Microcephaly is a condition involving the reduction of brain mass and head size with an intact bony calvarium, usually defined as a fetal head circumference of 3 SD, though some used 2 SD, below the mean of each gestational week.
- The head is small, with a fetal head circumference of 3 SD.
- The femur length/head circumference ratio is at 3 SD or higher.
- Doppler ultrasound may show reduced or absent flow in the intracranial arteries, suggestive of a vascular cause.
- Severe microcephaly may be difficult to differentiate from anencephaly.
- Microcephaly is diagnosed at a mean gestational age of 28 weeks, but may not be demonstrated until after 24 weeks, or even at late onset in the last trimester.
Causes of microcephaly:
- isolated microcephaly (16.7%)
- microcephaly due to holoprosencephaly (16.7%)
- microcephaly associated with chromosomal disorders (23.3%)
- microcephaly as part of a genetic syndrome (20.0%)
- microcephaly as part of multiple anomalies (23.3%)
- microcephaly as part of intrauterine infections (i.e.: Zika virus)
Intracranial calcifications have been rarely demonstrated prenatally. They most commonly result from intrauterine infection but are sometimes associated with intracranial tumor. The differential diagnoses of intracranial calcifications include
- intrauterine infection (often also related to microcephaly, or ventriculomegaly, or growth restriction)
- cytomegalovirus (most common): usually subependymal calcification
- toxoplasmosis: usually scattered
- varicella-zoster syndrome
- rubella virus (very rare)
- intracranial tumor (usually localized to tumor mass)
- a normal second trimester ultrasound does not always exclude intracranial calcifications.
Irregular echogenic mass in the area of lateral ventricle and cerebral mantle
Some specific anomalies have a characteristic shape of the head.
Lemon sign: a concave deformity of the frontal bones at the level of the coronal suture, which may be associated with
- normal fetus (1-2% of normal fetuses)
- spina bifida (common)
- occipital cephalocele (rare)
- thanatophoric dysplasia (rare)
- long-term oligohydramnios
Cloverleaf skull (found in 14% of cases of thanatophoric dysplasia)
- thanatophoric dysplasia (most common)
- homozygous achondroplasia
- Apert’s syndrome
- Pfeiffer’s syndrome
- Carpenter’s syndrome
Strawberry head shape (often associated with fetal trisomy 18).