Extensions of a (properly infinite) von Neumann algebra can be described by Q-systems, introduced by Longo. If the extension is ""small"" (it has finite Jones index), the theory of Q-systems with its application to Quantum Field Theory is worked out by Longo and Rehren. We present a way to suitably extend some results to the case of infinite Jones index. In particular we show applications to extensions and defects in Quantum Field Theory. Joint work with Luca Giorgetti.