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‘Dilatation’ and ‘dilation’: trends in use on both sides of the Atlantic
  1. Omar A Mahroo1,2,
  2. Zaid Shalchi3,
  3. Christopher J Hammond1
  1. 1Department of Ophthalmology, King's College London, St Thomas’ Hospital Campus, London, UK
  2. 2Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge, UK
  3. 3Department of Ophthalmology, Queen Mary's Hospital, Sidcup, Kent, UK
  1. Correspondence to Dr Omar Mahroo, Department of Ophthalmology, King's College London, St Thomas’ Hospital Campus, London SE1 7EH, UK; oarm2{at}

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The synonyms ‘dilation’ and ‘dilatation’ are used frequently in ophthalmology and other disciplines. The first recorded examples of early forms of the two words in English date from the end of the 14th century.1 ‘Dilatation’ is etymologically more sound,2 preserving the Latin dilatatio (corresponding to the verb dilatare, with the stem dilatat, from which the word ‘dilate’ also derives), but ‘dilation’ could be justified by arguing that –tion is a live suffix in English and so can be added to the verb ‘dilate’ to give ‘dilation’.2

We examined trends in usage in medical and wider English-language literature using three open access digital databases: the PubMed database ( provides data for citations per year; the Google Scholar search engine ( searches more widely, including non-medical literature and patents; and the Google Books Ngram Viewer ( searches corpora of several million digitised books, taken from over 40 university libraries, stretching …

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  • Contributors Drafting letter (OAM); critical revisions and final approval (all authors).

  • Competing interests None.

  • Provenance and peer review Not commissioned; internally peer reviewed.