IEEE 802.15.6 is a wireless body area network (WBAN) standard proposed to facilitate the exponentially growing interest in the field of health monitoring. This standard is flexible and outlines multiple basic medium-access control (MAC) protocols that are contention based and collision free to meet the WBAN quality-of-service (QoS) challenges. Typically, current research trends in WBAN MAC focus on designing a hybrid MAC that is a combination of basic MAC protocols. In this paper, we provide a first detailed survey of existing hybrid MAC protocols based on the IEEE 802.15.6, which would be useful for the related research community. First, this paper lists the design challenges of a WBAN MAC. Second, it highlights the significance of hybrid MAC protocols in meeting the design challenges while comparing them to standard MAC protocols. Third, a critical and thorough comparison of existing hybrid MAC protocols is presented in terms of network QoS and WBAN specific parameters. Finally, we identify key open research areas that are often neglected in hybrid MAC design and further propose some possible directions for future research.