The main considerations for single-shot spinal analgesia are the risk of postdural puncture headache and the choice of solution, given the requirement for maximal analgesia and minimal motor block and other side effects (see Chapter 26, Combined spinal-epidural analgesia and anaesthesia, p. 63).
Modern intrathecal catheters are very fine (e.g. 28-32 G) and thus may be difficult to handle and insert. They are usually supplied in a kit with a spinal needle; originally these needles had cutting tips, but they are now available with pencil-point tips in an attempt to reduce the incidence of postdural puncture headache. However, even with fine catheters, 22-26 G spinal needles are required. Some catheters include a removable wire to make them stiffer for insertion. A catheter-over-needle kit also exists, in which a 27-29 G needle protrudes from the distal end of a 22-24 G catheter; the catheter is slid over the needle into the subarachnoid space whilst advancement of the needle is prevented by a wire attached to its end.
A continuous catheter technique may also be used with a standard epidural kit (e.g. 16G or 18G catheter), either because specialist kits are unavailable or when an accidental dural puncture has occurred during attempted epidural block. A reduced incidence of headache after placement of an intrathecal catheter has been claimed when this is done, possibly related to inflammation around the dural puncture site, which leads to faster healing; however, this is uncertain since evidence is mostly anecdotal.
The main factor that has led to the withdrawal of microspinal catheters in the USA and that has contributed to the technique's unpopularity in the UK is the association between their use and the development of subsequent cauda equina syndrome. This is thought to be caused by a combination of factors, including the use of lidocaine (more common in the USA), the known neurotoxic effect of high concentrations of lidocaine on neural tissue experimentally (more so than bupivacaine), the pooling of drug around the sensitive nerves of the cauda equina associated with very fine catheters placed caudally, and the use of excessive doses of drug in an attempt to extend an inadequately extensive block (resulting in more pooling around the nerves).
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