Chronic Unexplained Thigh Pain from Saphenous Nerve Entrapment due to a Leiomyoma

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Dear Editor, Lower extremity entrapment syndromes are seen less commonly than those pertaining to the upper limbs and, despite the use of various diagnostic tools, their diagnoses can really be challenging [1] . Likewise, entrapment of the saphenous nerve is quite rare [2] and has been reported to ensue mainly due to local trauma (e.g., surgery and fracture) or soft tissue (i.e., mass, scar) compression [2–4] . In this report describing a patient with unexplained chronic severe thigh pain, we would like to underscore the complementary roles of ultrasonography and electrodiagnostic evaluations for the prompt diagnosis of a rare scenario of saphenous nerve entrapment. A 40-year-old man was seen for left medial thigh pain for the last 5–6 years (worse in the last 4 months). His pain was stabbing and very severe (9/10) while walking. He described that the pain was worse while walking and during sleep as well. The medical history was otherwise unremarkable. Although he had previously visited several medical centers with the aforementioned complaint, hip and knee radiographs, lower limb Doppler ultrasonography and electromyography, and left knee magnetic resonance imaging were all noncontributory. As such, he still remained undiagnosed, and various analgesic treatments had failed to improve his pain. In physical examination, spinal and lower limb motions were normal. Palpation over the subsartorial canal 10 cm proximal to the medial femoral condyle elicited his pain. Sensory deficit with pinprick and light touch was determined above the medial femoral condyle. There was no motor deficit or paresthesia. Laboratory tests were noncontributory. As ultrasonography with sonopalpation (“sono-tinel”) and electrodiagnostic evaluation were suspicious for saphenous nerve entrapment, targeted magnetic resonance imaging was performed. A hyperintense and enhancing lobulated mass compressing the adjacent femoral vasculature and saphenous nerve in the adductor hiatus was detected (Figure 0001 ). Finally, the patient underwent surgical removal of the mass and the pathological diagnosis was vascular leiomyoma. Postoperatively, the patient was pain-free, and he stayed painless for 8 months during follow-up. Coronal STIR (A) and coronal T1-weighted fat suppressed post-gadolinium (B) sequences demonstrate hyperintense and enhancing lobulated mass (arrows) compressing the adjacent femoral vasculature and saphenous nerve in the adductor hiatus. Saphenous nerve entrapment usually manifests with burning and aching pain around the knee (90%), thigh (7%), leg (3%), and ankle (2%) [3,4] . The clinical scenario might vary according to the level and etiology of compression. The most common location for entrapment is around the knee, and various causes including trauma, total knee replacement surgery, hydatic cyst, popliteal artery aneurysm, postmeniscectomy, and articular ganglion cysts have been reported in the pertinent literature [4,5] . By placing a patient with suspected saphenous nerve entrapment into a position of hip extension and abduction with full knee flexion, the clinician may be able to create further tension along the nerve in an attempt to recreate symptoms and assist clinicians with making a difficult clinical diagnosis. Once a clinical diagnosis of saphenous nerve entrapment has been made, further diagnostic testing such as nerve conduction studies, diagnostic injections, or advanced imaging may be warranted [3,4] . To our best notice, there is only one saphenous neuropathy due to an angioleiomyoma [6] ; thus, our patient exemplifies another very rare cause of entrapment in the adductor canal. Considering the fact that the former case actually mimicked a neurinoma of the infrapatellar branch of the saphenous nerve, it is noteworthy that correct diagnosis of these cases is quite challenging. As such, multidisciplinary approach with complementary use of electrodiagnostic and imaging methods plays a paramount role in the diagnostic algorithm [7] . Similarly, after prompt physical examination, convenient/sensitive use of sonopalpation, subsequent electromyography, and targeted magnetic resonance imaging, our patient (with chronic unexplained pain for 6 years) could have been appropriately diagnosed and treated. In this regard, we strongly believe that especially the complementary use of ultrasonography and electromyography for the management of peripheral nerve entrapment syndromes needs further attention in the daily practice of pain physicians [8] .