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"Peroneal Tunnel" Neuropathy?
Abstract & Commentary
By Michael Rubin, MD, Professor of Clinical Neurology, NewYork-Presbyterian Hospital, Weill Cornell Medical Center. Dr. Rubin is on the speaker's bureau for Athena Diagnostics, and does research for Pfizer and Merck.
Synopsis: Idiopathic peroneal neuropathy may be associated with entrapment in a tunnel formed by the short head of the biceps femoris muscle.
Source: Vieira RLR, Rosenberg ZS, Kiprovski K. MRI of the distal biceps femoris muscle: normal anatomy, variants, and association with common peroneal entrapment neuropathy. AJR 2007;189: 549-555.
For many patients with common peroneal neuropathy, the most common entrapment neuropathy in the leg, no specific cause can be found. Anatomic variability, hitherto undescribed, of the distal biceps femoris muscle may be the underlying cause in some of these idiopathic cases. In this study, 100 consecutive 1.5-T magnetic resonance (MR) examinations of the knee were reviewed by 2 musculoskeletal radiologists, to determine the normal anatomy of the region, any anatomic variation of the biceps femoris muscle, and the relationship of the common peroneal nerve to the muscle. Patients were excluded if there was evidence for signal abnormality or deformity of the posterolateral knee, including the joint capsule and ligaments, or any history of peroneal neuropathy. MR slice thickness was 4 mm, and MR protocol incorporated axial T1 and T2 fat-saturated, coronal proton density and T2 fat-saturated, and sagittal proton density and proton density fat-saturated images. Parameters recorded included the presence of any denervation edema in muscle, any signal abnormality of the common peroneal nerve, and any variation of the distal biceps femoris muscle. Statistical analysis was performed using the Mann-Whitney test.
In 77 % (n = 77), abundant fat posterior to the short head of the biceps femoris and superficial to the lateral head of the gastrocnemius surrounded the common peroneal nerve. In the remaining 23% (n = 23), the lateral head of the gastrocnemius and the short head of the biceps femoris surrounded a narrow tunnel, averaging 2.4 cm in length (range 1.5 - 4.0 cm), which was relatively fat free. The presence of this tunnel was significantly predicted by a closer distance between the joint capsule and short head of the biceps femoris (mean 0.05 cm vs 0.6 cm, p < 0.001), and longer posterior extent of the short head of the biceps femoris muscle at the level of the femoral condyles (mean 1.50 cm vs 1.02 cm, p = 0.005). No patient had an accessory biceps femoris tendon. This tunnel is newly described and has not previously been referred to as a cause of peroneal neuropathy. Its relative lack of fat offers a possible relationship between the 2.
Among 67 consecutive patients with peroneal mononeuropathy enrolled in an Italian multi-center study from November 2002 to January 2004, 16% remained idiopathic despite intensive investigation (JPNS 2005;10:259-268). Overall, men outnumbered women by a 4:1 ratio, mean age was 47.9 years, 97% were unilateral, and the common peroneal nerve was involved in 89.9%, compared to 8.7% and 1.4% affecting the deep or superficial branch, respectively. Sensory deficits were found in 87.9%, accompanied by pain in 19.7%. Causes included prolonged compression due to awkward posture (23.1%), surgery, including hip replacement, tibial osteotomy, thoracic, abdominal, thyroid, and prostate surgery (20.3%), weight loss (14.5%), trauma (11.6%), prolonged bed confinement (7.3%), compression from casting (5.8%), and arthrogenic fibular cyst (1.4%). Perioperative and idiopathic peroneal neuropathy were equally likely to be either axonal or demyelinating in nature, whereas trauma caused exclusively axonal injury in 60%. Older patients (> 60 years) experienced greater disability and pain, and longer duration correlated with greater disability. Those with idiopathic peroneal mononeuropathy tended to have a better quality of life.