 Morphine will be used as the primary analgesic agent this is within the standard of care. Samples will be collected at specified time intervals for 5 days or until the CSF drain is removed whichever comes first. Samples collected will be analysed for morphine, its 3- and 6- glucuronide metabolites, inflammatory cytokines, markers of CNS injury and anatomical integrity of the BBB. Area under the concentration vs. time curve will be calculated and the effect on morphine metabolism and penetration across the BBB will be determined using a repeated measures analysis of variance technique as used in our previous study. 