Assessment of Intrathecal Ziconotide Antalgic Efficacy for Severe Refractory Neuropathic Pain Due to Spinal Cord Lesions.

Spinal cord injury (SCI) has an average prevalence of 50 per 100.000 in general population (30.000 patients with SCI in France) with estimates of the overall prevalence for severe neuropathic pain ranges from 30 to 51% (up to 10.000 patients in France).

Patients with such spinal lesions may develop neuropathic pain called sublesional pain as perceived in an area below the level of injury. A second type of pain is at level of injury, i.e. perceived in a segmental pattern within the dermatome corresponding the spinal cord and nerve roots. These two types of pain are very harmful and are notoriously difficult to treat probably because of complex pathogenic mechanisms due to abnormal functioning of deafferented spinal and supraspinal nociceptive neurons.

Opioids, whatever be the route of administration, had demonstrated their inefficacy for these patients as well as several surgical techniques. So, chronic pain in relation with spinal lesion can be defined as real refractory pain.

Synaptic release of neurotransmitters is dependent on calcium intake trough voltage dependent channels. Type 2.1 or N-Type channels are specific for nociceptive system and can be blocked by a peptic neurotoxin: Ziconotide. Blocking these specific calcium channels neuromodulates nociception. Intrathecal use of Ziconotide, bringing the active molecule close to its receptors, has a proven clinical impact for a wide variety of pain (4). The intrathecal Ziconotide (ITZ) infusion using an implanted pump is validated for treatment of pain refractory to systemic analgesics (HAS, avis du 14-27 mai 2008). Meanwhile, no data are available in literature on positive effects of ITZ on specific spinal neuropathic pain.

A pilot study was performed by the coordinator team using ITZ on 12 patients with spinal pain: 8 patients had > 40% decrease of pain on numeric scale, 6 patients beneficiated from implanted pump allowing chronic ITZ treatment inducing 60% numeric scale decrease in average with 1 year follow-up.

Therefore intrathecal Ziconotide could be an excellent candidate for the treatment of spinal pain where the pain generators may be difficult to target by other available treatments.

This study is the first to assess ITZ (as IT antalgic monotherapy) versus placebo with a randomized controlled study with long follow-up. Trials have already been performed but not specially targeted spinal pain, and did not exceed three weeks follow-up.

Long term effects of Ziconotide on memory, cognition and mood have not been evaluated. In fact even though short term adverse effects on higher level functions have been described they have not been assessed in a placebo controlled situation.

Moreover, treating (successfully or not) patients with spinal pain could bring valuable insights both into the mechanisms of pain production in SCI patients and in the mechanisms of Ziconotide action: a positive result on pain below the injury level would imply action on the second or third order synapses of the nociceptive pathways. Similarly an effect at the level of pain, in absence of an effect below the level pain would argue discussion against such action. The impact of ITZ on the different clinical components of pain experienced by the patients, could also give some data on neuromodulation mechanism induced by the therapy.