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Spinal anaesthesia (first part)

Spinal or subarachnoid anaesthesia consists of the administration of a local anaesthetic, alone or in combination with other drugs, in the subarachnoid space (i.e into the CSF), where neural response occurs. The subarachnoid administration of an opioid alone is called spinal analgesia or intrathecal opioid analgesia.

The first spinal anaesthesia in humans was performed by August Bier in Berlin in 1899, using cocaine. In the veterinary field, Cuille and Sendrail were the first ones (1901) to perform spinal anaesthesia in the dog, also using cocaine, and to make a description of the procedures. The technique did not become very popular and was abandoned soon. So far, the necessary technique to carry out spinal anaesthesia in small animals has not been described yet, even though the access to the subarachnoid space is very well described, because it is used for diagnostic purposes (myelogram, cerebrospinal fluid sampling). Only ISVRA, in the last few years, has made some suggestions in performing spinal anaesthesia which follow the existing guidelines in human medicine. In veterinary medicine the subarachnoid injection of anaesthetic and/or analgesic drugs is suggested when CSF is detected in the hub of the needle as result of a failed epidural approach (Lumb and Jones’ Veterinary anaesthesia; Hall, Clarke, Trim: Manual of small animals anaesthesia and analgesia; Kona-Boun et al., Vet Anaesth Analg 2003). Consequently, clinical studies on drugs, doses, and volumes to be used have not been performed, and suggested doses do not take in any consideration mechanisms which rule the effects of intrathecally administered drugs (Lumb & Jones’ Veterinary anaesthesia; Torske and Dyson, Vet Cl N Am 2000; Hansen, JVECC 2001). A little news of some interest on veterinary spinal anaesthesia can be found in a retrospective study which analyses 182 clinical cases of epidural catheterization in the dog and the cat: this paper reports some cases of incidental subarachnoid catheterization (Hansen, JVECC 2001).

A diagnostic lumbar puncture (myelography, cerebrospinal fluid sampling) is usually performed by reaching the ventral subarachnoid space. This technique, even when applied at T13 – L1 level, has not been associated with clinically relevant permanent neural damage (McCartney, Vet Rec 1997). However, the technique has been related to a high incidence of spinal cord puncture, resulting in haemorrhage, gliosis and axonal degeneration (Kishimoto M et al., J Vet Med Sci 2004). For this reason, it is desirable to perform spinal anaesthesia in the dorsal subarachnoid space; by the way, this allows a metameric blockade, which means performing the local anaesthetic injection at different levels, blocking only the nerves supplying the surgical site.

The opioid to be used with the local anaesthetic for spinal anaesthesia is another source of controversy in veterinary medicine. Morphine, which is hydrophilic, is usually preferred to lipophilic opioids, but also this can be explained with poor knowledge of the technique.

Infact, lipophilic opioids should be the preferential drugs because, unlike morphine, they have a quick onset, allowing a reduction of the local anaesthetic doses, and finally they do not generate risks of rostral migration and late respiratory depression. Different principles are enforceable if only an opioid is administered (i.e. intrathecal opioid analgesia): in this case, a hydrophilic opioid like morphine can have some advantages which should be carefully evaluated and compared with possible disadvantages and complications.

[to be continued ...]

 

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