The most general model of elliptical birefringence in an optical fiber is extended to describe a transient Brillouin interaction including both gain and loss. The effects of elliptical birefringence cause a Brillouin spectral shape distortion, which is detrimental for fiber sensing techniques. The model investigates the effects of birefringence and the corresponding evolution of spectral distortion effects along the fiber, and also investigates regimes where this distortion is minimal.