This work investigates the dynamics of a single bubble rising in a polymer soltition confined in a vertical Hele-Shaw cell. Different mixtures of PEO (polyethylene oxide) in water have been used, which exhibit a Newtonian viscosity plateau, followed by a shear-thinning behavior above a shear rate of 0.1 to 1, typically. Depending on the bubble volume, different regimes are reported. When increasing the bubble volume, a transition is observed from a small, round bubble to a cusped bubble, which exhibits a singularity at the rear. Below a critical volume, the bubble rises vertically and does not exhibit any apparent shape deformation. However, above a critical volume, the cusped bubble develops a peculiar instability. Its front flattens at a given angle respect to the horizontal, leading to either a deflection in its trajectory, or its fragmentation. We characterize these two dynamics and interpret the fragmentation process in terms of a viscous fingering recalling the Saffman Taylor instability. Interestingly, the finger growth is directly controlled by the bubble size. (C) 2017 Elsevier B.V. All rights reserved.