ISO 14155 does not make this mandatory, nor up to now any medical device regulatory agency in Europe. The situation is slightly different in other parts of the world although you may find MedDRA not always practical to use for medical device AE coding. The choice is up to you, on the one hand it may introduce some consistency in your coding and it may be easier to write, some companies have taken MedDRA and adapted it to their typical device/treatment issues. But nothing is mentioned in the ISO 14155 about using MedDRA. There is a new ISO that was just published or is about to be published on adverse event coding ISO 19218, although primarily focused on post market, it gives a start for coding.