Check out this mouse by Swiftpoint. Firstly it’s so small you only control it with a thumb and finger. Secondly it is made to be able to slide over the keyboard of notebooks, therefore increasing the space available for mouse movement.

Simtrix Swiftpoint Slider Mouse

The Swiftpoint Slider Mouse stays attached to your thumb while you use the keyboard and it allows for the quick switching between keyboard and mouse input, therefore reducing the chances of muscle strain and carpal tunnel syndrome.

Video demo of the Swiftpoint mouse:

via Techpin

Simtrix Swiftpoint Slider Mouse Product Page