= EXENCEPHALY

= developmental anomaly characterized by partial / complete absence of membranous neurocranium + complete but abnormal development of brain tissue

Incidence: 25 cases reported

Cause: impaired migration of mesenchyme to its normal location under calvarial ectoderm → failed development of dura mater + skull + musculature

Time: develops after closure of anterior neuropore during 4^th week

May be associated with:

cleft lip, bilateral absence of orbital floors, metatarsus varus, talipes, cervicothoracic spina bifida

• ± elevation of maternal serum AFP
• absence of calvarium
• normal ossification of chondrocranium (face, skull base)
• hemispheres surrounded by thin membrane

Prognosis: uniformly lethal; progression to anencephaly (brain destruction ← exposure to amniotic fluid and mechanical trauma)

DDx: encephalocele, anencephaly, osteogenesis imperfecta, hypophosphatasia