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It is 1 or 2 words? I find data to support both. The context in which it is used is "brain stem infarction."
If Stedman is a reference you trust and go to, did you know you can search it on-line?
http://www.drugs.com/medical_dictionary.html#a2z
Maybe that will be a bit quicker, until you get the hard copy book.
Good luck.