Spectroscopic and Diffusion Weighted Analysis of the Effects of Dexamethasone on High Altitude Cerebral Oedema (HACE)

The exact mechanisms by which AMS develops remains poorly understood. Interestingly, brain and spinal cord swelling due to low oxygen levels can also occur in the period following surgery to treat thoracic and abdominal aortic aneurysms, dangerous swellings of the major blood vessel in the body. Therefore, if we find a therapeutic benefit of receiving a dose of Dexamethasone in a controlled, reversible setting of hypoxia, it is possible that this could be useful in the treatment of post-operative hypoxia as well. Work with MRI imaging has demonstrated reduced measures of water movement in patients suffering from cerebral or spinal ischaemia, due to swelling. Specific water channels in brain cells (astrocytes) are involved in the movement of water, and Dexamethasone has been shown to reduce expression of these channels in animal models. Dexamethasone already plays a role in lowering pressure in the brain in the setting of brain tumours. Although high doses are typically used in this setting, there is evidence that lower doses may be equally effective, especially in patients with less severe swelling. Subjects will be consented and randomised in the weeks before the actual study. Before entering the tent, the following data will be collected: * Lake Louise Acute Mountain Sickness self-assessment questionnaire * Pulse oximetry * Non-invasive cardiac monitoring (ECG) * End tidal CO2 * Venous blood collection (Full blood count, renal function, S100 and GFAP) * Finger-prick blood collection (Purines) * Magnetic Resonance Angiography Non-invasive monitoring will continue every 2 hours at the start of the study and around the time of administration of the study drug. They will continue at less frequent intervals throughout the study period. This includes ECG trace and an AMS self-assessment questionnaire. Venous sampling will be performed on 5 occasions throughout the study. Finger prick sampling will be done at the same time points Each subject will have 5 MRI scans during the course of the study. Subjects will be begin hypoxication 1 hour after entering the tent. They will be returned to normal oxygen levels after 24 hours.