Topic 5 - the fetal head Flashcards
You are performing a routine morphology scan at 19 weeks. You identify that the BPD is on the 2nd percentile and the head circumference is on the 20th percentile. How do you report this finding?
The finding of a small BPD is common. This is called dolichocephalic. If the head circumference is within the typical range, then this finding can be dismissed as normal. You may want to glance up and see whether the maternal or paternal heads are also narrow. Fetal lie can also create this appearance.
You are performing a routine morphology scan in 19 weeks. You identify that the BPD is on the 2nd percentile and the head circumference is also on the 2nd percentile. How do you report this finding?
the head is small for dates. Careful assessment of the other biometry should be performed to ensure that this fetus is not globally small for dates. If there are other growth issues, IUGR and chromosomal abnormality should be considered. The other obvious response is to check the dates of the pregnancy to make sure that this is not just a dating error.
Very importantly, a fetal head is only micro-cephalic when it is less than the 1st percentile. This is very very small. While it is important in this instance to draw attention to the size of the head, it is just as important not to label this head microcephaly
What are some cranial measurements to obtain in the second trimester?
Biparietal diameter Head circumference Lateral ventricles Nuchal fold Cerebellum transverse diameter Cisterna magna (Optional : Occipito-frontal diameter)
What are the three standard transaxial planes of the head?
(thalamic, ventricular, and cerebellar)
Where is the ambient cistern and what view is it seen?
behind the thalamus and between the occipital lobes is the ambient cistern (arrow), which contains cerebrospinal fluid (CSF) but is rendered echogenic because of strands of meninges supporting the brain structures
What is a key finding in CNS abnoralities?
Ventricular abnormality, especially enlargement
Measurement of the lateral ventricle should be…
Inner to Inner
Include the glomus of the choroid
Measurement of the widest part
Perpendicular to falx
What size should the cerebellum be up to 24 weeks?
diameter corresponds to gestational weeks
What is the normal measurement of the cisterna magna?
2-10mm
What does cisterna magna obliteration indicate?
Chiari II malformation and spina bifida
What might cause enlargement of the cisterna magna?
mega–cisterna magna, Blake pouch cyst, vermian dysplasia, Dandy-Walker syndrome, and arachnoid cysts.
How do you identify a cavum veli interposti?
small cystic collection in the midline
Doppler exclude vascular abnormality such as vein of Galen aneurysm
large cysts can distort the brainstem and adjacent brain, causing obstructive hydrocephalus (HC), and need treatment by unroofing.
What are some differentials for CVI?
dilated cavum Vergae, glioependymal cysts, arachnoid cysts, cystic tumors (mainly cystic teratomas), vein of Galen aneurysm, pineal cyst, and hemorrhage.
What is hydrocephalus?
refers to enlarged ventricles associated with increased intracranial pressure and thus is typically associated with head enlargement
What is mild ventriculomegaly?
atrial width of 10-12 millimetres
most cases it will result in the birth of healthy infants
can be isolated but is associated with an increased risk of heterogeneous CNS and non-CNS anomalies
What is moderate ventriculomegaly?
12-15mm
also has good outcome
similar to mild
What is overt ventriculomegaly?
> 15mm
can be isolated or associated with other cerebral anomalies
most frequently associated with neural tube defects and midline anomalies
What is aqueductal stenosis?
The most common cause of obstructive hydrocephalus in the neonate is aqueductal stenosis, which results from the narrowing of the aqueduct of Sylvius.
What is the aqueduct of sylvius?
a narrow channel that connects the third and fourth ventricles
What is the sonographic appearance of aqueductal stenosis?
Dilated third ventricle
Hydrocephalus
Ventriculomegaly can be severe and progressively worse later in the pregnancy. Can even be normal at the 2nd trimester scan
Normal fourth ventricle and posterior fossa
Parenchymal thinning
What are the 4 scenarios ventricular enlargement generally occurs in?
1) obstruction of CSF flow in the brain
a. usually at the aqueduct (intraventricular obstructive hydrocephalus).
b. Alternatively, the site of blockage may be outside the ventricular system
2) excess CSF secretion with choroid plexus papillomas.
3) following cerebral destruction and brain shrinkage as a result of abnormal brain development or diverse insults (hydrocephalus ex vacuo)
4) may be a feature of generalized cerebral malformation.
What are three neural tube defects
anencephaly, encephaloceles and spina bifida.
What are anencephaly and exencephaly?
Anencephaly follows failure of closure of the rostral neuropore leading to failed cranial vault development and unprotected exposed brain tissue (exencephaly)
What are some common associations of anencephaly?
Associated spinal and non-CNS abnormalities and polyhydramnios are common.
Chromosomal abnormality is seen in 2%, and additional abnormalities are common including cleft lip, cardiac defects, club feet, and abdominal wall defects
Why is detection of anencephaly before 14 weeks difficult?
relatively normal-appearing brain structure can be present
can be missed unless the examiner specifically looks for ossified cranial bones
What can raise MS-AFP mean
spina bifida (62%) and anencephaly (92%).
What are cephalocele and encephalocele?
A cephalocele is a herniation of intracranial structures through a defect in the cranium or skull base
encephalocele when it contains brain tissue.
Where do encephalocele occur?
Most encephaloceles occur in the midline in the occipital (75%), frontal (13%), or parietal (12%) regions
What chromosomal abnormalities are strongly associated with cephaloceles?
trisomies 18 and 13
Why is ultrasound important in the diagnosis of cephaloceles and encephalocels?
Unlike spina bifida and anencephaly they are usually skin covered.
So only 33% have a raised MS-AFP unlike spina bifida (62%) and anencephaly (92%).
This makes ultrasound the primary diagnostic tool.
What are some differentials for cephaloceles and encephaloceles?
cystic hygroma hemangioma scalp edema cephalohematoma epidermal scalp cyst branchial cleft cyst dermoid cyst dacryocystocele epignathus cervical teratoma
What is amniotic band sequence/limb body wall complex?
variable collection of disruptive abnormalities
associated with adherence of the fetus to bands in the amnion.
typically have normal karyotype.
Small cystic appearances within the midline will most likely represent
Small cystic appearances within the midline will most likely represent a cavum vergae, cavum septum interpositum, herniating third ventricle or a tiny arachnoid cysts
What is a cavum vergae?
The cavum vergae is the posterior extension of the cavum septum pellucidum, posterior to the anterior columns of the fornix,
What is holoprosencephaly?
cerebral malformations resulting from incomplete cleavage of the primitive forebrain into two cerebral hemispheres.
Most common abnormality of brain development
Midline facial anomalies occur in 80 percent of cases.
What are the three types of holoprosencephaly?
Alobar, semi lobar and lobar
Describe alobar holoprosencephaly
lacking two hemispheric lobes, no cerebral separation into two hemispheres:
· single ventricle
· no falx or interhemispheric fissure
· thalamic fusion
· absent corpus callosum
Describe semi lobar holoprosencephaly
partial formation of lobes, partial cleavage into hemispheres:
· absence of the falx
· thalamic fusion
Describe lobar holoprosencephaly
two hemispheres are present:
· absence of the septum pellucidum
What are the factors associated with holoprosenphaly?
Environmental teratogens: alcohol, smoking, retinoic acid, salicylates, anticonvulsants
Metabolic: insulin-dependent diabetes (1% risk)
Infections: cytomegalovirus, toxoplasmosis, rubella
Syndromes with normal karyotype (about 25% of holoprosencephaly)
Chromosomal abnormalities
Trisomy 13 (70% have holoprosencephaly)
Trisomy 18
Triploidy
Gene mutations
What chromosomal anomaly is most highly associated with holoprosencephaly?
Trisomy 13
What are the sonographic signs of holoprosencephaly?
a single ventricle
single orbit
proboscis (trunk nose)
1. Cyclopia with single eye, with or without proboscis
2. Ethmocephaly (hypotelorism and proboscis between the eyes)
3. Cebocephaly (hypotelorism, nose with single nostril)
4. Median cleft lip and palate and hypotelorism (eyes close together)
5. Single central incisor
How do you diagnose dandy walker malformation sonographically?
marked enlargement of the cisterna magna (≥10 mm)
complete aplasia of the vermis
a trapezoid-shaped gap between the cerebellar hemispheres
Sagittal view to check
Antenatal ultrasound may falsely overdiagnose the condition if performed before 18 weeks, as the vermis has not properly formed.
If you see an dandy walker appearance what are differentials? What do you do?
The next step when you think that the cerebellar vermis is absent or hypoplastic is to perform a sagittal image. In this plane, the cranio caudal size of the vermis can be measured.
An important differential diagnosis in this context is that there is significant rotation of the cerebellar vermis secondary to a Blakes pouch anomaly. This can give the appearance of cerebellar vermial hypoplasia.
What is the normal size of the cerebellar vermis?
12mm
What is the normal angle off the brainstem of the cerebellar vermis?
> 40 dandy walker
<30 blakes pouch
What are craniofacial anomalies often associated with?
They are often associated with chromosomal aberrations, syndromes, other abnormalities and polyhydramnios.
What are the definitions of micro and macro cephaly?
measurements are three standard deviations (SDs) below the mean, microcephaly is diagnosed
measurements are greater than 3 SDs above the mean, macrocephaly is suggested
What is an abnormally long narrow cranium?
Dolichocephaly
What is an abnormally round head?
Brachycephaly
What does a lemon shaped skull suggest?
with indentation of the frontal bones, is often seen in association with open neural tube defects and the Chiari II malformation of the hindbrain
may also be seen in normal fetuses
What does strawberry shaped skull suggest?
flattening of the occiput and narrowing of the bifrontal portion of the cranium—may be seen in association with trisomy 18.
What does a cloverleaf skull suggest?
seen with some dwarfs, especially thanatophoric dysplasia, and in some fetuses with craniosynostosis
What are some conditions associated with hypotelorism?
1. Abnormalities of the brain Holoprosencephaly Microcephaly 2. Chromosomal abnormalities ·Trisomies 13, 18, and 21 Chromosome 5p deletion 3. Head shape abnormalities Trigonocephaly 4. Syndromes
What are some associations with hypertelorism?
1. Chromosomal abnormalities Trisomy 9p 45,XO (turner syndrome) 2. Single-gene disorders 3. Developmental abnormalities Craniosynostosis Anterior encephalocele Median facial cleft Megalencephaly Agenesis of the corpus callosum Orbital teratoma Cleft lip 4. Teratogens Dilantin Valproate
What are some causes of mid face hypoplasia?
Syndromes Apert Crouzon Treacher Collins Wolf-Hirschhorn Pfeiffer Turner trisomy 21. facial clefts craniosynostosis skeletal dysplasias
When can a hypoplastic or absent nasal bone be seen?
seen with increased incidence in fetuses with trisomy 21
What are the different types of cleft lip and palate?
isolated cleft lip
unilateral or bilateral cleft lip/cleft palate
isolated cleft palate
midline cleft lip/palate
asymmetric clefts caused by bands, related to amniotic band syndrome.
What is the most common variant of clefts
cleft lip cleft palate
What is median cleft lip a classic finding in?
Alobar holoprosencephaly
What are sonographic signs of cleftlip/cleft palate variants?
abnormally high position of the fetal tongue
hypertelorism
deviation of the vomer (a triangular bone in the nasal septum forming the posterior and inferior portion of the septum)
micrognathia.
May interfere with swallowing and cause polyhydramnios
What is micrognathia?
Small chin
What is anagnathia?
Absent chin
What can cleft lip/palate cause in neonates?
feeding, hearing and speech problems.
What is an abnormal nuchal fold measurement?
> 6mm at 18-20 weeks
When does the nuchal fold measurement become unreliable?
> 22 weeks
What are the most common anterior neck masses?
a goiter, a haemangioma or a teratoma.
WHat are the most common posterior neck masses?
a haemangioma and a cystic hygroma.
How does a cervical teratoma appear on sonography?
complex masses, both cystic and solid elements, often associated with regions of calcification
There is often vascular flow within the solid portions of the mass
can cause hyperextension of the neck
What causes fetal goiter?
maternal thyroid disease, such as Graves disease or Hashimoto thyroiditis
Maternal use of thyroid blocking agents
primary fetal thyroid dysfunction
How does a cystic hygroma appear on ultrasound?
Septated fluid collection behind the foetal neck
What is cystic hygroma highly associated with?
Turner syndrome
