CPT® defines certain operative procedures as including the use of an operating microscope, code 69990.

69990 is an add on code, indicated by the plus sign in front of it in the CPT® book.  It is billed as a second procedure, without modifier 51.  At the start of the section about this code there is a list of CPT® codes which include already the operating micrsocope. If the service being performed is on that list, do not report 69990 in addition to it. 

In 2009, 69990 had total RVUs of 5.81.

It won't surpise anyone to know that has a different set of rules regarding 69990. Below, are citations from the Claims Processing Manual and the NCCI edits.  Source documentation is listed under citations.

CMS only allows payment of the operating microscope with these codes:

20.4.5 - Allowable Adjustments
(Rev. 1, 10-01-03)
B3-15055
Effective January 1, 2000, the replacement code (CPT® 69990) for modifier -20 -
microsurgical techniques requiring the use of operating microscopes may be paid
separately only when submitted with CPT® codes:
61304 through 61546 
61550 through 61711
62010 through 62100
63081 through 63308

63704 through 63710 
64831
64834 through 64836
64840 through 64858
64861 through 64871
64885 through 64891
64905 through 64907.
 

Here's what they say in the NCCI manual, Chapter 1 about 69990:

1. Three or more HCPCS/CPT® codes may be reported on the same date of service. Although the column two code is misused if reported as a service associated with the column one code, the column two code may be appropriately reported with a third HCPCS/CPT® code reported on the same date of service. For example, CMS limits separate payment for use of the operating microscope for microsurgical techniques (CPT® code 69990) to a group of procedures listed in the online Claims Processing Manual (Chapter 12, Section 20.4.5 (Allowable Adjustments)). The NCCI has edits with column one codes of surgical procedures not listed in this section of the manual and column two CPT® code of 69990. Some of these edits allow use of NCCI-associated modifiers because the two services listed in the edit may be performed at the same patient encounter as a third procedure for which CPT® code 69990 is separately reportable.