Skeletal dysplasia

ABC of Skeletal dysplasias:- Birth   prevalence   of skeletal   dysplasias is  estimated as 2.4/10,000  births . If one cannot estimate many important diameters  necessary to confirm the diagnosis of skeletal dysplasia / due possibly to time constraints then a femur/  abdominal   circumference ratio  less than 0.16  have to be measured as a minimum. In fact  F/ AC  is the  main  discriminator  among fetuses  with lethal skeletal   dysplasias and   that this   measurement   has  still a  better  performance  than a)  femur  shortening b)  thoracic circumference and c) thoracic circumference/ abdominal circumference  ratio.

Which  gene has undergone mutation?? :-- Till date it is known that about    25%   of all bone   disorders  the mutated gene   has not been  yet identified –

 What is the classification of constitutional disorders of bone ?? Ans:- Skeletal dysplasias   are a heterogeneous group  of disorders affecting the development   of chondro-osseous tissues leading  to abnormalities in the size ,   mineralization  and   shape of various  segments of  the skeleton. Despite recent advances in   imaging and molecular genetics accurate   prenatal diagnosis  of  skeletal   dysplasias remains a clinical   challenge. In the most recent   revision   of the International Nosology and Classification of Constitutional disorders of bones , it  is mentioned that in approximately   25%   of all bone   disorders  the mutated gene   has not been  yet identified.

What does  International  skeletal  Dysplasia   Registry tells us??  :--It is   important  to acknowledge  the contribution  of the International  skeletal  Dysplasia   Registry in the  identification   and study  of skeletal   anomalies   assisting  providers  and  patients  in the diagnosis   and clinical  management  of skeletal  disorders. This registry informs us that a)

Birth   prevalence   of skeletal   dysplasias was estimated as 2. 4/10,000  births.

b) Outcome of such  siblings:-Twenty three percent   of the affected infants  were stillborn  whereas 32%  died during the first  week of life. The overall   frequency  of skeletal  dysplasias among perinatal  deaths was 9.1/1000. c) Classification of skeletal dysplasia:-The four   most common kinds of skeletal   dysplasias were    A) thanatophoric dysplasia   B) achondroplasia   C) osteogenesis imperfect and D) achondrogenesis . Thanatophoric  dysplasia    and achondrogenesis accounted  for 62%  of all   lethal  skeletal  dysplasias and  the most common  nonlethal skeletal dysplasia was achondroplasia.

How significant is  incidental  solo findings of “short femoral length at late trimester”  ?? Will  we be  worried?? Concern to the Obstetrician is “Am I   dealing with normal biological variant/ real pathology of bone so called –“skeltal dysplasia” ?? Ans:- Significance of short   femoral length if noticed at  mid to late  third trimester “” This is  not unusual  for the femur  to be significantly shorter than the other  measurements   used to estimate   gestational age. In many cases  the femur is approximately 2 weeks   less than   the head circumference and menstrual age. The cause of this is often uncertain   and the short femur is often   attributed to biologic variation or ethnicity.  If the bones appear normal   morphologically and no other anomalies   are seen it is likely a normal variant.

One week less is permissible. Short F L . But my dear members this even 1 week lag may rarely be due to 1)  a sign  of trisomy   21 or 2) an  early manifestation of a short    limb   skeletal   dysplasia.  :-- One should acknowledge that a fetus from white  parents of normal height   and middle   socioeconomic   status will have a standard FL which one should not compare with that of FL of Indian parents of low socioeconomic  status .  Although   the femur can be   smaller than other   measurements in the third trimester   it is not acceptable for it to be greater    than 1 week less than the other measurements in the late first   and early to mid second   trimesters. Although   this deviation  may be nothing  more than   a temporary  growth lag or  normal   variation   at this stage , it may also be  1)  a sign  of trisomy   21 or 2) an  early manifestation of a short    limb   skeletal   dysplasia. This  may be  a case  of trisomy 21 as well . One should    not take   comfort   in comparing the femur measurement to the estimated   gestational   age by ultrasound imaging   . Remember that   unless excluded the femur measurement    is included   in the sonographic estimation of   gestational  age.