  In two dimensions the non-interacting density of states displays a Van Hove
singularity (VHS) which introduces an intrinsic electron-hole asymmetry, absent
in three dimensions. We show that due to this VHS the strong-coupling analysis
of tunneling spectra in high-$T_c$ superconductors must be reconsidered. Based
on a microscopic model which reproduces the experimental data with great
accuracy, we elucidate the peculiar role played by the VHS in shaping the
tunneling spectra, and show that more conventional analyses of strong-coupling
effects can lead to severe errors.
