We present self-consistent calculations of the properties of ruthenium dioxide in its cubic structures. The calculations were performed within local density (LDA) and generalized gradient (GGA) approximations using the augmented plane wave plus local orbitals (APW + lo) method. The structural and electronic properties of both structures have been determined in details. A good agreement is found with other previous calculations and experimental data. We have also extended our study to the optical properties: The refractive index, extinction coefficient, reflectivity and both imaginary and real parts of dielectric function are well described. When exposed to a pressure of 13 GPa, the material distorts from the fluorite CaF2 type Fm3m structure to the modified-fluorite Pa (3) over bar structure. This distortion affects the filling of the Ru-d orbitals in both structures and consequently the band structure energy gap that is necessary for many applications in industry. Our purpose is also to emphasis on the impact of the coordination polyhedron distortion resulting from the displacement of oxygen atoms (Jahn Teller effect) on the electron energies loss spectra (EELS) since the ability to 'fingerprint' different forms of the same element like the Ru-d orbitals is a strong advantage of EELS.