Efficacy of distal peripheral nerve blocks in lumbar radicular pain: A randomized placebo-controlled triple-blinded study.

Context: Inflamed nerve root leads to mechano-sensitization and upregulation of sodium channels in nerve endings far distal to the site of inflammation, leading to tenderness of these nerve endings. Due to pseudounipolar nature, the blockade of sodium channels at peripheral nerve endings leads to relief in radicular pain.

Aims: The aim of this study was to assess the efficacy of bupivacaine in improving pain and straight leg raising test (SLRT), 30 min after injection around tender nerves near the ankle, in patients with unilateral radicular pain.

Design: This was a randomized placebo-controlled triple-blinded study.

Methods: One hundred patients of single-level lumbar disc prolapse and unilateral radicular pain were randomized into two equal groups. Three nerves (sural nerve, lateral branch of deep peroneal nerve and posterior tibial nerve) were assessed for tenderness around the ankle. Bupivacaine (0.5%) was injected around each tender nerve in the case group whereas equal volume of normal saline was injected in the control group around each tender nerve. Pain Numerical Rating Scale (NRS) and SLRT were checked before and 30 min postinjection. A proportion of patients achieving minimum clinically important difference (MCID) of NRS ≤4 and MCID of SLRT ≥30° were compared.

Results: A mean decrease in NRS among cases was from 8.06 ± 1.236 to 3.90 ± 2.013 (P < 0.001) with 78% achieving MCID. A mean decrease in NRS in the control group was from 7.88 ± 1.023 to 7.66 ± 1.171 (P = 0.084), and only 2% of patients achieved MCID. There was a statistically significant improvement in SLR in the case group (40.90°-60.90°, P < 0.001), and 54% showed a clinically significant improvement by crossing MCID threshold. The control group showed nonsignificant improvement in SLRT (38.10°-39.10°, P = 0.351), and only 2% achieved MCID.

Conclusions: In single-level lumbar disc prolapse with unilateral radicular pain, injection of bupivacaine 0.5% (a sodium channel blocker) around tender nerves near the ankle provides clinically significant relief in pain and SLRT at 30 min when compared to placebo.