A theory is proposed for the treatment of the effects of distortion in electron-impact double ionization of atoms at large momentum transfer. The eikonal wave impulse approximation and the semiclassical post-collision interaction model are used to analyse the momentum distortion in symmetric (e,3-1e) reactions at intermediate energies. The effect of a strong interplay between electron-electron spatial correlation and momentum distortion is determined. The results of the analysis provide a guide for symmetric (e,3-1e) experiments which are being carried out or are underway.