To limit the computation time and the computer resource, 2D numerical approaches such as 2D Parabolic Wave Equation (2D-PWE) associated with 1D Multiple Phase Screens (1D-MPS) are classically used to estimate ionospheric scintillation effects on radio wave propagation.However, in the ionosphere, the turbulent fluctuations of the electron density responsible for the scintillation effects are clearly a three-dimensional process. This paper quantitatively assesses the errors potentially induced by the 3D to 2D dimensional reduction to predict ionospheric effects in terms of log-amplitude and phase variances. To that purpose, the ionospheric electron density fluctuations are described by an anisotropic turbulent spectrum with 3 axes of anisotropy. On the one hand, considering 4 typical configurations (2 equatorial and 2 polar radio links), scintillation effects are evaluated numerically, using 3D and 2D PWE-MPS numerical techniques. Some consequences of the dimensional reduction are then qualitatively discussed. On the other hand, an analytical framework that solves the 3D and 2D propagation equations in the Line-Of-Sight (LOS) coordinate system is proposed. Under weak scattering assumption, it allows assessing analytically the consequences of the dimensional reduction whatever the geometry of the transionospheric radio link, and, finally, allows 2D numerical schemes to be used advisedly for the prediction of ionospheric scintillation effects.