Entrapment of middle cluneal nerves as an unknown cause of low back pain



In April 2013, a 48-year-old woman presented complaining of LBP and buttock pain radiating to both legs that had gradually developed over 10 years. L4-5 discectomy performed at another hospital two years before resulted in no improvement. The pain was continuous and severe even with long-term daily use of tramadol 225 mg/acetaminophen 1950 mg, pregabalin 50 mg and loxoprofen sodium 180 mg. The visual analog scale (VAS) score was 67 mm and the Roland-Morris Disability Questionnaire (RDQ) score was 18. A neurologic examination revealed no sensory or motor disturbance in her legs. Lumbar motion was greatly limited in all directions because of pain. The finger floor distance in flexion was 50 cm. Palpation of the superior SCN tender point, located 7 cm laterally to the midline on the bilateral iliac crest, replicated the postero-lateral aspect of calf pain. She also had significant tender points approximately 1.5 cm caudal to the palpable margin of the bilateral posterior superior iliac spine, by the lateral sides of the long posterior sacroiliac ligament (LPSL). These loci were along the running course of the MCN as described in an anatomical report by Tubbs et al. Palpation of MCN tender points provoked mid-posterior thigh pain. Repetitive infiltration of a local anesthetic, Lidocaine, into each tender point consistently resulted in clear improvement of symptoms for three hours.

The patient was informed that release was previously performed exclusively for SCN entrapment and had never been applied for MCN entrapment. She gave theirinformed consent to undergo surgical decompression. In May 2013, microscopic SCN and MCN releases were attempted. Surgeries were approved by the Institutional Ethics Committee of our institution. Surgery was performed bilaterally under general anesthesia with the patient in the prone position. An oblique 10 cm skin incision was made over the iliac crests. Being careful not to injure nerve branches passing through subcutaneous tissues, the superficial layer of the thoracolumbar fascia was opened. Two branches of the SCN were identified within 5 cm above the iliac crest and were seen to emerge from the lateral margin of the deep layer of the thoracolumbar fascia. These SCN branches were traced in a caudal direction until they passed over the iliac crest. In agreement with a recent anatomical study, the two medial branches of the SCN where they pierce the thoracolumbar fascia over the iliac crest were found to be entrapped in adhesions. A thin branch of the MCN perforating the thoracolumbar fascia was identified just medial to the posterior superior iliac spines. Although obvious entrapment was not observed, the perforating orifices were opened.

Within one week following surgery, the patient reported that her pain had completely disappeared around the upper iliac crests, but remained around the LPSL on both sides. Palpation on the LPSL consistently induced LBP and leg tingling radiating from the buttocks to the calves on both sides. Injections around the LPSL were repeated every month. Each time, the patient reported reappearance of leg tingling during the block procedure and, soon after, complete improvement in LBP and leg tingling that continued for three days. Consequently, blocks were repeated six times over six months without substantial permanent change in LBP. The VAS score was 50 mm at six months after surgery. Near-full range of flexion was obtained with no pain reappearance, but lumbar extension was still severely limited.

In an attempt to eliminate remaining pain, in December 2013, revision surgery was done. Previous operative incisions were reopened. After gluteal muscle splitting approach, the bilateral MCNs were explored where the nerves penetrate the LPSL. Proximally, the nerves possibly arose from the S2 foramen. The MCNs were decompressed by excising the LPSL where the nerve penetrates the ligament. After revision, pain dramatically improved, precluding need for any medication. The patient had no limitation in lumbar motion. The VAS score at eight months after revision was 0 mm and the RDQ score was 1.

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