  In media with strong spatial dispersion the electric displacement vector and
the electric field are typically linked by a partial differential equation in
the bulk region. The objective of this work is to highlight that in the
vicinity of an interface the relation between the macroscopic fields cannot be
univocally determined from the bulk response of the involved materials, but
requires instead the knowledge of internal degrees of freedom of the materials.
We derive such relation for the particular case of "wire media", and describe a
numerical formalism that enables characterizing the electromagnetic response of
arbitrarily shaped spatially dispersive bodies formed by arrays of crossed
wires. The possibility of concentrating the electromagnetic field in a narrow
spot by tapering a metamaterial waveguide is discussed.
