Abnormal Spine Position

Anne Kennedy, MD

DIFFERENTIAL DIAGNOSIS

Common

Positional
Spina Bifida
Body Stalk Anomaly
VACTERL Association
Vertebral Anomaly

Less Common

Amniotic Band Syndrome
Thanatophoric Dysplasia
Achondroplasia
Asphyxiating Thoracic Dysplasia

Rare but Important

Conjoined Twins
Iniencephaly

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Is fetal position fixed or variable?
- If variable, less likely to be significant problem
- If fixed, evaluate global fetal movement
  - Any evidence of arthrogryposis/akinesia sequence
  - Serial evaluation important as akinesis sequences may be progressive
- Look at abdominal wall and cord insertion site to exclude body stalk anomaly
- Scan through amniotic fluid looking for bands
Are bones of spine normal?
- Use high frequency transducer for better resolution
- Assess vertebral body height, mineralization, presence of 3 ossification centers at all levels
Is spine length normal?
- Caudal regression sequence may cause spine position to look unusual as loss of normal sacral curvature
  - Make sure vertebra are present between the iliac wings
- Scan through cervical, thoracic, lumbar, and sacral spine systematically

Helpful Clues for Common Diagnoses

Positional
- Most commonly idiopathic due to fetal stretching movements
  - Fetus seen to move extremities
  - Normal amniotic fluid
  - Change in position on follow-up scans
- May be due to fetal crowding
  - Multiple gestations, late pregnancy
- Associated with oligohydramnios
  - Renal agenesis/other bilateral renal anomalies
  - Twin-twin transfusion syndrome donor
- Look for synechiae
  - Fetal parts oriented around large synechia
- Large uterine fibroids may distort cavity → unusual fetal position
- Müllerian duct anomalies may result in decreased cavity size/unusual fetal lie
Spina Bifida
- Depending on level of neural tube defect (NTD) may see sharp kyphosis
  - Thoracic NTD most likely to result in abnormal spine position
  - Iniencephaly is high cervical NTD causing neck hyperextension due to shortening of cervical spine
- Look for associated brain findings of Chiari II malformation
  - Ventriculomegaly, “banana” cerebellum, “lemon-shaped” head
- Look for splayed/absent posterior elements
- Look for associated myelomeningocele/myeloschisis
Body Stalk Anomaly
- Result of embryologic maldevelopment with interruption of normal embryonic folding process
  - Open abdominal wall defect
  - Peritoneum in continuity with amnion therefore fetus fixed to placenta
  - Body stalk/yolk stalk fusion fails: Short or absent umbilical cord
- Normal cord development allows fetus to move freely within amniotic sac
- If no cord, fetus tethered to uterine wall
  - Extremities and cranium move
  - Spine growth/elongation → hyperextension about fixed point where abdominal cavity is open and adherent to placenta
VACTERL Association
- Non-random association of seven core anomalies
  - Vertebral anomalies
  - Anal atresia
  - Cardiac anomalies
  - Tracheoesophageal atresia
  - Renal anomalies
  - Limb defects (radial ray)
Vertebral Anomaly
- Look for hemivertebra/fused vertebrae at apex of scoliosis/kyphosis
- Ask for history of maternal diabetes, alcohol use, drug exposure (e.g., valproate)

Helpful Clues for Less Common Diagnoses

Thanatophoric Dysplasia
- Lethal skeletal dysplasia with severe micromelia
- “Telephone receiver” femur in type 1
- Kleeblattschädel (cloverleaf) skull type 2
- Distinctive spine appearance
  - Marked platyspondyly with intervertebral disc height > vertebral body height (normally equal)
  - Lumbar kyphosis
Achondroplasia
- Thoracolumbar kyphosis
- Narrowed interpedicular distance
- Progressive rhizomelic limb shortening
- Macrocephaly with frontal bossing
- Trident hands
Asphyxiating Thoracic Dysplasia
- Thoracic lordosis, lumbar kyphosis
- Short, horizontal ribs with small thorax

Helpful Clues for Rare Diagnoses

Conjoined Twins
- Contiguous skin covering between twins
- Spines hyperextended due to fixed anterior point of union
- Bridging tissue may be pliable → orientation of twins can vary from scan to scan
Iniencephaly
- Lethal extensive open neural tube defect
  - Defect in occiput and inion
  - Occipital encephalocele + spinal dysraphism
  - Fixed cervical hyperextension → “stargazer” fetus

Other Essential Information

Twin gestation
- Determine chorionicity
- Conjoined twins are monochorionic
- Differential diagnosis for fixed scoliosis in one twin of dichorionic twins is as for singleton
Apparently isolated hemivertebra causing abnormal spine position may be “tip of the iceberg”
- Perform formal fetal echocardiogram at 18-22 weeks
- Look carefully for other stigmata of VACTERL association
- Assess spinal cord with high resolution transducer if possible
  - Tethered cord
  - Lipoma
  - Diastematomyelia

Image Gallery

Sagittal ultrasound shows prominent but transient thoracic kyphosis. This was a transient finding and is seen with normal fetal movement.

Sagittal ultrasound shows a 6 cm fibroid

compressing the fetal spine

. This was exacerbated by transducer pressure. The fetus moved normally.

(Left) 3D ultrasound shows acute angulation

of the thoracic spine in a fetus with a high thoracic neural tube defect. Origins of some of the upper ribs

are seen; the rest of the torso was twisted out of plane. (Right) Axial oblique ultrasound in the same case as previous image shows the myelomeningocele sac