A friend of my wife’s recently (like 6 days ago) had a baby girl. Today the woman who we’ll call Amy, called my wife frantically worried that the baby has “slight DS.”
Apprently, and this is obviously all third party information, so forgive the gauzy details, some clinician told Amy the the baby has a mild DS that never registered in the prenatal tests.
IANA Dr. (hence the question) but I thought prenatal DS testing was pretty damn conclusive and a “slight” form of it was along the lines of being slightly pregnant; either you are or you aren’t.
My wife and I have chalked it up to a new mother (it’s her first kid) who is probably in the throws of emotion that I’ll never comprehend, but I thought I’d put it to the experts here.
“Slight” DS, does such a thing exist? And would it show up in prenatal testing?
I don’t have anything approaching a medical background either, but I understand Down Syndrome to be a host of symptoms, not a single condition. In my experience in working with Down kids, they’ve ranged from almost normal to profoundly delayed. Here’s more from everyone’s favorite cite, Wiki:
Not the news I was hoping for, but that you very much. I hadn’t thought to look on Wiki, accoerding to this the chances of missing DS in a test are pretty low, so I thought she was worried over nothing.
DS results from having an extra copy of chromosome 21. This usually happens prior to conception (with one of the gametes having 2 copies instead of the usual 1). If it happens after conception, and after the first cell division, this is called Mosaic Down’s Syndrome. Some, but not all, of the cells in the baby’s body will have 3 copies of chromosome 21. I’m not sure what the prognosis is for Mosaic Down’s Syndrome, but you might ask your friend if this is what she means.
There are two issues here: one is the extent of the expression of Down Syndrome; the other is why it might have been missed on a prenatal test.
The usual cause of Down is a trisomy of chromosome 21, that is, there are three copies of this chromosome instead of the normal two. This should presumably be easy to pick up on prenatal screening, and should not have been missed. Expression of Down characteristics is variable, and it may not be possible to predict how they will later develop at birth.
However, sometimes only part of chromosome 21 is duplicated, or only some body cells contain three copies. I don’t know how easy this latter condition is to detect, or whether this is correlated with a milder form of Down.
In addition, according to my biology textbook, sometimes pieces of an extra chromosome 21 have been found attached to other chromosomes. This might produce something that could be described as “slight” Down’s syndrome, and would not be easily detectable in a standard karyotype, as the piece can be rather small. Fortunately, the smaller the piece attached, the less likely there is to be any effect at all on the bearer of said altered chromosome, so chances are, if it wasn’t detected in a standard karyotype, it’s probably pretty tiny and might be totally inconsequential.
So yes, it’s possible, and yes, it might be horrible, but no, it’s probably not.
The thing is, unless you are in a high risk group, the prenatal tests they give you only measure what your risk levels are for your baby getting DS (and other diseases) - they aren’t testing for DS itself. Then, if it looks like you are above a certain risk level, they’ll offer you further testing. The test is called a “triple screen” and, to keep with the trend in this thread, I’ll offer the Wiki page that explains it: http://en.wikipedia.org/wiki/Triple_screen
It all depends on whether or not the genes that cause DS are found in the extra piece of chromosome 21. If they are, then it doesn’t matter whether or not it’s attached to another chromosome. I’m not sure if we know where all the genes are, or whether we can say that if certain portions of an extra chromosome 21 are present, we can say anything definitive (from just that evidence) about the prognosis of the child. Does you textbook discuss that at all?
Like I said, according to the textbook, the smaller the piece of chromosome 21 is attached, the less likely that important genes will be duplicated on the extra piece. Since something like ~90% of the human genome is introns (noncoding segments), it’s possible that no genes have been duplicated on a piece. Then again, though, we have no idea if this semi-trisomy is even what the OP is referring to. The textbook does state that the outer “arm” sections of the chromosomes are more likely to become attached this way, but it’s not exclusive to them. Also note that this isn’t just a phenomenon occuring with chromosome 21; this can happen with any chromosome.