  Let $\mathcal{\scriptstyle{O}}_K$ be the ring of integers of an imaginary
quadratic number field $K$. In this paper we give a new description of the
maximal discrete extension of the group $SL_2(\mathcal{\scriptstyle{O}}_K)$
inside $SL_2(\mathbb{C})$, which uses generalized Atkin-Lehner involutions.
Moreover we find a natural characterization of this group in $SO(1,3)$.
