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. 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.
1. Klein R, Klein BEK, Moss SE, et al. The relation of retinal vessel caliber to the incidence and progression of diabetic retinopathy, XIX: the Wisconsin Epidemiologic Study of Diabetic Retinopathy. Arch Ophthalmol. 2004; 122(1):76-83.