An adapter for coupling a plug of an implantable neurostimulator lead extension to a connector port of a neurostimulator device includes an elongate body, a connector including at least four contacts coupled to a first end of the elongate body, and a housing including at least two ports coupled to a second end of the elongate body. The adapter connector contacts provide for electrical coupling within the device port, and the adapter housing ports are arranged to receive side-by-side connector terminals of the lead extension plug. Housing contacts within each port provide for electrical coupling with corresponding contacts of the connector terminals of the lead extension plug and are coupled to corresponding contacts of the adapter connector via conductors extending within the elongate body between the housing and the connector.