  We describe a diagrammatic technique for non-Hermitian fermionic systems that
is applicable in the steady state, and which allows addressing correlations
effects by systematic expansion. Applying this method to exceptional points or
rings, we find that nodal objects in non-Hermitian systems are generically
displaced in momentum-space due to interactions. This in turn can be connected
to the fact that exceptional points invariably break a class of orthonormal
symmetries that are generally present for nodal points in Hermitian systems,
and which protect the integrity of the node at low energy scales.
