An adaptor for connecting a syringe to an aspiration and/or irrigation device, typically a medical device such as a catheter, cannula or needle, is described herein. The adaptor is comprised of two components: a deformable elastomeric expansion sleeve and a relatively rigid insertion body. The expansion sleeve is essentially an elastomeric plug that forms a tight seal about the inner and outer surfaces of the syringe tip when deformed by the relatively rigid insertion body. The adaptor assembly described herein allows for full visualization of the syringe contents when in use and is compatible with multiple syringe components. The adaptor assembly is also relatively easy to manufacture and maintain. It provides a fluid-seal that adequately tolerates the mechanical loads such as torsion, mechanical moment, shear, and longitudinal loads expected during normal use.