Acrania

What It Is, Causes, Diagnosis, and More

Author: Emily Miao, MD, PharmD

Editor: Alyssa Haag, MD

Editor: Ian Mannarino MD, MBA

Editor: Kelsey LaFayette, DNP

Illustrator: Abbey Richard, MSc

Copyeditor: David G. Walker

Modified: Jan 06, 2025

What is acrania?

Acrania, also known as fetal acrania, refers to the partial or complete absence of the cranial vault (i.e., the top portion of the skull) due to an abnormality during embryonic development. As a result, the cerebral hemispheres of the brain are partially or fully formed; however, there is a lack of a fetal skull that normally protects the brain tissue. Acrania is considered a neural tube defect, which is a congenital abnormality resulting from improper closure of the neural tube. The neural tube is an embryonic structure that typically forms the early structures of the central nervous system (i.e., brain and spinal cord). This malformation is estimated to occur in 1 per 2,500 to 3,000 live births, and it is not compatible with life.

An infographic detailing the background, causes, signs and symptoms, diagnosis, and treatment of acrania.

What causes acrania?

Acrania is caused by the failure of the neural tube to close properly during embryonic development. Neurulation (i.e., the embryological process that forms the neural tube) usually occurs in the third week of gestation. During this time, one of the primary germ layers called the ectoderm undergoes neurulation to form a hollow neural tube. Failure for the preliminary hollow neural tube to close causes an absence of meninges and cranial bones that normally form the fetal skull, thereby resulting in acrania and an exposed brain.

The acrania-anencephaly sequence, also known as the acrania-exencephaly-anencephaly sequence, explains the progression from acrania to exencephaly (i.e., protruding amorphous brain tissue due to lack of fetal skull) and anencephaly (i.e., missing brain tissue and fetal skull). Without a layer of protective meninges and a skull, the fetal brain protrudes outward and is exposed to amniotic fluid, known as exencephaly. Over time, the exposure of brain tissue to amniotic fluid causes destruction and degeneration of the brain, resulting in anencephaly.

While the exact causes of why neural tube defects occur are not well understood, several environmental factors may increase the risk of these developmental defects. These factors include folic acid deficiency, which is essential for proper neural tube development, and exposure to medications and toxins during maternal pregnancy, such as valproic acid, retinoic acid, thalidomide, and alcohol.

What are the signs and symptoms of acrania?

Acrania is typically diagnosed prenatally; therefore, early signs may be seen during routine ultrasound examinations. These signs include the absence of a cranial vault or the presence of exposed brain tissue. Acrania may also be associated with the presence of craniofacial and spinal abnormalities and polyhydramnios (i.e., excessive amniotic fluid). In rare cases, because acrania is not compatible with life, fetal demise or stillbirth may occur.

How is acrania diagnosed?

Diagnosis of acrania begins with a thorough review of symptoms, medical history, and evaluation for risk factors. It is often detected in early pregnancy during routine prenatal ultrasound assessments. When there is high suspicion for the presence of neural tube defects seen on initial abdominal ultrasound, further specialized imaging, such as 3D ultrasound or fetal magnetic resonance imaging (MRI), may be used for more detailed visualization of fetal structures and associated anomalies. These imaging modalities may also be able to detect body wall or limb abnormalities, such as amniotic band syndrome, which is a condition that occurs when fibrous bands of the amniotic sac become tangled around the developing fetus. If acrania is diagnosed postnatally or after birth, genetic testing via chromosomal array (i.e., examining the entire genome for chromosomal abnormalities) may be offered to assess for underlying genetic abnormalities that may have contributed to the development of acrania.

How is acrania treated?

Acrania is not compatible with life or long-term survival, so management consists of supportive care and counseling for affected families. If acrania is detected prenatally, genetic testing and counseling may be offered to the parents to provide information related to pregnancy-related complications and delivery planning. Delivery options can include induction of labor or elective cesarean section to minimize distress associated with labor and vaginal delivery or continuation of pregnancy with vaginal delivery if no immediate risks are identified. If the fetus is delivered at term, palliative care measures, including pain management and fluid hydration, are provided to ensure comfort for the fetus. Regardless of the delivery option chosen, supportive care and counseling should be offered to families to assist with the psychosocial effects.

What are the most important facts to know about acrania?

Acrania, also known as fetal acrania, refers to the partial or complete absence of the cranial vault that occurs due to an abnormality during embryonic development. As a result, normal brain tissue is formed but lacks a protective fetal skull. The acrania-anencephaly sequence explains the progression from acrania to exencephaly and then to anencephaly. Without a layer of protective meninges and a skull, the fetal brain protrudes outward and is exposed to amniotic fluid, known as exencephaly. Over time, the exposure of brain tissue to amniotic fluid causes destruction and degeneration of the brain, resulting in anencephaly. Diagnosis of acrania typically occurs in early pregnancy during routine prenatal ultrasound assessments and is confirmed with more specialized imaging techniques, such as 3D ultrasound or fetal MRI. Acrania is not compatible with life and, therefore, management consists of supportive care and counseling for affected families.

References

Amin MU, Mahmood R, Nafees M, Shakoor T. Fetal acrania - Prenatal sonographic diagnosis and imaging features of aborted fetal brain. J Radiol Case Rep. 2009;3(7):27-34. doi:10.3941/jrcr.v3i7.271

Doležálková E, Unzeitig V. Kyselina listová a prevence rozštěpových vad centrálního nervového systému [Folic acid and prevention of the neural tube defects]. Ceska Gynekol. 2014;79(2):134-139.

Medical Task Force on Anencephaly. The infant with anencephaly. N Engl J Med. 1990;322(10):669-674. doi:10.1056/NEJM199003083221006

Padmanabhan R. Etiology, pathogenesis and prevention of neural tube defects. Congenit Anom (Kyoto). 2006;46(2):55-67. doi:10.1111/j.1741-4520.2006.00104.x

Weissman A, Diukman R, Auslender R. Fetal acrania: Five new cases and review of the literature. J Clin Ultrasound. 1997;25(9):511-514. doi:10.1002/(sici)1097-0096(199711/12)25:9<511::aid-jcu10>3.0.co;2-7

Yamaguchi Y, Miyazawa H, Miura M. Neural tube closure and embryonic metabolism. Congenit Anom (Kyoto). 2017;57(5):134-137. doi:10.1111/cga.12219