Intracranial Pressure Change

The optic nerve sheath is a continuation of the dura mater, extending along the subarachnoid space surrounding the optic nerve. Consequently, changes in intracranial pressure cause an augmentation or contraction in the diameter of the optic nerve sheath, which is coreleted with intracranial pressure. Participants undergoing lower extremity operations (ankle, distal tibia and fibula surgeries) using a tourniquet under spinal anesthesia will be investigated. Spinal anesthesia will be performed with a 25 G Whitacre, 25 G Quincke, or 27 G Quincke needle, depending on the study group. Optic nerve sheath diameter (ONSD) measurements will be performed before spinal anesthesia; and 5 minutes, 15 minutes,,24 hours after spinal block, respectively.The 4th measurement will be performed after tourniquet is opened. Measurements will be done with 15 MHz linear ultrasound (US) probe in B mode, 3 mm behind the posterior globe in the transverse plane from both eyes, on upper eyelid. Values will be recorded numerically. The impact of spinal anesthesia administered using various needle designs (Quincke and Whitacre) and sizes (25G-27G) on intracranial pressure will be investigated.