The child demonstrates a short-limbed dwarfism with predominant shortening of the proximal limbs (rhizomelia). The chest wall is very tall and narrow, almost bell shaped and the ribs are short with flared ends. Overall bone density is normal, particularly of the calvarium and there are no fractures making hypophospahasia or osteogenesis imperfecta (the type II lethal variant) unlikely. Achondrogenesis should also be associated with calvarial and vertebral bodies that often appear absent due to poor mineralization. In this case the vertebral body epiphyses are not well seen, due to prematurity but ossification is otherwise normal. Also in achondrogenesis the chest shape is usually barrel-shaped. Campomelic dysplasia is another consideration although once again there is often poor ossification, bone shortening is not as prominent, although a bell-shape chest may occur and there is proximal bowing of the more gracile femurs, tibias and 5th digits.
What other features lead to the diagnosis? Well asphyxiating thoracic dystrophy, thanatophoric dwarfism and campomelic dysplasia may all have a bell shaped narrow elongated chest. ATD and TD may also have superficially similar appearances in the pelvis – the trident appearance of the inferior acetabulum due to a inferolateral spur from the sciatic notch in ATD and the notched appearance of the acetabulum in TD due to the narrow sciatic notch demonstrated in this case. Now pay attention to the shape of the femurs. This is the key feature in this case. They are curved distally, in a so called telephone receiver configuration. This narrows the diagnosis down to one entity – thanatophoric dwarfism. Thanatophoric dwarfism or dysplasia (for those not blessed with a classical education or Greek heritage, thanatos means death, thanatophoric- carrying death) is by definition a lethal dysplasia. Death occurs due to pulmonary hypoplasia. The diagnosis may be made antenatally. Additional features not seen in this case include flattening of the vertebral bodies with more constriction of their midportions and wide intervertebral disc spaces causing an appearance likened to an inverted U or H. As seen in this case the pelvis may demonstrate small, rectangular iliac bones, small sacroiliac notches, and short and wide pubic and ischial bones. Occasionally thanatophoric dwarfism is associated with a clover-leaf skull. The condition may be occasionally indistinguishable from homozygous achondroplasia, although in such cases the diagnosis is obvious in view of both parents demonstrating the usual heterozygous form of achondroplasia.