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ULTRASOUND OF
ANENCEPHALY |
- Symmetrical, partial or total absence of the cranial vault above the orbits
- Holocrania = most or all of the cranium above foramen magnum is absent.
- Merocrania = partial or incomplete median cranial defect with ectopia of the brain. The foramen magnum is normal and there is no cervical lordosis. May be confused with a cephalocele but merocrania does not have skin covering the ectopic brain.
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- Variable degrees of degenerating brain tissue depending on the stage of development. This results in different appearances in the 1st and 2nd trimester:
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- Mickey mouse sign - cerebral tissue still present in the 1st trimester (1,2).
- Frogs eye sign - no cerebral tissue cephalad to the orbits.
- Missing parts of the brain include:
- Prosencephalon.
- Mesencephalon.
- Rostral part of rhombencephalon.
- Sagittal image:
- Chin, lips, nose and orbits appear normal in profile.
- Forehead and the calvarium are absent.
- Coronal image:
- Prominent orbits with preservation the skull base and facial features.
- Absent cranium.
- Prominent bulging eyes ("frogs eyes").
- Other abnormalities of the eyes and orbits have been described pathologically; coloboma, corneal dermoids, and anophthalmia. Eyes although they appear structurally normal, may have no connection to the brain centrally.
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- Polyhydramnios is present in up to 50% of cases, usually developing in the late second trimester. It is thought to result from impaired fetal swallowing.
- Associated anomalies include anomalous folding of the ears, cleft lip and/or palate, diaphragmatic hernia, omphalocele, limb and cardiac malformations and hydronephrosis.
- Anencephaly may be caused
by amniotic bands
- A rare cause of absent fetal cranium is fetal decapitation in utero.
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Fetal demise – Spalding sign with cranium in
uterine cavity and spine and body in cervix - ?? amniotic band syndrome |
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REFERENCES |
- Nishi T, Nakano R. First trimester diagnosis of exencephaly by transvaginal sonography. J Ultrasound Med 1994;13:149-151.
- Chatzipapas IK, Whitlow BJ, Economaides DL. The Mickey Mouse sign and the diagnosis of anencephaly in early pregnancy. Ultrasound Obstet Gynecol 1999;13:196-199.