Common Structural Oddities in the Human Body
May 29, 2009
Common Structural Oddities in the Human Body
May 29, 2009
There are a number of common anatomic variations and developmental changes related to aging. Some of these structural "oddities" can be mistakenly interpreted as abnormalities, or worse, a neoplastic disease of some sort. Usually these anatomic variations are nothing more than the result of normal developmental changes, daily activity patterns, or some genetic-based multivariate characteristic. Examples of each of these will now be discussed.
One common phenomenon is the presence of "extra" muscles or absence of common muscles. Perhaps the most well-known variation in this category is the palmaris longus in the anterior compartment of the forearm. This muscle lies superficial to the flexor digitorum superficialis and is roughly in the exact center of the forearm. Its tendon is long and thin and travels over the carpal tunnel to blend with the palmar fascia. You can easily determine if you have one or both muscles by flexing your wrist against resistance and looking for the prominent centrally located tendon.
This muscle is absent about 11 % of the time either bilaterally or unilaterally. More interesting for our purposes here is the fact that the position of the muscle belly can vary quite dramatically. The small muscle belly is usually situated proximally along with the muscle mass of the common forearm and digital flexors. Occasionally, however, the muscle belly of palmaris longus is situated distally just proximal to the flexor retinaculum. In this position it can be confusing. No muscle should be there -- what is wrong with this person? Nothing's wrong; these are just thoughts that might run through the mind of the naïve examiner.
Another similar example is the peroneus (fibularis) tertius. This muscle is commonly present but very variable in its presentation. Arising from the extensor digitorum longus, it runs to the lateral aspect of the ankle and foot and attaches at any number of places, usually including the fifth metatarsal. Palpation in this region of the foot will lead one to discover nothing; a rather dramatic muscle mass; and everything else in between. Unless these sorts of variations are accompanied by other clinically significant findings, they are usually just the result of normal anatomic variation.
Recently in a palpatory anatomy course I teach, one of the students became concerned because of a large and significant bulge right in the middle of her fellow student's popliteal fossa. She called me over, and I examined the young healthy male. My first impulse was correct. After asking a number of questions related to his medical history, I learned that he had a third head of gastrocnemius. This is often referred to as the gastrocnemius tertius. It was particularly prominent in this muscular young male.
Just yesterday in my medical gross anatomy course, I had two students approach me somewhat concerned about unusual muscle masses in the anterior forearms. In the first case, a young healthy male showed me large bilateral masses just medial to the tendon of the palmaris longus and three fingerbreadths proximal to the flexor retinaculum. They certainly did not look pathological at all. I simply asked what he did that might lead to this dramatic hypertrophy of the medial sides of both the flexor digitorum superficialis and the flexor digitorum profundus. At first, he couldn't think of anything; then suddenly he said, "Lacrosse -- I played lacrosse." The repetition of "twirling" the stick led to the muscular hypertrophy we were observing.
Not 10 minutes later, a young woman approached me and showed me the exact same situation, only just on her right forearm. I jokingly asked if she played lacrosse with one arm. She looked puzzled. Then I explained the situation with her classmate; she then saw the humor. I asked her if there was anything unusual about daily activities. She immediately put together her "anomaly" with the fact that her job as a pharmacy technician required her to inject solutions out of a 30ml syringe into a container. This was done hours on end and resulted in the observed muscular hypertrophy. I have found that many if not most of the muscle "abnormalities" I have witnessed over the last 25 years are in this category.
Of course, not all palpatory findings that might concern someone involve muscles. Another very common tissue that can fool us is lymph nodes. Normally lymph nodes are very difficult to palpate unless they are inflamed or the site of neoplastic disease. There are some exceptions to this, however. The superficial inguinal nodes lying along the inguinal ligament and surrounding the cribrifrom fascia in the femoral triangle can sometimes be very large. I distinctly remember a young female student in palpatory anatomy who had herself and her fellow students in a fit over her lymph nodes. Not only were they readily palpable, they were visible! After calming the group down, I discussed variability of lymph nodes with them. It turns out that her inguinal nodes were the largest I had ever seen in a healthy individual, but as the conversation continued, the "patient" described an interesting situation. She had been concerned about these "bumps" for several years, but was too scared to have a physician check them out. Had they been pathological, this could have been a terrible mistake. Palpable nodes can sometimes be felt in the axilla. A physician should always check these in female patients, but by far the most common nodes I have palpated are associated with nearby insect bites (we do have plenty of mosquitoes in Maine) or a recent cold.
As an example of a dramatic developmental change that can lead to unnecessary concern, let us consider the xiphoid process. This structure is found at the inferior end of the sternum. It begins as a pliable cartilage structure with an osseous core. Upon palpation in the epigastric fossa, one can feel the xiphoid give against digital pressure. However, as we age, the bony core enlarges at the expense of the cartilage. This can lead to a structure that feels like a bone growth -- which it is -- but it is part of the normal aging process. I always try to remember to tell the students in my palpatory anatomy class to be wary of such changes in their more mature classmates.
I never tire of the rich variation I experience in the cadaver lab or in the palpation lab. I always try to pass on this experience to my students and clients.