Congenital abnormalities of cranium of foetus Anencephaly & acrania

Anencephaly  : What we need to know ? Anenencepahaly is characterized  by the absence  of the cranial  vault and telencephalon. The necrotic remnants of the brain stem and rhomboencephalic structures are covered   by a vascular membrane. Associated malformations are quite common and include   spina bifida , cleft lip / palate,  clubfoot and  omphalocele .  Polyhydramnios is frequently found. The diagnosis is easy in the second and third trimester and relies on the demonstrations of the absence  of the cranial vault . Anencephaly is considered to be the final stage of acrania as a consequence of disruption of abnormal brain  tissue unprotected by  the calvarium . Although  the term  acrania  is often used in conjunction with anencephaly the term  acrania  implies  absence of the  entire skull including  the skull  base which is not true in anencephaly. But the  skull base is present in anencephaly and the calvarium is absent . Although the fetal head can be positively identified by vaginal sonography  as early  as the 7^th week of gestation the diagnosis may be difficult in the first trimester . Absent   calvarial mineralizataion before  10 to 12  weeks gestation makes  the differentiation of normal  and abnormal  brain  difficult. Therefore a cephalic pole albeit overtly abnormal is usually present in early gestation and may be difficult to identify and differentiate from the normal brain before  11 weeks gestation

 . Acrania-Exencephaly-Anencephaly Sequence  🔰

A) ACRANIA occurs during 4th week when normal mesenchymal migration doesn't occur to form cranial vault: Absence of cranial vault .ACRANIA exposes brain to amniotic fluid with resultant damage and destruction due to friction resulting in progressive disappearance of brain
on US: Mickey Mouse Sign during first trimester, with 2 semi circular hemispheres floating above fetal face like ears of Mickey mouse

B) EXENCEPHALY :In this anomaly  brain disappear completely or almost completely by 14 weeks
ON US: Frog Face Sign, with prominent eyes and no brain tissue above fetal face

C) Anencephaly : Imaging of the cranial vault is reliable at 11 weeks making the diagnosis of anencephaly possible at this early gestational age.

D)    

E)  In the first trimester the cerebral hemispheres have not yet eroded away but with absence of the cranium in the Sagittal view the “ Mickey Mouse sign” has been shown to be 100 % sensitive and specific for lethal anomalies associated with a missing cranium including anencephaly and acrania.

Anencephaly is a kind of  Neural Tube Defects

The prevalence  , cause and recurrence risk of neural tube defects are variously reported  There are several different forms of neural tube defects .Anencephaly , as mentioned earlier is characterized by the absence of the cranial vault and telencephalon. The diagnosis is made by ultrasound in the second and third trimesters and relies upon demonstration of the absence of the cranial vault . Most cases can be confidently identified by 11 to 13 weeks gestation . At this time the fetal head can be recognized as overtly abnormal owing to lack of an ossified calvarium. Quite often the terms acrania and exencehaly are loosely applied. The last two quoted terms are also abnormalities of NTD  have been used to describe this appearance these represent early stages in the development of anencephaly. The outcome of anencephaly is uniformly fatal.

TABLE 9-1  Incidence of Neural  Tube Defects
Geographic  Area	Spina Bifida incidence per 1000 Births	Anencephaly Incidence per 1000 Births
South Wales	4.1	3.5
Southampton	3.2	1.9
Birmingham ,UK	2.8	2.0
Charleston
White	1.5	1.2
Black	0.6	0.2
Alexandria	0	3.6
Japan	0.3	0.6

Modified from Brocklehurst G: Spina bifida . In vinken PJ. Bruyn GW : Handbook of clinical Neurology . Amsterdam , Elsevier/ North Holland Biomedical Press 1978 ,vol 32 pp 519-578.

What ate the  Recognized Causes of Neural Tube Defects

Multifactorial Inheritance Anencephaly myelomeningocele meningocele and encephalocele Mendelian Syndromes Pallister Hall syndrome Meckel gruber syndrome ,autosomal recessive (phenotype includes occipital encephalocele and rarely anencephaly ) Median cleft face syndrome possibly autosomal dominant (phenotype includes anterior encephalocele) Robert syndrome autosomal recessive (phenotype includes anerior encephalocele) Syndrome of anterior sacral meningomyelocele and stenosis ( dominant either autosomal or X- linked ) Jarcho-Levin syndrome autosomal recessive (phenotype includes meningomyelocele ) HARD (E) syndrome (hydrocephalus agyria retinal dysplasia +  encephalocele ) autosomal recessive (phenotype includes encephalocele ) Chromosome Abnormalities Trisomy 13 Trisomy 18 Triploidy Other abnormalities such as unbalanced translocations and ring chromosome Probably Hereditary but Mode of Transmission Not Established Syndrome of occipital encephalocele myopia and retinal dysplasia Anterior encephalocele among Bantus and Thais Teratogens Valproic acid (spina bifida) Phenytoin Carbamazepine Aminopterin/ amethopterin (anecephaly and encephalocele ) Exposure to high heat caused by fever Monozygotic twinning Maternal Predisposing Factors Diabetes mellitus (anencephaly more frequently than spina bifida ) obesity Methyltetrahydrofolate reductase (MTHER) gene carrier Specific Phentoype without Known Cause Syndrome of craniofacial and limb defects secondary to amniotic bands (phenotype includes multiple encephaloceles) Cloacal exstrophy (phenotype include myelocystocele) Sacrococcygeal teratome (phenotype includes myelomeningocele)

The 11 auditable conditions and detection rates (reproduced with permission from NHS FASP)
Condition	Detection rate (%)
Anencephaly	98
Open spina bifida	90
Cleft lip	75
Diaphragmatic hernia	60
Gastroschisis	98
Exomphalos	80
Serious cardiac abnormalities	50
Bilateral renal agenesis	84
Skeletal dysplasias	60
Trisomy 18 (Edwards syndrome )	96
Trisomy 13 (Patau syndrome )	95

  What we need to kno