British Journal of Radiology (2006) 79, e129-e132
© 2006 British Institute of Radiology
doi: 10.1259/bjr/83389292
Symptomatic accessory soleus muscle: diagnosis and follow-up on magnetic resonance imaging
N Doda, MD
W C G Peh, FRCPG, FRCPE, FRCR
and
A Chawla, MD
Department of Diagnostic Radiology, Changi General Hospital, Singapore
Correspondence: Prof. Wilfred C G Peh, Programme Office, Singapore Health Services, 7 Hospital Drive, #02-09, Singapore 169611. E-mail: wilfred{at}pehfamily.per.sg.
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Abstract
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The accessory soleus muscle is a rare anatomical variant which, although congenital in origin, may manifest in the second and third decades of life as a painful ankle mass or an asymptomatic ankle soft tissue swelling. We report a symptomatic accessory soleus muscle in a 21-year-old male soldier that was diagnosed and followed-up on MRI. Initial MRI showed a mass with signal characteristics of normal muscle, but in an abnormal location. There was increased intrafascial fluid and perimuscular oedema around the accessory soleus muscle. Following conservative treatment, repeat MRI showed resolution of this intrafascial fluid collection and perimuscular oedema, concurrent with relief of the patient's painful symptoms.
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Introduction
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The accessory soleus muscle is a congenital anatomical variant that was first described by Cruvelhier in 1843 [1, 2]. This anomaly, although more commonly presenting as an asymptomatic mass, may give rise to symptoms such as pain with exertion, during running and jumping [2, 3]. We report the MRI features of a symptomatic accessory soleus muscle.
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Case report
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A 21-year-old male soldier presented with the complaint of a sprained right ankle of 2 months duration. He also complained of intermittent pain around the same ankle for 1 year that was exacerbated during running and jumping. On examination, there was tenderness over the tibialis posterior tendon and the anterior talo-fibular ligament. No obvious swelling was seen. There was full dorsi flexion and plantar flexion of both ankles.
A lateral radiograph of the ankle showed normal bones with no evidence of fracture or bony lesion (Figure 1
). There was an area of soft tissue density in the pre-Achilles fat pad (Kager's triangle).
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Figure 1. Lateral radiograph of the ankle shows a soft tissue mass(arrow) in the region of the Kager's triangle, anterior to the Achilles tendon.
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MRI was performed to investigate the cause of the patient's ankle pain. T1 weighted axial sequences show a homogeneous soft tissue mass located posteromedial to the flexor hallucis longus tendon and anteromedial to the Achilles tendon. Signal intensity of the mass was identical to that of normal muscle on all sequences (Figure 2a). T2 weighted axial fat-suppressed sequences show a thin rim of fluid within the fascial plane around the mass. There was increased signal intensity adjacent to the mass, consistent with oedema (Figure 2b). T1 weighted sagittal sequence shows the mass to extend anterior to the Achilles tendon along the medial surface of the calcaneum (Figure 2c). Based on these findings, the diagnosis of an accessory soleus muscle with perimuscular oedema and intrafascial fluid was made.
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Figure 2. MR images of the ankle.(a) Spin echo T1 weighted axial image shows a soft tissue mass (arrow) of normal muscle signal intensity in an abnormal location, anterior to the Achilles tendon. Surrounding intralipomatous septations are seen. (b) Fast spin echo T2 weighted fat-suppressed axial image shows areas of moderate high signal intensity around this mass, consistent with perimuscular oedema. There is a thin rim of very high signal intensity corresponding to the fascial sheath, representing an intrafascial fluid collection (arrow). (c) Spin echo T1 weighted sagittal image shows the accessory soleus muscle (arrow), corresponding to the soft tissue mass seen on the lateral radiograph.
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The patient was put in an ankle cast for 6 weeks. He was given a non-steroidal anti-inflammatory drug, diclofenac sodium 50 mg thrice a day for a week, and then to be taken as required. The patient was also excused from exercises such as jumping, running and route march, and underwent physiotherapy sessions. Repeat MRI performed after 4 months showed resolution of the intrafascial fluid collection and oedema surrounding the accessory soleus muscle (Figure 3a,b). The patient's painful symptoms had also subsided and he was able to perform his routine duties, although he was still excused from heavier forms of exercise.
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Figure 3. Follow-up post-treatment MR images obtained 4 months later. (a) Spin echo T1 weighted and (b) fast spin echo T2 weighted fat-suppressed axial images sequences show resolution of the abnormal intrafascial fluid and oedema around the accessory soleus muscle (arrow).
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Discussion
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The incidence of accessory soleus muscle, found on cadaveric studies, is about 0.7–5.5% [1, 2, 4]. The soleus muscle originates from a single anlage embryologically, and the early splitting of this anlage during development is thought to be the most likely cause for the supernumerary musculature [1, 2, 3, 5]. The proximal origin is typically on the distal posterior aspect of the tibia, with other origins being deep fascia of the normal soleus or other flexor tendons [1, 2]. The distal insertion is typically via a separate tendon on the calcaneus, anteromedial to the Achilles tendon. Other insertions include the Achilles tendon and superior calcaneus [1, 2, 5, 6]. The accessory soleus is generally enveloped within its own fascia and derives its blood supply from the posterior tibial artery [1, 2]. The posterior tibial nerve supplies both the soleus proper and the accessory soleus muscle [5].
In a review of the literature, Brodie et al found that patients with accessory soleus muscle typically present at an average age of 20 years and it is more common in males than females (2:1) [4]. The cause for the presentation seen in young patients is likely a reflection of response to an increase in muscle mass during adolescence as well as increase in physical activity [1, 2, 3, 5].
With the increasing application of MRI for investigation of ankle disorders, the incidental discovery of accessory soleus muscle in asymptomatic patients is well recognized [5, 7, 8]. However, symptomatic accessory soleus muscle is a rare entity. These symptoms typically include a painful swelling (most common), painless swelling, or association with clubfoot or equinus deformity (rare) [1]. Painful swelling is thought to be due to either muscle ischaemia or a compressive neuropathy involving the posterior tibial nerve [1, 2, 5, 9]. To our knowledge, the MRI finding of intrafascial fluid surrounding an accessory soleus muscle has not been previously described. This can be considered similar to the intratendon sheath fluid collection typically seen in patients with tenosynovitis. Perifascial fluid has also been described in athletes with acute hamstring muscle injuries, although in this series there were also intramuscular signal changes [10]. In our patient, we postulate that the intrafascial perimuscular signal changes without intramuscular abnormality may be due to an impingement type effect or minor injury.
Imaging features are diagnostic, but the lack of familiarity with this variant may lead to a misdiagnosis. Lateral radiograph of the ankle may show obliteration of the pre-Achilles fat pad (Kager's triangle) by a soft tissue mass [1, 2, 5]. Ultrasound shows a mass with the same echogenicity as the normal muscle [1, 5, 7]. CT shows a soft tissue mass with characteristics of normal muscle [1, 2, 5]. MRI is the diagnostic modality of choice, as normal muscle has a distinctly different signal intensity from abnormal muscle and soft tissue tumours [1, 2, 5, 7]. It also demonstrates the muscle in all planes, its relationship to adjacent structures, and its origin and insertion [2, 5]. Prior to MRI, accessory soleus muscle was often diagnosed only during surgical resection for a suspected soft tissue tumour [3, 4].
The differential diagnosis of a soft tissue mass in the posteromedial aspect of the ankle includes ganglion, lipoma, haemangioma, encapsulated haemangioma, synovioma, sarcoma and haematoma [1, 4]. The key to the differentiation from other lesions is the finding of MRI signal characteristics that are identical to those of muscle, the well-encapsulated nature of the mass and the typical anatomic location [1, 5]. In our patient, the presence of intrafascial fluid and perimuscular oedema around the accessory soleus muscle, with lack of abnormal signal within the muscle itself, is consistent with some form of impingement effect or minor injury. This subsequently subsided on the follow-up MRI after conservative treatment. The diagnosis can be made fairly confidently based on these findings and should obviate the need for any invasive procedure for diagnostic purposes.
Treatment usually depends on the presence or severity of the symptoms. Asymptomatic patients can be reassured with no active treatment being required. For symptomatic patients, conservative treatment such as orthoses, physical therapy and activity modification may be tried [2–4]. Surgical approaches include fasciotomy or excision of the muscle. Both have been found to be equally effective [1, 4, 11]. Patients generally do well after surgery and generally become asymptomatic. Our patient showed improvement after suspension of physical exercise, non-steroidal anti-inflammatory drugs, rest and physiotherapy.
In summary, MRI was useful in the diagnosis and follow-up of a symptomatic accessory soleus muscle in our patient, with the resolution of intrafascial fluid and perimuscular oedema corresponding to patient's symptomatic improvement.
Received for publication July 27, 2005.
Revision received November 28, 2005.
Accepted for publication November 29, 2005.
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References
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Abstract
Introduction
