From by qthomasbower
Radiologists depend on two things to detect abnormalities that otherwise blend into the adjacent structures of the body: 1) abnormal contrast and 2) abnormal morphology. Not surprisingly, we are much better at detecting abnormalities when the pathology has differential contrast. When the abnormality is simply a variation in morphology, the detection rate falls.
In this case, a 17-year-old girl was sent to a hand surgeon because of a painful, bluish mass in the palm. She came to the appointment with an MRI scan, which was interpreted as normal. The clinical examination clearly revealed a palpable mass in the palm. Given the discrepancy between the MRI and clinical findings, the MRI scan was reviewed again.
In the area of clinical question, an axial T1-weighted image reveals a morphologic abnormality that is the same signal as muscle:
The mass is isointense to muscle on T2-weighted images:
Here is a comparison axial image from a normal patient. The normal flexor digitorum profundus (yellow arrow), flexor digitorum superficialis (green arrow) and lumbrical muscles (blue arrow) are identified:
The shape of the lesion in our patient is best seen on a coronal T1-weighted image, where a fusiform structure that is isointense to muscle is well-seen:
The MRI findings are diagnostic of an anomalous flexor digitorum superficialis (FDS) muscle. This is an uncommon accessory muscle that can present as a painful mass or pseudotumour within the palm. Stephens et al. (Can J Plast Surg, 15:44-46, 2007) described a similar case, and noted that there are three anomalies of the FDS that supplies the index finger:
1) Muscle belly originates in the forearm and extends distally into the carpal tunnel but not further into the palm.
2) Digastric form exhibiting continuity by way of a tendon between the palmar and forearm bellies. Patients with this form may present with a palmar mass or carpal tunnel syndrome.
3) Proximal tendon of the muscle attaches either to the base of the thenar eminence or the transverse carpal ligament.
If the anomalous muscle is causing symptoms (typically carpal tunnel syndrome or pain due to local pressure effects), then a total or partial resection can be performed.
Take away points from this case:
--> If a patient presents with a palpable mass, and you do not see anything on initial review of an MRI examination, consider the possibility of an anomalous muscle, especially in the hand and wrist, where anomalous muscles are relatively common. In other areas of the body, consider the possibility of a nonencapsulated lipoma or a fascial herniation, two other entities that are detected by morphologic changes, rather than by differential contrast
--> One of the strengths of MRI is the superb tissue contrast it offers. The vast majority of pathologic conditions will exhibit differential contrast, but strict reliance on tissue contrast to detect pathology will inevitably lead to a missed diagnosis. It is also important to consider morphologic disturbances, which can be much more subtle.