We here present fetal cases of Achondroplasia,
Thanatophoric dysplasia and Achondrogenesis with different manifestations in antenatal ultrasound.
The antenatal USG was done on all three patients and further confirmed on MRI.
All three cases were followed postnatally.
There are certain common findings which are commonly found in all three types like decreased chest to abdomen circumference ratio and few are specific to one of the types like clover shaped head in Thanatophoric dysplasia.
Thanatophoric dysplasia and Achondrogenesis are lethal in utero however Achondroplasia is of non-lethal variety with frontal bossing and rhizomelic type of dysplasia.
There are criteria to indicate lethalness in the fetus that includes: ( Lethality markers) (5)
- FL/AC < 0.16
- Hypoplastic Thorax
- Clover shaped head
The first four figures(Fig.1-4) demonstrates the classical case of Achondroplasia with macrocephaly,
short femur length,
narrow thorax and short humerus corresponding to the 16-week fetus in a 32-week gestation age.
Achondroplasia is the most common Rhizomelic chondrodysplasia.
•Mostly de-novo; AD inheritance-FGFR3 (4p)with same base pair substitution.
•US(Ultrasound) markers-:
- Short femur length measurement: often well below the 5th centile.
- Trident hand: 2,3 and 4 fingers appearing separated and similar in length.
- Protruding forehead: frontal bossing with narrow skull base.
- Narrowed chest with the protruded abdomen.
CT/MRI(Computed tomography/Magnetic resonance imaging)findings:
- Cranial-(Hypo and Pseudoachondroplasia-normal craniofacial features.)-:
- Large cranial vault with narrowed foramen magnum.
- Cervico-medullary kink,
prominent forehead with a depressed nasal bridge with midface hypoplasia.
- Relative elevation of the brainstem resulting in a large suprasellar cistern and vertically-oriented straight sinus.
- Communicating hydrocephalus (due to venous obstruction at sigmoid sinus).
- Spinal
- Gibbus: thoracolumbar kyphosis with bullet-shaped / hypoplastic vertebra with posterior scalloping.
- Progressive decrease in the inter-pedicular distance in the lumbar spine with short pedicle and canal stenosis.
- Laminar thickening.
- Widening of intervertebral discs.
Thantophoric dysplasia(Fig.5-8) is the next major differential which can be suspected on grey scale imaging due to its peculiar features.
It is one of the most common lethal rhizomelic type of skelatal dysplasia followed by osteogenesis imperfecta type2.
Genetic cause: AD (FGFR3 gene defect).
Two types-:
•Type 1(TD1)-MC,
Curved femur with very flat vertebral bodies,
marked shortening.
•Type2(TD2)-Straight femurs with taller vertebral bodies,
milder shortening.
Hallmark: Kleeblattschadel anomaly (Clover leaf crania).
Nasal bridge flattening and proptosis may be seen.
May be a/w Polyhydramnios.
Other findings:
- Cns: Temporal lobe dysplasia,
megalencephaly,
Hydrocephalus.
- Limbs: rhizomelic appearance typical "telephone handle" bowing with metaphyseal flaring,
sausage fingers.
- Iliac bones: usually hypoplastic squared iliac wings •may show a "trident" acetabular roof.
- Chest: narrow chest with short horizontal ribs.
- Small scapulae.
- Spine: Platyspondyly with wide disc space,
lack of caudal widening of the spinal canal.(7)
The other differential for skeletal dysplasia spectrum;micromelic type,
is Achondrogenesis.
(fig.
9)The major indicator,
apart from the usual narrow thorax and short bones,
being the unossified or minimally ossified spine due to hypomineralisation.
Facies:(8)
- Frontal Bossing
- Flat face
- Anteverted nares
- Long philtrum
ACHONDROGENESIS
-
Type 1 |
Type 2 |
More severe |
Less severe |
Type 1a-Houston Harris-Rib#,
proximal femoral metaphyseal spikes,
Congenital heart disease(ASD,
VSD,
PDA)
|
Langer Saldino-AD-Chondrogenesis defect-Normal cranial ossification.
(floating head). |
Type 1b-Parenti Fraccaro-Ribs with cupped ends,
No rib#,
Distal femoral metaphyseal spikes. |
Distal ulna metaphyseal irregularity,
Cystic hygroma,
Polyhydramnios,
Micrognathia.
|