  One loop corrections to the neutrino mass matrix within the MSSM with
Bilinear R Parity Violation are calculated, paying attention to the approach in
which an effective $3\times 3$ neutrino mass matrix is used. The full mass
matrix is block diagonalized, it is found that second and third order terms can
be numerically important, and this is analytically understood. Top-stop loops
do not contribute to the effective $3\times 3$ at first order, nevertheless
they contribute at third. An improved $3\times 3$ approach that include these
effects is proposed.
