It is always interesting to me when I see an older child with underdeveloped thumbs, aka hypoplastic thumbs. Most of the time, children with small thumbs are identified early in life as small thumbs are typically part of a larger issue, radial longitudinal deficiency (RLD). However, when isolated, underdeveloped thumbs can be easy to “miss” early in life and may only be noticed when children face increased demands on their thumbs in school or sports.
There are 3 parts to the underdeveloped thumb.
1) The thumb looks different due to a lack of the muscles at the base of the thumb, the thenar muscles. While this may be clear to someone with training, this deficiency in muscles may not always be easy to appreciate.
2) The thumb MCP joint (connecting the thumb to the hand) is unstable. This may be the most important part of the underdeveloped thumb from a functional standpoint. The reason this is important is the lack of joint stability affects pinch strength and, eventually, this can cause functional limitations. Basically, a weak pinch is not ideal and can absolutely limit what a child or adult can accomplish.
3) The web between the thumb and index finger may be shallow in the underdeveloped thumb. While not always a problem, when this web is really shallow, it can limit the size of the object that the child can grab (such as a ball or soda can).
As mentioned above, the three signs are usually identified early as part of the bigger problem of RLD. But sometimes, the thumb is an “isolated” problem. We see that when one arm has severe RLD and the other seems normal. But with close examination, the thumb on this "normal" side is really underdeveloped. Or, as noted above, the child comes to clinic at age 10 or so complaining of different appearing thumbs that lack strength. That is the case for this child.
|Underdeveloped or hypoplastic thumbs. Not the lack of muscle at the base of the thumb.|
|The underdeveloped thumb looks slightly different here but no obvious findings.|
|The underdeveloped thumb does not have good stability as shown in this stress picture.|
|X-rays of an underdeveloped thumb. While not obvious, there is a clear difference in the thumb development compared to normal.|
And why do we like to make this diagnosis? Well, the underdeveloped thumb is a part of the spectrum of RLD. So, even if the underdeveloped thumb is isolated, the patient still has RLD. And, as I have blogged about Here, this carries risks for other abnormalities such as VACTRL, TAR syndrome, Holt- Oram Syndrome, and others. If the child is properly identified, these other issues can be properly assessed.
In addition, if we identify the underdeveloped thumb, we can treat it to maximize function. I have previously written about this Here, although that post discusses pollicization (not appropriate here) and reconstruction of the joint and the muscles (what we would consider for this patient).
Charles A. Goldfarb, MD
My Bio at Washington University