Predictive value of a diagnostic block in focal nerve injury with neuropathic pain when surgery is considered

Abstract

Object: In patients with focal nerve injury and neuropathic pain cutting the nerve to obtain permanent pain reduction can be considered. Surgery is indicated only if a diagnostic nerve block provides temporary pain relief. We evaluated the predictive value of a block on the outcome of surgery.

Methods: In total, three blocks were performed at two week intervals. Patients were blinded to injections containing lidocaine 1% and a placebo was included. Surgery was offered regardless of the effect of the blocks. Twenty-four patients received 72 blocks. Sixteen patients opted for surgery, 5 patients refrained from surgery, and in 3 the blocks provided permanent pain relief. The predictive ability of the block on the outcome of surgery was assessed by calculating the area under a Receiver Operating Characteristic curve (AUC).

Results: The AUC of the first lidocaine block was 0.35 with a 95% confidence interval from 0.077 to 0.62. At 95% confidence (two-sided), the AUC is less than 0.62, and hence the predictive ability of the block was poor. The outcome of the second lidocaine block and saline block did not change the conclusion of the first block.

Conclusions: We conclude that the use of blocks to select patients for surgery should be critically appraised.

Perspective: A pain relieving response to one open block is currently considered mandatory before patients with focal nerve injury and neuropathic pain are offered surgery. Blinded blocks including a placebo show that responses for selection should be carefully interpreted because they may not be as predictive as generally presumed.

Fig 1. Current selection algorithm for nerve surgical treatment of patients with neuropathic pain due to a focal trauma to a nerve.

The decision whether to operate or not is based on one “open” nerve block. Adequate temporary pain relief following the block is considered mandatory before surgery is undertaken. The nerve is then cut proximal to the painful area to obtain permanent pain reduction.

Fig 2. The LUMC protocol applied exclusively to patients with persistent postsurgical or nerve injury–induced probable neuropathic pain related to sensory nerves.

All patients had positive sensory signs confined to the innervation territory of the damaged nervous structure and had insufficient pain reduction or severe side effects from pharmacotherapy. The involved nerve is blocked three times in 4 weeks (with a 2-week interval between injections) and consists of two single-blind injections with lidocaine and one with placebo. All patients are given the opportunity of surgery irrespective of the outcome of the block injections.

Fig 3. The effect on pain of three blinded nerve blocks in 24 patients with persistent postsurgical or nerve injury–induced probable neuropathic pain related to sensory nerves and results of nerve surgery.

Two weeks after each injection the response to the injection was documented. Two weeks after the third block, the content of all three provided injections was revealed to the patient and the effect of the injections was subsequently discussed. Regardless of the outcome of the three blocks, all patients were offered nerve surgery. A permanent positive effect on the pain following surgery was seen in 7/16 (44%) of the patients. No beneficial effect or an increase was reported by 9/16 (56%) of the patients. Five of the 16 patients (31%) were (as good as) pain free, and 2 (12%) had partial pain reduction. In 6/16 (38%) there was no effect of the surgery, whereas a temporary pain reduction was seen in 2 (12%) of the patients and an increase in 1 (6%).

Fig 4. Case illustration of a 63 year patient (number 8, Table 1) in whom blocking a nerve had no pain relieving effect, but nerve surgery had.

Following the current selection algorithm, this patient would not have been operated. The patient had complaints befitting ankle arthrosis. An isolated subtalar arthrodesis was performed by placing compression screws. Immediate postoperative, the patient had severe neuropathic pain with allodynia in the sural nerve area limiting the walking distance to around 150 meters. Conservative treatment failed. A: At inspection 38 months after the onset of the pain, a scar of the screw placement was seen around 4 centimetres below the lateral malleolus. Percussion in the scarred damaged area provoked irradiating painful sensations in the sural nerve area. The sural nerve was blocked three times in 4 weeks (with a 2-week interval between injections) consisting of two single-blind injections with a volume of 4 cc. lidocaine 1% and one with placebo. There was no effect of the lidocaine injections and following saline injection the pain increased temporarily. B: At surgery, the sural nerve was identified in undamaged area (1^st incision) and dissected free subcutaneously. A second incision was made and the sural nerve was followed distally towards the scar. Subsequently, a third incision was made over the scar and a damaged sural nerve was identified. C: The damaged sural nerve was cut and the abnormal looking nerve tissue was resected. Pathological examination of the abnormal tissue showed traumatic neuroma. The resection plane of the proximal stump showed normal myelinated fibers and fascicles. D: The proximal stump of the sural nerve was loosely buried in fat proximal to the ankle joint. Postoperatively, his pain decreased significantly. The area with allodynia disappeared and became hypesthetic. The patient could walk again for at least one hour.

Fig 5. The area under the Receiver Operating Curve (AUC) of the first lidocaine block.

The AUC was 0.35 with a 95% confidence interval (CI) from 0.077 to 0.62. The AUC is less than 0.62 at 95% confidence (two-sided), and hence the predictive ability of the lidocaine nerve block is poor.

Fig 6. The AUC of the second lidocaine block was 0.63 (95% CI 0.36 to 0.9).

Fig 7. The AUC of the saline block was 0.53 (95% CI: 0.26–0.82).