SPINAL OPIOIDS

Spinal is a collective term meaning epidural or intrathecal. Opioid receptors in the brain and spinal cord were identified in 1973. Wang reported the use of spinal morphine for analgesia in humans in 1979. By delivering morphine close to the receptors much lower doses are required.

The main indication is severe pain (visceral, soft tissue and bone pains respond best) uncontrolled by oral or systemic opioids. The spinal route, which is rarely needed, has the single advantage over recommended systemic routes (oral, subcutaneous, rectal, IM) of fewer side-effects such as constipation and drowsiness.

Spinal opioids occasionally cause dose-dependent nausea, itching and urinary retention, but the incidence of respiratory depression is very low, and tends to occur only in opioid-naive patients. Tolerance can develop if high doses are used in excess of the required analgesic dose.

The usual starting dose of epidural morphine is 50% of the systemic dose. For intrathecal morphine the starting dose is 5% to 10% of the systemic dose. For example, if the patient requires 10mg morphine by IM injection, an equi-analgesic dose by epidural injection would be 5mg, and an equi-analgesic dose of morphine intrathecally would be 0.5mg to 1 mg. The effect lasts 8 to 12 hours.

If the patient is morphine-naive (i.e., has not been on regular doses of morphine) the recommended starting dose for epidural morphine is 2mg to 3mg every 12 hours. This would be an unusual situation because most patients will already have been on regular oral morphine well prior to considering the epidural route.

The epidural route is preferable to the intrathecal (no headaches, less risk of infection or neurological damage, safer if an inadvertent overdose is given), but pain can occasionally occur on injection, probably because the catheter tip is near a nerve root. This is sometimes overcome by slightly withdrawing the catheter. The position of the catheter tip is not critical because the analgesic effect is obtained by morphine suffusing into the CSF and is non-segmental.

The catheter is usually inserted at L1, and the free end tunneled under the skin. (A long IV catheter can be used to form the tunnel for the epidural catheter.) This ensures that any skin infection is trivial, and does not track down into the epidural space. The catheter entry point can be covered with a small piece of transparent membrane (Opsite). Vigilant nursing care is needed to observe for signs of infection.

Morphine for spinal injections (epidural or intrathecal) should be preservative-free and given through a filter attached to the free end of the catheter.

Various advanced opioid delivery systems are available. An indwelling subcutaneous access portal can be implanted. This allows injections through the skin into the portal and reduces the risk of infection. Implantable reservoirs have also been developed to give a continuous infusion of morphine. These systems are expensive. The reservoirs are large and although they can function for months, they tend to block or become infected and need replacing.

In summary, the use of spinal opioids is rarely indicated. The technique has few advantages over oral medication that is carefully titrated. It has the major disadvantage of requiring the patient to have a permanent in-dwelling catheter in his back.

Note – Intraventricular opioids have been used to control intractable head and neck pain. Under local anesthesia an intracranial reservoir can be stereotactically placed with the catheter top in the frontal horn of one lateral ventricle. A usual dose range would be 0.25mg to 1 mg of morphine by injection into the reservoir every 24 hours. The theory is to reach the cerebral opioid receptors with morphine.

However, since the CSF has a continuous circulation, drug flow would normally occur to the brain within 30 minutes of a lumbar intrathecal injection. The indications for intraventricular opioids are therefore very, very few indeed.

The author and publisher have taken precautions to ensure that the information in this book is error-free. However, readers must be guided by their own personal and professional standards of good practice in evaluating and applying recommendations made herein. The contents of this book represent the views and experience of the author, and not necessarily those of the publisher.