  Here we provide a uniqueness result for viscosity solutions to sub-Riemannian
mean curvature flow. In this setting the uniqueness cannot be deduced via
comparison principle, which is known only for graphs and for radially symmetric
surfaces, due to the presence of characteristic points. Here we prove that any
viscosity solution is limit of a sequence of solutions of Riemannian flow, and
obtain as a consequence uniqueness and comparison principle already known in
the approximating riemannian setting. The results are provided in the settings
of both 3-dimensional rototranslation group SE(2) and Carnot groups of step 2,
which are particularly important due to their relation to surface completion
problem of model of the visual cortex.
