dilemma for parents when they receive the initial diagnosis during the ultrasound examination. The couple should not be forced to make a decision at that time. They should be encouraged to discuss their feelings with each other and come to a decision that they are comfortable with. It should be explained that if the pregnancy is continued, post-maturity can occur and if fetal distress is noted during labor (at any gestational age), a C-section would NOT be indicated. It is very important to assure the parents that humane care and comfort measures will be provided by the nursing staff to the anencephalic newborn after delivery. Counseling for future pregnancies should include increasing the patient’s intake of folic acid to 4 mg per day at least 4 to 6 weeks before the next conception to minimize the risk of recurrence.
If the diagnosis is not made until the third trimester, and the patient has an unripe cervix, cervical ripening techniques can be employed and / or laminaria placement can by utilized. (Laminaria is compressed seaweed that when moistened will slowly expand to several times its size over a period of hours.) Organ donation from anencephalic pregnancies has been suggested by some as a possibility but this issue is widely debated.
1, 2, & 3 Examples of anencephaly revealing the "frog-like" appearance of the face and absence of the cerebral hemispheres and skull.
4 & 5 Ultrasound images of acrania revealing the absence of the fetal skull but the cerebral hemispheres are present.
References and or Suggested Reading:
- Chervenak FA, Isaacson GC, Campbell S. Human Embryology in Health and Disease, Ultrasound in Obstetrics and Gynecology Volume I, pg. 675, 825-26, 1993.
- Jones, KL. Meningomyelocele, Anencephaly, Iniencephaly Sequences, in Smith’s Recognizable Patterns of Human Malformation, pg. 548, 659-660, 1988.
- Keeling JW. Central Nervous System Anomalies: Fetal Pathology, pg. 33-34, 1994.
- Sanders RC, James AE. Congenital Anomalies – Neural Tube Defects, The Principles and Practice of Ultrasonography in Obstetrics and Gynecology, pg, 301, 1985.
- Callen PW. Alpha-Fetoprotein screening programs: What every obstetric sonologist should Know, Ultrasonography in Obstetrics and Gynecology, pg. 24-26, 1994.
- Romero R, Pilu G, Phillippe S, Alessandro G, Hobbins J. Anencephaly, Prenatal Diagnosis of Congenital Anomalies, pg. 43-45, 1988.
- Harrison MR, Globus MS, Filly RA. Characteristics of the major fetal defects via MSAFP screening, The Unborn Patient – Prenatal Diagnosis and Treatment, pg. 66-67, 426-428, 438, 1990.
- Benacerraf BR. Syndromes and Conditions Associated with Neural Tube Defects, Ultrasound of Fetal Syndromes, pg. 277, 1998.
- Chervenak FA, Kurjak A, Comstock CH. Transabdominal Sonography of the Fetal Forebrainj, Ultrasound and the Fetal Brain, pg. 44-45, 1995.
- Fleiscger, A, Romero R, Manning F, Phillippe J, James AE. Neural Tube Defects, Principles of Ultrasonography in Obstetrics and Gynecology, pg. 216-218, 1991.
Susan Riordan received her Bachelors degree in nursing in 1979 and has been a full time nurse for 21 years. She has worked in labor and delivery, postpartum, and the newborn nursery. She has been a high-risk obstetrical sonographer for the past 10 years and received her RDMS in Obstetrics and Gynecology in 1997. She currently works at Norton Suburban Hospital in Maternal-Fetal Medicine under the direction of Dr. Jonathan Weeks.