 The investigators hypothesis is adding dexmedetomidine to pribulbal blocking LA will prolong anaesthesia duration and improve globe akinesia and decrease intraoperative supplementation of LA.There are Many studies had described the effects of dexmedetomidine on peripheral nerve blocks, spinal but up to date no knowledge is available on the impact of dexmedetomidine adjuvant to local anaesthetic in ophthalmic surgery 