  Weak gravitational lensing changes the angular power spectra of the cosmic
microwave background (CMB) temperature and polarization in a characteristic way
containing valuable information for cosmological parameter estimation and weak
lensing reconstructions. So far, analytical expressions for the lensed CMB
power spectra assume the probability density function (PDF) of the lensing
excursion angle to be Gaussian. However, coherent light deflection by nonlinear
structures at low redshifts causes deviations from a pure Gaussian PDF. Working
in the flat-sky limit we develop a method for computing the lensed CMB power
spectra which takes these non-Gaussian features into account. Our method does
not assume any specific PDF but uses instead an expansion of the characteristic
function of the lensing excursion angle into its moments. Measuring these in
the CMB lensing deflection field obtained from the Millennium Simulation we
show that the change in the lensed power spectra is only at the 0.1% - 0.4%
level on very small scales (below 4 arcmin) and demonstrate that the assumption
of a Gaussian lensing excursion angle PDF is well applicable.
