Based on clinical and imaging findings, the diagnosis of an accessory distal muscle belly of the palmaris longus muscle was established.
The patient, manual laborer, presented with a painful swelling at the volar aspect of the right wrist. The pain sensation was intermittent and elicited by intense use of his forearm muscle. There was a normal appearance of the overlying skin.
Sagittal and axial ultrasound of the volar aspect of the wrist-forearm shows a welldelineated mass medial to the right flexor carpi radialis tendon. This soft tissue mass is homogeneous and isoreflective with adjacent muscles.
Axial SE T1-weighted MR images of the right forearm demonstrates a fusiform mass medial to the flexor carpi radialis and superficial to the flexor retinaculum (arrows). The mass is homogeneous and isointense with adjacent muscles on T1-and T2-weighted (not shown) images.
Based on clinical and imaging findings, the diagnosis of an accessory distal muscle belly of the palmaris longus muscle was established. No surgery or biopsy was performed.
The palmaris longus muscle is a small, fusiform muscle originating from the medial condyle of the humerus. Its distal part is a single tendon inserting at the palmar aponeurosis superficial to the flexor retinaculum. Phylogenetically it is classified among the retrogressive muscles.
The palmaris longus is the most variable muscle of the hand. Agenesis is the most common variation and occurs in 12.8%. The muscle belly may be centrally placed, digastric, bifid, or extend over the entire course of the palmaris longus. Variations in origin and insertion, or duplication are quite rare. An accessory muscle slip is a frequent variation and can cause ulnar nerve compression. Carpal tunnel syndrome with compression of the median nerve may be provoked by the muscle inserting deep to the flexor retinaculum or by an additional distal muscle belly. A hypertrophied distal muscle belly of the palmaris longus has only been described three times in the last 50 years in the radiological literature. The presence of a thin rim of muscle tissue around the palmaris longus tendon recently has been described in three (7%) out of 42 normal wrists. However, we believe that these three cases can be considered as a digastric palmaris longus, which is less uncommon.
The diagnosis of palmaris longus variant can be made by ultrasonography or MRI based on location and signal characteristics. Clinical complaints and associated nerve compression determine the need for surgery.
Palmaris longus
In the ultrasound and MRI images, abnormal muscle tissue is observed in the distal forearm region near the flexor carpi radialis and flexor groups. The muscle belly is closely associated with the conventional palmaris longus tendon and exhibits a similar signal intensity to the surrounding muscle group. On the MRI cross-sectional view, a thickened muscle band (indicated by the arrow) can be seen near the midline of the forearm, which differs from the usual purely tendinous form of the palmaris longus tendon, suggesting the presence of an accessory or variant muscle belly. Across different imaging sequences and planes, no obvious bony destruction or ligamentous abnormalities are noted.
Based on the imaging features and the patient’s overall status, consider the following diagnoses or differential diagnoses:
Combining the patient’s background as a young male, the imaging findings indicating muscle-like signal and location consistent with an anatomical variant of the palmaris longus, the most likely diagnosis is:
“Accessory distal muscle belly of the palmaris longus.”
If the patient’s symptoms correlate with this variant, further intervention may be considered. If there are no significant clinical symptoms, periodic follow-up is sufficient.
Management of the accessory distal muscle belly of the palmaris longus depends on symptom severity and any nerve compression:
Below is a concise rehabilitation and exercise prescription example (modifiable according to patient tolerance and recovery progress):
Note: Throughout rehabilitation, if recurrent pain, numbness in the carpal tunnel area, or worsening discomfort occurs, promptly consult the healthcare team and adjust the training program accordingly.
This report is based on the patient’s history and imaging data provided. It is for reference only and does not replace an in-person clinical diagnosis or treatment. For a definitive treatment plan, please consult an orthopedic surgeon, hand surgeon, or rehabilitation specialist.
Palmaris longus
