Imaging of accessory soleus muscle: A case report with review of the literature

The accessory soleus muscle is an uncommon anatomical variant. It usually manifests as ankle pain and swelling or as an asymptomatic soft-tissue mass along the posterior and medial aspect of the ankle. In a few cases, it is sometimes misdiagnosed as a mesenchymal lesion. The idea of this case report was to highlight the existence of accessory soleus muscle mistaken as a mesenchymal tumor on fine needle aspiration cytology (FNAC). A 32-year-old male patient presented to the orthopedics department with a soft swelling in the distal one-third of the left leg posterior to the medial malleolus. The patient noticed this swelling about 5 years back. The swelling was non-tender and non-pulsatile. Before the imaging, FNAC from the swelling was done which raised suspicion of a mesenchymal lesion. He was referred to the radiodiagnosis department for imaging evaluation of the swelling. A radiological diagnosis of accessory soleus muscle was made; ruling out the possibility of a mesenchymal tumor. This accessory muscle is a less common congenital anatomic variant rather than a soft-tissue mass lesion. The characteristic imaging appearance and location of this muscle give a definite diagnosis. Further, it highlights this rare variation pertaining to its calcaneal insertion and relation with the flexor retinaculum and tarsal tunnel, which should be kept in mind by radiologists.