Intrathecal Medications

The nature of back pain and the conjunction of nociceptive and neuropathic symptoms frequently reduce therapeutic margin of single or even complex medication, therefore, many FBSS patients fail to respond to oral or transcutaneous drug administration.

Nerve blocks have also limited efficacy, for these precise diagnostic tools do not always have a corresponding treatment. More invasive therapies must be cautiously examined for, as previously discussed in this review, the failure rate increases with the number of spinal re-operations and unless a specific target has really been identified recurrent surgery is not an option.

Intrathecal drug infusion is now well accepted as a treatment option when all conservative and etiologic treatment failed. These therapies have failed either because pain relief is inadequate or due to intolerable side effects.

When it comes to neuromodulation therapies, the choice between intra-thecal medication and spinal cord stimulation is an important issue. SCS and Intrathecal drug infusion share common indications, but while SCS applies mainly to neuropathic symptoms, Intrathecal drug infusion also covers important nociceptives aspects of pain.

Once all other treatments have failed, a careful screening process of the candidates to an implantable therapy is needed. This screening can be divided in three steps.

The characteristics and localization of the pain must first be established. Low back versus leg pain and nociceptive versus neuropathic pain help in choosing the most appropriate approach between SCS and Intra-thecal drug infusion. Hassenbusch et al. in a retrospective study in 1995 estimated that intrathecal infusion may be best for bilateral leg and back pain as compared to spinal cord stimulation (Hassenbusch et al., 1995). No evidence has yet determined the adequacy of a particular treatment modality to select between spinal infusion and SCS, however, clinical practice is helpful in this regard. Although Intrathecal drug infusion may be efficient in a wide range of pain patterns and share common indications with SCS, the latter is easier for the patient and the physician. With SCS, no refills are needed, the patient may manage some stimulation parameters and there are no side-effects. Intrathecal drug delivery pumps need refilling and side-effects may be important. For these reasons, in common indications, it is only when SCS has failed that Intrathecal drug delivery should be used. For other indications like mixed pain patterns, Intrathecal drug infusion comes first.

Once the indication to Intrathecal drug delivery is determined, in a second step, patients must follow a medication trial and the most appropriate drugs must be tested.

The main principle is to first choose the most appropriate agent to the characteristics and localization of the pain. If not sufficient, it should be associated with a second medication. This second drug should be from another class of drugs. It should enhance the effect or complete the effect of the first drug by acting on other pain mechanisms like, for example, a local anesthetic if the first drug is an opioid.

Association of drugs may be required to achieve adequate analgesia but it will also complicate adaptations and changes of the medication as each drug concentration depends on the other. For example, to increase the delivery of one of three drugs mixed in the reservoir, the concentration of the others will need to be modified to keep their delivery flow constant. These sometimes complex therapies are needed and may be extremely efficient.

In most patients, morphine comes first. In a review of current practices Hassenbusch et al. determined that 98% of pain physicians who answered the questionnaire recalled using intrathecal morphine (Hassenbusch et al., 2000). The national outcomes registry for low back pain collected prospective data on 136 patients with chronic low back pain treated using intraspinal infusion via implanted devices, 81% of whom received morphine. Oswestry Low Back pain disability scale ratings after 12 months improved by 47% in patients with back pain and in 31% in patients with leg pain (Deer T et al., 2004).

In Intrathecal drug delivery, besides side-effects of the infused drugs one may face other associated complications. Recent studies have confirmed the clinical observation that intrathecal morphine infusion was responsible for catheter-tip inflammatory masses. Coffey has recommended positioning the catheter tip in the lumbar thecal sac to minimize opioid dosage and concentration to the extent possible. It was also proposed to provide an attentive follow-up of patients to encourage early diagnosis and to reduce the risk of neurological injury in these patients (Coffey R J et al., 2002).

Bupivacaine used mostly in association with opioids is a local anesthetic agent. Its use and safety in neuropathic pain syndromes has been widely recognized.

Up to maximum doses of 30-35 mg/day side effects are rare. Beyond 30 mg/day, and according to the place of the catheter tip, hypotension and motor weakness may be severe.

Less frequently used than morphine are mixtures: morphine+ bupivacaine (68% of pain physicians), hydromorphone (58% of pain physicians), morphine-clonidine, morphine-bupivacai'ne-clonidine. Fentanyl and sulfentanyl are also used alone or in mixed solutions. Combining drugs maximizes the effects and reduces the side-effects.

Although the above medications are used in a majority of patients new agents are in the pipeline and will soon be applied in clinical practice.

No definitive strategy has been established and the choice of the drug or the choice of the combination of drugs is specific to each and every patient. However, general principles are shared by pain specialists and guidelines have been proposed after reviewing current literature and practices by an expert panel in a polyanalgesic consensus conference in 2000, updated in 2003 (Bennett et al., 2000 a and Hassenbusch et al., 2004).

Although the acute cost of these implantable devices is high, the long term therapy is not more expensive than the conventional approaches (De Lissevoy et al., 1997).

New intra-thecal agents currently studied include midazolam, ketamine, neostigmine, gabapentine, ziconotide among others (Hassenbusch et al., 2004). These agents may be particularly helpful in the treatment of difficult neuropathic pain syndromes.

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