ACRANIA (Absent Cranial Vault)

	Anencephaly	Exencephaly	Iniencephaly
Flat bones of calvarium	Absent	Partial or Complete absence	* Partial or complete absence. * Defect in squamous occipital bone: Apertus = with encephalocele Clausus = without encephalocele
Skull base	N	N	* Dysraphic defect in occiput. * Enlarged foramen magnum.
Fetal neck	N	N	* Persistent hyperextension. * Occiput close to or fused with back.
Cervical Spine	N	N	* Hyperextended * Shortened by block vertebrae. * No laminae or spines.
Cerebral tissue	Absent - occasional remnants of forebrain	Present but: - abundant - disorganized - covered by a thin membrane.	Normal or absent if associated with anencephaly or abundant and disorganized if associated with exencephaly.
Brainstem	Partially present esp medulla.	Present.	Usually present.
Other signs	* Frogs eye: * bulging eyes * large tongue * short neck +/- polyhydramnios
Pathology	Primary non closure of ant. neuropore or secondary degeneration of closed neuropore	? Failure of mesenchymal migration	Unknown

REFERENCES

1. Ekici E, Gulmezoglu AM. Sonographic Diagnosis of Fetal Acrania. J Clin Ultrasound 1991, 19:363-366.
2. Yang YC, Wu CH, Chang FM et.al. Early Prenatal Diagnosis of Acrania by Transvaginal Ultrasound. J Clin Ultrasound 1992,20:343-345.
3. Kennedy KA, Flick KJ, Thurmond AS. First-trimester diagnosis of exencephaly. Am J Obstet Gynecol 1990, 162:461-463.
4. Cox GG, Rosenthal SJ, Holsapple JW. Exencephaly: Sonographic Findings and Radiologic-Pathologic Correlation Radiology 1985, 155:755-756.
5. Mannes EJ, Crelin ES, Hobbins et.al. Sonographic demonstration of Fetal Acrania AJR 1982, 139:181-182.
6. Shere DM, Hearn-Stebbins B, Harvey W et.al. Endovaginal Sonographic Diagnosis os Iniencephaly Apertus and Craniorachischisis at 13 Weeks, Menstrual Age. J Clin Ultrasound 1993, 21:124-127.