Compressive Neuropathy of the Posterior Tibial Nerve at the Lower Calf Caused by a Ruptured Intramuscular Baker Cyst
A 49-year-old man with left sole paresthesia and calf pain for 2 weeks was admitted to the department of rheumatology. This pain had a sudden-onset and caused the excruciating calf pain at nights. His medical history was significant for rheumatoid arthritis diagnosed 6 months prior to this and significant also for intermittent low back pain. The patient had bilateral knee joint arthroscopic synovectomy at a local orthopedic clinic 3 months before to manage synovial hyperplasia caused by rheumatoid arthritis. Magnetic resonance imaging (MRI) study of the lumbar spine, at the time of admission, revealed bilateral foraminal spinal stenosis at L3-4 and L4-5 levels. Thus, he was referred to the department of anesthesiology and received diagnostic epidural steroid nerve block at the corresponding levels. However, the symptoms did not improved after the block, and the patient was referred to our physical medicine and rehabilitation department for electrodiagnostic study and further evaluation.\nNeurological examination at the time of referral visit showed no significant muscle atrophy but manual muscle test revealed a grade-IV muscle strength in left ankle plantar flexors and a grade-I muscle strength in left great and little toe abductors. Paresthesia was noted at the left sole area. Nerve conduction study showed significantly reduced amplitudes of compound muscle action potential in the left tibial nerve. Mixed nerve studies of the medial and lateral plantar nerves showed no sensory nerve action potential (SNAP) responses. Sural and superficial peroneal nerves showed normal latencies and SNAP amplitudes (). Denervation potentials were observed at the left abductor hallucis and abductor digiti minimi muscles with discrete recruitment patterns, but no denervation potentials were observed in the gastrocnemius or tibialis posterior muscles (). On the basis of these results, a preliminary diagnosis of posterior tibial entrapment neuropathy around the tarsal tunnel was made.\nTo rule out any structural lesion or mass at the tarsal tunnel, a musculoskeletal ultrasonography (US) was performed. However, this failed to identify any discrete mass around the tarsal tunnel area or any echo intensity change of the posterior tibial nerve. Additional proximal ultrasonography scans along the left tibial nerve from the proximal knee did identify a cystic anechoic compressible mass around the left posterior tibial nerve at the mid one-third area of the lower leg (). The cross-sectional area of the left posterior tibial nerve at the location of maximal nerve swelling measured 0.21 cm2. The cross-sectional area of the contralateral posterior tibial nerve at the same location measured 0.12 cm2. Subsequently, a MRI study of left lower leg revealed a Baker cyst with intermuscular and intramuscular dissection. The cystic lesion extended from the popliteal fossa, between the two heads of the gastrocnemius muscle to the deep posterior compartment of the middle one-third of the lower leg. Its longitudinal length was about 18.7 cm, and there was no connection with the intra-articular space. Upon gadolinium-infusion, deep flexor muscles near the cyst and at the periphery of the cyst were enhanced indicating rupture of the Baker cyst. Therefore, we concluded that posterior tibial neuropathy was caused by a direct compression from the ruptured intramuscular Baker cyst ().\nCareful re-examination of the leg muscle function showed mild weakness of the left great toe and digit flexor muscles. Additional needle electromyography study was performed, which showed denervation potentials in the left flexor hallucis longus and flexor digitorum longus muscles; however, no denervation potentials were observed in the left tibialis posterior, gastrocnemius, or soleus muscles. He was ultimately diagnosed with a posterior tibial entrapment neuropathy between the tarsal tunnel and popliteal space, distal to the tibialis posterior muscle branching site, secondary to a ruptured Baker cyst.\nThe patient was treated with a regiment consisting of gabapentin Neurontin cap 1,200 mg (Pfizer Korea Ltd., Seoul, Korea), transcutaneous electrical nerve stimulation, electrical stimulation therapy, and physical therapy. Additionally, a referral was made to the department of orthopedic surgery for possible surgical intervention. However, the patient's symptoms improved substantially after non-invasive treatment. There being no evidence of infection or other complications, and because the small cyst was located between the deep flexor muscles and blood vessels, the risk of an aspiration was deemed too great. Therefore, the department of orthopedic surgery and the patient decided to continue the non-operative course of management, and after two months of medical and physical therapy, the patient expressed significant improvement of symptoms.