Abnormal Calvarium

Anne Kennedy, MD

DIFFERENTIAL DIAGNOSIS

Common

Abnormal Shape
- Poor Scan Technique
- Dolichocephaly
- Brachycephaly
- “Lemon-Shaped”
- “Strawberry-Shaped”
- Round
- Spaulding Sign
- Craniosynostosis
Calvarial Defect
- Exencephaly, Anencephaly
- Encephalocele
- Amniotic Band Syndrome
Abnormal Size
- Macrocephaly
- Microcephaly

Less Common

Decreased Ossification
- Osteogenesis Imperfecta
- Achondrogenesis
- Hypophosphatasia
Scalp Masses

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Assess calvarial size, shape, and mineralization in all cases
- Size
  - Is size concordant with gestational age and other biometric parameters?
- Shape
  - Can you see standard scan plane anatomy?
  - If not, is it because of fetal position or maternal habitus?
  - Use transvaginal sonography for better resolution
  - 3D ultrasound allows volume acquisition
  - Data manipulation allows reproduction of true axial plane
- Mineralization
  - Skull is formed after 10 weeks; use EV sonography from 10-14 weeks for better resolution if questions
  - If brain seen “too well” consider conditions with poor mineralization
  - Transducer pressure cannot deform a normally ossified cranium
Is there a bony defect?
- Essential to look at skull vault from several scan planes
- Refraction of beam may create an apparent defect where none exists
  - Cystic hygroma may be mistaken for an occipital encephalocele
- Must know normal anatomy: Do not mistake metopic suture for frontal encephalocele

Helpful Clues for Common Diagnoses

Poor Scan Technique
- Make sure thalami and cavum septi pellucidi are visible
Dolichocephaly
- Boat shaped: Long back-to-front, narrow side-to-side
  - Seen with breech presentation, oligohydramnios, myelomeningocele
Brachycephaly
- Short back-to-front, wide side-to-side
- Described in trisomy 21
“Lemon-Shaped”
- Bifrontal concavity seen with Chiari II malformation
  - Resolves in third trimester in all cases
- Occurs in various other conditions and 1% of normal fetuses
“Strawberry-Shaped”
- Triangular configuration described in trisomy 18
- Most fetuses with trisomy 18 have multiple other anomalies
Round
- May be technical if measurement obtained in wrong scan plane
- If normal anatomic markers are not identified and head shape appears round from multiple acoustic windows, underlying brain is usually abnormal
  - Look carefully for signs of aprosencephaly/holoprosencephaly spectrum
Spaulding Sign
- Bones of skull vault overlap as brain collapses following demise
Craniosynostosis
- Abnormal head shape secondary to premature closure of sutures
- Look for features of associated conditions (e.g., Crouzon, Pfeiffer, Apert, skeletal dysplasia)
Exencephaly, Anencephaly
- Exencephaly: Lack of cranial vault but brain tissue present
- Anencephaly: Cranial vault absent, no brain tissue, skull base contains gelatinous angiomatous stroma
Encephalocele
- Occipital: Herniation of intracranial structures through an occipital defect
  - Look for other anomalies/signs of aneuploidy
- Frontal: Herniation of intracranial structures through an anterior skull defect
  - Look for associated hypertelorism, callosal dysgenesis, midline lipoma
Amniotic Band Syndrome
- Look for linear echoes from bands in amniotic fluid
- Look for associated extremity amputation or constriction defects
- “Anencephaly” with asymmetric orbits or facial cleft bands highly likely
Macrocephaly
- Enlarged head: Biparietal diameter (± head circumference) > 2 SD above mean
- Look for underlying abnormalities (e.g., hydrocephalus, tumor, megalencephaly)
Microcephaly
- Small head: Biparietal diameter (± head circumference) > 2 SD below mean
- Seen with infection, ischemia, syndromes, malformations

Helpful Clues for Less Common Diagnoses

Osteogenesis Imperfecta
- Associated with fractures in long bones, beaded ribs
Achondrogenesis
- Hallmark is lack of vertebral ossification
Hypophosphatasia
- Associated with micromelia and thin, bowed bones in perinatal lethal form
Scalp Masses
- Calvarium normal
- Mass (e.g., lymphangioma, hemangioma) arises from scalp

Other Essential Information

Technique very important in head measurement and evaluation of calvarial contour
Biparietal diameter (BPD)
- Measure at level of thalami and cavum septi pellucidi
- Cerebellar hemispheres should not be visible
- Midline echoes in center of oval-shaped cross-section
- Measure outer edge proximal skull to inner edge distal skull
Head circumference: Measure at outer edge of skull in same plane as BPD