In this prospective pilot study, patients who will fulfill the inclusion criteria will be recruited at the time of admission in HDU by the investigators. The ONDS will be measured before the commencement of Magnesium sulphate infusion, 1hr after finishing the loading dose, 6 and 24 hrs after infusion started. Patients will be in supine position with the head of the bed less than 20 degrees. High frequency (7.5 MHz) linear probe will be used; it will be placed gently over both closed eye after liberal application of sterile gel, the patient will be instructed to look forward to align the optic nerve directly opposite to the probe. The ONSD will be measured 3mm behind the optic disc in both the sagittal and transverse planes and an average will be calculated and recorded. In the absence of complete visualization of the optic disc, the largest viewed diameter will be taken as the maximal ONSD. Hemodynamic variables & oxygen saturation will be recorded at the same times.