Tarsal tunnel syndrome due to aberrant muscle

Clinical History

A 31-year-old man experienced pain at the medial side of his left talus during plantar flexion of his left foot. Symptoms started three months before when he began playing basketball. His symptoms were attributed to tarsal tunnel syndrome (TTS) and he was referred for an MRI.

Imaging Findings

An MRI of his left ankle joint was performed on a 1.5 Tesla scanner. There was a soleus muscle with a lower than usual tendon insertion into the achilles tendon [Fig.1-3]. Besides that, a volume of tissue was disclosed encircling the belly of the flexor hallucis longus (FHL) muscle at the level of the ankle joint [Fig. 1b], where the belly of the FHL is expected to end. This tissue, distally to the ankle joint, seemed abutting the FHL tendon, and followed the tendon in front of the flexor retinaculum (FR) into the tarsal tunnel until the sustentaculum tali [Fig. 2, 3]. The nerves and vessels of the tarsal tunnel were displaced medially and backwards [Fig. 1b]. The signal intensity equalled that of muscle in all sequences [Fig. 2, 3]. There were no low signal intensity structures in this volume of tissue resembling tendons and/or vessels. The findings were interpreted as a low extending FHL muscle belly causing TTS.

Discussion

The tarsal tunnel is bounded ventrally by the FR and anteriorly by the distal medial cortex of the tibia, and the medial aspect of the talus and calcaneus. In the tunnel run tendons (FHL, flexor digitorum longus, tibialis posterior), vessels (posterior tibial artery and veins), and nerves (posterior tibial nerve and its terminal branches). The entrapment of these nerves in the tarsal tunnel causes tarsal tunnel syndrome (TTS) [1]. The TTS is idiopathic in about 50% of cases [1]. In the remaining cases various lesions have been described as causes. They include bone deformities after injury, tenosynovitis of the flexor tendons, tumours and tumour-like lesions, synovial hypertrophy and accessory or anomalous muscles [2].
Accessory muscles that may cause TTS are [3]: a) Flexor Digitorum Accessorius Longus (FDAL): FDAL has a prevalence of about 7%. The tendon descents in close relation to the posterior tibial nerve into the tarsal tunnel, beneath the FR, causing forward displacement of the nerve. In our case the neurovascular bundle was situated just beneath the FR excluding the presence of FDAL [Fig. 1]. b) Peroneocalcaneus Internus (PCI): PCI is uncommon. The tendon descends posteriorly and laterally to the FHL, displacing it medially, and adheres to the calcaneus. In our case there was no tendon attaching to the calcaneus [Fig. 1]. c) Accessory Soleus (AS): AS has a prevalence of 0.7-5.5%. It is located in front of the achilles tendon behind the FHL. Its insertion may be anywhere between the calcaneus and the gastrocnemius tendon [4]. It may compress the posterior tibial nerve, if hypertrophied. In our case there was a soleus muscle with a low-lying belly [Fig. 1-3], located away from the neurovascular bundle [Fig. 1, 2, 3]. d) Tibiocalcaneus Internus (TCI): TCI is rarely found. Its tendon descends deep to the FR and posterior to the neurovascular bundle, inserting onto the medial cortex of the calcaneus. In our case the neurovascular bundle was displaced medially and backwards, and there was no tendon insertion to the calcaneus [Fig. 1].
Anomalous muscles have been reported in the literature that may cause TTS [5, 6]. In these patients with TTS a hypertrophied belly of the FHL muscle was depicted extending into the tarsal tunnel. CT was used in the first case and MRI in the second. In our case MRI depicted accurately both the low belly of soleus muscle and the low extending belly of the FHL. The TTS was attributed to this latter finding.

Differential Diagnosis List

Final Diagnosis

TTS due to low extending FHL muscle belly.

URL:	https://www.eurorad.org/case/11566
DOI:	10.1594/EURORAD/CASE.11566
ISSN:	1563-4086