AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020

Subtitles
References

Style for subtitles follows that for titles (see 3.9, Titles) for spelling, abbreviations, numbers, capitalization, and use of italics, except that for journal articles the subtitle begins with a lowercase letter. A colon and space separate title and subtitle, even if a period was used in the original. Do not change an em dash to a colon. When the title ends with a question mark, the question mark is retained in the reference and replaces the colon as the delimiter. If the subtitle is numbered, as is common when articles in a series have the same title but different—numbered—subtitles, use a comma after the title, followed by a roman numeral immediately preceding the colon. If the title or subtitle ends with a closing quotation mark, the ending period should appear after the quotation mark because the end punctuation is not part of the original content.

1.Musch DC, Janz NK, Leinberger RL, Niziol LM, Gillespie BW. Discussing driving concerns with older patients, II: vision care providers’ approaches to assessment. JAMA Ophthalmol. 2013;131(2):213-218. doi:10.1001/2013.jamaophthalmol.106

2.Champigneulle B, Merceron S, Lemiale V. What is the outcome of cancer patients admitted to the ICU after cardiac arrest? results from a multicenter study. Resuscitation. 2015;92(7):38-44. doi:10.1016/j.resuscitation.2015.04.011

3.Jackson LG. Prenatal diagnosis: Down syndrome or more? Hum Mutat. 2017;38(7):749. doi:10.1002/humu.23242

4.Ahn HS, Kim HJ, Welch HG. Korea’s thyroid-cancer “epidemic”. N Engl J Med. 2014;371(19):1765-1767. doi:10.1056/NEJMp1409841

Note: Capitalization is retained if the first word of the subtitle is a proper noun, as shown in example 3 above.