In "The Westing Game," what is Chris diagnosed with?

Although Chris's disability is never given a specific name, Denton Deere offers "pyramidal tract involvement" as a general, informal diagnosis in Chapter 5.  The pyramidal tract is the part of the nervous system that transfers motor commands.  As Chris is able, with Deere's help later in the book, to gain access to treatment which does improve his symptoms, there is a good probability that Deere is correct in his evaluation.

Chris's disease manifests itself in spasms and a general inability to control muscle movements consistently or efficiently.  When asked directly by Sydelle Pulaski in Chapter 13 if he can stand on his legs at all, Chris responds that he cannot.  Chris is confined to a wheelchair and needs virtually constant care.  Although his ability to speak clearly is compromised, his intelligence is definitely not.  Chris is an avid birdwatcher, and is quite knowledgeable in the science of ornithology, he observes and analyzes clues in "The Westing Game" better than most, and has a wonderfully positive attitude.