Michigan Subscriber Answer: If the surgeon performs a procedure that spans several interspaces, you should select the "region" in which the surgeon performs the majority of the work.
For example, if the surgeon performs diskectomy at C7/T1 and T1/T2, treat the C7/T1 interspaces as "thoracic" and report 63077 (Diskectomy, anterior, with decompression of spinal cord and/or nerve root[s], including osteophytectomy; thoracic, single interspace) and +63078 (... thoracic, each additional interspace [list separately in addition to code for primary procedure]).
Some coding consultants recommend that if the surgeon operates at C6/C7 and C7/T1, treat the C7/T1 interspace as "cervical" and report 63075 (... cervical, single interspace) and +63076 (... cervical, each additional interspace [list separately in addition to code for primary procedure]).
The question is more difficult if only a single "inter-region" interspace (such as C7/T1 or T12/L1) is involved. In such a case, most experts suggest coding from the "top to the bottom."
That is, you should treat the C7/T1 interspace as cervical and the T12/L1 interspace as lumbar. Therefore, if the surgeon performs a diskectomy at C7/T1 only, for example, report 63075.
Other coding consultants advise reporting the code with the higher relative value units (RVUs), and not "top to bottom." Ask your payer which method it prefers that you use.