  Weak lensing effects are known to introduce non-linear couplings in CMB
temperature maps. In inflationary scenario, the primary CMB anisotropies are
expected to form a 2D Gaussian map, for which, the probability distribution
function of the ellipticity defined from the local temperature curvature matrix
has a very specific shape. I show that lenses alter significantly the shape of
this PDF, inducing an excess of elongated structures. The precise functional
form is computed for both the field points and the temperature extrema.
  These analytical results are confirmed by numerical experiments on 10x10
square degree maps. These numerical results allow to investigate the effects of
smoothing and to estimate the cosmic variance. For the best resolution and sky
coverage of the Planck mission the signal to noise ratio for the statistical
indicators presented here is about 3 to 6 depending on the cosmological models.
A marginal detection should therefore be possible.
