The accessory soleus muscle is rare anatomic variation (in up to 6-10% of the population) that is usually asymptomatic. Pain from the accessory soleus muscle is a less common cause of leg pain in athletes in the anteromedial region of the achilles tendon and was first described as a cause of pain by Dunn in 1965. Accessory soleus pain is more commonly unilateral than bilateral; more common in males than females. It may also occur as a 'low' soleus rather than an accessory soleus.
Clinically: pain around the medial posterior aspect of the ankle, anterior to the achilles tendon; pain with exercise; often accessory muscle is a palpable mass in the medial posterior malleolar region that enlarges with exercise; the mass is contractile.
May present with symptoms of tarsal tunnel syndrome (pain associated with paraesthesia that radiate)
Source of pain is unclear, but proposed mechanisms include:
- exercise induced intermittent claudication
- tibial nerve compression (tarsal tunnel syndrome)
- local functional compression syndrome (as pain only tends to be present during exercise); though intramuscular pressure measurement do not support this (Kouvalchouk et al; 2005)
- partial tear or strain the the accessory soleus muscle
X-ray: rule out bony pathology; may see increased opacity in Kager's triangle and a soft tissue shadow in region.
MRI: to confirm muscle nature of mass.
Rest and/or activity modification; NSAID's; stretching, deep tissue massage; eccentric loading.
Botulinum toxin has been shown to reduce muscle mass and symptoms (in a case series of 5 by Isner-Horobeti et al, 2015)
In worse case: fasciotomy or surgical removal of accessory soleus (Kouvalchouk et al; 2005)
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