  The flux of ISM neutral atoms surrounding stars and their environment affects
the motion of dust particles in debris disks, causing a significant dynamical
evolution. Large values of eccentricity and inclination can be excited and
strong correlations settle in among the orbital angles. This dynamical
behaviour, in particular for bound dust grains, can potentially cause
significant asymmetries in dusty disks around solar type stars which might be
detected by observations. However, the amount of orbital changes due to this
non--gravitational perturbation is strongly limited by the collisional lifetime
of dust particles. We show that for large values of the disk's optical depth
the influence of ISM flow on the disk shape is almost negligible because the
grains are collisionally destroyed before they can accumulate enough orbital
changes due to the ISM perturbations. On the other hand, for values smaller
than $10^{-3}$, peculiar asymmetric patterns appear in the density profile of
the disk when we consider 1-10 mum grains, just above the blow-out threshold.
The extent and relevance of these asymmetries grow for lower values of the
optical depth. An additional sink mechanism, which may prevent the formation of
large clumps and warping in the disks is related to the fast inward migration
due to the drag component of the forces. When a significant eccentricity is
pumped up by the ISM perturbations, the drag forces (Poynting-Robertson and in
particular ISM drag) drive the disk particles on fast migrating tracks leading
them into the star on a short timescale. It is then expected that disks with
small optical depth expand inside the parent body ring all the way towards the
star while disks with large optical depth would not significantly extend
inside.
