As a US citizen (if 21 years of age or older) you may petition for immigrant visas your "Immediate Relative" parents either initially through filings stateside with USCIS by means of the applicable Service Center or by consular processing through the US Embassy auspices in Nicaragua. You may wish to contact a consular information officer in Managua regarding their recommended procedures and processing times for same.
Answered on Aug 10th, 2012 at 3:51 PM