The sympathetic pathways to the heart are extremely variable in their   topography, and the diversity of arrangements encountered accounts for   the morphological contradictions in the literature. So numerous are  the  possible variations that the outcome of a sympathectomy is   unpredictable. Where denervation is incomplete, collateral sprouting and   regeneration of nerves could even lead to hyperstimulation via the   sympathetic pathways.
http://onlinelibrary.wiley.com/doi/10.1002/aja.1001240203/abstract