A group of 90 children aged 10-16 years scheduled for vitreoretinal surgery who received retrobulbar block were randomly assigned to 1 of 3 groups : group L ( retrobulbar ropivacaine ) , group LD1 ( ropivacaine plus 0.5 g.kg-1 dexmedetomidine ) , or group LD2 ( ropivacaine plus 1 g.kg-1 dexmedetomidine ) .