Diagnosing Silver-Russell Syndrome
Getting a diagnosis of a rare condition, such as Silver-Russell Syndrome can be a drawn-out process. In the UK, it can take over four years, on average, to get an accurate diagnosis of a rare condition. This can be a frustrating time for parents, and all concerned. It can also be a time of wrong-turns, dead-ends and misdiagnoses. It is crucial to get the diagnosis right, as the diagnosis often determines management and support options.
Diagnosing SRS poses particular problems as there is often a large overlap of SRS symptoms with many different conditions. A crucial breakthrough in Poblet, Spain, in 2015 saw 36 leading international SRS specialists agree, by vote, and representatives from patient support groups, including the CGF, the elements that are distinctly SRS. And a year later the conclusions of this meeting were published as the First International Consensus Statement regarding Silver-Russell Syndrome. Consensus statements are vital for increasing understanding and awareness of rare conditions, which greatly improves the diagnosis process. The consensus statement regarding SRS recommended using Netchine-Harbison Clinical Scoring System (NH-CSS) as a tool for clinically diagnosing SRS.
Consensus Statement
A consensus statement is a public statement on a particular aspect of medical knowledge, agreed by a representative group of experts to be evidence based, they are vital for increasing understanding and awareness of rare conditions, which greatly improves the diagnosis process. The consensus statement regarding SRS recommended using Netchine-Harbison Clinical Scoring System (NH-CSS) as a tool for clinically diagnosing SRS.
What is the NH-CSS/How Does it Work?
The scoring system identifies six factors (clinical criteria) that are considered statistically strong indicators of Silver-Russell Syndrome. It actually works by statistically ruling out Silver-Russell Syndrome and therefore, if it can’t be statistically ruled out then SRS becomes a strong possibility. If the patient scores 3 or less out of 6 they are determined not to have a clinical diagnosis of SRS (however a score of 3 can result in continued clinical suspicion). The scoring system is relatively straight forward, it is a score of 1 if that indicator is present, and it is a score of 0 if it isn’t. The end result is a score out of 6, which makes it easy for a physician to interpret and is flexible enough to allow for some missing information.
What are the Six Clinical Criteria?
SGA (small for gestational age – birth weight and/or birth length)
SGA is a term given to a baby that is born smaller than normal compared to other babies born at the same gestational age (read more).
Score a point if: the baby is equal to/or less than 2 standard deviations (SDS) below average. Standard deviation is a measure of how spread a measure is around the average (or mean), in this case weight and/or length of babies born at the same gestational age. This would mean the baby is in about the smallest 2% of all babies born at that gestational age.
Postnatal growth failure
Is a term (and not a great one, because you haven’t failed at anything!) used to describe babies that are not growing as expected/normal following birth.
Score a point if: the baby’s height at around 24 months is less than 2 standard deviations below the average for babies at the same age. OR the baby’s height is 2SDs below the mid-parental target height.
Standard Deviation
Standard deviation is a number used to tell how measurements for a group are spread out from the average.
The lower the number, the closer it is to the average, the higher the number the further it is from the average.
The bell curve above shows that 2SDs (the dotted lines) below the middle point represents around 2% of the population.
Relative Macrocephaly at Birth
Macrocephaly means your baby’s head is larger than other babies of the same sex and same age, measured by head circumference. Relative macrocephaly considers the size of your baby’s head against their weight or length.
Score a point if: your baby’s head circumference at birth is equal to/or greater than 1.5 SDS ABOVE their birth weight and/or length SDS. For example, if your baby’s birth weight was -2SDS and their head circumference was -0.5SDS then they would score a point.
Protruding Forehead
Also known as frontal bossing, this is where your baby’s forehead sticks out beyond their face when looking at them side on.
Score a point if: your child has a protruding forehead between the ages of 1 to 3.
Body Asymmetry
This is where all, or part of one side of the body is smaller than the other.
Score a point if: there is a leg length discrepancy (LLD) of 0.5cm or more or, there is arm or leg discrepancy that is less than 0.5cm but two other parts of the body are asymmetrical.
Feeding Difficulties and/or Low BMI
Body mass index gives a figure that compares weight against height, and in children also considers age and gender. It is used to give an idea of how healthy your weight is.
Score a point if: the BMI score is equal to/or less than -2 SDS at 24 months old. Or, score a point if your child is using a feeding tube.
Clinical Diagnosis
If the child being examined scores 4 or more on the NH-CSS then it is recommended they have genetic testing. The genetic testing may confirm Silver-Russell Syndrome. However, the testing might come back “normal”, which means no genetic cause has been found.
If this happens then the score of 4 or more on the NH-CSS can be used to give a clinical diagnosis of SRS, BUT only if the clinical criteria that scored a point includes BOTH protruding forehead and relative macrocephaly at birth. The large head in relation to weight at birth is a distinguishing characteristic of SRS that almost all SRS children have. But macrocephaly can be present in other conditions too, which is why it should be used as an indicator of SRS unless other clinical criteria have also been identified.
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