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Algorithm for 3-Tesla MR Neurography-guided Nerve Blocks to distinguish Sitting Pain Origin from either Pudendal Nerve or Posterior Femoral Cutaneous Nerve
Lena Sonnow, M.D.1; A. Lee Dellon, M.D., Ph.D.2; Jan Fritz, M.D.1; (1)The Johns Hopkins University School of Medicine, Baltimore, MD, (2)Plastic Surgery, Johns Hopkins, Baltimore, MD

Introduction:

Inability to sit is a significant disability. A neural origin for this disability can be from either the pudendal nerve (PN) or the posterior femoral cutaneous nerve (PFCN). While surgical approaches to resect or perform neurolysis of these injured nerves have been described, the diagnostic approach to identify the source of the pain has not. It is the purpose of this report to describe the algorithm we have been using successfully for the past two years.

Materials and Methods:

A total of 20 patients who had pain with sitting and no history of injury to the coccyx were included. Each patient had undergone 3-Tesla MR neurography of the pelvis that was negative for nerve entrapment and neuroma formation. The differential diagnostic block of the PN and the PFCN were performed under 3-Tesla MR neurography guidance. For each block, 3 ml of 0.5% ropivacaine were used. PN blocks were performed in the Canal of Alcock. PFCN blocks were performed in the subgluteal space near the ischial tuberosity. After each block, the area of numbness and relief of pain with sitting were noted. Pain relief of 50% or more was considered a positive pain response. Based on the results of the nerve blocks, either the PN or PFCN or both were operated upon. For the PN, either the sacrotuberous ligament was divided if there were rectal symptoms, or the perineal branch of the pudendal nerve was resected if there were no rectal symptoms. The PFCN was resected and proximally implanted into the gluteus muscle. Successful surgery was defined as pain relief of at least 50% and ability to sit again.

Results:

Of the 20 patients, 18 had a positive response to either PN, PFCN, or both blocks, whereas 2/20 (10%) patients had a negative response to either block. The differential nerve block identified the PN as the source of pain in 8/20 (44%), PFCN in 8/20 (44%), and both in 2/20 (12%). Based upon the surgical outcome, the differential blocks had a sensitivity of 90% (95% confidence interval, 68-99%), positive predictive value of 90% (68-99%), and accuracy of 83% for diagnosing the pain generating nerve and predicting pain relief after surgery.

Conclusions:

An algorithm is presented for the diagnosis of sitting pain of neural origin that has a sensitivity and positive predictive value of 90% in managing this difficult clinical problem.


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